TRIUMF: Canada's national laboratory for particle and nuclear physics

Proceedings of the ISAC Workshop, Lake Louise, Alberta , February 17, 18 and 21, 1994 D’Auria, J. M. 1994

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TRIUMFPROCEEDINGSOF THE ISAC WORKSHOPLAKE LOUISE, ALBERTA FEBRUARY 17, 18 AND 21, 1994CANADA’S NATIONAL MESON FACILITY OPERATED AS A JOINT VENTURE BY:UNIVERSITY OF ALBERTA SIMON FRASER UNIVERSITY  UNIVERSITY OF VICTORIA UNIVERSITY OF BRITISH COLUMBIAUNDER A CONTRIBUTION FROM THE NATIONAL RESEARCH COUNCIL OF CANADAASSOCIATE MEMBERS:UNIVERSITY OF MANITOBA  UNIVERSITE DE MONTREAL UNIVERSITY OF TORONTO  UNIVERSITY OF REGINA. T R I-9 4 -1TRI-94-1PROCEEDINGSOF THEISAC WORKSHOPLAKE LOUISE, ALBERTA FEBRUARY 17, 18 AND 21, 1994Editor: J.M. D’AuriaPostal Address: TRIUMFPublications Office 4004 Wesbrook Mall Vancouver, B.C. Canada V6T 2A3PREFACEThe Lake Louise ISAC Workshop (LLIW) was held on February 17, 18 and 21, 1994 at the Chateau Lake Louise, in Lake Louise, Alberta. The organizing committee consisted of J. D’Auria (SFU), K.P. Jackson (TRIUMF), and P. Stewart (TRIUMF). Funding for the workshop was provided entirely by TRIUMF.Approximately 30 scientists from various Universities and National Laboratories in Canada and the United States participated in the meeting which consisted of 26 invited talks and several contributions (over three days). The LLIW was held sequentially with the annual Western Regional Nuclear and Particle Physics Conference (February 18-20) and the Lake Louise Winter Institute (February 21-25). The overall objective of the workshop was to allow prospective users of the proposed ISAC (Isotope Separator and Accelerator) Exotic Beams Facility, proposed for installation at TRIUMF, to discuss the scientific opportunities with accelerated radioactive beams, to ensure that the specifications which are being used to design the facility are appropriate to perform forefront research, to allow scientists with experience using such facilities to share their knowledge, and to provide an open forum to discuss in detail the types of experimental facilities needed at ISAC.Overall, there was lively discussion following each talk and the group focussed on the questions related to the type of science that could be performed at ISAC, particularly in the energy regime from 1.5 to 10 MeV/u. An open discussion period on the specifications and layout of the proposed ISAC facility was of value to the designers of the facility and there was consensus approval for the general layout and concepts being pursued. It is clear that there is considerable support for a facility like ISAC to be based at a high intensity, intermediate energy proton facility which is ideal for producing a wide range of exotic radioisotopes and which can be further accelerated to desired energies. The method of the subsequent post acceleration was a topic of some debate.The organizers wish to thank all those who contributed to the planning, running and successful operation of the Workshop. Special thanks go to Maria Freeman who assisted with various aspects of the planning and to the staff of Chateau Lake Louise who contributed significantly to the success of the Workshop. The work of Jana Thomson and Bev Ward with the preparation of these proceedings is also gratefully acknowledged.J.M.D.vCONTENTSThe Role of ISAC in TRIUMF’s FutureJ.-M. Poutissou............................................................................................  1The Proposed TRIUMF-ISAC FacilityJ. D ’Auria and J. Beveridge....................................................................... 4Status of the Accelerator Design for Unstable Ion Beams at TRIUMFP.G. Bricault, H.R. Schneider and L. Root.........................  16Multiple Charged Ions for Radioactive BeamsM. Dombsky.................................................................................................. 3 3Symmetry-tests in Nuclear Beta-decay: Status and ProspectsJ. Deutsch..................................................................................................... 3 7Experiments with Radioactive Beams at Louvain-la-Neuve and Prospects at an ISAC FacilityW. Galster....................................................................................................  4 7Parity Violation in Beta-delayed Alpha EmissionG. Roy..........................................................................................................  62Spectroscopy of Exotic Nuclei with Resonant and Direct Reactions at 1.5-10 MeV/fx at ISACU. Giesen and K.P. Jackson....................................................................... 7 7Ion Traps at RNB Facilities: Mass Measurements and Future PossibilitiesG. Savard....................................................................................................  91Decays of Nuclei Far From StabilityE. Hagberg...................................................................................................  9 9Radioactive Beam Experiments Using the Fragment Mass AnalyzerC.N. Davids.................................................................................................. 108Isomers Near 100Sn and Other Experiments Along the Proton Drip LineW.B. Walters................................................................................................  H 3Study of Transuranium Nuclides with Radioactive Nuclear Beams From ISACW. Loveland.............................................................................................. H 5Coulomb Excitation Studies with Radioactive Nuclear Beams (not presented)R.F. Casten..........................................   12 2Beta-delayed Alpha Emission from Neutron-rich Light-mass NuclidesP.L. Reeder............................................................................................  1 2 5Physics with Radioactive Ion Beam FacilitiesS.M. Austin............................................................................................  1 3 4Nuclear Astrophysics with Radioactive Beams at Oak RidgeN.P.T. Bateman.......................................................................................  1 4 4A Storage Ring for Radioactive BeamsD. Moltz...............................................................  152Tilted Foil Polarization and Magnetic Moments in Mirror Nuclei (not presented)M. Hass........................................................................................  I5 0Detailed Description of ISACJ. Beveridge and J.M. D ’Auria..................................................................  162Fundamental Symmetry Tests with Trapped Neutral AtomsO. Hdusser................................................................................... 1 9 2viiMeasurements of Cross Sections of Interest to Primordial NucleosynthesisR.N. Boyd...........................................................   207Cross Section and S-Factor for the 13N(p,7 ) 140  ReactionJ.D. King.................................................................. ...................................  223Strategies for Nuclear Astrophysics Studies with Radioactive Ion BeamsJ. Gorres........................................................................................................  231Nuclear Genesis and Radioactive BeamsR.A. Malaney...............................................................................................  243Nuclear Astrophysics with Radioactive BeamsL. Buchmann.................................................................................................  244APPENDIXWorkshop Program..................................................................................................  265LLIW Planning Document.......................................................................................  268Status of the ISAC Post-Accelerator Design StudyH.R. Schneider, P. Bricault, L. Root......................................................... 276List of Participants.................................................................................................... 287viii1T he R ole o f ISAC in T R IU M F ’S FutureJ-M. PoutissouTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. Canada V6T 2A31. HISTORICAL PERSPECTIVEIt should not come as a surprise to this audience tha t an isotope accelerator (ISAC) would figure very high on the list of new facilities tha t TRIUMF would like to develop in the future.In 1985, a proposal was put forward to the Natural Sciences and Engineering Research Council (NSERC) for the funding of a radioactive beam facility a t TRIUMF.A special site visit committee was set up to review the proposal and advise the granting committee on Subatomic Physics on the merit of the case. I quote directly from their report:“JUSTIFICATIONThe committee considered that the research potential of an ISOL post-accelerator facility at TRIUMF was high enough to justify the preparation of a soundly based proposal in the near future. The committee recognized the large number of problems in various areas that could be investigated through the use of an accelerated radioactive beam. Although the astrophysics which would be investigated might not be as im portant as the work which has been done on accelerators in the past twenty years, the committee recognized tha t it would definitely be the most interesting nuclear physics contribution to astrophysics to be done in the near future. The committee agreed tha t another im portant justification for such a facility was the use of radioactive ion beams in areas such as solid state physics, production of radionuclides, deep im plantation and other applied work which showed potential for industrial applications.TRIUM F would be an excellent location for such a facility, not only because of the characteristics of the TRIUMF accelerator (high intensity, variable energy), but also because of the extensive technical expertise and massive infrastructure tha t characterize the laboratory. The facility would be and would remain unique so tha t international interest and usage would be high and its construction would provide both short and long-term economic benefits to Canada. At a cost of some $20 million it could well be a very worthwhile investment.”The NSERC grant selection committee could not take such a bold step into the future 10 years ago and did not fund the project but “encouraged” TRIUM F to go ahead with the test facility which we have nicknamed TISOL.TISOL (the test isol facility) was supposed to help TRIUM F users test some of their ideas and cement a core group of physicists behind such activities. I must say that, tha t mission has been accomplished very successfully under the strong will of Professor J. D ’Auria who has not deviated one iota from his original objective.In the meantime, very exciting physics results have been obtained with TISOL, witnessing the recent measurement of 12C (a,7) cross sections via the reverse process of N(/5,o:) decay chain which was one of the highlights of nuclear physics research in 1993 and has been reported in prestigious “scientific” journals, i.e. the Toronto Star and the New York Times.The requirement for developing a user community for such beams of isotopes has also been met with several new proposals being put forward which make use of the high fluxes of exotic nuclei available or soon to be available at TISOL, coupled with the development of neutral atom trapping techniques using laser technology.The original interest expressed in 1984 in pursuing the determ ination of cross sections of astrophysics has strengthened and accelerated beams of isotopes far from stability and are becoming the focus of attention at several facilities worldwide.Canada has gone through a period of procrastination in deciding the future of TRIUM F and mainly the possibility of providing a KAON factory for the world.This meeting is taking place at a crucial time and the non-KAON options for TRIUM F must be formulated in a very effective way, hence the purpose of this workshop which is to crystallize the science program to which an isotope accelerator should be able to  cater.*During the recent planning exercise commissioned by our funding agencies for the subatomic physics facilities, the Nuclear and Particle Physics Advisory Panel rec­ommended th a t an ISAC type facility be considered to  develop new opportunities at TRIUM F, reflecting the wish of a large segment of the Canadian community for such a scientific program. In the executive summary of its report, the panel said:“NPPAP recommends tha t a world-class radioactive beam facility (ISAC) be sited at TRIUM F to  serve the nuclear physics and nuclear astrophysics communities.NPPAP further recommends the expansion of TRIUM F’s infrastructure role for the support of major new initiatives in subatomic physics.”2. Why ISAC at TRIUMF?First and foremost, the science program of ISAC is very promising indeed and I hope tha t this workshop will demonstrate it emphatically. I shall leave it to the specialists to  develop their rationale for ISAC.From TRIUM F’s point of view, it is clear that such a facility is an ideal match to our expertise and tha t we should be able to provide a world-class facility.• TRIUM F has the medium energy, high-intensity beams best suited to  the produc­tion of exotic isotopes via the thick target ISOL concept and TISOL has demon­strated it. The expertise developed in the exploitation of TISOL during the last few years is exactly what is needed to push ahead.• TRIUM F has established a very strong ion source group which is recognized world­wide for its success in high-intensity negative ion sources (polarized or not).• TRIUM F has the accelerator expertise required to  produce energetic beams of radioactive species.• TRIUM F has operated high-intensity beams for meson production for 20 years and has developed the expertise in remote handling necessary to  handle the very high level of radioactivity and contaminations which will be generated a t ISAC.*The week following this presentation showed that the workshop’s timing was ominous for KAON.3• Finally, TRIUMF has built up a user community eager to do the physics of ISAC.About 20 Canadian physicists are presently involved in the TISOL program orits extension, a number of U.S. collaborators have generated proposals for TISOLand have run experiments at the facility, and a large shift of interest is occurringamong groups of nuclear physicists towards the science of ISAC.In other related fields, the potential of an ISAC facility has also been expressed (condensed m atter, life sciences ...) and a broad based user community could easily be developed once the basic facility is available.3. Where would such a facility be built at TRIUMF?J.D ’Auria and J. Beveridge will describe our latest thoughts on this m atter, but let me just point out that the TRIUMF cyclotron is such a versatile machine that providing a specific extraction port for a high-intensity, variable energy beam is rather easy without even affecting other traditional users of TRIUM F’s beams.The current line of thinking would see a new beam extracted from the 2A port, the one tha t would have been used for KAON.4. When?You can see tha t as my talk progresses, I am becoming more brief and this reflects our present state of knowledge of TRIUM F’s long-term future. We are at this moment developing a full proposal with detailed cost estimates and time schedule. Depending upon the scope of the facility and the funding profile which is achievable, one could see some physics program initiated within the next five years. But, first, the physicists who want to use radioactive beams must tell us what they want to  see in terms of specifications of the complex... and this is why you are here.ISAC will play a critical role in TRIUM F’s future operation by providing a new unique facility catering to a large segment of the community. This will justify the con­tinued operation of one of the most reliable cyclotron in the interm ediate energy domain and consequently maintain several other activities in particle and nuclear physics, con­densed m atter physics, etc. By providing a new facility, the existing pool of talent which has accumulated at TRIUMF will be retained and also made available to support other activities, in particular the subatomic part of the program based at other accelerators. In fact, it is the only way to maintain such a valuable infrastructure for the benefit of all. This was the vision expressed in the NPPAP report cited above and is also the vision endorsed by the TRIUMF users in the absence of KAON.In conclusion, I think tha t TRIUMF has a lot to offer to your community and it is very timely tha t you participate in the exercise which will define your future tool. I hope tha t your advice will be taken into account in our final plans, so tha t you will be able to do your experiments at our site in the not too distant future.THE PROPOSED TRIUMF-ISAC FACILITYJohn M. D’Auria, Department of Chemistry, Simon Fraser University, Burnaby, B.C. V5A 1S6 Jack Beveridge, TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6 T 2A3I. INTRODUCTIONRadioactive beams can be produced using a wide variety of nuclear reactions including fission, spallation, fragmentation, fusion evaporation, deep inelastic collisions and nucleon transfer reactions. Many different facilities using a variety of techniques have been constructed and used over the last 25 years, each with its range of applicability and limits of production. The need to select and separate the radioactive species of interest from those produced and to deliver these as a usable beam in an experimental area has led to the two main approaches for radioactive beam production. The Projectile Fragmentation Method, which is characterized by a peripheral interaction of a projectile with a target nucleus which leaves the fragment with much of the initial momentum, can be utilized at heavy ion accelerators and is most useful for the production of high energy beams (>25  MeV/u). In the Isotope Separator On-Line (ISOL) technique, radioactive species are produced in a target material by a variety of nuclear reactions and then transferred continuously to a suitable ion source where they are ionized, accelerated and mass analyzed to form a radioactive beam. Many different production beams have been used in this method and each has its advantages and disadvantages. In general, however, it has been found that high energy protons (0.5-1 GeV) are the most effective projectiles for the production of a broad range of radionuclides due to a combination of production cross section, available intensities and low dE/dX in materials. The beams produced by this method are of low energy (60 keV) and must be further accelerated if higher energies are required.The options for post acceleration of ISOL produced beams include tandems, cyclotrons and linacs. A tandem seems an attractive option as it is well matched to the dc low energy beams available, has a low energy spread and easy energy variability. Unfortunately, only negative ions can be accelerated and these must be stripped in the high voltage terminal. The efficiencies of negative ion production and losses on stripping will limit the application of this accelerator option. A cyclotron presents the attractive feature of simultaneously accelerating and mass separating the beam. However, the final beam energy is proportional to the square of the charge to mass ratio of the accelerated ions and high charged state ions are required to realize this option. The combination of a linear accelerator (linac) preceded by a radiofrequency quadrupole (RFQ) bunching and preacceleration section seems the most attractive option for post acceleration of presently available ISOL beams. The RFQ has a large acceptance for the low velocity ISOL ions and produces a beam that can be efficiently matched to a following conventional or superconducting linac.The existing TRIUMF cyclotron is capable of delivering intense beams of protons (0-200 /xA) of variable energy (200-500 MeV) and, due to the acceleration of H" ions, numerous beams can be simultaneously and independently extracted. This capability gives TRIUMF an excellent opportunity for the production of a world class radioactive beam facility based on the ISOL production method. This opportunity was recognized ten years ago when the first proposal for5an accelerated radioactive beams facility was put forward at TRIUMF.In the intervening period, TRIUMF’s priorities have been directed toward the development of a proposal for a major kaon facility based on the cyclotron as an injector. The production of radioactive beams has been restricted to the successful construction and operation of a test on­line source and separator (TISOL) on BL4A. This facility, although limited in its capabilities due to remote handling, radiation damage and shielding considerations, presently supports a small but active user community.II. A SECOND GENERATION FACILITYA schematic diagram of a radioactive beam facility (ISAC) based on an ISOL produced beam followed by an RFQ and linac post accelerator is shown in Fig. 1. A list of desired specifications is given in Table 1. Protons from the TRIUMF cyclotron bombard a thick production target and the resulting radioactive nuclei are formed into a low energy beam by the ISOL method. Ion beams with masses up to 240 amu can be separated and delivered to a low energy experimental area for use in nuclear, atomic and applied physics experiments. Alternatively, ion beams with restricted charge to mass ratio (q/m) and fixed velocity can be injected into an RFQ and accelerated to an energy of 60 keV/u. At this energy a gas stripper can be used to increase the q/m of the ions if required. The beam is then injected into a series of linear accelerators and accelerated to the energy required by the physics program. For high energy beams the ions may have to be further stripped to reduce the total voltage required in the accelerating structures and thus maintain a reasonable facility cost.A possible realization of such a radioactive beam facility at TRIUMF is shown in Fig. 2. A variable energy (200-500 MeV) proton beam is extracted from the 2A port of the TRIUMF cyclotron and transported in a tunnel directed to the north of the existing facilities. This beam can be extracted independently of the present TRIUMF beams and can have intensities up to 100 ^Am. Bending magnets are situated along this beam line to deflect the protons to one of three possible production target locations. Two target stations are envisaged for initial operations and space made for a third to allow future expansion or upgrading. The radioactive ion beams produced in the target/source region are extracted at nominally 60 kV and delivered to the mass separators by a magnetic matching section. Two separators are provided; one of medium resolution (resolving power of 5,000) and a second of high resolution (resolving power 20,000). Mass separation is done in the horizontal plane and beams from these separators can be transported by an electrostatic system to either a low energy experimental area, situated at proton beam level, or to a system of accelerators located at ground level.The thrget/ion source facility presents the most difficult problems for such a high current ISOL system. High levels of radioactivity will be produced by the interactions of the protons with the production targets making effective handling techniques a requirement. The three target stations are housed in a long heavily shielded building as shown in Fig. 3. All highly activated components such as production targets, beam dumps, ion sources and initial focusing devices will be located in this building along with their primary radiation shields. Services to operate these components will be located in a more accessible service tunnel. A repair center consisting of a hot cell, a warm cell and decontamination and storage facilities is directly connected to thenorth end of the target building. The highly active target area components will be constructed as modules and placed in heavily shielded canyons as shown in Fig. 4. These modules will be accessed vertically by means of an overhead crane. Required repairs or modifications will be made by transferring a module from the target area to the repair facility where they can be dealt with in a controlled environment. This approach to the target facility design is based on successful experience in target handling at TRIUMF and at the other operating meson factories with provision for larger modules and much higher levels of mobile activity. Further details of the ISOL section of ISAC are provided in a later report at the Workshop.The initial accelerator stage of ISAC increases the beam energy to 1.5 MeV/u. It is composed of a 25 MHz RFQ, capable of accelerating ions with charge to mass ratios as low as 1/60 up to 60 KeV/u, followed by two superconducting linac structures similar to those presently employed at the ATLAS accelerator in Argonne. The low frequency RFQ must operate cw and is technically the most challenging part of the accelerator. The beam from the RFQ is passed through a matching section containing a gas stripper which increases the q/m of the ions to a minimum of 1/20 before injection into the linacs. The beams from this first acceleration stage are variable in energy and can be delivered to a medium energy experimental area or further stripped and directed into a second stage of acceleration. The second stage, a superconducting drift tube structure similar to the JAERI post tandem accelerator, accelerates the beam to a maximum energy of 10 MeV/u for delivery to a high energy experimental area. Further details of the ISAC accelerator can be found in a later presentation by P. Bricault.III. EXPERIMENTAL AREAS AND FACILITIESThe concepts of the experimental areas and facilities at ISAC are presented below, with reference to the experimental programs to be accommodated. The philosophy followed in setting up these experimental areas is that the sequence of construction will provide radioactive beams in the order of low-energy (60 keV), medium energy (E<1.5 MeV/u), and high energy (1.5 MeV/u <  E <  10 MeV/u) beams, respectively. Thus, the scientific program for the ISAC facility will be enabled at the earliest possible opportunity, depending upon the progress in constructing the heavy ion accelerator.Ill.a. Low-Energy Area (nominally 60 keV)The low-energy area shown in Fig. 5 contains experimental stations available for studies using the mass-separated ion beams directly from the mass separators. There are, in general, several types of experimental purposes that define the characters of the individual stations; nuclear astrophysics measurements, nuclear decay studies (including perturbed angular correlation studies), on-line nuclear orientation studies, atomic and hyperfine interaction laser spectroscopy, measurements using atomic and ionic traps, collection of longer-lived radionuclide samples (for off-site studies or potential commercial use), and various materials science (both surface and bulk) studies using implantation and ultra-high vacuum capabilities.Ill.b. Medium-Energy Area {E/A <1.5 MeV)This area is directly downstream from the 75 MHz linac section, and uses the undeviated beam7(the stripper for subsequent further acceleration in the high-energy linac section is removed, and the sector magnet for the first part of the matching section leading to the high-energy accelerator section is not energized). Singly-charged ions with 1.5 MeV/u up to mass 60 have a large magnetic rigidity, about 3.5 T-m. Thus, the use of a switching magnet would involve a very large magnet. It is proposed that one experimental station be placed in the undeviated line and that a switching magnet be used to deliver the beam to two alternative experimental stations. This medium-energy area will serve the astrophysics reaction studies and efforts in the use of penetrating radioactive beams for materials science.Ill.b.i Scattering chamber and general purpose stationA general purpose scattering chamber where detector arrays can be placed around a target to determine specific reaction charged-particle distributions is envisaged as one of the required experimental facilities for the 1.5 MeV/u beam. The large scattering chamber also invites other uses, such as collection of samples for off-line studies. A beam dump is provided after the chamber to allow the primary, radioactive beam to be deposited away from the chamber detection systems.Ill.b.ii Recoil spectrometerThe second station, located on the direct beam from the accelerator, is a recoil separator for high-separation-power studies of reactions of astrophysical interest (see Section 2.5). The reactions to be studied typically result in the product of interest being less than ten orders of magnitude as intense as the incident beam. Inverse kinematics are used to be able to improve the spectrometer efficiency. The location of the station in the beam line is chosen so that the beam can be bunched at the target to allow time-of-flight possibilities.The recoil spectrometer consists of a gas target (a foil target can also be inserted) surrounded by a gamma array, and followed by a Wien filter (tuned to the incident beam), an electrostatic deflector, and a magnetic analyzer with detector system. In this system, extreme selectivity is expected for (p,y) reaction products, for example. The detector system following the magnetic sector is selected for high selectivity of scattered particles according to energy loss, total energy, or even decay mode. The spectrometer system will be followed by a beam dump for the undeviated (unscattered incident) beam well shielded from the detector system after the sector magnet. This recoil spectrometer is expected to find heavy use in the early experimental program, after the construction of the 1.5 MeV/u accelerator is completed, and while the high- energy accelerator is under construction.Ill.b.iii Tilted foil and high-energy implantation stationThis station, placed in another switched beam, will consist of a scattering chamber into which a tilted foil array can be placed to polarize the beam. A magnet will be stationed at the exit of the scattering chamber so that NMR studies on aligned nuclei can be carried out. Another possible use for this station will be for implantation of energetic ions for bulk materials modification studies. This usage could have profound commercial possibilities.III.c. High-Energy Area (1.5 MeV <  El A <10 MeV)The high-energy beam from the 125 MHz linac section is transported to a series of two switching magnets which deflect the beam to a number of target stations. Among these stations is a scattering chamber (and high-energy implantation station), a magnetic analyzer with detector station, a general-purpose station allowing, for example, Coulomb excitation studies, and a station for a gamma detector array. It is intended that these stations, combined with the recoil analysis capabilities of a recoil spectrometer, will satisfy the experimental needs of the physics described in Sections 2.3 and 2.6. The placements of the components of this experimental area have been made using reasonable conceptual approaches; detailed ion optical designs have not been made but are not anticipated to result in grossly different placements. Each station after the switching magnets is equipped with a beam stop that is shielded from the detector systemsemployed.The recoil spectrometer, consisting of electrostatic and magnetic analysis sections plus an appropriate target station, is designed to provide spectroscopic capability for reaction studies including nucleon transfer reactions, sub-barrier reactions, and particle identification. This spectrometer has features similar to recoil spectrometers in use at Oak Ridge and Argonne National Laboratories.III.c.i Gamma array stationSpace is provided that should be adequate to set up a full, granular gamma-ray array. Details of the array cannot be provided at this time, and when developed, will reflect the state-of-the-art at other facilities. An example of the use of this array will be for high-spin studies of nuclei either far from stability or for "complete" spectroscopy of nuclei near stability (formed using radioactive beams as projectiles).III.c. ii General purpose stations (2)These stations provide for large-acceptance devices, such as solenoidal filters, as well as for Coulomb excitation studies of radioactive beams. Detector arrays, such as for neutrons, can also be accommodated.IV. TRIUMF OPPORTUNITIESAt TRIUMF, the presence of the TISOL radioactive beams facility and its scientific successes have increased Canadian awareness of this field. Despite the original intent of the TISOL ISOL system to be a prototypical test facility, approximately fifteen scientific experimental proposals have been considered by the TRIUMF EEC and a number of these proposed experiments have been completed.The existence of the ISAC proposal testifies to the recognition by TRIUMF of the opportunities present in an enhanced radioactive beams facility. Such an extension of TRIUMF capabilities is natural, and in keeping with TRIUMF’s experience and record of supporting multiple experiments concurrently. The users of an ISAC facility are expected to be drawn from all over9Canada and also from other nations, and the infrastructure TRIUMF has built up will serve this new community of users well.At present, there are about 280 members of the TRIUMF Radioactive Beams Users Group, and annual general meetings are held in tandem with the TRIUMF Annual General Meeting. Further, there are over 25 grant-eligible Canadian scientists who have indicated their support for a facility such as ISAC, and would be prepared either to mount an experimental program at ISAC, given appropriate funding, or, in the case of theoretical scientists, are very interested in the results of studies with radioactive beams. This list, available upon request, includes scientists both with the TRIUMF organization and external to TRIUMF.TRIUMF offers several capabilities not easily found elsewhere. It is only one of four machines (TRIUMF, PSI, LAMPF and ISIS) presently operating which are capable of producing high- energy protons with intensities in excess of 100 nA. Extraction of the beam at TRIUMF by stripping the H ion makes the delivery of multiple beams of variable energies and intensities a practical and proven possibility. Thus, TRIUMF is capable of delivering a high intensity, high quality CW (good beam emittance) proton beam to the ISAC facility without a large perturbation to its ongoing scientific program.TRIUMF has a proven record of stable and reliable operation, having operated for many years delivering proton beams >100 /xA to pion production targets. TRIUMF personnel have extensive experience, ranging from the cyclotron operation, the usee of high-power meson production targets, beam dumps, and handling techniques and designs for equipment in high radiation environments. This experience, plus the experience of the TRIUMF Safety Group in radiation monitoring, shielding, and waste disposal, will be invaluable in addressing the difficult design problems which will arise in the harsh environments of the ISOL production targets.TRIUMF staff, over the years, have developed valuable experience in accelerator and ion source technologies, and can be devoted to the challenges associated with the ISOL systems and post- acccelerator designs. Facilities are available for fabrication, assembly, and testing of many of the required components, as well as for engaging in prototypical activities.Finally, TRIUMF personnel have experience with the only thick-target ISOL facility in North America. The successful operation of this facility for scientific studies is a considerable advantage in looking toward the challenges in the design and construction of a major facility such as ISAC; gaining experience by operating an existing ISOL is invaluable. The existing TISOL facility can also be considered a test bed for ISAC prototype target and ion source designs; it is scheduled to be upgraded to 10  /xA production beam capability.V. SUMMARYThis facility represents a true second generation radioactive beam facility as proton beams 50 times more intense than those available at present facilities are envisaged and optimal accelerating structures at the forefront of technology are proposed. The estimated cost for such an installation is $62 million, exclusive of TRIUMF salaries, with a further $28 million required for buildings and services. The minimum construction time is estimated to be 5-6 years.Table 1 Specifications of ISACPRODUCTION SYSTEM Production Accelerator Projectile Energy Intensity Beam Size Time Structure Target System Form LengthPower Deposited Temperature RangeEXTRACTION SYSTEM Ion Sources TypesSeparator Systems TypeExtraction Energy Mass/Mass Resolving Power Transmission VacuumACCELERATOR SYSTEM First Stage TypeInput Energy Input q/A Emittance Transverse Longitudinal Output Energy Transmission Second Stage Type Input q/AOutput Energy Range Resolution A E/E Energy Increment Transmission Third Stage Type Input q/AOutput Energy Range Resolution A E/E Energy Increment Transmissionproton1 8 0 -5 0 0  MeV<; 100 fiA<  cm2 -  CWfoils, powders, molten metals 20 cm<  40 kW 25 - 2600 C°Surface, Laser, CUSP, ECR Plasma (FEBIAD, Bemas-Nier)Magnetic for mass analysis <; 60 kVA <240; <10,000  >95%^  2 x 10“7 torrRFQ LINAC, Room Temperature 2 keV/u1/60; (A >  60 for q >  1)<0 .25  7r mm-mr (normalized) 1007r keV-ns 60 keV/u >90%LINAC - SC 2: 1/200.2-1.5 MeV/u < 0.1%< .0 2  MeV/u>95% excluding stripper effectsLINAC-SCs l/3 -1 /41.5 MeV/u - 10 MeV/u <0.1%  desirable < .1  MeV/u>  95 % excluding stripper effects11Fig. 1LOW ENERGY NUCLEAR AND ATOMIC PHYSICSFig. 2 The Proposed ISAC Facility“ '0 -13Fig. 3 ISAC Target Facility — Plan ViewFig. 4 Target Station -  Plan View15Fig. 5 Low-Energy AreaStatus of the accelerator design for unstable ion beamsat TRIUMFP. G. Bricault, H. R. Schneider and L. Root TRIUMF, 4004 Wesbrook Mall Vancouver, B. C. CanadaTalk presented at the ISAC Workshop February 1994 Lake Louise, Alberta, CanadaAbstract:Preparation of a proposal for the installation at TRIUMF of an unstable ion beams facility (ISAC) is in progress. The physics program calls for acceleration of ions with charge to mass ratio greater than 1/60, to energies up to 1.5 MeV/nucleon, in one beam line, and to energies up to 10 MeV/nucleon in a second beam line. The current accelerator concept is a three stage linac with strippers between stages to increase the ions q/A. The first stage is composed of an RFQ, while the second and third stages are a series of independently driven superconducting quarter wave resonators, similar to those used for ATLAS positive ion injector at ANL, and the post tandem booster at JAERI. Current status of the design study for this accelerator configuration is given.Introduction:The construction of an heavy ion linear accelerator complex for acceleration of unstable ion beams has been under consideration since 1984, [TTS85]. These unstable species are produced during the interaction of a high energetic proton beam with a target nucleus. The reaction products formed are thermalized in the target material. The atoms or molecules are then transferred to an ion source from which they are injected into the acceleration stage of the mass separator to form an ion beam.The post-accelerator for unstable ion beams must fulfill the following mainrequirements;1) since the nuclei of interest are in the mass range of 6  to 240 and have velocity and very low q/A, it should be able to accelerate efficiently such particles,172 ) losses of beam intensity should be very small to preserve the beam intensities and to avoid radioactive contamination of the accelerator must be avoid. Typical beam intensities will be in the range of 1 0 3 to 1 0 11 p/s,3) the final energy must be variable continuously in the range of 200 keV/u to 1.5 MeV/u. Considering the production method described previously, the energy changes must be done in a reasonable time period,4) since the lifetime of some of the most interesting element can be very short, sometime less then 1 0  ms, the post-accelerator must be operated with 1 0 0 % duty cycle to preserve the intensity.Table 1 gives a resume of the major specifications for the ISAC post-accelerator.Since the most efficient ion source used with on line isotope separator provides single charge ion 1+ or 1-, the post-accelerator must be designed for acceleration of single charge ions. The consequence will be a limitation of the mass range for singly charge ions. In regard to the physics motivation the mass range for singly charged ions can be restricted to 60, keeping in mind that the use of multiply charged ion can increase the mass range. Some attempt to use an ion source providing multiply charged ion are made in two different laboratories, Louvain-la-neuve and GANIL, but they are restricted to gaseous or very volatile elements. The efficiency for on line operation is not yet well proven.In view of the physics program two different experimental areas are very well defined, the first is the nuclear astrophysics area where the maximum energy is fixed at 1 .5  MeV/u and the second for higher energy nuclear physics. Two beam lines are therefore defined, one at medium energy providing a continuously energy variability from 0 .2  to 1 .5  MeV/u and a second providing beams from 1.5 to 10 MeV/u.Choice of the post-acceleratorAmong the different choices for the post-accelerator we have the tandem, cyclotron, linear accelerator and a combination of linear accelerator and cyclotron.TandemA tandem offers good beam transmission, if stripping process are not taken into account, good energy resolution and can accelerate dc beams. But, an electrostatic tandemTable 1ISAC post-accelerator basic specificationsInput BeamsEnergyMassIon charge Beam currentBeam emittance (normalized)60 keV/u A <60 1+ or 1- < 1 |iA0.25 7t mm mradAccelerated Beam Output EnergyAE/EDuty Factor0.2 < E < 1.5 MeV/u 1.5 < E <  10 MeV/u 10-3 100%requires negative ions and most of the ion sources developed for on line application produce positive ions. Therefore, negative ions have to be produced using a charge exchange channel. The efficiency of this process is element dependent and for some species it is very low and even not feasible. To reach high energy the tandem must be very large. It seems to be impractical to use a tandem to reach 10 MeV/u for mass 60. To reach such high energy a booster must be added to the tandem. The tandem booster can be either a linear accelerator like the ATLAS booster or a superconducting cyclotron like the Chalk River tandem-superconducting cyclotron combination. In this case particularly attention has to be made on the choice of the various parameters. The tandem solution seems to be a good solution for certain elements if it is already installed.CyclotronThe cyclotron has been used in nuclear research for many years. The main advantages of the cyclotron are its relatively low cost and the possibility to use it as a mass spectrometer, offering a good beam purity. But, cyclotrons suffer disadvantages. If singly charged ions are injected into a cyclotron it must be very large to provide even 1 MeV/u for mass 60. The energy per nucleon of a cyclotron is given by the following relation assuming that the machine operates on the first harmonic,19E = K ( q /A ) 2 ,where q is the charge state of the ions, A the mass and K the maximum energy of the machine for proton. We can see for example that for mass 60 and charge 1 the K of the cyclotron must be 3600 for 1 MeV/u. This is a really huge machine, and the advantage of the cyclotron disappears. To be really cost effective the q/A must be greater than 1/10. This means that any cyclotron used as a front end for an unstable ion beams facility will rely on a high charge state directly from an ion source. The only ion source providing such a high charge state is the Electron Cyclotron Resonance ion source (ECR). Unfortunately, for the moment the use of such an ion source producing high charged state has not been proven for on line application. Only gaseous element or very volatile species can be produced with an ECR on line. Due to its large volume and sophisticated construction the high radiation field will impose severe limitations on this type of ion source.Linear acceleratorThe main advantages of the linear accelerator are the following, high transmission, very good energy/time resolution, can accept ions with very low charge to mass ratios. The front end of the linear accelerator complex must be a RFQ which is the key part of the linear accelerator solution. The RFQ has the merit that it can accelerate very low velocity ions with good efficiency, > 90%. RFQ's have important applications in the low velocity part of many accelerators. They provide several necessary functions in a continuous manner to produce a final beam suitable for injection into a conventional accelerator. These functions are the following, 1) acceptance of a dc beam and radially match to the next accelerating section, 2 ) bunching this beam adiabatically with high capture efficiency > 90% and 3 ) accelerating the beam up to several hundred keV/u if necessary. Since the charge to mass ratio is very low the RFQ must be operated at low frequency, say between 10 and 30 MHz. At such low frequency the normal four vanes RFQ is no longer practical since its size will be prohibitive. Four rod RFQ’s have been developed for low frequency operation. Research and development are under way at TRIUMF to develop a RFQ operating at 25 MHz with a duty cycle of 100%.Post-RFQ linear accelerators can be of various type. Room temperature or superconducting structure are commonly in used. In the 1984 proposal the RFQ was followed by eight tank Widereo linac operated at 23 MHz at room temperature. The power need to reach 1 MeV/u was estimated to be 1 MW. Since the commissioning of the PositiveIon Injector (PII) at ATLAS Argonne National Laboratory using superconducting structures we have looked at that kind of technology for the ISAC post-accelerator.LINAC-CyclotronThere are other options that can be used to increase the energy, but they have not yet been fully explored. One of them is very attractive since it combines the requirements for the nuclear physics at low energy and at high energy. This solution use a cyclotron for the high energy beams. As mentioned previously, a cyclotron for the nuclear astrophysics and applied physics program is probably not practical since achieving the required continuous energy variability is difficult with a cyclotron, but for higher energy the normal tunability of cyclotron such as for example, at the Chalk River superconducting cyclotron, is acceptable. In this case we might consider acceleration of the singly charge ions to an energy where stripping injection into the cyclotron is practical. This energy is around 300 keV/u and at this energy the use of a carbon foil stripper becomes feasible and a stripping injection scheme similar to that used on the Chalk River cyclotron could be employed. With a K=900 cyclotron, operating at 4 Tesla, a maximum energy of 25 MeV/u would be possible.Further studies would be necessary to check the viability of this concept. For the present therefore we adopt the linear accelerator solution consisting of a RFQ followed by a series of independently phased superconducting quarter wave resonators as the reference design to produce beams for of the two experimental areas, 1.5 and 10 MeV/nucleon.Conceptual designThe block diagram in fig. 1 illustrates the three stages of the linear accelerator that would satisfy the ISAC specifications given in Table 1. Initial acceleration of the singly charged ion beam delivered from the mass separator is accomplished in a RFQ. As in the case for all fixed frequency linear accelerators, the RFQ in this case is designed for a specific input ion velocity. Since the extraction voltage on the ion source will be fixed at 60 kV, the ions will be delivered from the mass separator with velocities that are mass dependent. Therefore, to accommodate the RFQ input requirements, it is necessary to place it on a high voltage platform, and operate it with a dc bias, adjustable between ± 60 kV, so that the ion input energy can be in all cases be 2 keV/nucleon for A < 60. After acceleration to 60 keV/u in the RFQ, the beam passes through the first of two strippers and matching sections where the ion charge state to mass ratio is increased to greater than 1/20. The beam21is matched in both transverse and longitudinal phase space and injected into a two stage superconducting drift tube linac consisting of a series of independently driven accelerating structures operating successively at 50 and 75 MHz. at the exit of the 75 MHz stage the beam energy is 1.5 MeV/nucleon.Here the beam is either transported to the medium energy experimental area or it passes through a second stripper for a further increase in q/A to greater than 1/4, before being deflected 90 degrees in an achromatic and isochronous beam transport system that matches it to the next accelerating section. The final energy is 10 MeV/nucleon.Choice of the stripper locationsSince the unstable ion beams extracted from the ion source are most abundant in the singly charged state, the post-accelerator must be designed for such low charge to mass ratios. It would be however impractical to carry out the entire acceleration of the particles in the singly charged state since this implies, in view of the specifications above, that the total effective acceleration voltage required would be at least 600 MV. To limit the accelerator size stripping at one or more intermediate energies, with the concomitant loss in intensity is a practical necessity and must be accepted by the users. The charge state distribution and particularly the mean charge state of ions after passing through a stripper, depends on the ion velocity, the atomic number of both the ion and the stripper medium, and the state of the medium ( gas or solid).There are therefore optimum energies at which stripping should be done to minimize the total effective voltage required to achieve a given final energy. Thus, for example, using only a single carbon foil stripper we find, for Z=26 and A=60, that the total effective accelerating voltage required to achieve an ion energy o f  10 M eV /u  has a minimum  at 71 MV when the stripper is located at an energy of 300 keV/u. On the other hand when two strippers are used, the minimum effective accelerating voltage required can be reduced to 48 MV, if the first stripper is located at 150 keV/u and the second at 1.5 MeV/u. The price paid is about a factor four in beam intensity.For practical reasons we have located the first stripper just after the RFQ accelerator. The stripping energy is then 60 keV/u. At such low energy only gas strippers can be used, since the equilibrium target thickness at this energy is ten time smaller than the practical solid target thickness, say 5 |ng/cm2. Depending on the beam dynamics two different solutions can be used. One is the low pressure gas canal and the other one is the gas jet perpendicular to the beam axis.We have looked at a low pressure gas canal similar to the one installed in the terminal of the Daresbury tandem, [AIT8 6 ]. The effective stripper length is about 44 cm and the overall length is about lm. The conductance is limited by two holes of 5 mm diameter. Nitrogen gas is bled into the canal via a series of regulators and valves. If the transverse beam emittance is too large this solution is no longer a practical solution. To avoid losses the gas canal will have to be shorter and/or the exit holes will have to be larger. The resulting effect will be a larger pressure drop and a shorter effective stripping length. This gas canal will demand a very powerful pumping system. The other solution for such large emnittance can be a gas je tThe desirable properties of a gas jet stripper should be short length and high pressure gradient in the direction perpendicular to the ion beam axis. They are realized with a supersonic gas jet perpendicularly crossing the ion beam. Target thickness of few tenths of (ig/cm2 to hundredth pg/cm2 can be achieved. Such gas jets are described in ref. [FRA72]. Jet densities between 5 1014 and 1.5 1017 molecules/cm3 or target thickness between 2 .5  1 0 14 and 7 .5  1016 molecules /cm2 are obtained by adjusting the inlet pressure.The choice of the gas stripper system will depend on the beam size at the stripper location. If the emittance is smaller than 0.25 n mm mrad a gas canal of 30 cm long can be used with 5 mm aperture at both ends. For larger transverse emittances the only possible solution is the gas jet, but this solution will demand some studies since the beam size is much larger than the usual beam size when gas jets are employed.For the second stripper the energy is fixed at 1.5 MeV/u since this is a natural break in the accelerator dictated by the nuclear astrophysics and applied physics program. At this energy a carbon foil stripper can be used without any problem.Current conceptual linear accelerator designIn the mid 1980's Sheppard et al. at ANL reported on the development work of low frequency, low (3 superconducting accelerator structures, for a proposed positive ion injector (PH) for the ATLAS accelerator [SHE87]. Subsequent studies at TRIUMF showed that the PII linear accelerator concept, which was developed for ion beams with charge to mas ratios greater than 1/ 1 2 , could be extended to a linear accelerator capable of accelerating ions with even lower charge to mass ratios, as required for ISAC. In our design we base the first accelerating section after the RFQ accelerator on the ANL structures developed for the positive ion injector for ATLAS. These structures are illustrated in fig. 2. They are basically quarter wave resonator capacitively loaded with a bifurcated drift tube and a counter drift tube forming four accelerating gaps. The structure,23made of Niobium and Niobium clad Copper, is cooled with a pool boiling liquid Helium in the center conductor. Three models for mean particle velocity of (3=0.009,0.016 and 0.025 were developed at ANL for operation at 48.8 MHz. A fourth model, for (3=0.037, was designed to operate at 72.75 MHz. For the ISAC design, nominal operating frequencies of 50 and 75 MHz have been chosen for acceleration from 60 keV/u to 1.5 MeV/u.Because of their relatively large sizes, these resonators are subject to microphonic vibration which, in view of the high Q values (109) involved, causes shifts in the resonator eigenfrequencies much larger than the intrinsic bandwidth. Fast control of the structure resonant frequency to compensate for this mechanical motion, is essential to achieve a stable phase of the RF accelerating fields. This is accomplished in the ANL structures by coupling a voltage controlled reactive load made up of a circuit that includes an array of PIN diodes, to each resonator. The loads, cooled with liquid Nitrogen, are attached to the resonators by a high thermal impedance thin wall stainless steel tube, and coupled electromagnetically with a loop. At operating fields of about 3 MV/m approximately 100 Watts of RF power is dissipated in each reactive load. This is the major RF power load. Dissipation in the resonators themselves at 4.5 K is only 5 Watts.Because of the use of four accelerating gaps in each resonator, transit time effects limits the particle velocity range over which a particular resonator design can be used for efficient acceleration. To cover the particle velocity range of the ISAC post-accelerator it is anticipated, although the design calculation are not complete , that four resonator models will be necessary for the 50 MHz section and three or four models for the 75 MHz section.For acceleration to higher energies we base our design on the JAERI superconducting post-tandem accelerator structure [TAK90], illustrated in Fig. 3. This is also a capacitively loaded quarter wave resonator, but with only two accelerating gaps. The operating frequency is 129.8 MHz, and it is designed for a particle velocity , (3=0.1. For ISAC the nominal operating frequency would be 125 MHz with a design particle velocity range from 0.057 to 0.146, corresponding to the energy range 1.5 MeV/u to 10 MeV/u.Fig. 4 shows a block diagram of the latest conceptual design of the ISAC post­accelerator, incorporating the superconducting structures discussed above. It is a three stage accelerator with strippers located between stages to increase the charge state of the ions.For the ISAC accelerator design studies to date, only an idealized linac geometry has been used for the superconducting drift tube. That is, it is assumed that cell lengths ( gap to gap distances ) vary correspondingly to the velocity of the synchronous particle so that ln=(3nA,n/2. With this simplification then, fifty six resonators units would be required inthe three stages to reach 10 MeV/u. Table 2 summarize the main parameters of the first two stages and Table 3 summarize the parameters of the third stage.Table 2Parameters of the first and second Drift Tube LinacStage 1 Stage 2Structure 4 gap QWR 4 gap QWRFrequency 50 75(M Hz)Number of 8 2 2resonatorsEacc (MV/m) 3-5 3-5Os (Deg.) -2 0 - 2 0Eout (MeV/u) 0.382 1.5Pout 0.028 0.056Focusing Superconducting solenoidsSolenoid 15-25 cm 30-105 cmlengthBSol 6.5 T 6.5 TFocusing Res-Sol-Res 3Res-Sol-3ResperiodicityTable 3Parameters of the third Drift Tube LinacStage 3Structure 2 gap QWRFrequency 125(MHz)Number of 30resonatorsEacc (MV/m) 5O s (Deg.) - 2 0Eout (MeV/u) 10Pout 0.145Focusing Superconducting solenoidsSolenoid 13 cmlengthBsol 6.5 TFocusing 6 Res-Sol-6 Resperiodicity25Particle tracking calculations through the whole linac were done with the aid of the computer code PARMILA. This code allows for designs with multiple accelerator tanks and inter tank beam transport elements. In this case a tank is either a two gap or four gap QWR and the inter tank beam transport elements are drift distances and when required , solenoids or quadrupoles or dipoles. The transverse and longitudinal emittances of the beam at 1.5 MeV/u are 0.29 jc mm mrad and 177 it keV ns, respectively. Fig.4a shows the transverse emittances, and fig. 4b shows the longitudinal emittance of the beam at 1.5 MeV/u. After passing through the second stripper and matching section and accelerating the beam to 10 MeV/u, the calculated beam phase space corresponding to a contour of 95% are0.31 7t mm mrad and 290 7t keV ns, respectively. Fig. 5a shows the transverse emittances, and fig. 5b shows the longitudinal emittance of the beam at 10 MeV/u.Based on experience at the ANL ATLAS accelerator and at the JAERI booster, the static heat at 4.5 K would be about 250 Watts for both superconducting stages with RF drive dissipation in each resonator of about 4 Watts, adding another 270 Watts to the load, allowing about 230 Watts reserve capacity means that a refrigerator providing at least 750 Watts of refrigeration would be required.Reference[TIS85] The TRIUMF-ISOL facility, TRIUMF report, June 1985.[FRA72] B. Franzke, et al. IEEE, NS-19, no 2, 266. and A. Gruber et al. Nucl. Instr. and Methods, (1989) 87.[AIT8 6 ] T. W. Aitken et al. Nucl. Inst, and Meth. A244 (1986) 183.[SHE87] K. W. Sheppard, Proc. 1987 Part. Acc. Conf. IEEE ns 34, 1812.[TAK90] S. Takeuchi et al. Nucl. Instr. and Meth. A287 (1990) 257.J S f f C  c  o->ic&p£u.a.e Z ? C 5 7 J H27BEAM0.009La (cm 10.2f (MHz) 48.516.548.525.448.525.472.75OOmr-oinmc\jSCALE (cm)p-aw130UHz 1M WAVE Nb JAERJ29The ISAC Post Accelerator Conceptx -p x  d is tr ib u tio n  at the ex it o f the  75 MHz LINAC y - p y  d is tr ib u tio n  at the exit o f the  75 MHz LINA0.0060.004  ■^ 0.002  ■ o <io.ooo ■X^ 0 .0 0 2  M -0 .0 0 4Q<cc-0 .0 0 6- 1.0 - 0.6  - 0.2  0.2  0.6 X (CM)0 .006 - 0.0C4 0 .0 0 2 -  0.000 J- 0.002-  -0 .0 0 4-0 .0 0 6,<*> <? o 11.0 - 1 . 0  - 0 . 6  - 0 . 2  0 .2Y (CM)0.6- y  d is tr ib u tio n  at the exit o f the  75 MHz LINAC $ -A E  d is tr ib u tio n  at the  exit o f the 75 MHz LINAx - y1.0 -0 .6 -^  0 .2 :  2> - 0 .2 - - 0.6 :- 1.00o O <po o oJ?1.5-1.0-10.5S'<u0 .0 -UJ<-0 .5  - 1.0- 1.0 - 0.6 - 0.2  0.2 X (CM)0.6-1 .5   - i ■ ■ ■ ■ i1.0 - 5 0  - 3 8  - 2 6  -1 4$ (DEG)PY(RAD) PX(RAD)31X -P X  DISTRIBUTION AT EXIT OF 125 MHZ LINAC0.003  -!—   1-----------1-----------1-----------1 l .0.0020.0010 . 0 0 0- 0.001- 0.002-0 .003-0 .00490 % OF POINTS ARE WITHIN € =  .26  71 MM*MrtAD (n o rm a lize d ^e AT RFQ ENTRANCE =.3 77 MM*MRAD (NORMALIZED)i -------------r0 .0030.0020.0010 . 0 0 0- 0.001- 0.002-0 .0030.8 -0 .6  -0 .4  -0 .2  0.0 0.2 0.4 0 6 o ’:X(CM)Y -P Y  DISTRIBUTION AT EXIT OF 125 MHZ L1NACJ-------------- 1-------------- 1-------------- 1________I________I_______ L9 0  % OF POINTS ARE WITHIN e =  .24  77 MM+MRAD (NORMALIZED)€ AT RFQ ENTRANCE =.3 77 MM*MRAD (NORMALIZED^- 0 .6  - 0 .4  - 0 .2 0.0 0.2 Y (CM)03AE(MEV)LONGITUDINAL EMITTANCE AT EXIT OF 125 MHZ LINAC6 -4 - 2 - 0  ■ -2 - 4  - 6  - 88 - X*POINTS AT EXIT OF 125 MHZ LINAC9 0  % OF POINTS ARE WITHIN e =  13 7T MEV*DEG =  290  7r KEV*NSEC>fe*xXX8 0  % OF POINTS ARE WITHIN t  — 3 .4  it MLV + OLG =  185 tt KEV*NSECx xAX X- 3 0  - 2 5 — 90 - 1 5  - 1 0PHASE( 125 MHZ DEG)033MULTIPLE CHARGED IONS FOR RADIOACTIVE BEAMSM. Dombsky, Department of Chemistry, Simon Fraser University, Burnaby, British Columbia and Department of Physics, University of Alberta, Edmonton, Alberta, CanadaREQUIREMENT OF MULTIPLE CHARGED IONSTo date, the conventional ISOL facilities have not had a great need for multiply charged radioactive species. Ion source development has, mainly, been directed towards increasing the efficiency of singly charged ion production. The primary advantage of a multiply charged species has been the ability to avoid isobaric contamination;; a 2 +  ion of a given element may provide a cleaner beam if the 2+  ionization of a contaminant element is hindered. However, with the addition of a post-accelerator to an ISOL for production of energetic radioactive beams, multiple charged ionization can greatly enhance the range of beams available. For example, a post-accelerator designed for a mass range of 6  <  A < 60 for ions of charge Q = 1 + becomes capable of a mass range 6  < A < 240 for ions with Q >  1 + . An ion source capable of efficient production of Xe2+ and Rn4+ would allow acceleration of essentially the entire range of elements.ECR ION SOURCESThe current choice for efficient multiple-charge ion production is the electron cyclotron resonance (ECR) ion source. In this type of source, electrons excited by RF power are confined axially by a magnetic mirror generated by solenoid coils. Addition of permanent magnets parallel to the plasma chamber provides radial ion confinement and enhances multiple-charge production. The advantages of ECR ion sources are that they have been shown to provide high efficiencies for gaseous species [1], they do not have filaments that can burn out or form refractory compounds with elements such as oxygen, nitrogen and carbon, and they have been shown to provide both molecular ions and multiply-charged ions in on-line application withThe chief disadvantages of ECR sources are size and cost. The magnitude of the solenoid field (hence the size of the magnetic coils) is determined by the operational RF frequency, which also determines the size of wave guide required to service the source. With the required additional radial magnetic confinement, multiple-charge ECRs tend to be much larger than the traditional ISOL plasma sources. Clearly, though a high radiation environment such as an ISOL target station is best served by a small inexpensive and disposable ion source, it may not be practical to discard an entire ECR source due to its replacement cost.A further disadvantage of ECR sources is their requirement of low pressure ( ~  10" 6 torr) for both high efficiency and multiple-charge production. In on-line situations, the vapour pressure of the target material can severely limit the observed efficiency of the source. At the TISOL separator at TRIUMF, source pressure has been the most critical parameter of ECR operation; pressure fluctuations seem to be the determining factor in on-line operational stability.An additional disadvantage of an ECR source is that the distribution of a desired product over its possible charge states seems to be mainly a result of the intrinsic design of that source. While it is possible to observe enhancement or suppression of higher charge states, it does not appear practical to control the charge state distribution while maintaining high overall efficiency in an on-line situation. Simply put, ECR sources are not easily "tunable" on-line.Lastly, beam extraction from an ECR source takes place in a magnetic field due to the requirement of the confining solenoid coil. Consequently, the beam properties are not the best with regards to the requirements of injection into a post-accelerator. Normalized beam emittances from ECR sources tend to be of the order 0.25-1 x mm mrad.A POSSIBLE ALTERNATIVE ION SOURCEIn a discussion of on-line sources, Shubaly [4] has suggested that a hot cathode plasma source with ion confinement by a multi-cusp magnetic field would exhibit enhanced ionization efficiency and plasma stability. Such "multi-cusp" sources indeed exist and are used for off-line ion generation. (For a review see Ref. [5]) Multi-cusp sources consist of a plasma chamber surrounded by an even number of permanent magnets alternating in their north-south pole orientation. Such a configuration results in a "minimum B" confining field at the center of the source volume, with increasing field strength closer to the chamber walls similar to the radial field employed in high-charge ECR sources. Ionization is achieved by either RF power introduced into the chamber or by means of an arc discharge from a filament. As with an ECR, the lack of a filament can be advantageous in source lifetime or element selectivity, however, with a filament driven source, the arc potential can be selected to suppress higher charge state production. For example, an arc voltage can be selected to ionize a species up to a charge of 2+ but not to 3+  or higher. This could effectively reduce the dilution of a low yield radioactive species among a range of undesired charge states.Multi-cusp sources have been demonstrated to produce both molecular species [5] and high-charge state ions [6 ]. In Table 1, the Xe charge state distribution of a multi-cusp source is compared with the on-line 125mXe distribution measured with the TISOL ECR source. As well, multi-cusp sources can be made compact, with a chamber diameter as small as 2.5 cm [7], A small size multi-cusp source could be disposable and also offer both cost and size advantages over an ECR for on-line application. A further advantage of multi-cusp sources may be their ability to operate at pressures on the order of 1 0 " 4 torr allowing the use of higher vapour pressure target materials. Perhaps the most significant advantage of a multi-cusp source over an ECR for application with a post-accelerator is the fact that the ion beam is not extracted in a magnetic field; normalized emittance for a multi-cusp source is on the order of 0 . 0 2  x mm mrad [6 ].While the multi-cusp ion source may be a viable alternative to an ECR for multiple- charge ion production, it has the failing that it has never been tested on-line using radioactive species. Ultimately, such questions as confinement time, wall effects and overall efficiency for short-lived low-yield radioactive species will have to be answered by on-line testing. To this end, TRIUMF has entered into a collaboration with a group at the Lawrence Berkeley Laboratory to design and test a multi-cusp ion source both off and on-line.35REFERENCES[1] V. Bechtold, H. Dohrmann, S. A. Sheikh, Proceedings of the 7th Workshop on ECR Ion Sources, Jiilich, (1986), 248.[2] M. Dombsky, J.M. D’Auria, L. Buchmann, H. Sprenger, J. Vincent, P. McNeely, G. Roy, Nucl. Instr. and Meth. A295 (1990) 291.[3] P. Decrock, M. Huyse, P. Van Duppen, F. Baeten, C. Dom, Y. Jongen, Nucl. Instr and Meth. B58 (1991) 252.[4] M.R. Shubaly, Nucl. Instr. and Meth. B26 (1987) 195.[5] K.N. Leung, Rev. Sci. Instrum. 65 (1994) 1165.[6 ] K.N. Leung, R. Keller, Rev. Sci. Instrum. 61 (1990) 333.[7] L.T. Perkins, P.R. Herz, K.N. Leung, D.S. Pickard, Rev. Sci. Instrum. 65 (1994) 1186.TABLE 1Comparison of Xe Charge State Distributions for ECR and Multi-Cusp Ion Sourceslarge State TISOL ECR Multi-Cusp(on-line) [Ref. 7]125mXe Xe+  1 67.1 % 36.4%+ 2 21.7% 36.4%+3 8.9% 19.1%+4 1.4% 5.8%+5 0.7% 2 .0 %+ 6 0.08% 0.3%+7 0 .0 2 % -+ 8 0 .0 1 % -37Sym m etry-tests in Nuclear beta-decay : status and prospectsJ. D e u ts c hUniversity Catholique de Louvain, Institut de Physique Nucleaire,B -1 3 4 8  L ouvain-la-N euve, B elg ium1 .  I n t r o d u c t i o n  : o n  th e  in t e r f a c e  b e t w e e n  n u c le a r  a n d  p a r t i c l e  p h y s i c sRadioactive ion beams, intense and pure as they w ill be produced at the upgraded T r iu m f/T iso l  fa c i l i ty ,  w i l l  op en  n ew  o p p o r tu n it ie s  for  sym m etry-tests  perform ed w ith  n uc le i .  As th ese  e f fo r ts  seek  to gain  in fo rm a tio n  on the fu n dam en ta l  c o n s t i tu a n ts  o f  m a tter  and their  interactions using  low -energy probes, it seem s w orthw hile  to situate these  efforts in the realm o f  Particle Physics and also to face the arguments raised to question their relevance.As well known, the Standard SU (3)C x SU (2)L x U ( l )  Model assumes that the fundam ental constituants  o f  matter c o n s is t  o f  three generations  o f  ferm ions : three generations o f  quark-doublets o f  three co lor  quantum  number each and three generations o f  lep to n -d o u b le ts .  T h ese  ferm ions  interact ex ch a n g in g  ga u g e-b o so n s .  the W + /- ,  Z° and the photon w hich  mediate the electroweak force and - in addition, for the quarks alone - the gluons which  mediate the strong force. The gau ge-b oson s  and the fermions  acquire m ass by coupling  to a hypothetical H iggs-sca lar. The m asses  and interfermion co u p lin g s  are arbitrary parameters w h ich  are not accounted  for by the model. Other unpleasant features o f  it are : a) that the neutrinos take the very particular mass value o f  zero, b) that the W +/- bosons couple  only in a left-handed helicity-combination and c) that CP-violation is simply  allowed in the model but not accounted for.Though some o f  the ingredients o f  the Standard Model such as the top- quark, the H iggs-scalar and the boson-boson couplings were not observed as yet, the m odel is remarkably successfu l.  B ecau se  o f  its shortcom ings wementioned h ow ever ,  a good  part o f  modern particle  p h y s ics  con s is ts  in attempts to uncover deviations from the Standard M odel and to find new  particles which would show  the way to higher unification-schem es.„ S u ch k Partic les can be searched for at co l l id er -en erg ies  by "brute torce  m ethods or m low -energy  precis ion  exp er im en ts  lo o k in g  for tiny  effec ts  their exchange contributes to various processes .  In this respect thenucleus o ffers  a variety o f  w e ll-d ef in ed  quantum -states p rov id ing  various  liters for p ieces  o f  this hoped-for new physics and also the possib ility  to perform experim ents o f  high statistical accuracy, e sp ec ia l ly  at fac il it ies  like the T n u m f T iso l,  tailormade for such purposes.rnnt, , It,  is ^ j e c t e d  s o m e t im e s  that in su ch  ex p e r im e n ts  the sm all  contributions which  would signal new physics  beyond the Standard Modelare obscured by ambiguities due to nuclear structure. For beta-decay, sucham biguities  cou ld  e f f e c t iv e ly  be introduced by nuc lear  matrix e lem ents  o f  reco il-order  and so o n e  should p r iv i led ge  "fast" b eta-transitions  in which  the relative contribution o f  these terms is n eg lig ib le  at our actual level o f  precision, as d iscussed  e .g .  in a recent review -paper1) . The transitions to be con s idered  for ex p lo ra t io n  at T iso l  and d isc u sse d  in this n o te  are all su pera llow ed  b eta-transitions  within isosp in  m uitip le ts  and so  escap e  this c r i t i c i s m .Theory does not provide any reliable prediction for the sca le  at which  physics  beyond  the Standard M odel w ill  be eventua lly  ob served . So tests-  ex p er im en ts  o f  v a r io u s  k ind  sh ou ld  be a lw a y s  attem pted  i f  they can  s ign ificant ly  improve upon the ex is t in g  lev e ls  o f  prec is ion  obta ined  in the same type o f  lo w -en erg y  sym m etry-tests .  L inks to constra in ts  obtained at high en erg ies  are o ften  m od el-d ep en d en t and so these  constraints,  even ifthey ex is t ,  should not d iscourage  us from g o in g  ahead w ith  com petit ivep rec is io n -ex p er im en ts  at lo w  en erg ies .W e shall not d iscuss  in this note tests w hich  cou ld  be performed at T iso l in the neutrino-sector or on atom ic parity-violation , both important in searches for physics  b eyon d  the Standard M odel,  and shall concentrate on the sem i-leptonic sector, i .e . on the decay o f  radioactive nuclei which  w ill be produced cop iously  at T iso l.2 . T h e  l o w - e n e r g y  b e t a - d e c a y  h a m i l t o n i a n  a n d  th e  S M - p r e d i c t i o nTo set the stage, w e  rewrite here the beta-decay probability as givenby J.D. Jackson et a l .2 ) , i .e . assum ing that the polarization  o f  the finalnucleus is unobserved :The notations used are obvious.V arious aspects o f  this exp ress ion  w il l  be tested  at T iso l  and are discussed  b elow .2 .1 ) C o u p lin g - s t r e n g thT he c o u p l in g - s tr e n g th  o f  b e ta -d e c a y  ca n  b e  m e a su r ed  in thesu pera llow ed  pure Fermi transitions and com pared to that o f  the purely  leptonic muon-decay. This comparison a llow s to extract the (u ,d )-p iece  o f  theK o b a y a sh i-M a sk a w a  m atrix  and to ver ify  i f  the W -c o u p l in g  is reallyex h a u sted  by the three pairs o f  quarks. O thers co n tr ib u t io n s  to this  workshop w ill  com m ent on  the im portance o f  radiative corrections in thisgame and the interest o f  h igh-Z  superallowed transitions, to be produced at T isol, to study them.392 .2 ) R elative phase o f  the coup ling-constantsA relative im m aginary phase between co u p lin g  con stan ts  results in n o n - v a n is h in g  c o r r e la t io n s  odd under t im e -r e v e r s a l .  A  n o n - v a n is h in g  correlation-coefficient D w ould indicate, in first order, an imm aginary phase between the vector and axial coupling constants and signal the ex istence o f  a new  vector gauge-boson  sim ilar to the W. A non-vanish ing term o f  the R- type in a Fermi-amplitude w ould signal the presence o f  a scalar gauge-boson  (such as an electrically charged Higgs beyond the Standard M odel). A term o f  the sam e type in a G am ow -T eller  amplitude could not be accomodated in a usual gauge-theory and would require leptoquark exchange g iv in g  rise to an interaction which, p henom enolog ica lly ,  would be o f  the tensor type.This is displayed in the expressions below :D i  = 23m[<?j,jMFMGT^j^y (GvG^ + GvGa)]Rt  = 29fm[TAwMjT(GAG^ + G^G^)+*w MfMot^ j |^ ( G vG¥' +  GvGt -  GaGs" -  G'AGS*)]Let us note again that a non-vanish ing  R-term in a pure G am ow -T e ller  transition would require a tensor interaction w hich  can not be accomodated  in a usual gauge-m odel and w ould  require e .g . lep toquark-exchange. If it observed in a mixed transition, it could arise also from a scalar interaction  ( o f  re lative im m aginary phase) and signal the e x is te n c e  o f  an extended  (ch a rg ed ) H ig g s -s e c to r .The D-term was found to be zero with a precision o f  4x1 O’4 in the decay o f  1 9 Ne and this experiment is still under im provement at Princeton. It may be difficult to do better at T isol ; at this level o f  precision one has to consider  m oreover final state e lec trom agnetic  interactions w hich  cou ld  m im ic  T-odd  effects .  As for the R-term, the observation o f  which increases the difficulty  o f  the exp er im en t by an order o f  m agnitude b eca u se  it requires the m easurem ent o f  an e lectron  (positron)-polarization , it was constrained with a precision o f  7 x l 0 ' 3 on the practically pure G am ow -T eller  decay o f  L i8 3) and this experiment is still under improvement at PSI. N o  measurements exist  how ever up to now on the R-term o f  a mixed Fermi G am ow -Teller  transition which would be o f  rs&J interest and should possibly be attempted at Tisol.For such experiments the highest sensitiv ity  w ill  be obtained in beta- transitions where the Fermi and G am ow-Teller amplitudes are about similar. For the s u p e r a l lo w e d  tr a n s i t io ns w e  c o n s id e r  h ere  th e  s e n s i t iv i t ys -  | 2 (C y  Mp) (C a M q t )  Vj/CJ+l) /  [ (C y  Mp)2 +  (C a  M g t ) ^ ]  I can be obtainedfrom the compilation o f  0 . Naviliat-Cuncic et al.4 ) ; it is displayed, for some of  the interesting cases, in the Table 1 below.Table 1 : Sensitivity s o f  the correlation-coefficients D and R (defined in thetext) to a VA» ctSA and phase angles o f  Im C \  and Im Cs with respect to Ca- We assumed Q  = C'i and C j  = 0n u c l e u s sen sit iv ity  sn 0.45N e 19 0 .70N a 21 0.73K 57 0.682.3) H e l i c i t v - s t r u c tu r eTwo types o f  questions are o f  interest in this sector :2 .3 .1 )  eventual scalar (tensor) admixtures to the v ec to r  (a x ia l  v ec to r )  interactions which would require to extend the Standard M odel by a charged  H iggs ( or by leptoquarks) and2 .3 .2 )  eventual deviations from the pure V -A  coupling (i .e .  maximal parity- vio la tion) which  would indicate the interplay o f  a n ew  (r ight-handed) W  g a u g e - b o s o n .2 . 3 . 1 )  The interest o f  T iso l-type installations for the first type o fexperiments is exem plified  by the work o f  Schardt and R iisager at Iso ld e5 ) on Ar32 and Ar33 where beta-decay feed s  a narrow ( iso sp in -fo rb id d en )  proton-decay leve l.  E.G. Adelberger used the shape o f  the resulting proton-  peak to obtain constraints on scalar admixture to these  dom inantly  v ec tor  i n t e r a c t i o n s 6 ) and plans to persue further these experim ents at Isolde. It w ould  be interesting to search for similar transitions at T iso l p oss ib ly  to alpha- or proton- unstable lev e ls  o f  measurable intrinsic width which  w ould  help to avoid the interplay o f  this additional parameter. Such could be the  case  for decays in T=2 multiplets where the width o f  the p-unstable lev e ls  co u ld  be co n tr o le d  o b serv in g  re so n a n ces  w ith  r a d io a c t iv e  b ea m s on  hydrogen  (M g 20 ?)2 . 3 . 2 )  In the oral version o f  this talk w e  expanded on the interest to search for deviations from m a x im a l p a r i tv -v io la t io n  in sem ilep ton ic  decays  and on the inconsistency o f  som e o f  the ex isting results with the predictions  o f  the Standard Model. This inconsistency on the 2.3 a - l e v e l ,  in tro d u c ed  principally  by a recent decay-asym m etry  m easurem ent on the neutron, is further illustrated in Fig. 1 by te 90 % C.L. solution region noted "Beta-decay  1994". For further details w e refer the reader , in addition to the review -  paper already quoted, to 0 .  Naviliat-Cuncic et al.4 ) , to A.S. Camoy et al. 7) and N. Severijns et a l .8 ) ; w e stress only the need to clarify the inconsistency  with the help o f  novel experiments.W e noted also that the co o lin g -p a ttem  o f  the supernova S N 1 9 8 7 A  p ro v id e s  l im its  on r ight-handed  currents w h ich  a l lo w e d  Barbieri and  M o h a p a t r a 8 ) to exclude the ~  0.42 to 31 T eV /c2 range for the mass o f  a new ,  p red o m in a n t ly  r ig h t-h a n d ed ,  g a u g e -b o s o n  so  a s s ig n in g  to  la b o r a to ry -  experiments the task to explore a mass-range down to only M 2 ~  420 G eV /c2 .41This w e ll-d ef in ed  goal should be confirmed by recent evaluations o f  the problem by M. Turner and coworkers®).We d iscussed  first a b s o l u t e  m easurements o f  quantities odd under  parity transform ation  such as e lec tro n -p o la r iza t io n  or asym m etry  with  respect to the spin o f  the (polarized) emitting nucleus. Such measurements  are difficult in the absence o f  sufficiently good calibrations o f  the degree o f  polarization o f  the em itting nucleus and o f  the geometrical acceptance o f  the detector. As discussed by O. Naviliat-Cuncic et al.4 ), a relative precision o f  0.5 % or better on these quantities would be required. Table 1 o f  this work4 ) ind icates  a lso  the error-limits on the positron em iss ion -asym m etry  A as predicted by the pure V-A helicity-structure o f  the Standard Model, due to the uncertainty o f  the corresponding ft-values. In order to obtain useful  constraints on the m ass o f  the new gauge-boson (M 2 ~  300 G eV /c2 ), the precis ion  on the Standard M odel prediction a  a / A  has to be better than -  2x10*3.We d iscussed  then the r e l a t i v e  m easurem en ts  w h ich  seem  more  promising in this respect.R e la t iv e  F e r m i /G a m o w -T e l l e r  p o la r iz a t io n  m e a s u r e m e n ts  w e re  performed to a high level o f  precision*®) but as they are sensitive to the P r o d u c t  o f  the left-right mixing angle (£ ) and the left/right boson squared mass ratio (5), they lo o se  sensitivity to the right-handed gauge-boson mass i f  the mixing is vanishingly small as it seems to be the case.A novel type o f  relative measurement, that o f  the electron (positron) polarization emitted in two opposite directions with respect to the nuclearspin was proposed by Quin and Girard**) and readily performed by N.Severijns et al**). This measurement requires a polarization-m easurem ent o fthe electrons (positrons) emitted by polarized nucle i and so is more intricate than a pure asym m etry-m easu rem en t ; b e in g  - h o w ev e r  - a re lat ive  m easurem ent, it d o es  not require a prec ise  k n o w le d g e  o f  the nuclearpolarization and is free o f  many sources o f  system atic errors which plague  absolute m easurem ents.  M ost importantly, it d o es  not require a precise  know ledge o f  the corresponding ft-value neither.For a superallow ed m ixed transition, the polarization ratio (Ra) can be written as :(Ra) =  [P(0j.p = O)/P(0j.p = jt)] = 1 +  Se . SN . 82where w e neg lec ted  £ ,  the left/right mixing angle , very near to zero, and8 = (8O.2/M 2 ) 2 . M2 being the mass o f  the n ew , predominantly right-handed,gau ge-b oson , expressed  in G eV /c2 .0j .p . is the em ission  angle o f  the positron with respect to the nuclear spin o f  the emitter.S n  is a transition-dependent factor which w e do not have to know  precise ly  :SN = [A - 2X Vj/(J+1) + X2 (A+1/(J+1))] /(1+X2).J is the nuclear spin, A the em ission-asymmetry adequately approximated by the Standard Model prediction A 0 and X = ( 1/Y 0 ) = CA MG T/ C V MF ; for the superallow ed transitions w e consider all these quantities can be found inTable 1 o f  ref. 4. In a note under preparation12),  w e shall display the Sn -values for all the transitions considered in ref. 3.S e = 4P /[1 -(P A )2 ] is an important enhancement factor which is related to the asymmetry PA, as e f fec t ive ly  measured : P stands for the e ffec tive  nuclear polarization m ultiplied  by the mean value o f  the em iss io n -a n g le s< c o s 0 j .p >  and <P > the positron velocity ; w e stress that the product PA is measured in the experim ent and does not have to be known precise ly  (cfr. also ref. 11). Se can becom e very large for transitions with A ~  1 i f  the emitting nucleus can be polarized near 100 %. A survey o f  sensitiv it ies  willbe presented in ref. 12 ; a glance on Table 1 o f  ref. 4  shows already that F 1 7 and Sc41 would be, in this respect, particularly interesting candidates. In the fo llow ing w e shall discuss the probabilities offered by K 3 7 .3. K37 : an outlookThe upgraded Tisol will produce a sizeable amount o f  K 37 (~ 10®/s for a 10 p.A b ea m 13)); plans exist to confine K 3 7 , as neutral atom, in a m agneto­optical trap and to polarize it practically to 100 %13). It seemed worthwhile to discuss briefly the prospects offered by this important developm ent.3.1) The. ..P-iermAs discussed above, a search for this term, forbidden in the Standard M odel, requires the observation  o f  the positron and that o f  the recoil ingnucleus. I f  non-zero, it would signal the presence o f  an immaginary phase a  a  V *  0 between the vector- and axial-vector coup lings.  This phase isconstrained to be ( a A V ) n = (0.07 ± 0.18)° and ( a A V ) N e 19 = (0.008 ± 0 .051)°  (ref. 1 and Table 1).The obtention o f  a comparable precision o f  e.g. (o a v )K 37 ~ 0 .03° would  require a precision o f  -  4  x 10'4 on ( D ) k 37 which may be difficult to obtain in the first phases o f  the experiment.3.2) T.h.e.._R.denilThe prospects to observe this term, forbidden by the Standard Model,or to obtain usefu ll constraints on it already in the first phases o f  the experiment may turn out to be better than those to search for the D-termsd iscussed  above. This experim ent does not require the observation  o f  the recoiling  nuc leus but, instead, requires the m easurem ent o f  the positron's  transverse polarization . H igh-lum inosity  polarim eters for a s im ilar  purpose  on L i8 -d e c a y  w ere  re a d ily  d e v e l o p e d 1 3 ) and could  be used for this e x p e r im e n t .A ssu m in g  Im (C A C x ) ,  constrained with improving precision by the L i8 - experiment, to be zero, an upper limit on R k 37 would constrain, as discussed  a b ove , any T -v io la t in g  im m aginary phase c c a S  b etw een  the axial and eventual scalar coup ling .The existing direct limit on a  a s  one can derive from a measurement o fthe parameter R in the N e 19-decay , assuming C j  = 0, is rather loose : < 8° 13).A comparable indirect limit can be obtained from constraints on | C s |2 6 ). R e fe r in g  to ta b le  1, an o r d e r - o f - m a g n i t u d e  im p r o v e m e n t  on43“ AS (e.g. a  a s  * 1°) would require a precision o f  only ~  1 % on R k 3 7 , which -in v iew  o f  the limits readily achieved on the L i8 - d e c a y 3 ) - may be wellw ith in  reach.3-3) Th£ A-term (em ission  asymmetry)As discussed above a comparison o f  the beta em ission  asymmetry with the one predicted  by the Standard M odel p rov ides  constraint on any eventual deviation from the maximal parity-violation postulated by ' e  "V-A" character o f  the SM, i.e. on the mass (and m ixing) o f  right-handed gauge-bosons introduced in left-right symmetric extensions o f  it. This comparison  requires a s u ff ic ie n t ly  p rec ise  determ ination  o f  the K 37 p o la r iz a t io n  (including unpolarized background) as w ell as that o f  the m ean-velocity  and em iss io n -a n g le  o f  the detected  positron. It requires a lso  a su ff ic ien t ly  precise determination o f  the asymmetry predicted by the SM, which depends  on the precision available o f  the reduced transition probability (ft) o f  K3 7 .This is known actually to a relative precision o f  0 .66 %4 ) , due mainly to theimprecision o f  the corresponding life-time (0 .57 %)14).To illustrate the constraints one may hope to obtain from a precision  asymmetry-measurem ent, w e assume that the precision o f  the ft-value can  be increased to 0 .25 % (the best value amongst the m ixed  superallowedtransitions) leading to a relative precision o f  0 .57 % o f  the SM-prediction. We  assum e further that the experim ental asym m etry (cfr .  a b o v e)  can be measured to a relative precision o f  0.3 %. The corresponding constraint isshown (at the 90  % C.L.) in Fig. 1, where we displayed also the 90 % C L  - contour o f  the 1994 world-average obtained from beta-decay. (Let us recallthat the disturbing deviation from the SM-model prediction 5 = £ = 0 is due to a new  neutron-asym m etry measurement c la im in g  a dev ia tion  from the SM-  prediction at the 2.4 a  leve l)7). As can be seen, a precise measurement o f  theK asymmetry will confirm or strongly -exclude the anomalous region o f  s o lu t io n s .  t ^  ' c £th • ^  ulllUjStjatl° n’ WC sh° W als0 in Fig 2 (assuming C =  0) the evolution o fthe right-handed gauge-boson  m ass-lim it at 90 % C.L. with the relativeprecision o f  the asymmetry measurement. The l e v e l in g -o f f  o f  the impactwith increasing precision is due to the assumed imprecision o f  the ft-value.3 4) T he rglatiyg longitudinal polarization (R al o f the pn^itmn^We stressed already the interest o f  these measurements in search fordeviations from the V-A" character o f  the SM : their high sensitivity  forstrongly polarized emitters and their character o f  a r e l a t i v e  m ea su rem en telin?inates  a great number “ f  p o ss ib le  sy s tem a tic  errors w h ichtn  v  abf olute measurements. They require, how ever ,  a measurement o fnote h qUar tlty’ 3 at 0 f  the lonSitudinal positron-polarization. Let us note, h o w ev er  that e f f ic ien t  po lan m eters  readily ex is t  for this type o f  m e a s u r e m e n t s 8 ).In Fi,g ‘ \ we sbow also the impact, at the 90 % C.L., o f  such a relative easurement performed at a relative precision o f  only 0 .6  % (the double o fthe one we assumed for the asymmetry measurement) assum ing an e ffec tiveS a n f atlon P (poianzation  x mean velocity  x mean co s0  o f  em ission) o f  0 8L i t  * 9 0  I T  C L  w L  V hC eVOlmit>" ,0 f  the nght-handed gauge-boson mass-  limit at 90 % C.L. with the increasing relative precision o f  (Ra) for P = 0.8.D eta iled  evaluation  o f  the system atic errors prone to this type o f  measurements and a careful optimization o f  the figure-of-merit as a function  o f  the angular acceptance o f  the polarimeter w ill  be required. It seem s,  how ever, that the potential o f  these measurements m akes it an interesting  candidate for the use, in its second phase, o f  the unique in ten se  highly  polarized trapped K 3 7 -source under developm ent at the upgraded Triumf- Tisol facility .I w ish  to thank my collaborators J. Govaerts, O. N aviliat-C uncic , R. Prieels, P. Quin and N. Severijns for valuable d iscussions, M. K okkoris for numerical evaluations and - m ost o f  all - J.M. Poutissou, J. d'Auria and P. Jackson for having given  me the opportunity to contribute with this note to their ISA C -project.R e f e r e n c e s1) J. D e u ts c h  and P. Q uin , S y m m e try -te s ts  in s e m i l e p t o n ic  w eak  interactions : a search for new physics, to be published in Precision Tests  o f  the Standard Electroweak Model, World Sc ien ce  A dvanced Series on Directions in High Energy Physics, Paul Langancker, editor2 )  J.D. Jackson, S.B. Treiman and H.W. Wyld, Phys. Rev. 1 0 6 . (1957) 517 and Nucl. Phys. 4 ,  (1957) 2063 )  M. Allet et al., Phys. Rev. Letters 68 (1991) 5724 )  O. Naviliat-Cuncic et al., J. Phys. G. Nucl. Part. Phys. 12. (1991) 9195 )  D. Schardt and K. Riisager, Zeitschr. fur Physik A 3 4 5  (1993) 2656 ) E.G. Adelberger, Phys. Rev. Letters 2H (1993) 2856)7 )  A.S. Camoy et al., J. Phys. G. Nucl. Part. Phys. 18  (1992) 8238 ) N. Severijns et al., Phys. Rev. Letters 7 0 .  (1993), 40 4 7  and erratum, to be published ; F. Gimeno-Nogues, Ph. D. Thesis, UCL 1994, unpublished.9 )  A. Burrows et al., Phys. Rev. Letters £ 8. (1992) 383410) V.A. Wichers et al., Phys. Rev. Letters 5JL, (1987), 1821 and A .S . Camoy et al., Phys. Rev. Letters £5., (1991), 324911) P.A. Quin and T.A. Girard, Phys. Letters B 2 2 9 .  (1989), 2912) M. Kokkoris et al., to be published1 3 ) 0 .  Hausser, Triumf Research Proposal n°715 and private com m unication ; sim ilar important experim ents  are in preparation for N a 21 in LBL (S. Freedman et al., LBL preprint, 1994) and W isco n s in  U niv . (P. Quin, private com m unication) as w ell as for K37 in W isconsin  U niv . (P. Quin and T. Walter, private com munication)14 )  private com m unication  from N. Severijns.456  = (M 1/ M 2)2 M 2Sfc-1Excluded  m ass reg ions o f  a new  (predom inantly  right-handed) gauge boson  W 2 as function o f  its m ix ing  angle with the know n, predominantly right-handed one (M i = 80.2  G eV /c2) (ref. 1).The region below  the line "H" is excluded by p lausib le assumptions on theH iggs-sector  (ref. 1 and refs, quoted) and that be low  the line  "SN" by the energetics o f  S N 1 9 8 7 A  (mean value from ref. 8 ; cfr. texte). The line "SN" indicates the limits to be reached by laboratory-experim ents.The region delim ited by the line "Beta-decay 1994" is the region allowed at 90 % C.L. by the actual world-average o f  experim ents  on beta-decay ; the exclusion  o f  the Standard M odel prediction 5 =  £ =  0 is due mainly to neutron-decay asymmetry m easurem ents and should be clarified  by n ew  experiments. T he constra in ts  p rov id ed  by future asym m etry  and re la t iv e  polar isation  m easu rem en ts ,  perform ed to the ind icated  re la t ive  p rec is io n  on asd iscussed  in the text, are a lso  shown ;eventua lly  they w il l  confirm , strongly constrain  or ex c lu d e  the anom alous  region which actually cha llenges the validity o f  the Standard Model.F ig. 2 ,90  % C.L. constraints on M 2 , com pared to the S N -e x c lu s io n - le v e l  to be r e a c h e d ,  p r o v id e d  by fu tu re  a sy m m etry *  and r e la t iv e  p o la r iz a t io n  m easurem ents  on K 37 as a function o f  their relative precision. W e assum ed  £ = 0 and the validity o f  the Standard Model.47Experim ents with Radioactive Beams at Louvain-la-N euve  and Prospects at an ISAC FacilityW. GalsterU C L , I n s t i tu t  de P h y s iq u e  N u c l6 a ir e ,  C h e m in  du  C y c lo t r o n ,  2 ,  B - 1 3 4 8  L o u v a i n - l a - N e u v e ,  B e l g i u mA b s t r a c tThe main aspects o f  an ISOL based RB facility  are described. Thed iscu ss io n  focuses  on experim ental techniques in RB exper im ents .  Firstexperim ents with 1 3 N and 1 9 N e beams at L ou v a in -la -N eu v e  are presented  and prospects at a large scale dedicated RB facility are analysed.I. The Radioactive Beam FacilityEarly accelerators for research applications w ere co n ce iv e d  by thescientists who planned and carried out the experiments. S ince then w e haveseen an imm ense increase in accelerator based experim ents in the f ie ld s  o fsc ience  and technology. One o f  the consequences o f  this developm ent hasbeen specialization. The scientist has become user o f  the facility  and, evenamong users working in different domains, a m eaningful d iscussion  is often  d i f f i c u l t .An ISOL based radioactive beam (RB) facility constitutes a new start. It is not simply a continuation/extension o f  the sc ientific  work that has been  done with stable ion beams in the past. We will have to take a fresh look at the techniques and at sc ience itself. The experimenter using RB has to havea clear understanding o f  the main aspects o f  a RB facility.The quality o f  the RB, its intensity, purity, em ittance and spread inenergy, is determined by quantities that are not entirely independent. The  three stages o f  an ISOL-based RB facility are : (i) primary production y ie ld  and extraction effic iency  ; (ii)  transport to the ion source, ionisation o f  therad ioactive  sp ec ie s  and injection  into the (post)  acce lerator ,  ( i i i )  post  acceleration and optics o f  the RB.A high extraction y ield  may adversely affect the ionisation e f f ic ien cy ,a chem ical and/or cryogenic purification stage should be introduced. Beamemittance, spread in energy, the intensity and purity o f  the accelerated RBdepend largely on the injection into the mass analyser and into the post-  a c c e l e r a t o r .Primary production o f  the rad ioactive sp ec ie s  is a ch ieved  by an accelerator providing intense beams o f  several tens to several hundreds o f  microamperes. Most fac il it ies  (planned or in operation) use  proton beam s,but He, 1 C beams or an intense neutron source (e.g. reactor) are promisingo p t io n s .  r  6T he L o u v a in - la -N c u v e  f a c i l i t y 1) uses 30 M eV H" beams o f  150-200  microamperes ; (p,n) reactions have typically  a conversion  rate per incidentproton o f  ~  10' 3 , but (p,2 n), (p ,a n )  and (p,2 p) also o ffer  usable conversation  rates o f  10‘4 to 10' 5 RB species per incident proton. A proton beam o f  170microamperes and a conversion  rate o f  10' 3 provide 1 0 13 RB particles persecond (pps). With < 50 % extraction, < 40  % ECR source ionisation efficiency  and a transmission o f  3-5 % through the post-accelerator, w e obtain up to 109 pps on target. In our case, the bottleneck is clearly the cyclotron used as post-accelerator. 1 0 1® pps on target might be possible in the future, but one  cannot sim ply multiply the e f f ic ie n c ie s  obtainable in the three s tages,  as they interact to some extent. The merit o f  using such a low energy as 30  M eV  protons for production lies in the low  overall radioactivity be ing  produced,  the drawback is that only a few  p-rich and very few  n-rich RB can be  produced. The overriding reason o f  course was that the 30 M eV  H' cyclotron  was available. The Louvain-la-Neuve RB facility consists  o f  two pre-existing  cyclotrons for production and acceleration coupled via a gas link and an ECR  ion source. To date intense RB o f  13N and 19Ne are available for experiments  at the Louvain-la-Neuve facility. Beams o f  ^He, ^ C ,  130 ,  ^ F ,  ^ N e  and 3 3 A r  are in preparation and should be available soon 3 ). The a ccess ib le  energy  range is 0.6 to 9 MeV/u in the harmonic 6 and harmonic 3 modes o f  the K =  110 cyclotron, with the higher energ ies  requiring high charge states (q > 3 + ). The highest intensity on target is achieved' for low  charge states (q < 2+ ) o f  the RB.Fig. 1 : RB facility at Louvain-la-Neuve.The RB facility o f  C ERN-Isolde3) utilises a high energy (E = 0.6, 2 GeV)  proton beam for production. W h ile  the production process  (fragm entation ,  f iss ion , spallation) again provides ~  1 0 13 RB particles per second, a w ide  ranee o f  p- and n-rich nuclei is produced. R B ’s o f  < 60 keV are available,49how ever, the fac il ity  lacks a post accelerator. A proposal has been putforward for an Iso lde post accelerator that is very sim ilar to the ISAC  proposal (which first surfaced in 1984).The scientific  programme at a large scale RB facility (such as CERN- Iso ld e ,  ISA C , IS O S P IN ) should  co v er  the f ie ld s  o f  n u c lea r  p h y s ic s ,  astrophysics, applied, material and m edical sc ien ces .  This versatility  canbest be ach ieved  with  a high energy production accelerator, several ion  source stations (well shielded from one another to allow  coo ling  o f f  o f  the act iv ity  in one so u rce ,  w h ile  operating  another) and a l in e a r  post  accelerator providing a high quality beam o f  low emittance (< 10 m m tcm rad)  and narrow energy p rofi le  (A E/E < 0 .5  %). I f  a cyclotron were used,  emittance and energy resolution o f  the RB will be ~  5 times worse. A  pulsed beam is desirable as a tim e-of-flight trigger.Useful intensities o f  a p-rich RB are 108 - 10^® pps on target in most cases. Too low and too high intensities will cause severe problems with the detection techniques. In the former case, the event to background rate is too small, in the latter the background rate itse lf  is too high. Experimental techniques have to be considered on equal footing with the more prominent  aspects (production and acceleration) o f  a RB facility  from the beginning.  Som e proven techniques can be adapted to RB experim ents^) with lowintensities o f  a few 108 pps by upgrading effic iency  and solid angle to near 100 % and 4tc, respectively. In general, new ideas are needed and techniques have to be found that can cope with the intense p /y  and/or thermal neutron fluxes encountered in RB experiments.The prospective user has to be aware o f  these technical aspects, which  determine the scop e o f  experim ents that can be carried out in the first operational phase o f  a RB fa c il i ty .  The d if f ic u l t ie s  sh o u ld  not beunderestimated, but th ings that are in it ia lly  "im possible" m ay w e ll  be feasible later on.II. Ex p er im e nt a l  T e ch n i qu esRB experiments have two problems : a low intensity and the em ission  o f  an intense radiation due to the decay o f  the beam particles. The latter is most severe in the case o f  p-rich nuclei. In this section, the main points  (targets, detectors, e lectron ics ,  data acquisition) are discussed briefly .1. T a r g e t sDue to its low intensity the RB poses few problems. H owever, a few  points merit mention. Targets for nuclear studies should have a rather large diameter and be mounted on a thin frame to avoid build up o f  activity. For the same reason an open geom etry o f  the vacuum chamber is preferable.  Plastic targets (C H 2 )n are an attractive alternative to H-gas targets for studiesin n u c le a r  a s t r o p h y s i c s ^ ) .  P o ly e th y len e  (P .E .) is more resistant to irradiation than more com plex polym er chains like polypropylene. Static PEtargets resist to a few  108 pps with a low loss rate in hydrogen. A largediameter rotating PE target works w ell for intensities  o f  a fe w  1 0 ^  nns (stable beam).2 . D e t e c t o r sSeveral techniques for the detection o f  p, y, charged particles (CP) and heavy ions (HI) can be upgraded for the use with RB4 ).A solenoid can be very effective in suppressing low  energy p particles.  A suppression factor o f  at least 1 0 10 has been achieved for 2 0 N a / 1 9 N e P- decay, the P end point energies are 11.2 and 2.2 MeV, respectively. A stack of  plastic scintillators in coincidence serves well as p - c o u n te r .Large arrays o f  y-detectors w ill suffer from the intense background o f  annihilation  y associated with p-rich RB. A typical array for y - s p e c t r o s c o p y  con s is t in g  o f  Ge with active  BGO anticompton suppression can be made  operational by shielding the BGO ring with 2 cm Pb. In favourable cases 2n  geom etry  can be used for near sym m etric  entrance channels. In reverse  kinem atics a 37t geometry might be feasible. However, overall so lid  angles  will be considerably below the 2 n (3n)  limit, due to sh ie lding requirements.  Studies o f  high energy y-rays are affected to a lesser extent, large BaF2 or BGO arrays can be used in close geometry with ~  2 cm Pb shielding. A fast timing coincidence with the beam RF helps to suppress background counts.A high efficiency can be obtained in the case o f  CP detection. A  solid  angle o f  2 -3 ji is possib le  with large microstrip silicon  detectors. H ow ever,  the requ irem en ts  for  d e te c t io n  e le c tr o n ic s  and data a c q u is i t io n  arestringent. Such CP arrays can also be used as a trigger for y-ray arrays. Fasttim ing co in c id en ces  between y-counter, CP array and the beam can be very e f fe c t iv e .  The L ED A array6 ) has worked to our satisfaction  (se e  later  sections) .  The energy resolution for 5.5 MeV a-partic les  is 15-20  keV  per strip. A good position resolution is obtained for a double sided X -Y  array without com prom ising the high countrate capability o f  ~  10 kHz per strip. T im e-o f-f l igh t  information for mass (A) identification is readily available, i f  the accelerator is pulsed. By using the timing signals from the accelerator  RF and the microstrip array time resolutions < 1 ns are achieved. A stack o ftwo arrays can provide identification o f  the nuclear charge Z. Thus com pleteinformation on energy, mass A and charge Z o f  an ion is obtained. D ue to the small volum e o f  a strip, such detectors have a very low  sensit iv ity  to p /yradiation (and to thermal neutrons) and are thus ideally  su ited  for RBexperiments. In an X-Y array orthogonal strips on the front and back facedefine a small pixel. True pixel counters with electronics integrated into thedetector chip will offer even better performance and lower sensitiv ity  to p /yradiation and to thermal neutrons.HI detection in RB experiments also requires large detector arrays due to the low  beam intensities. The problems encountered are similar to thosein experiments with stable beams. Due to the short range and the higher  radiation damage induced by HI, solid state counters are less  attractive thanfor CP ; however, they are often a convenient means. Gas counters can be an alternative (ion chamber, Bragg curve spectrometer, avalanche counter). Toobtain  large so lid  angles ,  an array o f  segm ented  gas counters  can be em ployed. Scintillators (Phoswich , BaF2 , Csl) can be used where good timingbut poor energy and Z resolution is needed. The application is l imited toheavy ions o f  high energy (> 10 MeV/u). A recoil mass separator should also be considered in particular for reverse k inem atics  applications, where its sm a ll  e f f e c t iv e  o p en in g  an g le  cou ld  s t i l l  prov ide  a rea so n a b ly  h ighe f f ic ien cy .  For many nuclear physics  applications a CP trigger is moreefficient,  albeit less  accurate than a recoil mass separator. Fast timing does  not only provide the mass o f  the ion, but also serves to clean up the spectra.513 . E le c t r o n ic sH ighly e ffic ien t detector arrays that cover a large so lid  angle require a vast amount o f  e lectron ics. This is not really a problem , h ow ever, as am p lifica tion , shaping and tim ing o f  a signal can be integrated into one  channel. A s in g le  spectroscopy type channel con sists  o f  a pream plifier, a shaping am plifier and an ECL discrim inator^). If necessary , pulse shaping  and constant fraction tim ing can be added. Fixed gain settings are su ffic ien t  and fine adjustments are best done by softw are during the data analysis for a com plex system . Multiple AD C , QDC, TDC (8-32 in one m odule) can be used to set up a com pact system  for ~  300 channels. Each such channel can accept ~  10 kHz, hardware thresholds are used to elim inate p /y  background b elow  a sp e c if ic  energy eq u iva len t.A h igh ly  integrated d etector /e lectron ics array fo r  3 00  ch an n els can  run at 3 MHz. This is far too much for a state o f  the art data acquisition  system  in event-by-event mode. One day o f  data taking at 3 M Hz w ould take im m ense efforts to analyse with current tech niq ues. A lthough  there has been little  progress in principal in nuclear e lec tro n ics  in the last tw o  decades, integration  o f  ex is tin g  com ponents into com pact ch an n els so lv e s  m ost o f the problems. H ow ever, advances are highly desirable.4 . D a ta  a c q u is i t io nT he b o ttlen eck  fo r  ex p er im en ts  a im in g  at h igh  p r e c is io n  andeffic ien cy  appears to be the data acquisition, also in the case o f  RB studies. Ideally  one w ants to store all inform ation in ev en t-b y -ev en t m od e, eachevent w ill co n sist o f  severa l param eters attributable to real and virtual q u a n tit ie s , su ch  as e n e r g y , tim in g  e tc . and im p o se d  c o in c id e n c erequirem ents, real time etc ., respectively . Gain m atching, energy and tim ecuts and other m anipulations are carried out by so ftw are rather than by hardware operations in the o ff-lin e  analysis. It w ould  be m uch too tim econsum ing to adjust fine ga in , tim ing and threshold for 300  channels byhardware prior to an experim ent.At the bottom line is the human factor. How much data can actually be analyzed ? From this it appears that an acquisition system  o f  30 0  channelsrunning at up to 100 kHz in event-by-event m ode is su ffic ien t for all but a few  experim ents. These restrictions should be accepted in an experim ent.At L ouvain-la-N euve a CAM AC/VM E based data acquisition for up to 300  channels has been developed  for RB experim ents^). T w o full s ize  CAM ACcrates for 8-fold Silena ADC's and 16-fold LeCroy TDC's are connected to a VME  crate with fast processors and buffers connected  to the U N IX  system  via  ethernet. The system  is open in the sense that all its com ponents can be replaced/upgraded w ithout changing its structure. O peration is based upon  softw are rather than upon hardware (FERA). W e have com prom ised a bit on sp eed , but gained  in f lex ib ility . The acq u isition  so ftw are can ea s ily  be changed for d ifferent requirem ents. Total count rates up to 100 kH z are p ossib le  in event-by-event m ode with a high lev el o f  dead tim e. W e are running routinely at 30 kH z with 10-30 % dead tim e depending on  the acquisition  m ode. With faster processors and buffers in particular for the tape drive and histogram m ing tasks up to 100 kHz should be p ossib le  in the near future without excessive  dead time (at present 80 %).M on itorin g  the data tak ing in a co m p lex  sy stem  is  o f  u tm ost im portance. This w ill allow  to use recognition patterns (in the sim plest case  a trigger) to select good events that are m onitored independently. W e use a P C -b ased  sy s te m 9 ) that reads histogram s directly from the VM E buffer. A  la rg e  num ber o f  so ftw a re  c o n d it io n s  can  be u sed  on  a ll ch a n n e ls .  B id im en sion al and s in g le s  spectra can be d isp layed  with ease  as w e ll as m u ltip lic itie s, gates etc ... Proper m onitoring a llow s to reduce the event-b y-  even t rate to acceptable lev e ls  by m aintaining a large num ber o f  "control" histogram s. The system  is very com pact and inexpensive.III. Experiments with RB at Louvain-la-NeuveSin ce the RB facility  delivered its first intense (> 108 pps) 13N beam  by the end o f  1990, experim ents have been carried out in the field s o f  nucleara stro p h y sics  and low  energy nuclear p h y s ic s . T he m ain fo c u s  is  on  astrophysics. Over the last 3 years w e have investigated  the onset o f  and the esca p e  from the hot CNO cy c le s  through the rad iative capture reaction s  13N (p ,y )140  10) and 19N e (p ,y )2 0 Na, resp ective ly .In the first case capture y-rays could be m easured d irectly in large Gecrystals. The y cro ss-sectio n  integrated over the resonance reg ion  is  verylarge (~  100 p b ) in this case. The 1 9 N e ( p ,y )  rate on the other hand wasanticipated  to be ~  1000 tim es low er. G iven  that the y-ray energy is  inaddition more than 2 times low er, direct y-detection w as ruled out. Instead w e  developed  indirect detection  techniques. The reaction product 2 0 Na y decays  to its ground state fo llow ed  by p + decay to several excited  states in 2 0 N e ; a 79 % branch populates the 2 + [1.63 M eV] state in 2 0 N e and 16 % and 3 % branches feed the 2+ leve ls  at 7 .42 and 10.27 M eV, which a -d ecay  to the 1 6 0ground state. For one group o f  experim ents, w e constructed a so len oid  ; thelow  en ergy  (2 .2  M eV en d p oin t) p a ssocia ted  w ith the 1 9 N e beam  are suppressed and the high energy (11 .2  M eV ) p o f 2 0 Na are focused into a stack  o f  p lastic  scin tilla tors for counting. In another group o f  exp erim en ts w e  m easured the P -d elayed  a -d ec a y  from 20 Na D ) in very thin s ilic o n  m icro  strip detectors (30 pm ). These detectors are insensitive to very high flu xes o f  m inim um  io n iz in g  p -p artic les. An extrem ely sim p le  technique (so lid  state  track fo ils) has also been used su ccessfu lly .T h ese  first ex p er im en ts  w ith  RB o f  a stro p h y sica l in te r e s t1 0 ' 1 3 ) illustrate the need to use different techniques. It is  often not a priori clear,w hich  technique is the best to carry out a particular experim ent. In the fo llo w in g  I w ill concentrate on nuclear p h ysics exp erim en ts, although there is som etim es an in terd iscip linary interest.i) R e s o n a n t  s c a t t e r in g  in  r e v e r s e  k in e m a t ic sResonant scattering o f  a RB on a hydrogen target is a pow erful tool to investigate  broad (T ~ 1 keV) resonances in exotic nuclei. The y ield  obtained  with a thick polyethylene target (C H 2 ) reproduces the energy dependence o fthe scattering across the resonance5 ) accurately at a s ing le  shot. The heavy  RB loses a sizeable fraction o f  its initial energy in the target ; the recoiling  proton lo sin g  very little  o f its energy in the target, scans the energy region53o f  interest very p recisely . The parameters o f  the resonances in the energy  range defined  by the RB traversing the thick (C H 2)n  target are extracted  from fits to the scattering y ield  using the B reit-W igner form alism , the R- matrix and the K -m atrix approach. The results are con sisten t w ith in  very  small errors (< 1 keV) ; resonance energy Er and total width T are determ ined  to w ithin  ~  1 keV  absolute accuracy by u sing  isobaric stab le beam s for energy calibration. Spin and parity are unam bigously obtained and p o ssib le  contributions o f  h igher partial w aves (Z > 0) can be extracted from the angular d istribution . The L ED A  array a llo w s to m easure en ergy  range  ("excitation  function") and angular d istribution  sim u ltaneously . 16 s ilic o n  strips o f  5 mm width form a segm ent, 8 segm ents are assem bled to an array o f  concentric rings form ing an annulus o f  an outer diam eter o f  2 60  m m , the central hole is 100 mm in diam eter. The method is w ell su ited  to study  resonances in ex o tic  nuclei o f  interest to nuclear p hysics, astrophysics and applications. The low er detection lim it for the total width is T ~  0.5 keV. Two resonances with J71 =  0 + and 1+ in 20Na appear at Ecm ~  800 and 890 keV in 1 9 N e + p scattering. The well known resonances at Ecm = 8 8 8(1+ ), 829(2" ),8 0 1 (0 + )  and 637 k e V (l+ )  in the isobaric team scattering 19F + p are used for en ergy  ca lib ra tio n .Fig. 2 : Resonant scattering in reverse kinem atics using isobaric beam s.The absolute precision o f ~  1 keV is an order o f  magnitude better than what isach ieved  with ind irect m ethods u sin g  stab le beam s in (p ,n ) or ( 3 H e , t )reactions to populate resonances. A full publication  o f  the 1 9 N e / 1 9 F + presults is being subm itted.i i )  E la s t ic  s c a t t e r in g  u s in g  is o b a r ic  b ea m sT he e la s t ic  transfer o f  a lo o s e ly  bound p roton /neutron  b etw eenidentical cores was studied with 13N / 13C beams on 1 2 C targets. The effect o f  charge sym m etry on the n u c leu s-n u cleu s potential can be in vestiga ted  bycom paring e lastic  proton and neutron transfer in 1 3 N +  1 2 C and 1 3 C + 1 2 C scattering. The optical potential can be written as :V(r) = V0 (r) + (-1)* Vp(r) ;V o is  the central poten tia l, V p is  a parity term and t  is  the angular  m om entum . An enhanced parity e ffec t  is expected for 1 3 N + 12C due to the low er binding energy o f  the proton in 1 3 N (1 .94  M eV ) as com pared to the binding o f  the neutron in 1 3 C (4 .95  M eV ). The data are analyzed in this sim p le  optical m odel with parity term and in the LCNO (Linear Com binationo f  N uclear O rbitals) m odel o f  von Oertzen. The transfer o f  tw o va len ce  n u c leo n s betw een  identical 1 2 C was studied with 1 3 N + 1 3 C and 1 3 C + 1 3 C scattering. P osition  sen sitive  detectors were used coverin g  an angular range corresponding to a full angular distribution. The scattered and the recoil ion  w ere measured at forward angles < 9 0 ° , thin degrader fo ils  w ere m ounted at the front face o f  the silicon  detectors exp lo iting  the d ifferen ce in stopping  pow er. The analysis is being com pleted and results w ill be published so o n 14).i i i )  I so sp in  e f fe c t s  in  G D R  d eca yThe com pound nucleus 2 4 M g was formed at an excitation energy E* = 40  M eV  with channel isospin T = 0 and T = 0,1 using the fusion reactions 1 2 C + 12C and 13N + 11B. High energy y-rays were measured in a 4" x 4" BGO crystalin c lo se  geom etry. A relatively small crystal o f  high absorption density c lo seto the target o ffers the best y ie ld  to cosm ic background ratio. T he BGO  su rface to target d istance w as 10 cm . By com parison , a large N a l(T l)  spectrom eter o f  10” x 13" offers the same solid angle at ~  30 cm distance, but exceed s the BGO volum e by a factor 20. The fact that the N al offers a better energy response is not very important in the case o f  broad giant resonances. T he GDR y -y ie ld s for both reactions were detected. N aively  one exp ects astrong inhibition o f  E l y-decay for the T = 0 channel. The E l (GDR) y -d e c a yin a selfconjugate nucleus requires an isospin  flip  AT = ± 1 forcing a T = 0 lev e l (in the regim e o f  high level density) to decay to T = 1 lev e ls  at low  e x c ita tio n  en ergy  and low  le v e l d en sity . C o n seq u en tly , on e ex p ec ts  inhib ition  o f  E l y-decay fo llow in g  fusion  o f  a pure T = 0 entrance channel, u nless there is strong m ixing o f  isospin T in the com pound nucleus. This T- m ixing is generally assumed to be weak. A raw y-spectrum is shown in Fig. 3 for the 12C + 12C reaction at Ejab =  55 M eV. A fast coincidence with the RF o f  the cyclotron was used and the cosm ic background was reduced by a factor 3 by m eans o f  an active shield o f  Cerenkov counters. A low  energy threshold  o f  ~  4  MeV y energy eq u ivalent w as set in the tim ing channel (constant fraction discrim inator). The 1 2 C beam intensity was 3 .109 pps. A very clean  spectrum  is observed for the y yield up to Ep ~ 25 M eV  covering 6 orders o f  m a g n itu d e .557' rays are obscured by discrete y-lines ; above Ey > 25 M eV  cosm icbackground d om in ates.D iscrete lines are present in the region o f  the GDR that should be subtracted  before fitting the data with a statistical cod e (C A SC A D E). T he problem  is compounded in the case o f  13N + n B. The J* = 1+ (T = 1 and 0) levels in 12C at 15.11 and 12.71 M eV are strongly populated by a resonant transfer o f  the valence proton in 13N to hole states in 1 2 C ; the transfer Q -value is 14 M eV. No enhancement o f  the T = 1 over the T = 0 component is observed in the GDRy-yield induced by 13N + n B. A detailed study o f  this phenom enon using 8-10B a F 2 crystals in coincidence with a silicon  m icrostrip array (L E D A ) w ill beundertaken in March 1994. The aim is tw o fo ld . RB are em p loyed  toinvestigate isospin  purity and m ixing in m edium  heavy n uclei ; G DR decayfo llo w in g  RB induced reactions serves as a tool to gain  an in sig h t intoreaction m echanism s. A d econ volu tion  m ethod for y - s p e c t r a * 3 ) has been  developed  by a collaborator Csaba Sukosd (Budapest) that a llo w s to subtract (broad) discrete lines and to extract precise inform ation on the cross sectionso f  y y ields. This is essential for obtaining accurate values for isosp in  m ixing.The CERN M onte Carlo code G EAN T3, an interpolation technique and matrixinversion  accum ulate the response in the full energy region o f  a y-ray. In this way it is possib le to obtain spectroscopy like resolution for high energy  y-rays with high effic ien cy . Each elem ent o f  a BaF2 4ji array (5" x 6") withan inner radius o f  10 cm offers 2 % total efficien cy  for 15 M eV  y-rays. Theenergy resolution is lim ited by doppler broadening o f  about 200  keV  in the case o f  the reactions studied ( 12C + 12C and 13N + n B ).Fig. 4  : GDR y-decay follow ing the RB-induced 13N + J1B fusion reaction. The 15.11 y-ray in 12C obscures the giant resonance y ield . The cosm ic raybackground poses a problem as the RB intensity is low  (2.10^ pps).Other experim ents using 13N and 19N e beam s are scheduled  for Spring 94. Phil W o o d s16) has suggested to investigate the di-proton decay o f  1 4 0  with the LEDA array. The perhaps best candidate for this exotic  decay mode is the 7 .77 M eV level in 140  that can be reached with a 45 M eV 1 3 N  beam inreverse k inem atics. B ill G elle tly 17) and collaborators have proposed to studythe breaking o f  mirror symmetry at high spin (1 7 /2  - 31 /2  h )  due to thealignm ent o f  a proton (neutron) pair. The m irror n u cle i 3 5 N i / 3 3 Co arepopulated by 4 ®Ca ( 1 9 N e / 19F, 2p2n) with isobaric 1 9 N e / 19F 4+  beam s o f  70 M eV. 8 com pton suppressed Ge detectors from the Daresbury T essa  array are used in co incidence with the CP array LEDA. Channel selection  is less precise but the e ff ic ie n c y  is much h igher than with a reco il m ass separator. A  L o u v a in -la -N e u v e  K iev  co llab oration  w ill in v e s t ig a te  subbarrier ^H e (14  M eV ) scattering on 2 08p ^  t0 search for a 4 H e-2n  com pon en t in the w ave  function o f  6 H e .C o lla b o r a tio n s  w ith  ex tern a l u sers h a v e  g rea tly  en h a n ced  the  potential o f  the L ouvain -la-N euve RB fa c ility . In particular the Edinburghgroup o f  A lan Shotter and collaborators has helped  us im m en se ly  w ith a variety o f  sc ien tific  and technical problem s.Our current lim itations are m ainly due to m anpow er, ava ila b ility  o fbeam  tim e (w eek en d s on ly ) and (perhaps m o st im portant) the lim ited  number o f  RB. The two cyclotron concept delivers a good beam purity as the cvrlotron i*= ? trnod mass «enarator H ow ever careful tuning i<= necessary and57wc have suffered from isobaric beam contam inant at the percent lev e l in one experim ent using 1^ N e ^ + beams o f 20 M eV. Isobaric contam inants are difficu lt to detect, but resonant scattering on H is a very precise tool down to the subpercent lev e l. Beam  d iagn ostics is a d ifficu lt task w ith RB as intensities are too high for direct detection o f  the beam particles and too low  for indirect methods. U nlike intense p-rich RB which P + -d ec a y  p ro d u c in g  an im m ense flux o f  annihilation y-rays, n-rich RB p ose few er problem s to the detection .IV. Prospects at an ISAC facilityA large sca le RB fac ility  using high energy proton beam s a llo w s to produce a w ealth  o f  p- and n-rich  n u c le i through p -in d u ced  f is s io n ,  fragm entation and spallation  reactions. Fast and e ffic ien t extraction  fromthe hot production target is a com plex process, but one can draw on the vastexperience gained at CERN. General features o f  the extraction  chem istry  limit the scope o f an ISOL based RB facility to exotic nuclei o f  halflives > 0 . 1  s; for shorter halflives a fragm entation based RB facility  is better suited. Bothm eth ods are com plem en tary  in m ost resp ects  in c lu d in g  the s c ie n t if ic  program. The intensity and quality o f the RB is superior for an ISOL basedfacility , how ever, this doesn't com e cheap. Only a dedicated fac ility  w ith ahigh design aim can utilize the potential offered by the ISOL technique.Intensities o f  103 - 101® pps are useful for experim ents with p-rich RB,but higher intensities are desirable for n-rich RB. Very intense p-rich RB(> 1 0 1(  ^ pps) require novel detection  techniques and it is o ften  better toim prove d etectio n  e f f ic ie n c y  than to ch ase a s lig h t in crea se  in beam  in tensity . T herefore d etection  techniques have to be considered  on a par with production and acceleration  o f  the R B. The em ittance o f  the beam  should be small (< 10 mm n.mrad) and a narrow energy profile (AE/E < 0.5 %) is desirable ; this w ill make life  much easier for the experim enter. The ratioo f  m ass over m ass d ifference for isobars is typ ically  ~  6 .1 0 3 . H igh beam  purity dem ands a very good m ass se lection  (M /A M  > 104 ). C hem ical andcryogenic purification o f  the extracted radioactive sp ec ies  prior to ion isationim proves ion isation  e f fic ien cy  as w ell as beam purity by a con sid erab le  margin. A very attractive method is se lec tiv e  laser-induced ion isation  albeit only q = 1+ charge states can be produced. A n ice feature is the ease  o f  sw itching betw een isobaric beam s. At L ouvain -la-N euve this is done by a slight adjustment to a single trim coil. Intense beams o f  stable isobars can be em ployed to calibrate and setup the RB experim ent. From my point o f  v iew  the desirable energy range is < 10 M eV/u for nuclear research. At h igher  en erg ies RB based on the fragm entation technique are already a v a ila b le , although the intensities are low er than those that could be achieved w ith an ISOL based facility. A post-accelerator could be added later to a ISOL type RB fac ility  fo llow in g  strong demand.N uclear A strop h ysics dem ands an im m ense input o f  n uclear data.Rates for radiative capture are required for the netw orks o f  the hot CNO  cyc les, the rp process, the r and the p process always needing RB. Resonant capture (p .y) can som etim es be m easured d irectly , but often  requires new  techniques due to the low  cross sections (< 1 pb) in most cases. D irect capture rates can be determ ined by m eans o f  the (d ,n) reaction . R eso n a n ce  parameters are obtained by RB scattering in reverse k in em atics for broadresonances (T > 0.5 keV ). Spectroscopic information on nuclei c lo se  to the p and n dripline is needed where reaction rates cannot be m easured d irectly.RB o ffer  a unique opportunity to gain a better insigh t into N uclear  P h y s i c s .( i )  R esonant scattering u sing  isobaric beam s can be em p loyed  for the spectroscopy o f  exotic  nuclei. In som e cases lik e  + p —» m assm easurem ents o f  nuclear ground sta tes are a cc ess ib le  to RB o fferin g  very high precision (absolute error <  1 keV ). U sin g  isobaric beam s for an absolute energy ca libration , nuclear m asses o f  som e ex o tic  n u cle icou ld  be determ ined to w ith in  ± 100 eV , i f  le v e ls  in neighbouringisobars are known to within ~  10 keV  from y -sp ectro sco p y . A t h igherRB energies o f a few  M eV/u RB scattering on (C H 2)n and H 2 gas targets allow s to probe the surface o f  d iffuse p and n-rich n uclei. Hard spherescattering increases rapidly above the energy o f  the cou lom b  barrierand begins to saturate at a few  M eV /u (in reverse k in em atics), largely  exceed in g  the cross section  for pure coulom b scattering around 0 Cm =180°. From a first evaluation o f  data obtained at L ouvain -la-N euve oneexp ects a sen sitiv ity  below  the 1 % lev e l to deviation s from the rigid  sphere scatterin g . Very sm all d ev ia tio n s  (<  1 %) from  cou lom bscattering o f a point charge can be explored in the v icin ity  o f  a strong resonan ce at en erg ie s  b e lo w  the cou lom b  barrier. T he e f fe c t  is am plified  by the interference o f  the cou lom b and resonant am plitudesat the m inim um  and m axim um  o f  the in terferen ce pattern. Suchd ev ia tio n s  have p o ss ib ly  been attributed  to con tr ib u tion s o f  h igh erpartial w aves (£  > 0) in past studies using proton scattering on 1 2 »1 3 Ct a r g e t s .( i i )  E xotic decays o f  proton and neutron pairs from excited  p and n-richn u cle i are a cc ess ib le  to RB stu d ies  and even  ground-state d i-protonradioactivity may be observed. The proton and a -ra d io a ctiv ity  o f  p-rich  n u cle i can be in vestiga ted  sy stem a tica lly . T he p h y sics  in terest hasbeen d iscussed  in recent review  a rtic les18). A detailed know ledge o f  J3- d e la y ed  p a rtic le  d eca y  o f  e x o t i c  n u c le i is  a lso  req u ired  for  investiga tion s using th is phenom enon as a d etection  technique (e .g . in n u c lea r  a str o p h y s ic s ) .( i i i )  Reaction m echanism s with exotic  n ucle i. N uclear lev e ls  o f  high isospin  T = 2 in N = Z nuclei are reached through fusion o f  a p-rich RB with a n- rich target. Som e exam ples are :+ 10Be -> 20Ne ; 10C + 14C -> 24Mg ;140  + 14C -> 28Si ; 18Ne + 22Ne -> 40Ca etc.The level densities o f  T = 1 and T =  2 states in s e lf  conjugate nuclei can beinvestigated  by using the particle and y-decay o f  giant resonances as a too l. The transfer o f  a valence nucleon  or clu ster is exp ected  to be en h anced  or reson an t for the c a se  w here o n e  en trance ch an n el nucleus exh ibits a very low  binding energy, whereas for the other one  the b indi ng is high.  M o lecu la r  c o n fig u ra tio n s  (r e so n a n c es) and com pound nucleus doorw ay states can be exp lored . There m ight be a connection  to the co llec tiv e  p-h excita tion s in giant resonances.T ransfer reactions can be used to study rad iative capture u sin g  the nuclear rather than the coulom b break-up. Alan Shotter has su ggested59to use the Trojan horse" e f f e c t ^ ) .  A proton loosely  bound to a heavy  core is brought over the cou lom b barrier in the heavy ion entrance  ch an nel. It can than m ove rather free ly  and the e ffe c t  o f  the  penetrability on capture p + HI is strongly reduced. The sim ultaneous  m easurem ent o f  three final p artic les is required to in v estig a te  this  phenom enon in a L E D A -typ e d etector . T here is a n u c lear  and a stro p h y sics  in terest.( v i )  Excitation o f  giant resonances with RBAn important question is to what extent isospin remains a good quantum  num ber in hot n u c le i. T he purity and m ix in g  o f  iso sp in  in s e lf ­conjugate nuclei in the m edium m ass range (20 < A < 60) can best be investigated with RB. The particle and y-decay from giant reson an ces, induced with RB should exhibit a more sp ecific  pattern than in the case  o f nuclear levels populated by stable beams. The quenching o f  the GDR  strength at m oderate excitation  en erg ies can be investigated  u sin g  RB  by varying the entrance channel. The isosp in  dep en dence and the dependence on angular m om entum  can be studied. The co lle c tiv e  p-h excitations o f giant resonances can be regarded as doorway states in the com pound nucleus. Giant resonances are an apt tool to investigate  the sharing o f  ex c ita tio n  en ergy in the entrance ch a n n el. T he tw o  extrem es are equal sharing irrespective o f  m ass in fast direct p rocesses  and sharing a cco rd in g  to m ass fo r  fu ll eq u ilib ra tio n . E x c e ss  neutrons/protons in n/p-rich nuclei are lo o se ly  bound ; in m any cases  this configuration is w ell described by a sim ple picture (core +  valen ce  nucleons). T his feature can be explored to gain an insigh t into the details o f  the excita tion  process. In clusive m easurem ents o f  G D R  y yields can be carried out in coincidence with the core + nucleons.The effect o f  the neutron skin on co llec tiv e  states has been d iscussed  by D ave Warner20) at the 2nd RB Conference at L ouvain-la-N euve.V. Final RemarksMany facets o f  the physics with RB have been d iscussed  previously  in dedicated conferences and w orkshops. In a recent workshop at Oak R idgeCyrus B aktash21) has discussed aspects o f  nuclear structure studies w ith RB. J. W ilhelm y and P. M oller2 2 ) have proposed to use heavy exotic  n u c le i in reverse kinem atics (d, p fis) to investigate fission  o f  extrem ely n-poor heavy  nuclei. New  aspects w ill em erge at the ISAC workshop. I could only cover a sm all part o f  this new  field  in sc ien ce  and I w ill refrain from presenting  conclusions. This is just the beginning o f  research using RB. After the pilot experim ents with accelerated  RB, w e should focu s on first generation  RB experim ents that are feasib le  with the characteristics o f  the proposed largesca le  fa c ility  ; experim ental techniques play a vital role. The dream tim e will be over soon.A c k n o w l e d g e m e n t sT he w orks d escribed  here are part o f  the R a d io a ctiv e  Ion B eam  Program m e in L ouvain -la-N euve and are supported by a special grant (PA I) from  the B elg ian  governm ent. T he experim ental w ork referred to w as carried out in collaboration with R. Coszach, Th. Delbar, P. L eleux, I. L icot, E. L ienard, P. L ipnik, C. M ichotte, A. N inane, M .P. Sim onart and J. V ervier  (U n iversite  C atholique de Louvain) ; F. B inon, P. D uham el, J. V anhorenbeeck  and Ph. V ijghen (U niversite Libre de B ruxelles) ; C. B ain, T. D avin son , R. P age, A. Shotter and P. W oods (U niversity o f  Edinburgh) ; P. D ecrock , M. H u yse , G. V an craeyn est, P. Van D uppen (K .U . L eu ven ) ; M. W iesch er  (U n iversity  o f  Notre D am e) ; C. Siikbsd (T echnical U niversity  o f  B udapest). T ech n ica l support was provided by the Cenre de R ech erches du C yclotron  (CRC) and by P. C ollin, P. Demaret, Y. Longree and the technical workshop.I would like to thank P. Leleux, C. Sukosd and P. W oods for suggestions co n ce rn in g  the m an u scrip t.R e f e r e n c e s1) D. Darquennes et al., Phys. Rev. C42 (1990) R8042 )  M . L o ise le t et a l., in 3rd Int. Conf. on R ad ioactive  N u clear B eam s, M ichigan State U niversity, May 19933 )  H. Ravn in W orkshop on the Production and U se  o f  Intense R ad ioactive  Beam s at the Isospin Lab., Oak Ridge, Oct. 1992, conf-9210121, p. 1714 )  W . G alster in R ad ioactive N uclear B eam s, L ou va in -la -N eu ve, A ug. 1991,Adam H ilger (Bristol 1992), p. 3755 )  W. Galster al., Phys. Rev. C44 (1991) 27766 )  W . G alster in W orkshop on R esonant S catterin g , B ru ssels , D ec . 1992,Internal Report No 93-01 , p. 14A. Shotter, ib id p. 217 )  S. Thom as, T. Davinson and A. Shotter, Rutherford A ppleton Lab. reportRAL 89-063 and NIM A288 (1990) 212, T. Davinson et al., NIM A 288 (1990)2 458 )  A. Ninane et al., to be published9 )  Y. Longreee and P. Duham el, to be published1 0 ) P. Decrock et al., Phys. Rev. Lett. 67 (1991) 808P. Decrock et al., Phys. Lett. B304 (1993) 501 1 ) R. Page et al., in 3rd Int. Conf. on R NB, M ichigan State Un iversity, May 1993 and to be published1 2 ) R. Coszach el al., in N uclei in the Cosm os, Karlsruhe, July 1992, Inst, o f  P hysics Pub. (Bristol 1993) p. 295 and to be published1 3 ) W . G alster in N ew  N uclear Physics w ith A dvanced T echniques, Ierapetra (G reece), June 1991, W orld Scintific (Singapore 1992), p. 354W. G alster in Int. Conf. on the Future o f  N uclear Spectroscopy, H eraklion  (G reece), June 19931 4 ) D. Baye et al., in Radioactive Nuclear Beam s, L ouvain-la-N euve, Aug. 1991, Adam H ilger (Bristol 1992) p. 173 and to be published1 5 ) C. Sukosd et al., to be published1 6 ) P. W oods et al., proposition o f  experim ent at the L ouvain -la -N euve RB f a c i l i t y1 7)  W. Catford et al., ibid.1 8 ) K. Livingstone et al., PRC 48( 1993) R2151 ; A.C. M ueller and B. Sherrill, to be published Ann. Rev. Nucl. Sci.611 9 ) See A. Shotter in ref. 62 0 )  D. Warner in R adioactive N uclear Beam s, L ouvain -la -N euve, A ug. 1991, Adam Hilger (Bristol 1992) p. 1392 1 )  C. Baktash in Workshop on the Proc. and Use o f  Intense RB at the Isospin  Lab., Oak Ridge, Oct. 1992, conf-9210121, p. 172 2 )  J. W ilhelm y and P. Moller, ibid, p. 1512 3 )  planned large scale RB facilities such as ISOSPIN, ISACPARITY VIOLATION IN BETA-DELAYED ALPHA EMISSIONG. Roy University of AlbertaIntroduction:The leptonic and semi-leptonic weak interaction is well described by the Standard model of Glashow, Weinberg and Salam. In this model, weak processes are due to current-current interactions, where these currents are mediated by the W and Z intermediate vector bosons. The existence of semi-leptonic weak processes implies the existence also of the nonleptonic weak interaction, strictly between quarks. The GIM mechanism tells us that strangeness changing decays of hadrons (e.g. lambda decay) are due to charged currents only, while both charged and neutral currents are involved in flavour-conserving weak interactions between hadrons. Thus the only way to study the part of the interaction due to hadronic weak neutral currents is in non-strangeness changing processes. The presence of these weak currents will normally be masked by the strong interaction; in order to study them we require experimental situations where conservation laws suppress the strong interaction, allowing weak neutral currents to show their effects. Parity violation experiments are the only way to study flavour- conserving weak neutral currents. Unfortunately the hadronic weak interaction is difficult to understand quantitatively because it does not lie in the perturbative QCD regime.PARITY VIOLATION AT LOW AND MEDIUM ENERGIES:At low energies between the hadrons ( e.g. in nuclei and below 300 MeV in p-p scattering) a meson-exchange description is convenient, based on one and two meson exchanges between a parity conserving (PC) strong interaction vertex and a parity non­conserving (PNC) weak interaction vertex. The PNC interaction is parametrized by a set of six meson-nucleon coupling constants fT , hp0,1,2 and hu0'1 , where the superscripts refer to isospin changes. Desplanques, Donoghue and Holstein11 (DDH) synthesized various63approaches based on the quark model and SU(6)W to calculate the coupling constants. They presented theoretical "best values" and "reasonable ranges" for the coupling constants. Similar calculations were made by Desplanques2) independently and by Dubovik and Zenkin3). Feldman et al4^ extended the calculations by including the weak A-nucleon-meson and A-A-meson PNC vertices for 7r, p, and u mesons. Their predictions are summarized in table 1, where the quantities in brackets are reasonable ranges. In a review, Adelberger and Haxton5) fitted the best parity violation data to a two parameter expression based on the quark model formalism of DDH. However, their values for the weak meson-nucleon coupling constants were only marginally better constrained than the reasonable range estimates of DDH. The result of their fits are also included in table 1.Experimental situation:Many parity violation measurements have been performed in nuclear systems and in p-p elastic scattering, but they have not been sufficient to accurately determine the coupling constants. An ongoing experiment at TRIUMF (E497) should serve to uniquelyCoupling DDH1} d2> d z 3) FCDH4* a h 5)xlO '7 xlO ' 7 xlO '7 xlO ' 7 xlO '7fx1 +4 . 6 (0-*ll .4) + 2 . 7 +1.3 +2.7 (0-*6.5) +2 . 1-11.4 (- 31-*11.4) -6 . 1 + 8.3 -3 . 8 (-31-*11) -5.7V -0.19 (-0.38-»0) -0.4 + 0.4 - 0 . 4 ( -1. l-*0 . 4) 1 0 to HV -9.5 (-1 1 -*-7 . 6 ) - 6  . 8 -6.7 -6 . 8  (-9. 5-*-6 .1) -7.0303A-1.9 (-10.3->5.7) -6.5 -3.9 -4 . 9 (-10 . 6-»2 . 7) -6.5K 1 -1.1 (-1. 9-*- 0 . 8 ) -2.3 -2 . 2 -2.3 (-3.8-+-1.1) -2.4determine the hp coupling constants, and also the value of hu, whenthe results of other p-p parity violating experiments are foldedin. However p-p scattering does not include effects of the single pion coupling constant fx, as this would also violate CP conservation, since the 7r° is its own antiparticle. The isovector coupling constant fx is particularly interesting because 95% of its predicted value is attributed to neutral current diagrams in the quark model.In light nuclei, the PNC nucleon-nucleon interaction leads to small opposite parity admixtures in the initial and final nuclear states. Specifically, the nuclei 18F,19F and 21Ne have a low-lying doublet of states of identical spin but opposite parity; these states are mixed by the weak interaction which leads to PNC effects like circular 7 -ray polarization. Measurements of fT were performed by measuring P7 or Ay from parity-mixed doublets in 18F6), 19F7), and21Ne81, and by measuring the longitudinal analyzing power AL in polarized proton scattering on 4He9). The last three of these experiments are not only sensitive to fx but also to hp and hw. In these cases, the experimental results are expressed as Af ,,1 + B(hp° + 0.58hu°) , where the A and B coefficients are obtained from nuclear structure calculations. Note that the shell model calculations for 18F and 19F can be checked by comparison with the 18Ne and 19Ne first forbidden /3-decay to the appropriate states101. This leads to a reduction of the weak meson-nucleon coupling constants by a factor of three in the case of (l+ho?) shell model calculations. Such a procedure is not possible with 21Ne, as there is no proper /3-decay, but it is expected that the same reduction factor is valid and iscommonly applied.The 18F 7 -ray circular polarization experiments yield61 a mean value for fx of (0.3^.3+0-9)xl0'7. The average of the four above experiments91 gives fx = (3. 3±1.0) xlO’7. However, the fourexperiments do not have a common overlap, as is shown in figure 1 , where the 21Ne results were reduced by a factor of three, as mentioned. Holstein111 has found that this small value of fx would require current algebra quark masses to be increased by a factor of652 over the Weinberg values. Therefore it is important that a confirming experiment be done on this coupling constant.The mixing of states of opposite parity by the weakinteraction can also lead to PNC a-decay. Consider a simple two-level case, where we have a O', T=1 state which can a-decay byparity violation to a 0+ state. This O'-state mixes with a 0 + , T=0 state. Then the PNC a-decay width will be raPNC = |A|2 ra(0 + ), where ra(0+) is the parity-allowed a-decay width, and the amplitude A is related to the weak interaction potential VPNC(AT=1) by12):<0 + T=0 [Vm c {AT=l) ] 0T=1>(E.-Ej-li(T_-r+)where E+(.) and r+(.) denote the excitation energies and the total widths of the 0+('^ levels. It is clear from the above formula that it is important to have good shell-model information on the relevant state wave functions; also, small energy differences between the mixing levels and small total decay widths will enhance the r / NC width. Of course the presence of other mixing states will complicate the calculations.Parity violating a-decay has been observed previously in the decay of the 8.87 MeV 2* state in 160 to the ground state of 12C by Neubeck et al13); this state has a small y-width of 3xl0'3 eV, which enhances the PNC effects. However, there are several 2+ levels that must be included in the analysis of PNC, and the shell model picture for these states is not adequate. Neubeck at al measured a parity-violating width raPNC = (1. 03 + 0 . 28) xlO'10 eV! Note that this particular decay is not dependent on fx. PNC a-decay of the 11.26 MeV 1+ state in 20Ne has also been studied by detecting a parity violating resonance in the 16O(a,7 )20Ne reaction14). This case is sensitive to the isovector Al=l PNC interaction. Unfortunately the shell-model identification of the mixing 1' states is unclear andthus it is not possible to extract a PNC matrix element.Proposal:We propose to study the parity nonconserving (PNC) a-decay width of the O' state in 180 at 6.88 MeV. The energy-level diagram for 180 taken from Ajzenberg-Selove151 is shown in fig. 2. The 6.88 MeV state will be populated from /3-decay of 18N; this /3-decay has been studied by Olness et al16), where they measured a /3-branching ratio of 14.8% for the decay to this state. Brown and Gai17) have made a preliminary study of this a-decay. In this case, both isoscalar and isovector PNC interactions contribute and the PNC effect is proportional to the combination 3.If,.1 - 0.84hp°. Thus this decay would allow us to obtain a further measurement of fx. The 6.88 MeV state is similar in structure to the 0" state in 18F at 1081 keV: it has a fairly pure lS1/2-0P1/2 structure. The PNC a-decay width is dominated by interference with the 0" state from the 0 + ground state (predominantly 2p in structure) and the 0+ 3.63 MeV state (predominantly 4p-2h in structure). Unfortunately this is a relatively large energy difference between the mixing states, and decreases the r / NC width. Other states are less important and give a small destructive contribution. Note that uncertainties in the structure of the states involved could confuse the interpretation of the results. Brown and Gai17) predict a PNC width of approximately 10'11 eV, but this width could be reduced by as much as a factor of 1/3 depending on the shell model calculations used. This result is an order of magnitude less than the PNC width in 160 measured by Neubeck et al 131.Experimental technique:We intend to use the TISOL apparatus at TRIUMF to produce a separated beam of 18N. the TISOL apparatus and beamline are described in more detail in D'Auria et al18). The 18N atoms will be produced by the 500 MeV TRIUMF proton beam striking a target consisting of Zeolite powder, heated to several hundred degrees C. The 18N (in the form of 18N-14N) will be passed into the ECR ionizer,67where the mass-32 beam will be accelerated to 12 kilovolts and stopped in a self-supporting 10 /zgm/cm2 carbon foil. After a short period, the foil will be rotated to a position between two thin silicon surface barrier detectors, where we will look for a-14C coincidences in the proper energy range. After a suitable counting interval, the foil will be rotated out from between the detectors, and another catcher foil which was irradiated with the 18N beam while counting was going on will now be rotated between the detectors. Very thin (s7 /x) Silicon surface barrier will be used, in order to reduce the pulse heights of the /3-particles in the detector. The energy loss program TRIM19) was used to calculate the energy losses of the 508 keV a's and 145 keV 14C ions in the 10/xgm/cm2 carbon catcher foil and in the 40 /xgm/cm2 gold layer on the surface of the detectors; the results are that the a and 14C energies will be 475-495 keV and 114-135 keV respectively, where most of the energy spread comes from differential traversing of the carbon stopper foil. The ratios of the a-energy to the 14C energy will range from 3.5 to 4.3, depending on the location of the stopped 18N in the stopper foil.We have already looked at the /3-delayed a-decay of 18N in a preliminary run for the purpose of studying the background from this decay in the Red Giant experiment (TRIUMF expt 589), which studied the ^-delayed a-decay of 16N (t1/2=7.2 sec.) with three pairs of detectors D1D2, D3D4, and D5D6. Figure 3 shows the spectrum with a coincidence condition on the 14C recoils and a ratio cut so that the recoil energy is approximately 1/4 of the a-energy. In this case, D1 was 11/x thick, and D2 was 3 0/x thick. Note that the carbon recoils are almost completely eliminated by the coincidence condition and the ratio cut. Also two higher energy a peaks become visible, which have not been previously observed. The broad state seen at 2.18 MeV (9.03 MeV in excitation) might be a combination of several states20). The "tails" of the parity-allowed a-peaks will be of crucial importance for the success of this experiment. Neubecket al13), studying the a-decay of 160, extracted 9538±1810 PNC a's from a spectrum total of 2.5xl08 a particles. In our case, there will be much fewer PNC a's, and we must be confident about the number of PC a's in the region of the PNC a's.We have estimated the background in this spectrum by doing a K-matrix fit to the data. The K-matrix21) is a simple parametrization of nuclear reaction cross sections in terms of resonance parameters. One of our very preliminary fits is shown in figure 4 ; here, only the (a,7 ) channels have been taken as input. Twenty parameters were needed to describe seven states (including two subthreshold states), one echopole, and a constant background. No (a,a) or neutron channel information has been included. We note that there is insufficient information on the 14C(a,a) channels and on the neutron spectrum. Five neutron channels can contribute to the neutron spectrum, each with their own set of parameters. These latter channels may not affect the low energy behaviour of the cross-section very much. The results shown in figure 4 estimate that the "tail" of the parity allowed a-spectrum will be at least 7 orders of magnitude less than the maximum of the 1080 keV a-peak, in the region of the PNC a's. Recall that our estimate for the PNC a-decay was nine orders of magnitude smaller than the 1080 keV peak. We are continuing our calculations on the K-matrix; more data on the low-energy part of the a-spectrum will improve our fits.This broad state at 2.18 MeV is interesting in itself because the neutron emission threshold occurs at an a-energy of 1413 keV (8.0443 MeV in excitation), and yet a-emission seems to dominate.Expected count rate:The lifetime of the 6 . 8 8  MeV state in 180 can be estimated by assuming that most of the decays of this state are by 7 -emission, so that:^ _ TyNa ( 6 . 8 8 M eV)“ JVp (6  . 88M eV )69Now ^(7.62 MeV) should equal Na(7.62 MeV),so:Given that the branching ratio for the 6 . 8 8  and 7.62 MeV states are 15% and 5% respectively, and that the lifetime of the 6 . 8 8  MeV state is -25 femtoseconds, this yields:Na ( 6 . 8 8 MeV) =10~sNa (7.62MeV)An accurate knowledge of the Ml width of the 6 . 8 8  MeV O' state is important in order to improve the count rate prediction, and in order to obtain ra. Unfortunately the lifetime of this level is too short to measure by the Doppler shift attenuation method; Olness et al22) could only give a lifetime limit of < 25 fsec. Engeland and Ellis23) calculated a lifetime of 0.8 fsec (2.8 Weisskopf units for this Ml decay) using a simplified shell-model and a combination of 0h-2p and 2h-4p wave functions. As the values of Na and Ta are proportional to the lifetime of the 6 . 8 8  MeV state, it is crucial that a modern shell-model calculation of this lifetime be performed. Warburton and Brown24) have begun new shell-model calculations using cross-shell model space, and intend to calculate parity non-conservation matrix elements for A=16-21 nuclei.The previously observed maximum yield of 18N was approximately 2xl04/sec for a proton beam of 1 ^amp. A proposed upgrade of the TISOL beamline would enable us to increase the current limit to 20 jiamps, leading to a yield of 5x10s 18N/sec. Using a lifetime of 25 fsec for the 6 . 8 8  MeV state, the above yield should lead to 0.5 - 1 a ' s  per day from the PNC decay of the 6 . 8 8  MeV state, after folding in the counter solid angles and efficiencies. This is rather a small count rate, even with an extremely clean spectrum.It is clear that further improvements in the Nitrogen isotope production rates from TISOL will be necessary to obtain a value for ra with a reasonable error.Improved /3a data would also be significant for the low energy 14C(a,7 ) reaction, which is of considerable astrophysical interest. In highly degenerate matter, (p>107g/cm3) , the 14N can convert to 14C via electron capture, and heavier nuclei can be built up via 14C (a, 7 ) 180 (n, 7 ) 190 (/S') 19F(n,7 )20F(/3')20Ne (n,7 )21Ne (n, 7 ) 22Ne . This process is expected to occur on the surfaces of white dwarf and Wolf-Rayet stars. Thus it is important to be able to extrapolate this reaction to low energies, and a K-matrix study of the a-decay of 180* might well be the best method for studying this process. This reaction is also of interest for inhomogeneous big bang nucleosynthesis models25*.Studies are also planned on increasing the yield of 18N from the production target; a thallium-based zeolite production target might substantially increase the yield of neutron-rich isotopes like 18N.REFERENCES.1.B. Desplanques, J. Donoghue, and B.R. Holstein, Ann. Phys. 124 (1980) 4492.B. Desplanques, Nucl. Phys. A335 (1980) 1473.V.M. Dubovik and S.V. Zenkin, Ann. Phys. (NY)172 (1986) 1004.G.B. Feldman, G.A. Crawford, J. Dubach and B.R. Holstein, Phys Rev C43 (1991) 8635.E. G. Adelberger and W. C. Haxton, Ann.Rev. Nucl. Part. Sci., 1985,35, 5016 .H.C. Evans, G.T. Ewan, S.P. Kwan, J.R. Leslie, J.D. MacArthur,H.B. Mak, W. McLatchie, S.A. Page, P. Skensved, S.S. Wang, A.B. Mcdonald and C.A. Barnes, Phys. Rev. Lett. 55 (1985) 7917.K. Elsener, W. Gruebler, V. Konig, P.A. Schmelzbach, J. Ulbricht,D. Singy, Ch. Forstner, W.Z. Zhang and B. Vuaridel, Phys Rev Lett718 .K.A. Snover, R. von Lintig, E.G. Adelborger, H.E. Swanson and T.E. Trainor, Phys Rev Lett 41 (1978) 145; E.D. Earle, A. B .Macdonald, K.A. Snover, H.E. Swanson et al, Nuc Phys A396 (1983) 221c9.J. Lang, Th. Maier, R. Muller, F. Nessi-Tedaldi, Th. Roser, M. Simonius, J. Sromicki and W. Haeberli, Phys Rev Lett 54 (1985) 17010.E.G. Adelberger, M.M. Hindi, C.D. Hoyle, H.E. Swanson, R.D. von Lintig and W.C. Haxton, Phys Rev C27 (1983) 283311.B.R. Holstein, Can. J. Phys. 66 (1988) 50812.J.M. Davidson and M.M. Lowry, Phys Rev C18 (1978) 277613.K. Neubeck, H. Schober and H. Waffler, Phys Rev CIO (1974) 32014. L.K. Fifield, W.N. Catford, S.H. Chew, E.F. Garman, D.M.Pringle, K.W. Allen and J. Lowe, Nuc Phys A394 (1983) 115.F. Ajzenberg-Selove, Nuc Phys A475 (1987) 116.J.W. Olness, E.K. Warburton, D.E. Alburger, C.J. Lister and D.J. Misener, Nuc. Phys. A373 (1982) 1317.B.A. Brown and M. Gai, MSU Cycl. Lab. Annual Report (1988)18.J.M. D'Auria, L. Buchmann, M. Dombsky, P. McNeely, G. Roy, H. Sprenger and J. Vincent, Nuc Instr Meth B70 (1992) 75; M. Dombsky, L. Buchmann, J.M. D'Auria, P. McNeely, G. Roy, H. Sprenger and J. Vincent, Nuc Instr Meth B70 (1992) 12519. TRIM20.Z. Zhao, M. Gai, B.J. Lund, S.L. Rugari, D. Mikolas, B.A. Brown,J.A. Nolen and M. Samuel, Phys Rev C39 (1989) 198521.J. Humblet, Phys Rev C42 (1990) 158222.J.W.Olness, E .K .Warburton, and J.A. Becker, Phys Rev C7(1973) 223923.T. Engeland and P.J.Ellis, Nuc Phys A181 (1972) 36824.E.K. Warburton and B.A. Brown, Phys Rev C46 (1992)92325.M. Gai, Phys Rev C45 (1992) 254852 (1984) 1476Figure 1. Constraints on the isovector and isoscalar weak interaction.Figure 2. Energy level diagram for 180.Figure 3. Alpha-14C coincidence spectrum.Figure 4. K-matrix fit to a corrected spectrum.Name Institution Status % time0-58h°(10‘7)73F i g . l .  Constraints on the isovector and isoscalar weak interactions.Fig.2.Energy level diagram for 180. The 6.88 MeV level is the level of interest.COUNTS75Figure 3. Alpha-14C coincidence spectrum.COUNTS / 10 KEVr-JLAKJtiJi-r2 3 4 E-crm  [MeV]Figure 4. K-matrix fit to a corrected spectrum.77S p ectro sco p y  o f  E x o tic  N u c le i w ith  R eso n a n t and  D irect R ea ctio n s  at 1.5 - 10 M e V /u  at ISA CU .G iesen 1,2’3, K .P .Jackson1T R IU M F , Vancouver, B.C., Canada;2University of Toronto, Toronto, Ont., Canada;3University of Alberta, Edmonton, Ab., Canada;A b stract. Here we describe the physics motivation and experimental de­signs for the spectroscopy of exotic nuclei with radioactive beams of 1.5 - 10 M eV /u at ISAC. Resonance scattering on a hydrogen target is proposed to study the ground and excited states of proton-unbound nuclei using proton- rich beams and also to indirectly study neutron-unbound nuclei via isobaric analogue resonances using neutron-rich projectiles. Stripping reactions are described as a means to gain nuclear structure and spectroscopic informa­tion for specific nuclei and states of importance for nuclear astrophysics. All experiments will be performed in inverse kinematics and use arrays of silicon-strip detectors. In addition, the suitability of various light-mass targets is discussed.1. In troductionA primary motivation for the construction of an accelerated radioactive beam facility is to provide unique opportunities for the study of specific nuclei away from the valley of stability. The interest arises largely from the desire to achieve a deeper understanding of nuclear structure, but also from the need to determine the properties of specific levels of importance in other fields, particularly nuclear astrophysics.A variety of models of nuclear structure have been developed to account for the diverse properties measured for those nuclei currently accessible to experiment which tend to be concentrated near the valley of beta stability. The limited experimental data so far available for the “exotic” nuclei far from stability already reveal striking new features such as the existence of “neutron halos” (low density nearly pure neuton m atter at large radii) and possibly a related low-lying collective E l mode of excitation[1,2]. Also of note are the apparent collapse of the N=20 shell closure for Z=10-12 [3] and strong quadrupole deformation of the ground states of nuclei in the region Z  =  jV ~  40 [4]. Moreover, analyses with existing theories suggest one should expect other im portant new features to be seen in the properties of nuclei far from stability [5,6]. In the lighter nuclei (A  <  40) which are most readily accessible to large shell model calculations there have been detailed investigations of the degree to which existing data, particularly for the very neutron-rich nuclei, can be explained by models developed for nuclei nearer stability [7,8,9].The vital role of nuclear reactions involving unstable isotopes in nuclear astro­physics is discussed in detail in other contributions to this workshop [10,11]. In serveral aspects the specific objectives in this field coincide with those dedicated to a more gen­eral understanding of the structure of nuclei far from stability. The reactions of interest for proton-rich nuclei of Z  <  50 generally involve states in nuclei with Z  > N  which are therefore isobaric analogues of states in the usually better-known mirror nuclei. To this end, a better understanding of the breaking of isospin symmetry, including Coulomb displacement energies based on new experimental data, would be a very im portant element in predicting the properties of the relevant nuclei near the proton drip line for Z  <  50. Nuclear reactions such as those in the rapid neutron capture (r-) process re­quire knowledge of the properties of very neutron-rich nuclei. Additional experimental data are needed to refine the models used to predict the masses and half-lives of these nuclei.Interest in the properties of a specific nucleus far from stability usually extends well beyond the simple question of the mass, half-life and decay modes of the ground state. An understanding of the nucleus as a many-body quantum  system requires knowledge of the spins and parities of these states as well as those of the low-lying excited states; information ideally augmented by measurements of other spectroscopic properties such as 7-ray  tra n s itio n  p robab ilities  and  spectroscopic fac to rs  for one and  two-nucleon transfer. It is not realistic to anticipate that, for nuclei far from stability, one can duplicate the rich detail of the spectroscopic information currently available for nuclei closer to stability, but ISAC offers unique opportunities to obtain such in­formation in selected cases.A very significant part of the detailed spectroscopic information we currently have, particularly for the lighter nuclei (A<40), has been obtained from the study of nuclear reactions involving stable targets and relatively low energy (E/A<10 MeV) beams of protons, deuterons, 3He and 4He. For the purpose of the present discussion attention is focussed specifically ona) resonance reactions involving protons, which normally would be studied at energies below the Coulomb barrier andb) transfer reactions, particularly those involving one and two nucleon stripping from deuterons, 3He and 3H. Specifically, the neutron transfer reactions (d,p) and (t,p) allow one to probe farther towards the neutron drip line and could provide im portant information for (n,7 ) reaction rates in explosive astrophysical scenarios.The brief outline of the opportunities envisaged for ISAC in these two areas is presented m the following way. The im portant objectives that can be addressed by79the study of low-energy resonant scattering are discussed in Section 2, followed by a similar evaluation of the role of stripping reactions in Section 3. The experimental requirements to undertake these studies depend in large measure on the features of the inverse kinematics which are illustrated for both types of reactions in Section 4. Also included there are discussions of suitable targets, detection systems and the anticipated counting rates th a t may be observed in the initial experiments to be undertaken at ISAC. The final section contains a brief summary.2. R esonance R eaction sThe study of resonance reactions, particularly the elastic scattering and radiative cap­ture of protons on stable targets is a well-established tool of nuclear spectroscopy [12]. The im portant extension of these studies to reactions of direct significance in nuclear astrophysics using accelerated radioactive beams of proton-rich nuclei has been pio­neered at Louvain-la-Neuve [11,13,14]. Most relevant to the present discussion has been the development of a technique for one-step energy scanning of an elastic scat­tering resonance using a relatively thick polyethelene target [13,15]. Accurate values of the energy and width of the resonant state, as well as a unique determination of the spin can be derived by observing the energy spectrum of the recoil protons at spe­cific angles. In the broader context of nuclear structure, similar techniques involving hydrogen targets and low-energy (0.2 <  E /A  < 4 MeV) radioactive beams could have a significant impact on the study of nuclei away from the valley of stability in the following three catagories.2.1. Nuclei beyond the proton drip lineA variety of techniques have been used to identify, with one or two exceptions, all the proton-rich nuclei with Z < 28 which are stable with respect to proton emission[16]. Much of the limited information that exists on 4Li and 5Li, the lightest nuclei beyond the proton drip line, has been derived from R-matrix analyses of low energy elastic proton scattering on 3He and 4He respectively [17,18]. None of the heavier nuclei beyond the proton drip line can be formed as a compound nucleus using protons and a non-radioactive target. However, the properties of the ground state and low- lying excited states of many of these nuclei could be investigated as compound nuclear resonances with a hydrogen target and beams of proton-rich radioactive isotopes.Table 1 lists the cases with A < 22 for which the necessary radioactive beam could be produced at ISAC. The known or predicted separation energies are taken from a recent compilation [19]. Masses of some states have been measured for 4 of these nuclei by transfer reactions such as (3He,8Li) [16]. The precision with which the mass of the ground state (and in some cases excited states) is determined varies in the range from 10 to 200 keV and, because of the complexity of the reaction involved, very little additional information can be derived. In contrast to this situation, studies of these states as elastic scattering resonances in the manner demonstrated with beams of 13N and 19Ne [20] have the potential to provide unique spectroscopic data on energies,spins and decay widths of the lowest levels of these nuclei; similar examples could be cited with A > 22.Table 1. Light proton-unbound nuclei accessible as resonances.Isotope S p [keV][19] Projectile r£Projectile2i on -3500 (400) # 9C 0.1265ii IV -1970 (180) i°C 19.2614 jp -3210 (400) # 13 0 0.0089is F -1480 (130) 14Q 70.61i6jr -536 ( 8) 15 O 122.218 N  a -1540 (400) # 17 N e 0.10919N a -321 ( 13) 18N e 1.67221 Al -1260 (300) # 2 °M g 0.082# : value and uncertainty estimated from systematic trendsThe study of the nucleus 16F by the 150  +  p reaction is included in Table 1, although it is fairly well known. The widths of the ground state and the first 3 excited states (Er <  1 MeV, Tcm ~40 keV) have large uncertainties, which would make a better determination of these properties an interesting initial test case.In general the lifetimes of these unbound states are too short for them  to be of direct significance even in the most explosive stellar environment, but the properties of these very proton-rich nuclei are of interest in connection with such topics as mass models as well as the systematics of Coulomb energies over large isobaric multiplets, the influence of extended unbound proton orbitals and the general question of isospin purity.As specific examples it would be of interest to identify, and measure the energy and reduced width of the lowest J n = 1/2+ states in n N, 15F and 19Na for compar­ison with their known analogue states in the isobaric multiplets. In each case the properties of the state (including the Coulomb energy) should be strongly influenced by the extended nature of the wavefunction of the unpaired proton with a large 2s1/2 component.2.2. Specific excited states in proton-rich nucleiThe studies of the 5.17 MeV state in 140  at Louvain-la-Neuve are examples of detailed nuclear spectroscopy motivated by nuclear astrophysics. In this particular case the im portant cross section for radiative proton capture is unusually large and hence well suited to a first generation experiment with a radioactive beam. Unfortunately, most of the other experiments of greatest interest in the field of explosive stellar burning81[21] involve much smaller yields and hence present serious challanges even for a second generation facility such as ISAC. The 5.17 MeV state in 140  is, however, quite typical of many unbound states in nuclei with Z  > N  for which unique spectroscopic informa­tion could be derived by the study of proton-induced resonances involving radioactive beams. It is im portant to emphasize that in many cases the states of greatest interest for nuclear structure are those with large reduced widths which consequently appear as strong resonances. Two simple examples can be used to illustrate the possible experiments.The 3 /2" ground state of 9C is bound (5P =  1296 keV) and together with the known analogue states in 9B, 9Be and 9Li represent the T =  3/2 quartet exhibiting the largest known deviation from the isobaric mass multiplet equation [22]. The precision with which a similar test of the I M M E  can be made for the lowest excited T = 3 /2  states in these nuclei is limited by the uncertainty in the excitation energy of the state in 9C observed at Ex =  (2218 ±  11) keV in the 12C(3He,6He) reaction [18]. The properties of this state (including a more precise value of Ex and confirmation of the assumed J 7r= (l/2 )~  could best be studied as a resonance in 8B +  p at E# ~  922 keV (resonance energy in the center-of-mass).The study of resonances in the 17F +  p reaction could be used to provide detailed spectroscopic information on the unbound states with Ex > 4 MeV in 18Ne. There are 6 well-known states in 180  for which the analogue states in 18Ne should appear as resonances at E# < 2 MeV [23]. The corresponding T =  1 states in 18F have been studied in detail (all but the lowest as resonances in 170  +  p) and in 3 cases there are interesting examples of very strong isospin mixing [24], Careful comparisons of the reduced widths and of the Coulomb energy shifts for the 6 T=1 multiplets could provide new insight into the problem of isospin breaking in nuclei involving both simple shell model configurations (2 particle states) and strong configuration mixing (4 particle - 2 hole states). The properties of these states are also of astrophysical significance in connection with the 17F(p,7 )18Ne reaction [21].The inelastic scattering from low-lying states of the projectiles, for example in 19Ne +  p, should also be considered, both as a source of background in the studies mentioned above and as a means of studying these states using thin targets.2.3. Isobaric analogue states in neutron-rich nucleiThe determination of spins, parities and other detailed spectroscopic properties of even the lowest lying states in a specific nucleus (Z ,N ) near or beyond the neutron drip line is in general very difficult. Studies of resonance reactions induced by radioactive beams incident on hydrogen could in many cases be used to determine the corresponding properties of the isobaric analogue states in the (Z+1,N-1) nucleus as illustrated in Fig.l for 7He and 7Li. The lowest T = 3 /2  state in 7Li should appear as a very prominent resonance in 6He +  p at E#=1.27 MeV. (In this example the IAS in 7Li is already known because of proximity to the stable nuclei 6Li and 9Be, but this is an exception since all the heavier neutron-rich nuclei in question lie much further from the line of stability.)The systematics of the energy of such a resonance are simply given byF igu re  1. Level scheme of 7Li and 7He with respect to 6He+p [18].E r  = A E c -  S n (1)where A E c is the Coulomb displacement energy (7Li-7He) and Sn is the neutron separation energy (7He). It should be noted tha t most of the Coulomb displacement energies known for nuclei with A > 60 have been derived using this expression for isobaric analogue resonances observed with high resolution proton scattering from sta­ble targets for which the corresponding value of Sn is substantially greater than  zero [25,26]. As is the case for these heavier nuclei, the isobaric analogue resonances should be characterized by large reduced widths for the isospin-allowed formation channel (p0) and very small reduced widths for isospin-forbidden neutron emission (in Fig. 1 to the T=0 states of 6Li). The IAS of a level beyond the n-drip line (Sn <  0) will also have an isospin-allowed decay by low-energy neutron emission (in F ig .l to the 3.56 MeV state in 6Li).The simplest cases for which the study of isobaric analogue resonances could be used to make unique spin assignments for very neutron-rich nuclei would involve the use of accelerated beams of even-even nuclei for which J* =  0+. In addition to the example given in F ig .l, the isobaric analogue states corresponding to the ground states of 9He, 17C and 230  would be expected as strong resonances in 8He+p, 16C+p and 220 + p  at energies Eh ~  2.1, 2.0 and 0.7 MeV respectively.3. Transfer R eactionsTransfer reactions in inverse kinematics (Section 4.1) involving radioactive projectiles have been described previously [27,28]. For stripping reactions such as (d,p) the im-83portant center-of-mass (c.m.) forward angles generally correspond to backward angles in the laboratory. The light reaction products are then separated from the elastic scattering of the light target atoms (which is confined to 9iab < 90°). The reactions considered here are in this category and in many cases are well suited by ion energies up to 10 M eV/u. For pick-up reactions and studies involving large negative Q-values or high excitation energies, however, it has been pointed out [27] th a t the available energy should be at least 25 MeV/u.In this context it is interesting to compare the ISAC energy limit with typical tandem energies (9 MV), at which much nuclear structure data has been obtained. Table 2 lists, for the main tandem  projectiles (or ISAC targets), the tandem beam energy and the tandem  and ISAC c.m. energies. The energy values are in MeV and for an A =  10 target (beam). For heavy targets (A «  60) the c.m. energy approaches the tandem beam energy, and for 10 M eV/u ISAC beams it approaches 10 times the value of the target mass; ie. 20 MeV for a deuteron target, 30 MeV for a triton  target etc. On this basis (as well as because of the Q-values) the proposed ISAC energy is too low for most studies of (p,d), (p,t) and (p,3He) reactions. In many other cases, however, initial comparisons of the results from transfer reactions involving radioactive isotopes are most directly made with the abundant existing data for stable targets using beams within the ISAC energy range.Table 2. Comparison of center-of-mass energies for light ions from a 9 MVTandem and for a 10 M eV/u radioactive beam (A=10) at ISAC.B eamT arge tJ7'Tandem ■p Tandem  cm■&ISACcmp 18. 16.4 8.2d 18. 15.0 16.7t 18. 13.9 23.13He 27. 20.8 23.14He 27. 19.3 28.66Li 36. 22.5 37.57Li 36. 21.2 41.2The transfer reactions presently considered are to some extent motivated by as­trophysics and the spectroscopy of light, unbound nuclei. For instance, inhomogeneous Big Bang nucleosynthesis may lead to the production of observable quantities of heav­ier elements through the 8L i(a ,n)n B reaction [29,30,31]. However, 8Li can also be destroyed by the “leak” reactions 8Li(n,7 )9Li, 8Li(p,n)8Be and 8Li(d,n)9Be for which the reaction rates are quite uncertain. We therefore intend to measure the neutron and proton transfer reactions 8Li(d,p)9Li and 8Li(3He,d)9Be. Of interest for the Hot CNO-cycle and the rp-process [21] are measurements of key reactions such as the pro­ton transfer reaction (3He,d) on 13N, 19Ne and 72Kr (the latter to determine the proton separation energy for 73Rb). A challenging, though not first-generation, experiment would be the o-transfer on 150  with the 150 ( 6Li,d)19Ne reaction.In addition to resonance scattering described in Section 2.3, studies of the nuclear structure of light isotopes at the neutron drip line can be complemented and extended with the one- and two-neutron transfer reactions (d,p) and (t,p), using, for example, beams of 6He, 8He and 8Li for the spectroscopy of 7-8-9-10He and 9’10Li. Investigations of nuclear structure along the neutron drip line could of course be continued to heavier elements with the projected high beam intensities for C, N, 0 , Ne, Na and other isotopes [32]. Similar work with a triton  beam of 23 MeV on stable isotopes of helium to carbon [33] showed cross sections in the range of 10 to 1000 /ib /sr.4. E xperim ental C onsiderations4-1. Inverst KinematicsIf the projectile is heavier than the target atoms, the relations between quantities in the center-of-mass (c.m.) and the laboratory (lab) system are referred to as inverse kinematics. These have been calculated for the transfer reactions mentioned above and the essential features are illustrated in Figure 2a-e for the typical case of the stripping reaction d(6He,p)7He with a 6He beam incident at 10 MeV/u. The elastic scattering of the projectiles is, in this situation, confined to a narrow cone of less than  20°o6, and the recoiling light target nuclei, for example protons in the study of resonances in elastic scattering, are limited to lab angles of 0° to 90°, where 0° corresponds to 180°m and 90° to 0°m.The angular range of interest in the study of transfer reactions is usually between 0°cm and about 60°m, which, as shown in Fig. 2a, corresponds to 9iab in the range from 180° to 80°. The lab backward angles, 9tab > 90°, are free of background from elastic scattering from the light target atoms and thus it becomes possible to use “simple” arrays of Si-strip detectors (without particle identification). The collection of data  for reaction products in the region 0iab ~  90° would be complicated by the very large flux of elastically scattered target nuclei (for which 90°afc =  0°m). However, as the energies of the scattered target nuclei are only a few hundred keV for about 90 to 80°a6, the useful angular range could be extended, using appropriate trigger thresholds or thin absorbers.The display of proton energies as a function of lab angle in Fig. 2b is typical for the energies of light reaction products. The effect of the energy spread in the incoming beam is shown in Fig. 2c as the difference between the proton energies for a 6He-beam of 10 M eV/u and a 1% higher beam energy. Limiting the energy spread in the incident beam to less than 0.3% would keep its contribution to the overall energy resolution small compared to an assumed detector resolution of about 30 keV. Similarly, Fig. 2d which shows the differences in proton energies for a difference of 1° in the lab as a function of 9iab, is useful when considering the angular resolution required in thedE/dE dE/dE. (=1%) [MeV]" n85Figure 2. Kinematics for the d(6He,p)7He reaction at 10 M eV/u.Figure 3. Conceptual design, showing one element of an array of Si-strip detectors and two monitor detectors.detector array. Fig. 2e shows the difference in lab proton energies leading to the ground state and a state at 1.0 MeV excitation energy. For backward lab angles (forward c.m. angles) one observes a compression in energy scale by a factor of 2 to 4. W ith a detector resolution of 30 keV, this effect means th a t states with a separation of less than about 100 keV cannot be resolved for 6c,m. < 15°. This compression is reduced by only 10% at I75°ab if the beam energy is doubled to 20 MeV/u.4-2. Detector SystemsThe proposed reactions can be studied, as shown above, with relatively simple arrays of Si-strip detectors within an appropriate scattering chamber. The detector elements will be arranged in concentric circles, covering as much of one hemishere as possible in order to maximize the solid angle and minimize measuring times. This is illustrated conceptually in Fig. 3 for one ring of detector elements. The array will be positioned in the forward hemisphere relative to the beam  direction for the study of elastic scattering. The energy of recoiling light target nuclei increases as 9iab —> 0° (9cm —> 180°) requiring a detector thickness of up to  1000 pm  of Si for E cm—3 MeV (about 300 ptm for E cm —1.5 MeV). For the detection of the light reaction products from transfer reactions the array will be positioned in the backward hemisphere. Here, the particle energies increase towards 90^6. Fig. 2b indicates that an array of 300 pirn detectors would be adequate for this reaction, but in other cases detectors of up to 1000 pun would be required.The spatial resolution of the Si-strip detectors would be utilized to define 9iah and to limit the contribution of the kinematic broadening to the energy resolution. Fig. 2d87indicates that to limit this to less than 30 keV requires an angular resolution of 0.2° at 90°ab and of about 1° at 130fab. The corresponding maximum width (total solid angle) of one series of Si-strips at a distance of 15 cm is 0.5 mm (0.02 sr) at 90°ab and 2.5 m m  (0.08 sr) at 130°ai). In general it is sufficient to obtain angular distributions with a bin width A 9cm of 1 to 5°. The effective solid angle could thus be increased by adding the spectra from several adjacent elements, with appropriate kinematic corrections. For the actual experiments one would also have to consider the effect of the finite size of the incident beam on angular resolution, incomplete coverage of the azimuthal range with active detector area and the intensity ratio between c.m. and lab frame.Si-strip detectors are available in a large variety of shapes and sizes, at reasonable costs, to accommodate these requirements. Separate silicon detectors will be mounted at selected angles in the forward hemisphere during transfer studies to observe the elastic scattering and thus to monitor the ion beam flux, target density and target composition (Fig.3). This set-up could of course be extended by scintillator arrays for (p,n) or (d,n) studies by neutron time-of-flight, by arrays for gamma-detection, and by a spectrograph for recoil separation, detection and in some cases for further decay studies.4-3. TargetsA list of possible light-mass targets and practical maximum target thicknesses is given in Table 3. Foil targets containing hydrogen in the form of polymers of the type (CH2)n are probably the most convenient proton or deuteron targets. They can be obtained and prepared in thicknesses of about 0.05 to several m g/cm 2 and are fairly stable against irradiation with the relatively low intensities of radioactive beams [34], A foil of 1 m g/cm 2 will have a c.m. target thickness of 120 keV for a 6He beam of 1.5 MeV/u, which is typical of that required for energy scanning of resonances in elastic scattering. It will also have the highest density of target atoms compared to other materials in Table 3. However, for the study of stripping reactions one has to take into account the effects of variation in projectile energy (Fig. 2c) and energy loss of the light reaction products as a function of target thickness. To maintain the latter contribution to the overall resolution below about 30 keV would require a foil thickness of only 0.1 m g/cm 2, under the conditions depicted in Fig. 2b.Gases of hydrogen isotopes can be adsorbed by heated Ti-foils to produce p, d or t targets with a stoichiometry of TiH (1:1) [28]. A suitable thickness would be of the order of 0.1 m g/cm 2 which contains only 1.3*1018 hydrogen atom s/cm 2. These targets will produce large backgrounds from elastic scattering and reactions on Ti.Gas cells could be used for all hydrogen, helium or other gaseous isotopes. How­ever, it would not be trivial to maintain a large viewing angle in all directions with thin foils. In addition, these entrance and exit foils will lead to background from reactions (plastic foils :H,C) or scattering (Ni-foil etc). W ith a target length of 1 cm and a gas pressure of 0.25 bar one obtains a target density of about 1019 atom s/cm 2.Several gas-jet targets are discribed in the literature [35,36,37,38] and their appli­cation for radioactive beams were discussed in [39,40]. Compared to the other targetsT ab le  3. Compositions and thicknesses of possible light-ion targetsTarget Form Thickness Atom s/cm 2p, d CH2 (Polyethelene) 1 m g /cm 2 8.6 * 1019p, d, t in Ti (1:1) 0.1 m g /cm 2 1.3 * 1018H2, (He) in Gas cell / =  1cm, P  =  1/4 bar 1.4 * 10196Li,7Li evaporated 0.1 m g /cm 2 1.0 * 1019H2, He Gas-Jet-Target 3.0 * 1018in Table 3, gas-jet targets have the following advantages: they are pure, no compound materials, windows or carrier substrates; offer unrestricted viewing from nearly all di­rections; the target density is very uniform, stable and can be varied by adjusting the gas pressure. They also allow one to change the target gas or its composition with­out opening the vacuum chamber, and also to artificially increase the otherwise small target thickness (several keV) by adding heavy gasses such as Xe. The reported areal densities are about 3*1017/ cm 2 for a variety of gases [35,38,40] and up to l*1019/cm 2 with much larger compressors and a higher vacuum outside the jet [36,37]. A low outside vacuum, however, is especially crucial in the present application, where the target system will be connected to linacs or recoil separators. According to [39,40], their target density could be increased by a factor of 10, to about 3*1018/cm 2, through additional pumps and larger compressors, while maintaining an acceptable outside vac­uum. We will use this number for the estimates of reaction yields, although there are no technical reasons not to achieve higher densities. W ith a cost of about $ 200,000 the gas-jet is unfortunately the most expensive target system.4-4- Reaction YieldsIntensities of a variety of radioactive ion beams have been presented and discussed by J . D’Auria at this workshop and are in the range of 108 to 10n  per second for isotopes of interest here. The 6He beam intensity is expected to be about 108/s. For the proposed resonance scattering experiment, this leads to a count rate of about 8000 per hour from a 1 m g/cm 2 polyethelene target and the detector elements at 27° (125°m) subtenting0.02 sr. For He on protons, the Rutherford cross section at this angle is only 13 m b/sr, which will of course be much larger for isotopes with higher Z. Nevertheless, proton spectra with a statistical quality comparable to those obtained at Louvain for 13N-fp will be collected in only about 4 hours.Cross sections for transfer reactions vary between 0.01 to 100 m b/sr, depending on the type of reaction, transition and angle. For a general estim ate of reaction yields we use 1 m b/sr, an ion beam intensity of 109/s , a solid angle of 0.02 sr (Section 4.2) and a gas-jet system with a target density of 3*1018/cm 2. The count rate per transition89and angle or detector element would then be about 200 per hour. W ith an extensive detector array the accessible angular range is measured in once, and a complete angular distribution could be obtained in less than a day.5. Sum m aryIn this contribution we have outlined some unique opportunities in interesting areas of nuclear structure and nuclear astrophysics. These involve the spectroscopy of exotic light nuclei (A<40) with beams of radioactive nuclei accelerated to energies of up to 10 M eV/u incident on targets containing protons, deuterons and other light nuclei.Detailed investigations of resonant reactions, particularly the elastic scattering of radioactive nuclei from hydrogen, could be used to provde precise measurements of energies, partial widths as well as spins and parities of unbound levels in nuclei ranging from those beyond the proton drip line to the isobaric analogue resonances of states near the neutron drip line. The “inverse kinematics” permits the use of a technique of scanning the entire resonance in a single step using a large array of Si-strip detectors mounted in the forward hemisphere.Most of the existing data of stripping reactions on stable targets have been ac­quired with beams of less than 10 M eV/u. Similar studies utilizing radioactive beams and an improved gas jet target could extend this spectroscopic tool to the exotic nuclei of interest. The same array of Si-strip detectors mounted in the backward hemisphere would provide efficient collection of the required data.For first generation experiments in the start-up phase of the proposed ISAC- facility, one should begin with studies of processes with large cross sections, such as elastic scattering, and simple detection systems of high efficiency. Here we propose such experiments which require a modest amount of equipment and, therefore, can proceed as soon as the radioactive beams are available.R eferences[1] I. Tanihata, Nucl. Phys. A520, 411c (1990).[2] P.G. Hansen, Nucl. Phys. A553, 89c (1993).[3] N.A. Orr et al., Phys. Let. B258, 29 (1991). (and references therin)[4] W. Gelletly et al., Phys. Let. B253, 287 (1991).[5] R.F. Casten, Nucl. Phys. A557, 675c (1993).[6] J. Dobaczewski, I. Hamamoto, W. Nazarewicz, and J.A. Sheikh,Phys. Rev. Let. 72, 981 (1994).[7] E.K. W arburton, J.A. Becker, and B.A. Brown, Phys. Rev. C 41, 1147 (1990).[8] E.K. W arburton and B.A. Brown, Phys. Rev. C 46, 923 (1992).[9] N.A.F.M. Poppelier, A.A. Wolters, and P.W.M. Glaudemans,Z. Phys. A. 346, 11 (1993).[10] L. Buchmann, these proceedings.[11] W. Galster, these proceedings.[12] H.E. Gove, in “Nuclear Reactions” Vol.l,ed. P.M. Endt and M. Demeur, (Amsterdam, North-Holland, 1959) p. 259.[13] Th. Delbar et al., Nucl. Phys. A 542, 263 (1992).[14] Th. Delbar et al., Phys. Rev. C 48, 3088 (1993).[15] M. Benjelloun et al., Nucl. Instr. and Meth., A321, 521 (1992).[16] A.C. Mueller and B. M. Sherrill, Annu. Rev. Nucl. Part. Sci. 43, 529 (1993).[17] D.R. Tilley, H.R. Weller and G.M. Hale, Nucl. Phys. A541, 1 (1992).[18] F. Ajzenberg-Selove, Nucl. Phys. A490, 1 (1988).[19] G. Audi and A.H. W apstra, Nucl. Phys. A565, 1 (1993).[20] R. Coszach et al., Phys. Rev. C 50, 1695 (1994).[21] A.E. Champagne and M. Wiescher, Annu. Rev. Nucl. Part. Sci., 42, 39 (1992)[22] M.S. Antony, J. Britz, J.B. Bueb, and A. Pape,At. and Nucl. D ata Tables 33, 447 (1985).[23] F. Ajzenberg-Selove, Nucl. Phys. A475, 1 (1987).[24] W.E. Kieser et al., Nucl. Phys. A327, 172 (1979).[25] W .J. Courtney and J.D. Fox, At. and Nucl. D ata Tables 15, 141 (1975).[26] M.S. Antony, J. Britz, and A. Pape, At. and Nucl. D ata Tables 40, 9 (1988).[27] J.C . Hardy, Proc. Workshop on the Production and Use of Intense Radioactive Beams atthe IsoSpin Laboratory, Oak Ridge 1992; ed. J. G arrett, CONF-9210121, p .51.[28] P. Egelhof, Workshop on the Physics and Techniques of Secondary Nuclear Beams;Dourdan, France (1992), p. 217.[29] R.A. Malaney and W.A. Fowler, Astrophys. J., 333, 14 (1988).[30] R.A. Malaney and G.J. Mathews, Phys. Rep., 229, 145 (1993).[31] T. Rauscher, J.H. Applegate, J .J . Cowan, F.-K. Thielemann and M. Wiescher,Astrophys. J. 429, 499 (1994).[32] J. D’Auria, these proceedings.[33] F. Ajzenberg-Selove, E.R. Flynn and Ole Hansen, Phys. Rev. C 17, 1283 (1978).[34] W. Galster et al., Phys. Rev. C 44, 2776 (1991).[35] H.W. Becker et al., Nucl. Instr. and Meth., 198, 277 (1982).[36] W. Tietsch, K. Bethge, H. Feist and E. Schopper, Nucl. Instr. and M eth., 158, 41 (1979).[37] G. B ittner, W. Kretschmer and W. Schuster, Nucl. Instr. and M eth., 161, 1 (1979)and Nucl. Instr. and M eth., 167, 1 (1979).[38] T. Griegel et al., J. Appl. Phys., 69, 19 (1991).[39] H.W. Becker, Proc. Accelerated Radioactive Beams Workshop, Parksville Workshop 1985;TRIUM F Report TRI-85-1, p.257.[40] T.R. Donoghue, T.C. Rinckel and C. Rolfs, p.264, Tri-85-1 (see Ref.[39])91Ion  trap s at R N B  facilities: m ass m easu rem en ts and  fu tu re p o ssib ilitiesG. SavardAECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada, KOJ 1J0.A B S T R A C TIon traps installed on-line at accelerator facilities offer many new possibilities for precise measurement of the properties of unstable isotopes. We describe a versatile on-line apparatus which uses a new helium-jet coupled laser ion source to load ions into a system composed of an RFQ trap and a Penning trap. This system is intended for high-precision mass measurements on un­stable isotopes.In tr o d u c tio nThe Penning trap offers an ideal environment for high-precision measurements. In a Penning trap, charged particles can be stored for extended periods of time in a well con­trolled and stable environment, free from outside perturbations. These devices are par­ticularly well suited to mass measurements since the cyclotron frequency (cuc) of charged particles in the magnetic field of a Penning trap can be measured very precisely yield­ing an accurate mass determination from the simple relationship loc — This method is at the origin of most of the new measurements in the field of high-accuracy mass determination1-4. The Penning trap has been mainly used so far with light charged par­ticles and with species that can be created inside the trap. Extension of the techniques to radioactive heavy ions involves the development of fast and reliable methods for injecting unstable isotopes into the trap. The ISOLTRAP spectrometer3 was developed for this task and is now operating at the ISOLDE/BOOSTER mass separator of CERN. It is a tandem Penning trap system where the first trap collects and prepares the ions and the second trap  is used for the actual mass measurement. While the high accuracy achievable with such a system has now been demonstrated, the injection system used so far limits the isotopes that can be studied to surface ionizable elements.In the following, a Penning trap mass spectrometer concept with a new, more versatile, injection technique is presented. It is similar to the technique proposed for the Canadian Penning trap  mass spectrometer presently being built in Chalk River. It makes use of the high primary beam current capabilities of the RNB facilities which when coupled with an helium-jet system, as proposed at LAMPF, can produce a wide variety of isotopes for precision experiments in traps.G e n e ra l layou t o f  th e  sy s te mA schematic layout of the proposed system is shown in figure 1. The high-current primary proton beam impinges on a stack of thin targets enclosed in a helium-jet system. The unstable isotopes created by spallation or proton-induced fission experience a small recoil which kicks a fraction of them out of the thin targets. They are thermalized in the helium gas flowing between the targets and attach themselves to aerosol particles contained in the gas. They are then carried to a collection point outside the target area. A typical transport efficiency is of the order of 50% over distances exceeding tens of meters. For a 5 meter transport distance, transport times are of the order of 20-100 ms depending on the gas flow conditions. Isotopes of most elements can be transported in this manner. More importantly, extraction from the thin targets themselves is independent of93DETECTORPRECISION TRAP ) • (Figure 1: Schematic layout of the proposed systemthe chemical properties of the species of interest since it is a result of the kinematics of the reaction. This method is therefore more versatile than the usual thick target approach for producing short-lived isotopes. The maximum yield can be higher with thick targets, but only for isotopes of elements that diffuse rapidly out of the target material. The yields for various isotopes with the proposed technique have been calculated by Talbert5 for the 1 mA 800 MeV proton beam of LAMPF. They estimate a yield (at the end of the transport capillary) of roughly 109 ions/second per mb of production cross-section. Recent experiments at LAMPF6 confirm these predictions. These yields, when scaled down by a factor of 10 for the lower proton beam intensity and energy available at the proposed ISAC facility, would be more than adequate for precision measurements in traps.The hehum-jet system delivers the isotopes of interest to the end of the capillary in a collection spot which contains micrograms of aerosol with traces of isotopes from essentially all available reaction channels. The isotope selected for injection in the trap must now be extracted from the collection spot. This is accomplished through the high selectivity of multi-step resonant laser ionization. The collection spot is moved to a laser interaction chamber with a transport system operating through a differential pumpingsection. It is then desorbed by a pulsed Nd:Yag heating laser. The laser pulse deposits a short burst of heat on the surface, vaporising part of the collection spot and creating a plume of neutral atoms expanding from the surface. This plume is illuminated by two or three excimer-pumped dye laser beams tuned to resonantly excite and ionize the isotope of interest. The selectivity of this process allows the clean extraction of the isotope of interest from the mixture of activities and from the aerosol. The clean ion bunch created in this fashion can now be accelerated and transported to the rest of the system. This method of creating the ions is particularly well suited to the present purpose since a pulsed ion beam is required for proper injection into ion traps. It is also very efficient and, with an excimer pumped system, a suitable ionization scheme can be found for practically all elements in the periodic table.A radiofrequency quadrupole (RFQ) trap is inserted as a buncher between the laser ion source and the measurement trap. The purpose of this device is to match the 20 Hz repetition rate of the laser system for the helium-jet coupled laser ion source described above with the approximately 1 Hz loading cycle of the Penning trap when performing mass measurements. The collection spot is therefore heated by the Nd:Yag laser, ions of the specified isotope are created by resonant laser ionization, the ion bunch is transported to the bunching trap and captured, then cooled in this trap by buffer gas collisions. A second ion bunch can then be created and captured in the same fashion, the process repeating until a sufficient number of ions has been accumulated. The cooled ion cloud in the bunching trap is then extracted and transferred to the precision Penning trap where the mass measurement takes place. Injection of the ions into the Penning trap is best performed with an ion bunch of low energy spread; however a fairly long pulse (<  1 fxs) is acceptable. A ramped cavity synchronized to the extraction is therefore included to match the phase-space of the ion bunch extracted from the RFQ trap (which is usually a short ion pulse with considerable energy spread) to the acceptance of the precision trap. This allows the ions to be captured at the bottom of the trapping potential well. The transfer process is carried out essentially without losses because of the pulsed structure of the ion beam extracted from the bunching trap.We have followed the sequence of events to where the ions of the isotope of interest are now stored in the measurement trap. This precision trap is located in the stable and homogeneous 5.9 Tesla magnetic field of a superconducting solenoid. High-magnetic field homogeneity is obtained in the trapping region by careful selection of the materials inside the magnet bore and design of the trap electrodes so that their respective field perturbations cancel. The magnetic field at the trap location must be highly stable since it will ultimately limit the accuracy of the mass measurements. This stability is ensured by95control of the parameters which affect the field locally, such as the temperature of materials inside the strong magnetic field and the helium recovery pressure on the magnet cryostat. The trap is also shielded from the ambient field fluctuations (which are an important consideration in an accelerator laboratory) by a self-shielding superconducting solenoid which attenuates those external field fluctuations by roughly a factor 100.In this precision trap section, a vacuum of lower than 10~9 torr is obtained by differ­ential pumping in the transfer section between the two ion traps and a large cryopump placed below the magnet. The main electrodes of the trap consist of two endcaps and a ring electrode which are shaped like hyperboloids of revolution. They are made of gold- plated oxygen-free copper. Correction electrodes are added to compensate for the finite size of the main electrodes and the apertures in the endcaps for injection and ejection of the ions. The ring electrode is split in quarters to allow dipolar and quadrupolar az­imuthal excitations of the ion motions. The quadrupolar excitation is used to determine the cyclotron frequency as described in ref. 7.The cyclotron frequency (hence the mass) is measured by a time-of-flight technique7. This technique is selected because of its wide applicability and high-sensitivity. It is a destructive method in that it involves ejecting the stored ions from the trap, but for short-lived isotopes this is not a disadvantage since those ions would be lost by radioactive decay. In a Penning trap, ions are confined by the combined effects of an axial quadrupolar electrostatic field and an axial magnetic field. The ions have three basic motions inside the trap, the axial oscillation loz, the modified cyclotron motion and the magnetron motion u -  • These frequencies are affected by the electrostatic field and only the true cyclotron frequency ujc can be related to the mass via a precise knowledge of the magneticfield independently of the electrostatic field. This unperturbed cyclotron frequency can be related to the eigenfrequencies of the ions in the trap by two relationships:u ]  =  a?z +  u i  +  a;9 (jJc — +  u>+.The second relationship is the one that is used in the excitation scheme under con­sideration. The ions are initially trapped near the center of the Penning trap. A small magnetron motion is introduced by a dipole excitation at uj_. The ions are then excited by a quadrupole field at a frequency close to the cyclotron frequency. If this frequency co­incides with the true cyclotron frequency of the ions, the magnetron motion of the ions is converted to motion at the reduced cyclotron frequency, resulting in a significant increase in the radial energy of the ions. When the ions are ejected from the trap and allowedto drift towards a channel plate detector, this radial energy is converted to longitudinal kinetic energy in the fringe field of the magnet, accelerating the ions towards the detec­tor. If the above process is repeated over a range of frequencies, a time-of-flight versus frequency spectrum can be obtained. The resonance at the true cyclotron frequency of the ions is observed as a reduction in the time-of-flight.N e w  p o ss ib ilit ie sThe proposed system will offer many new possibilities for mass measurements. First, the versatility of the injection system will allow the spectrometer to be used on a wide range of stable and unstable isotopes. In particular, the isotopes of very refractory el­ements will become available since the historical difficulty of getting them out of the target/ion source combination is solved by the helium-jet coupled laser ion source. Since the number of ions required in a trap for a mass measurement is small (about 10), it is expected that a yield of the order of 103 per second will be sufficient to perform a mass measurement on an isotope with an half-life of 1 second or more. Many new isotopes should therefore become available for study, even within the elements accessible with standard ion sources. It should be noted that helium-jet systems can also efficiently cap­ture recoils from heavy ion reactions. The present system could therefore accept isotopes produced by reactions with accelerated radioactive ion beams if there were cases where this could be more advantageous.New opportunities will also come from the high accuracy that will be available for both stable and unstable isotopes. W ith the technique described above, an accuracy of — ~  10-7 — 10-8 will be available for short-lived isotopes, and about an order ofmmagnitude better for stable and long-lived isotopes. It should allow significant mass measurements to be performed in fields, such as double beta-decay or the superallowed 0+ —> 0+ decays, where these high accuracies are required.The proposed instrument also opens up many new possibilities beyond those of mass measurement. The helium-jet coupled laser ion source will provide pulsed radioactive ion beams of unsurpassed purity. Its applicability to the elements that are not amenable to on-line extraction from conventional ion sources make it particularly well suited to radioactive isotopes of refractory elements. The laser resonant ionization process could also be used to perform isotope shift measurements on heavy elements where the resolution experimentally achievable by this technique is sufficient to obtain valuable information. The traps connected to this unique ion source, both the bunching RFQ trap and the Penning trap, add to this potential. They will enable the storing of radioactive ions from a selected isotope in the well controlled environment of an ion trap. The properties of these97stored radioactive ions will then become observable under very favorable conditions and properties which were not amenable to observation under normal conditions might become apparent. For example, high-precision experiments to measure the hyperfine anomaly on chains of isotopes could be performed in this system with the nuclear magnetic moments measured in the strong magnetic field of the Penning trap and the hyperfine structure (HFS) splitting factors measured in the RFQ trap. The extremely low uncertainties (10 11) demonstrated in HFS splitting-factor measurements in RFQ traps8 could also be used to search for higher moments in nuclei. It will also open up totally new possibilities for the study of fundamental phenomena. For example, an essentially 100% polarized radioactive source at rest in a vacuum would clearly be an asset to the field of ,5-decay asymmetry studies. A method has also been suggested by Fortson9 to measure parity-non- conservation effects in atoms using single radioactive ions stored in traps. In these cases, the possibility of storing different species of radioactive ions allows the experimenter to select the nucleus combination best suited to isolate the specific effect under investigation.C o n c lu sio nA Penning trap mass spectrometer with a novel injection system has been described. It combines a helium-jet system collecting the reaction products obtained from a high- current primary beam on thin targets with a resonant-ionization laser ion source to load very efficiently a system of two ion traps. This system offers many new mass-measurement possibilities because of its universal injection system and of the high-accuracy that it provides for measurements on stable and unstable isotopes. In addition, this system will allow us to store unstable isotopes at rest in the well-controlled environment of ion traps opening many new possibilities for precision measurements in atomic, nuclear and fundamental physics.R e fe r e n c e s1. P.B. Schwinberg, R.S. Van Dyck and H.G. Dehmelt, Phys. Lett. A81 (1981) 119.2. R.S. Van Dyck, D.L. Farnham and P.B. Schwinberg, Phys. Scripta 46 (1992) 257.3. H. Stolzenberg, S. Becker, G. Bollen, F. Kern, H.-J. Kluge, T. Otto, G. Savard, L.Schweikhard, G. Audi and R.B. Moore, Phys. Rev. Lett. 65 (1990) 3104.4. W. Jhe, D. Phillip, G. Gabrielse, J. Groebner and H. Kalinowsky, Phys. Scripta 46 (1992) 268.5. W.L. Talbert Jr., Proc. of the Workshop on the Science of Intense Radioactive Ion Beams, Los Alamos, April 10-12 1990, Report LA-11964-C, p. 172.6. D. Vieira, private communication.7. G. Bollen, R.B. Moore, G. Savard, H. Stolzenberg, J. App.Phys. 68 (1990) 4355.8. G. Werth, Comments on At. and Mol. Phys. (in print)9. N. Fortson, Phys. Rev. Lett. 70(1993) 2383.99D E C A Y S  O F  N U C L E I F A R  F R O M  S T A B IL IT Y  E. H a g b ergAECL Research, Chalk River Laboratories Chalk River, Ontario, Canada KOJ1J01. IN T R O D U C T IO NThe study o f  nuclei far from  stability is n o w  a mature sc ien ce1"3-* but it still rem ains one  o f  the m ost exciting  and rapidly grow ing areas o f  m odem  nuclear physics. T he continuing  interest in such nuclides stem s ch iefly  from their location at the extrem es o f  the chart o f  nuclides, their unusually h igh  decay energies and the p ossib ilities they open for system atic studies.T he nuclides found c lose  to the drip lines have extrem e proton-to-neutron ratios. They  therefore offer the m ost stringent test possib le for m any nuclear theories and a lso  the last opportunity to fine-tune a m odel that m ust be used to predict the properties o f  even m ore extrem e nuclides such as those found in stellar system s and supem ovae.The large decay energies available to nuclei far from stability m ake a w id e range o f  states accessib le  in their (3-decay daughters and offer the prospects o f  m ore com plete spectroscopy in  areas such as beta-decay strength functions and G am ow -Teller quenching. The large decay  energies also lead to unusual decay m odes such as single or m ultiple (3-delayed particle em ission. The study o f  such processes are interesting in their own rights but they also provide n ew  tools  for studying sp ecific  nuclear properties.The capability to synthesize nuclei far from  stability generally m eans that num erous other nuclides closer to stability are accessib le as w ell. Thus, taken all together they provide a trem endous opportunity for w ide-ranging system atic tests o f  any nuclear m odel. Furthermore, this capability m eans that for m any sp ecific  problem s the nuclear decay m ost suitable as a stringent testing ground for that problem  can be chosen at w ill.E ffective  studies o f  nuclei far from  stability require specialized  equipm ent but no single  piece o f  m achinery w ill be ideal for all decay processes to be studied. Each set o f  equipm ent w ill be appropriate for som e areas o f  physics, but not for others. In this paper I w ill briefly  discuss som e o f  the physics know ledge obtained from  studies o f  nuclei far from  stability (Section2) and som e o f  the equipm ent used in such studies (Section  3). The m ain part o f  the paper (Section  4 ) consists o f  m y attempt to classify  the physics and the equipm ent according to  w hich  decay process studies they are best suited for and w hich  regions o f  the chart o f  n uclides they can access.2 . P H Y S IC S  IN T E R E S T  IN N U C L E I F A R  F R O M  S T A B IL IT YFor the purpose o f  this paper, I w ill categorize the know ledge w e obtain from  studies o f  nuclei far from  stability into 5 broad areas. They are listed b elow  together w ith som e com m ents  on the decay process studied and the m ost salient physics inform ation deduced from  th ese studies. B ecau se the topic o f  this workshop is a R adioactive N uclear Beam  facility  at T R IU M F, the p h ysics top ics d iscussed  in this section are restricted to those achievable w ith  studies o f  ground- state decay processes or reaction studies perform ed with beam s o f  less than 20A  M eV  energy.i) E xisten ceFrequently, the first inform ation on a n ew  isotope is sim ply that it exists; it show s up as a cluster o f  dots on a display. Thus w e can map out the driplines and deduce w hich  system s are bound.ii) M assD irect-m ass m easurem ents provide us w ith the m ass ex cess  o f  the isotope and the extent o f  nuclear binding. Such inform ation is important for the developm ent o f  m ass form ulae with  good  predictive p ow ers that are necessary in the field s o f  astrophysics and cosm ology .iii)  H a lf-lifeA  m easurem ent o f  the h a lf-life  o f  an isotope provides inform ation that is  frequently used  to test and develop  gross beta-decay theories. T hese theories provide predictions that are necessary for our understanding o f  the different astrophysical n ucleosynthesis processes. They  also serve as a gu ide to experim entalists in predicting o f  the branching ratios betw een  particle decay channels and |3 decay o f  as yet unknown isotopes.iv ) L evel StructureInform ation on the level structure o f  an isotope m ay be obtained from  in-beam  y-ray studies or transfer reaction studies. In this paper, the term in-beam  y-ray studies refers exclu sively  to low -sp in , low -excitation-energy work. Our first assessm ent o f  the deform ation o f  an isotope m ay com e from  in-beam  y-ray studies aim ed at providing the first data on  its low - ly ing  energy levels. R eaction  studies y ie ld  a wealth o f  nuclear structure inform ation such as spins, parities, isosp in s and spectroscopic factors.v ) D ecay  RadiationT he types o f  radiation studied may be particles, self-delayed  or beta-delayed, or y rays. Studies o f  se lf-d elayed  particle em ission  provide inform ation on m asses, hindrance factors and configurations. D ata from  [3-delayed particle-em ission  studies m ay provide inform ation on  resolved  excited  states such as the location o f  isobaric analogue states, G am ow -T eller quenching, as w ell as data to test the parameters o f  the fundam ental w eak interaction. Such studies also provide data on the properties o f  unresolved excited  states w hich  are used to deduce inform ation  on statistical theories on ensem bles o f  states, level densities, life tim es and decay w idths as w ell as the shape and m agnitude o f  the parent beta strength function. Studies o f  (3-delayed y rays y ie ld  nuclear structure inform ation on subjects such as configurations, co -ex isten ce , intruder states and shell-m odel com parisons.In broad term s, this is the type o f  know ledge w e obtain from  studies o f  nuclei far from  stability. In addition, w e  also gain know ledge in other areas that w ere not in itially the goal o f  our research. This additional inform ation com es about as a bonus because the equipm ent or techniques develop ed  for our studies o f  n ew  decay processes turn out to be very usefu l as n ew101too ls for the study o f  different nuclear properties. A m ong these bonuses resulting from  a program to study nuclei far from  stability are:1) Inform ation on fundam ental properties o f  the weak interaction. Precise studies o f  kinem atic shifts in 3-d elayed  particle groups have been used to verify  the constancy o f  the w eak vector  coup ling constant for J” *  0 + transitions45. Studies o f  the detailed peak shapes o f  such decays have been show n to be pow erful probes o f  possib le scalar contributions to nuclear 3  decay55.2) V ery short nuclear level life-tim es. The delayed Particle X -ray C oincidence Technique65 (PX C T ) has been used successfu lly  to m easure nuclear life-tim es at the 10'16 s level through  com parisons w ith w ell-know n atom ic lev e l life-tim es.3) The d iscovery o f  halos. Studies o f  the interaction cross section  for light nuclides75 have  y ield ed  ev id en ce for the existence o f  large neutron halos in som e very neutron-rich nuclei.4 ) Probes for the existence o f  dineutrons or diprotons. Studies o f  beta-delayed 2-neutron85 or 2-proton95 em ission  have provided a very sensitive m ethod to investigate the temporary ex isten ce o f  such objects.A nother advantage that arises from the study o f  nuclei far from  stability is  that it has led  to our present capability to synthesize a w id e range o f  nuclides, and w ith that capability com es  the possib ility  for pow erful system atic investigations. The particular nuclear physics problem  under study can be explored in many different regions o f  the chart o f  nuclides. Or, i f  this is more instructive, it can be investigated for a w ide scan o f  isotopes, isotones or isobars (see  Figure 1). In som e cases, such as mass form ulae, a single know n point situated at the extrem es o f  the know n nuclei w ill provide the m ost stringent test o f  ex istin g  m odels. For other problem s, tracking a parameter as a function o f  rapidly changing deform ation m ay be the m ost severe test. And, for m any problem s, there may exist one particularly w ell suited nuclide that is m ost sensitive to this problem  so w e would benefit from  the ability to study that nuclide selectively . A ll these p ossib ilities are available with the equipm ent and techniques developed  for the study o f  nuclei far from  stability.3 . E Q U IP M E N T  U S E D  F O R  S T U D IE S  O F  N U C L E I F A R  F R O M  S T A B IL IT YIn th is section , I w ill categorize the equipm ent used for studies o f  nuclei far from  stability  into broad areas, in the sam e manner as the physics know ledge w as treated in section  2. B ecause  I restrict m y se lf  to radioactive beam s w ith less than 20A  M eV  energy the categories w ill not include projectile fragm ent separators.i) First Stage ISOLA  first stage ISOL refers to a system  consisting o f  a target/ion-source com bination and a m ass separator m agnet coupled  to the primary production beam. A TR IU M F-based facility  w ould  m ake u se o f  the available high energy proton beam  and thus necessarily  in vo lve an ISOL  system  o f  the thick-target type. It w ould thus require the reaction products to d iffuse out o f  thedeformedj_ r ~SYSTEMATICSW ID E S P A NEXTREMESSCAN ACROSS RAPIDLY CHANGING CONDITIONPICK BEST NUCLIDE FOR PARTICULAR PROBLEMFigure 1 The benefits to physics interpretation of being able to study isotopes over a wide range o f the chart o f nuclides.bulk target to the ion source. The purpose o f  the ISO L is to transport the desired activity away  from  the hot interaction region and to ach ieve a reasonable degree o f  m ass separation. The end  product delivered  to the user is a low -energy beam o f  nuclides (occasionally  a sp ecific  isotope) within a g iven  isobar. T he drawback o f  the ISO L technique is an elem ent- and h alf-life  dependent effic ien cy . A n ISO L g iv es  y ou  purity at the expense o f  effic ien cy .ii) G as-iet/Tape-Transport SystemA  H e-jet gas transfer system , coupled to a fast transport tape also ach ieves the goal o f  rem oving the desired activity from the interaction region. It is done w ith no elem ental dependence and, for h a lf-liv es  longer than 100 ms, som e selectiv ity  can be realized  by m atching  the cy c lin g  tim e w ith the h a lf-life  o f  the desired activity. Such a system  can be coupled  to the primary TRIUM F beam  (and serve as the injector to the ISO L ) or to the accelerated radioactive  beam s. The advantages o f  such a system  are that it is cheap, efficien t, elem ent insensitive , easy to operate and rem oves lon g-lived  background. The disadvantages are the absence o f  purity with  the concom itant high counting rates, w hich  means that such a system  is m ost profitably em ployed  w hen operated together w ith another system  that enhances the selectiv ity .103iii) Post A cceleratorThis d ev ice  is  alw ays envisaged to be em ployed  together with the first stage ISO L. It w ill efficien tly  accelerate the low -energy radioactive beam s available from  the ISO L up to 10- 20A  M eV . T hese beam s can then be used to initiate and study nuclear reactions or to produce even m ore exotic  radioactive species.iv ) R ecoil M ass SeparatorT his d ev ice  is coupled  to the post accelerator and used to spatially isolate and identify  activities created by fusion-evaporation reactions betw een  an energetic radioactive beam  and a thin target. T ypical exam ples o f  such d evices are SH IP 10) at G SI and the F M A n) at A rgonne  National Laboratory. The strong com petitive points o f  such d ev ices  are their selectiv ity , elem ent insensitivity and speed.v ) D etector ArrayThe detector array is intended for y-ray detection and is envisaged  sim ply as a cluster o f  H PG e detectors, not a fu ll-fled ged  high-spin type array. The detectors could  be left overs from  the first generation o f  such arrays. The main purpose o f  such an array is to obtain a goodeffic ien cy  for y-ray detection w h ile  at the sam e tim e b eing  capable o f  handling high decay ratesthrough the use o f  m any detectors, each subtending a sm all so lid  angle. Such an array could  be located at the recoil m ass separator target location or focal p lane location  or it could be coupled  to som e o f  the other d ev ices m entioned in this section.v i) Second Stage ISO LThe purpose o f  a second  stage ISOL is to m ass analyze on-line products generated by reactions w ith accelerated radioactive beam s. This ISO L  is  therefore m uch different from  the first stage one in that it em ploys a thin target; is aim ed at speed  rather than bulk production rates and does not require the sam e precaution in areas o f  radiation protection and hot m aterials handling. It should therefore be much sim pler and cheaper. T he com petitive area for a second  stage ISO L w ill be in the production o f  reasonably pure sam ples o f  nuclei near the drip-lines. The inevitable drawback w ill be a sizeable loss o f  effic ien cy .4 . P H Y S IC S  A N D  E Q U IP M E N TIn this section  I w ill com bine the concepts defined  in the previous tw o sections. I w ill attempt to order and quantify the relation betw een the physics problem  selected  (as contained in the fiv e  broad areas o f  know ledge given  in section 2 ) and the types o f  experim ental devices  available to study this problem  (as listed in section 3). The object o f  this exercise is to get som e  understanding o f  h o w  useful certain experim ental d ev ices are for the study o f  a selected  physics  problem  or, conversely , what type o f  physics know ledge one can obtain from the em ploym ent o f  a sp ecific  p iece  o f  equipm ent. Figure 2 sum m arizes m y attempts at relating the physics and the experim ental equipm ent.F IR S T  S T A G E  ISO L  G A S -J E T  /  T A P E  T R A N S P O R T  P O S T  A C C E L E R A T O R  R E C O IL  M A S S  S E P A R A T O R  D E T E C T O R  A R R A Y  S E C O N D  S T A G E  ISO LI 1 W MEXISTENCEMASSDECAY T1/2LEVEL STRUCTURE in-beam y rays reactionsDECAY RADIATION g.s. particles delayed particles delayed y raysxXxXXxXxXXXxXxFigure 2 An illustration of the usefulness of certain combinations of equipment For more information see textThe left-hand side o f  the figure displays on top the six  broad equipm ent categories and b elo w  them  the fiv e  broad physics categories. H orizontal bars are show n on the top right-hand  side o f  the figure, next to the equipm ent categories. A  filled  bar indicates the equipm ent listed  to the le ft o f  the bar is em ployed. An open bar indicates that the equipm ent is  optional, i.e ., it m ay be ben eficia l but not essential. Underneath the bars are s ix  colum ns representing six  p ossib le com binations o f  the listed  equipm ent. The position o f  a cross in on e o f  th ese colum ns  sign ifies  that the physical process found on the sam e line as the cross can effec tiv e ly  be studied  with the equipm ent indicated by the horizontal bars above the colum n the cross is located  in. A  larger cross indicates the best physics-equipm ent match. A  sm all cross ind icates that the physics can also be studied w ith another equipm ent com bination, but not w ith  as h igh  a degree o f  efficien cy .A  sp ecific  exam ple w ill be used to further illustrate the use o f  the inform ation presented  in Figure 2. Suppose that one is  interested in a physics problem  that is best studied by in-beam  y-ray spectroscopy. On the sam e line as the category "in beam  y rays" in F igure 2 on e finds a large cross in the fifth colum n. E xtending this colum n upwards results in the interception o f  four filled  bars. This m eans that in-beam  y-ray spectroscopy is best perform ed w ith a com bination105o f  a detector array, a recoil m ass separator for identification, a post-accelerator and a first stage ISOL. There is also a sm aller cross in the second colum n o f  the "in beam  y-ray" line. This m eans that this type o f  physics can also be studied effectively  w ith the equipm ent corresponding  to the second  colum n, a post-accelerator coupled to a first stage ISO L with optional benefit from  a detector array.T he inform ation presented in Figure 2 does not tell the full story, how ever. E ven i f  one  accepts the prem ises underlying that figure and that it w ill indicate h o w  to best ach ieve certain physics goals, these goals cannot be achieved in all areas o f  the chart o f  nuclides. Certain com binations o f  equipm ent may be com petitive in areas c lose to the line o f  beta stability but not applicable in areas far from  stability. Figure 2  therefore needs to be augm ented by som e further inform ation and that is  attempted in Figure 3.F IR S T  S T A G E  ISO L B B SG A S -J E T  I T A P E  T R A N S P O R T  C Z D  P O S T  A C C E L E R A T O R  R E C O IL  M A S S  S E P A R A T O R  D E T E C T O R  A R R A Y  S E C O N D  S T A G E  ISO LFigure 3 Regions of the chart of nuclides accessible with certain combinations of equipment For more information, see textT he top o f  Figure 3 is identical to that o f  Figure 2 and is interpreted in the sam e manner. The s ix  colum ns show n in Figure 2 are also present in Figure 3, albeit in a much shorter form  and n o w  labelled  w ith the num bers 1 to 6. The bottom  part o f  Figure 3 depicts a "one-dim ensional chart o f  nuclides" with the sole purpose o f  defin ing regions o f  nuclei as a function  o f  their rem oteness from  the line o f  (3 stability. Horizontal bars, labelled  with num bers, signify  the region o f  n uclid es accessib le with the equipm ent corresponding to the colum n with that number and that this equipm ent is com petitive in that region.A s an exp lic it exam ple o f  the use o f  Figure 3 let’s select region  num ber tw o. T he top part o f  the figure indicates that in this case the equipm ent em ployed  is  a post-accelerator coupled to a first stage ISO L  and from  Figure 2 it is clear that this equipm ent is useful for reaction m ass m easurem ents, in-beam  y rays and general reaction studies. The inform ation displayed in Figure 3 then sp ecifies  that th ese studies w ill generally b e com petitive on ly  in a region fairly c lose to stability. T he reason is  that really intense radioactive beam s, necessary for these types o f  studies, are on ly  available in that region. Naturally it is  easy to find  excep tions to the very general conclusions presented in Figure 3. The drip-line nuclide n Li, on ly  available w ith  very lo w  intensities, has been used  in interaction radius and break-up studies, but such studies form  a very  small subset o f  the fie ld  o f  reaction studies and have thus been ignored in Figure 3.It is  apparent in Figure 3 that the w idest range o f  access to n uclides w ithin the driplines is ach ieved  w ith a first stage ISO L alone. In term s o f  bulk production rates o f  radioactive isotopes, other p ieces o f  equipm ent attached to such a ISOL w ill rarely, i f  ever, be able to match the rates available from  the ISO L alone. W hat the additional p ieces  o f  equipm ent achieve are m ainly increased sam ple purity, removal o f  potentially intense background and m ore specific  types o f  experim ents. Thus the added p ieces o f  equipm ent are m ost usefu l in the regions close  to the fringes o f  stability, as exem plified  by regions 3 to 6 in F igure 3, w here purity, low  background and sp ecific ity  are m ost needed.5 . S U M M A R Y  A N D  C O N C L U S IO N SThere is  nothing n ew  or startling presented in this paper on the physics o f  n uclei far from  stability or the p ieces o f  equipm ent that may be used at a radioactive nuclear beam s facility. W hat is  n ew  is  m y attempt to organize and classify  the physics and the equipm ent in such a way  that the connection  b etw een  the em ploym ent o f  a specific  set o f  equipm ent and its usefu lness and the physics k now ledge gained is more transparent. Naturally, at th is point, the linkages shown  are very subjective and open to debate. H ow ever, the fram ework show n in F igures 2 and 3 form  an objective structure that can be m odified  or added to. The final, desired outcom e o f  additional work m ay be a better appreciation o f  the physics know ledge attainable with a certain p iece o f  equipm ent and its price tag, i.e ., the nucleus o f  a cost-benefit analysis.F inally , a fe w  subjective notes on the strong points o f  a T R IU M F-based radioactive nuclear beam s facility .1. Y ield . O nly first stage ISO L needed.The h igh  intensity, energy and high repetition rate o f  the T R IU M F proton beam  m akes this laboratory capable o f  ach ieving the h ighest production rates o f  a very large range o f  radioactive nuclides. W ith the best y ield s available in the w orld , com petitive experim ents can be perform ed even  w ith simpler and cheaper equipm ent than w ould  be required elsew here.1072. R eactions w ith  chosen radioactive species. Add post-accelerator.The com bination o f  an ISO L and a post-accelerator w ill result in a facility  w ith  an unprecedented versatility in perform ing experim ents in the areas o f  astrophysics, nuclear structure and C oulom b barrier energy reaction m echanism s.3. A ccess  to the fringes o f  stability. Add recoil m ass separator.T his equipm ent com bination w ould result in the capability o f  observing  ground state and 3- delayed particle decays and, to a lesser extent, low -sp in  in-beam  y rays for nuclides at, or c lo se  to, the drip lines.REFERENCES1. Proc. 6th Int. Conf. on N uclei far from  Stability, B em k astel-K u es, G erm any, 1992 July 19- 24; IPP, Bristol and Philadelphia; Institute o f  P hysics C onference Series 132, p. 31.2. Treatise on H eavy-Ion Science, V ol. 8, Ed. D .A . B rom ley , Plenum  Press 1989.3. Particle E m ission  from N uclei, Eds. D .N . Poenaru and M .S. Ivascu, CRC Press Inc., 1989.4. E.T.H. C lifford, J.C. Hardy, H. Schm eing, R.E. A zum a, H.C. E vans, T. Faestermann,E. H agberg, K .P. Jackson, V.T. K oslow sky, U.J. Schrew e, K .S. Sharm a and I.S. Towner, Phys. R ev. Lett. 50 (1983) 23.5. E.G. A delberger, Phys. Rev. Lett. 70 (1 9 9 3 ) 2856 , errata in Phys. R ev. Lett. 71  (1 9 9 3 ) 469.6. J.C. Hardy, J.A. M cD onald, H. Schm eing, H.R. A ndrew s, J.S. G eiger, R.S. Graham, T. Faesterm ann, E.T.H. Clifford, K.P. Jackson, Phys. R ev. Lett. 37  (1 9 7 6 ) 133.7. I. Tanihata, H. H am agaki, O. H ashim oto, Y. Shida, N . Y osh ikaw a, K. Sugem oto,O. Y am akaw a, Y . Kobayashi and N. Takahaski, Phys. R ev. Lett. 55 (1 9 8 5 ) 2676.8. R.E. A zum a, L.C. Carraz, P.G. H ansen, B. Jonson, K .L. Kratz, S. M attsson, G. N ym an,H. Ohm , H .L Ravn, A. Schroeder and W. Ziegert, Phys. R ev. Lett. 43 (1 9 7 9 ) 1652.9. M .D . Cable, J. H onkanen, R.F. Parry, H.M . Thierens, J.M. W outers, Z.Y . Zhou and J. C em y, Phys. R ev. C26 (1982) 1778.10. G. M iinzenberg, W . Faust, S. H ofm ann, P. Armbruster, K. Guttner and H. Ewald, N ucl. Instr. and M eth. 1 6 1  (1979) 65.11. C. D avid s, these proceedings.Radioactive Beam Experiments Using the Fragment Mass AnalyzerCary N . D avids A rgonne N ational LaboratoryAbstractT he Fragm ent M a ss A n a ly zer  (F M A ) is a reco il m ass spectrom eter that has m any  potential app lication s in  exp erim en ts w ith  rad ioactive beam s. T he F M A  can  be u sed  for spectroscop ic stud ies o f  n u c le i produced in reactions w ith  rad ioactive beam s. T he F M A  is  also  an idea l to o l for studying radiative capture reactions o f  astrophysica l interest, u sin g  inverse k in em atics. T h e F M A  has both  m ass and en ergy  d ispersion , w h ich  can  be used to effic ien tly  separate the reaction reco ils  from  the prim ary beam . W hen u sed  w ith  radioactive  beam s, the FM A  a llo w s the reco ils from  radiative capture reactions to  be detected  in a lo w -  background environm ent.I. IntroductionThe F M A  is  a versatile  recoil m ass spectrom eter d esigned  to  separate reaction reco ils at 0° from  the prim ary b eam  and to d isperse them  by M /q  (m ass/charge) at the fo ca l p lane. Figure 1 sh o w s a sch em atic layout o f  the FM A . In norm al operation , the prim ary beam  is  stopped on  the anode o f  the first e lectric  d ip ole, w h ile  reco ils  w ith in  a broad range o f  M /q  and energy pass on  to  the foca l p lane.TGT Q1Q2 ED1 MD ED2 Q3Q4 DETFig. 1. Schem atic layout o f  the FM A .T he F M A  can be u sed  in con ju n ction  w ith  a C om pton-suppressed  G e detector array around the target p o sitio n  for m ass-gating  o f  prompt gam m a spectra, or to provide a beam  o f  reaction products for im plantation into different typ es o f  detectors p laced  at the foca l p lane. In this report w e  d iscu ss tw o  potentia l areas w here the F M A  or a sim ilar d ev ice  cou ld  be used for experim ents at a radioactive beam  facility.II. Nuclear Structure Studies With Radioactive BeamsA  reco il m ass spectrom eter w ill  be an important too l for nuclear structure studies using  rad ioactive beam s. B ec a u se  o f  background p rob lem s lik e ly  to  be en cou n tered  in  such  experim ents, each  param eter lik e  M /q  that can be m easured h elps to increase the strength o f  the desired  signal. It is  particularly important to strive for the h igh est detector e ffic ien c ies  for these m easurem ents.109The study of nuclei at high spin has been a topic of intense interest in recent years. We have learned much about the behavior of nuclei under conditions of high angular momentum and high excitation energy. With the advent of large arrays of gamma-ray detectors, unprecedented levels of sensitivity have been attained. A result of these capabilities has been the discovery of superdeformed nuclei in the A=150 and A=190 regions. Because of the available stable beam-target combinations, the observed cases of superdeformation have been limited to neutron-deficient nuclei. There are predicted regions of superdeformation closer to stability, and even hvperdeformation (axis ratios > 2 :1) has been predicted to occur in stable nuclei such as I66,l68£r and 170Hf[l]. Although the low- spin structure of these nuclei is well-known, the study of such nuclei at high spin can only be accomplished using heavy-ion beams. It is here that radioactive beams could be used effectively to complete the spectroscopy of such nuclei and possibly to observe hyperdeformation for the first time.An example of such an experiment would be the 124Sn(46Ar, 4n)166Er reaction at 160 MeV, which has a calculated cross-section of about 100 mb. Here 166Er production represents about 20% of the total yield. Using a beam of 46Ar with an intensity of only 106 particles/s and an array of 60 Compton-suppressed Ge detectors, more than 106 y_Y coincidences will be obtained in a 5-day run, which would be quite adequate to characterize the yrast structure and perhaps to give evidence for the presence of hyperdeformation. During the same run, using a device like the FMA, about 107 recoil-Y coincidences will be obtained, enough to cleanly identify transitions well below the 1 0 '2 relative intensity level.On the neutron-deficient side, prompt gamma-ray spectroscopy of nuclei near the N=Z line could be studied using 34Ar beams on targets of 54Fe, 58Ni, and ^Zn. Beams of intensity near 1 0 8 particles/sec will enable comprehensive spectroscopic studies of these interesting nuclei. One of the main interests is in observing the effect of nucleon pairing for nuclei where the valence neutrons and protons are filling the same shell-model orbitals. Pairing should also influence the ground-state masses in this region, and a recoil separator will be valuable for making such measurements.III. Radiative CaptureRadiative capture reactions such as (p,Y) and (a,y) play an important role in nuclear astrophysics. In many instances they are part of sets of reactions such as the CNO cycle and the pp chain, where the final results are the conversion of hydrogen into helium and the production of energy. In the CNO cycle, the constituents such as C, N, and O play the role of catalysts, since they are created and destroyed at equal rates. Typical energies needed for the CNO cycle are around 25 keV.Interest has recently been focused on the hot CNO cycle and the rp-process (see Ref. [2] for a review), which provide an opportunity to "break out" of the CNO cycle and lead to production of elements with higher atomic numbers. The astrophysical environments where these might occur have temperatures in the range 0.1 < T9 < 1.5 and densities of 102 -108 g/cm3, where T9 is the temperature in units of 109 K. Under these conditions the seed materials C and O are processed on time scales comparable to or faster than typical beta- decay half-lives. This means that reactions involving radioactive species such as 13N, 14-150 , 17.18f , I8,i9]qe  others will become important.III.(a) Measuring Radiative Capture ReactionsT he cross section s for radiative capture reactions are sm all, and very  strongly  en ergy- d ep en dent. O f prim ary in terest to  astrop h ysics is  the p resen ce  o f  reso n a n ces in  the  com pou n d  sy stem , w h ere the cross sec tio n  m ay reach up to  m illib a m  le v e ls . W here no  resonances ex ist, the reaction  p roceeds by direct capture. F igure 2  sh ow s the cross sec tion  for a typ ica l ca se , the 13C (p ,y)14N  reaction , w h ich  is dom inated  by  a resonan ce near 5 5 0  k eV . M easu rem en ts su ch  as that sh o w n  in F igure 2  w ere p a in sta k in g ly  o b ta in ed  by  observing the capture gam m a rays w ith  detectors w h o se  e ffic ien c ies  in  the 5 M eV  range are typ ica lly  a few  percent for large N a l(T l) detectors and about 10'3 for large G e detectors.F ig. 2 . C ross sec tion  for the 13C (p ,y)14N  reaction in the lo w  energy region .T h e first rad iative capture reaction  to be stu d ied  u sin g  a rad ioactive  b eam  is  the  13N (p ,y )140  reaction , perform ed u sin g  inverse k inem atics at L o u v a in -la -N eu v e[3 ]. In this  experim ent, a C H 2 target w a s bom barded by an 8 .2  M eV  b eam  o f  13N  (h a lf-life  9 .9 7  m ) having  an intensity o f  3 x l 0 8 particles/s. A  large G e detector located  near the target p osition  w as u sed  to  observe the 5 .1 7 -M eV  capture gam m a rays from  a resonance occurring at E cm =  5 2 6  k eV . T he fu ll en erg y  peak  e ff ic ie n c y  o f  the detector w a s ap proxim ately  1 0 '3 . In  addition , the detector w a s  lo ca ted  in  a h igh ly  rad ioactive environm ent produced  b y  the  d ecay o f  scattered b eam  particles in  the target cham ber.T he u se  o f  ra d ioactive  b eam s for stu d yin g rad iative capture reaction s m ean s that inverse k in em atics m ust b e used . B eca u se  the beam  inten sities are ex p ec ted  to be m uch  low er than those obtainable for stable hydrogen and helium  iso to p es, it is  very  im portant to  d evelop  h igh -effic ien cy  low -background detection  techniques. In this w ay the cross section s  for 13N (p ,y )140  and other important astrophysical reactions in v o lv in g  radioactive beam s canI l lbe determined over a broad energy range. One way to achieve this high efficiency is to use a recoil mass separator to detect the recoil products[4].III. (b) Using the FMA for Radiative Capture MeasurementsIn what follows, we describe the planned use of the FMA to study the 13C(p,y)14N reaction, the analog to the 13N(p,y)140  reaction. Instead of detecting capture gamma rays from the target, the 14N reaction recoils will be detected and identified at the FMA focal plane with a total efficiency of about 50%, when the charge state distribution of the recoils is taken into account.The key to making inverse kinematics measurements with the FMA is to prevent the primary beam from striking the anode of the first electric dipole, and instead intercept most if not all of it on apertures placed before and following the 40° bending magnet. In the test case 13C(p,y) 14N, the beam energy will be 7.2 MeV and the recoils will have an energy of 6.7 MeV and a peak charge state of 5. These recoils enter the FMA within a cone of half­angle 18 mr around the beam direction, due to 8 MeV y-emission and multiple scattering in the target. Unlike the reaction products from normal kinematics, these recoils have a narrow (~+2%) range in energy. As the recoil and primary beams traverse the first electric dipole, they diverge because of differing energy/q (E/q) values. When they reach the region between the first electric dipole and the bending magnet they have become physically separated.This separation of the 7.2 MeV primary 13C beam from the 6.7 MeV central 14N recoils is depicted in the optics diagram of Figure 3. In this figure, q=5 ions of masses 13 and 14 and energies of 6.7 and 7.2 MeV are shown leaving the target with angles of 0 and +18mr. The energies of the 2 groups of trajectories emerging from the first electric dipole are 7.2 MeV (upper group, E/q=1.44, both masses) and 6.7 MeV (central group, E/q=1.34, both masses). The desired E/q=1.34 14N recoils follow the central optic axis of the FMA. An aperture (energy aperture) placed in the space between the first electric dipole and the bending magnet will pass only the E/q=1.34 recoils, while the E/q=1.44 beam particles are stopped. Other charge states of the primary beam that are of concern are 4 and 6, with E/q values of 1.80 and 1.20 These will be deflected away from the optic axis and will also be stopped on the energy aperture, along with the q=4 and q=6 14N recoil components.To further enhance the removal of scattered beam background, a second aperture (mass aperture) will be placed between the magnet and the second electric dipole. This aperture, also shown in Figure 3, will help to remove any remaining primary beam which may have scattered inside the bending magnet. The central ion, 14N with q=5, E/q=1.34, will be transmitted through the FMA with 100% efficiency.At the focal plane a number of measurements will be made to identify the 14N recoils. The x-position (M/q) will be determined by the position-sensitive focal-plane detector. Then the recoils will be allowed to travel a distance of 40 cm behind the focal plane where a combination gas energy-loss and silicon total-energy detector will be located. Time-of-flight and energy measurements will allow A=13 and A=14 particles to be separated, and a Z determination will be made using energy loss. This is possible because at the energies near 0.5 MeV/u found in this experiment, 14N and 13C are at the peak of the Bragg energy-loss curve.The FMA additionally offers discrimination against the 1.5xl0 4 concentration of deuterium in natural hydrogen. The cross section for the contaminant reaction 2H(13C,14N)n can be orders of magnitude larger than the cross section for radiative capture on hydrogen.-i 0 .820  LLUi—i— i —i3 5 o oo olJlJ CCLCXLUUJ<- CL Q_ D Dc c o 'o 'Fig. 3. Ion optics for the F M A , set up for a central ion  w ith q =5, E = 6 .7  M eV , and A = 1 4 . T he trajectories represent em ission  angles at the target o f  0  and ±18m r, energies o f  6 .7  and 7 .2  M eV , and m asses 13 and 14.H ow ever, the contam inant reco ils  h ave quite d ifferent kinem atics, and w ill be elim inated  on  th is basis. T he 14N  reco ils  from  the contam inant reaction  2H (13C ,14N )n  h ave en erg ies o f  3 .15  and 9 .3 7  M eV , and on ly  the extrem ely  w ea k  q=7 com ponent o f  the latter group w ill  pass through the F M A . A n  energy m easurem ent w ill  then serve to  identify  them .I f  further back grou nd  red u ction  is  n ece ssa ry , a h igh  granularity, h ig h  e f f ic ie n c y  gam m a-ray detector array, sh ie ld ed  for annihilation radiation, cou ld  be p laced  at the target and operated in co in cid en ce w ith  the F M A  focal-p lane detector. H ow ever, th is w o u ld  reduce  the total detection  e ffic ien cy  to about on e-h a lf o f  that o f  the gam m a-ray array.H ydrogen targets w ill b e  L iH  on  C backings or thin C H 2. A  target w obbler for the F M A  target cham ber is  ava ilab le to  a llo w  for h igh  b eam  currents. For a 6 0  |Xg/cm2 target, the  p roduction  rate at the target is  2 .8 x l0 " 2 re c o ils /s /p n A /|lb . W ith  th e q=5 ch arge state  conta in ing  50%  o f  the to ta l, a 13C  b eam  o f  5 0  p n A  w ill y ie ld  a 14N  fo ca l p lan e  rate o f  approxim ately 100 /s at the peak  o f  the resonance.In co n c lu sio n , a reco il separator lik e  the F M A  has m any p rom isin g  ap p lica tion s to experim ents at a rad ioactive b eam  fac ility . Past exp erien ce has sh ow n  that n ew  w a y s o f  operating the F M A  w ill  be d iscovered , opening up  n ew  classes o f  experim ents w h ere it can  be used.W ork supported b y  the U .S . D epartm ent o f  E nergy, N uclear P h y sics  D iv is io n , under Contract W -3 1 -1 0 9 -E N G -3 8 .References[1] J. D udek, T . W erner, and L. L. R iedinger, P h ys. Lett. B  2 1 1 ,2 5 2  (1988).[21 A . E. Cham pagne and M . W iesch er, A nnu. R ev. N u cl. Part. S ci. 4 2 , 39  (1992).[3] P. D ecrock  e t a l., Phys. R ev . Lett. 67 , 808 ( 1 9 9 1 ) ,  P hys. Lett. B  3 0 4 , 50  (1 9 9 3 ).[4] M .S . Sm ith , C. R o lfs , and C .A . B a m es, N u cl. Instrum. M eth. A 3 0 6 , 233  (1 9 9 1 ).ISOMERS NEAR S n -1 0 0  AND OTHER EXPERIMENTS ALONG THE PROTON DRIP LINEWilliam B. WaltersDepartm ent of Chemistry, University of Maryland, College Park, MD 20742The prospect of being able to u se  Tz <0 beam s heavier than Ca-40 for the study of nuclear structure around the Sn-100 double shell closure is m ost attractive. There are wide variety of structure and decay phenom ena made possible by the high Q values whose study will reveal considerable fundamental shell model and nuclear structure information.In Fig 1 are show n possible level structures and isom ers in S n -100  and adjacent nuclides. For example, consider Sn-100, itself. Kris Heyde presented results from new  calculations at the Chicago ACS m eeting indicating an energy of ~ 4 .5  MeV for the first 2+ level and indicating the real possibility of a long-lived 7+ isomer. 1 That is, the excited states in S n -100  will be constructed by promoting a proton, or a neutron, from its position below the shell closure in a gg/2 orbital across the shell into a d s /2 orbital! The resulting multiplets will span sp ins 2+ to 7+. Promotion of the neutron to the nearby g7/2 orbital will produce another 1+ to 8+ multiplet. Depending on how these states mix, a 7+ to 5+ isomer transition could have a half life in the m icrosecond range, and if the 7+ gets below  the 5+ level, then a m uch longer lived isomer could be expected. The expected proton binding energy is about 3 MeV, hence an isomer at 6  MeV would be ~3 MeV unbound witha Coulomb barrier of about 5 MeV and a hindered, relatively high -I  proton decay could ensue. Beta decay competition to the ground state of In-100, w hich is expected to be either a 6 + state of 7+ state, would also be possible, and would have a large Q value and could be followed by the em ission of delayed protons.Another important nuclide is Sn-101. Again, there is the strong possibility of a high spin isom er arising from the coupling of the d s/2 ground state or g7/2 first excited state to the 7+ state in the S n -100  core. My hope is that a m icrosecond isom er would exist that would live long enough to pass through a recoil m ass separator of the type Cary Davids has ju st described to you and then undergo gamma decay, revealing the positions of the crucial single-neutron orbitals in Sn-101. These orbitals underlay the Ni-78 to Sn-132  shell and their determination in Sn-101 is critical to the description of the monopole shift in position of the single particle orbitals with changes in N, and Z. And, those orbital shifts, in turn, have important effects on the properties of the nuclides lying along the astrophysical r-process path. A number of such  isom ers have been identified in Sb-133, Sb-129, In-131, and Sn-131 w hose properties provide m uch usefu l information about the structure of the orbitals near the Sn-132 shell closure. 2*3-4 In Figure 2 are shown some PACE calculations for production cross sections using a G e-64 beam . Also shown is a value for Sn-101 production using a Cu-58 beam  on a Ca-40 target.For my own interest in odd-odd nuclides, it would be usefu l if there were an isomer in In-100 that undergoes internal decay down the yrast band to the ground state, thereby giving the positions and splitting of the high spin members of the two ground state multiplets. By combining those data with data from beta decay of S n-100  into levels of In- 100, a  full picture could be achieved for the m ultiplet splitting in In-100 which has a single proton hole and a single neutron beyond Sn-100. 5-6There are a num ber of spin gap isom ers with A < 100 for w hich detailed calculations for their energies have been published by Ogawa. 7>8 Perhaps the m ost interesting of these isom ers is the possible fully aligned isom er in Cd-96 show n in Figure 3. The analogous state in Po-212 is a well-known alpha emitter and there are also a number of other long-lived isom ers in adjacent nuclides. The actual decay m odes of these isomers will depend on the energy of the isomer itself, and the m asses and configurations of the possible daughter products. Examples of the charged particle decay of high spin isomers is found for H f-156 and W -158. 9 In both of these nuclides, isom ers with apparent spins and parities of 8 + that lie below 2 MeV have been identified that undergo alpha decay.It is  n o t c lear how  m an y  of th e se  isom ers c an  be reached  an d  identified  w ith  stab le  b eam s. In  F igure 4  a re  som e PACE calcu la tions for w h a t can  be p roduced  w ith  Ni-58 on  C a-40  (show n in  th e  r ig h t of each  box), along w ith  cross sec tions for th e  u se  of a  r a d io a c t iv e T u S s  b e ^ o n  C a -4 0 (sh o W  on th e  left in  each  box). H ie  v a lu es do illustraU  S e  relauve advan  “ g ^ o n f e r r e d  by  u s in g  th e  radioactive beam , b u t  w h e th e r  th a t  writ com pensa te  for lower b eam  c u rre n ts  an d  o th er problem s rem ain s to  be  seen .Thpre are  a  n u m b e r  of o th er nuc lides along th e  p ro ton  d rip  line u p  to  S n -1 0 0  th a t  a re  of in te re s t a n d  for w hich  m ea su re m e n t opportun ities are  lim ited w ith o u t radioactive b eam s. T hese include  Q va lues for ligh t nuc lides betw een Zr a n d  Sn, a s  well a s  levelenerg ies an d  decay properties .Below Z r-80  a re  th e  Tz = - 3 /2  nuc lides th a t  have been  s tu d ied  a t  A = 57 an d  61 a t  T RT w ith  add itional evidence for som e heav ier ones. 10 T hese a re  u n iq u e  n u c lid es th a t  have^b rong  b e ^  delayed p ro to n  b ra n c h e s  to  th e  T  = 3 /2  isobaric  analog  in  th e  d au g h te r n u ch d e  S o t iS r ^  im p o rtan t re su lt  h a s  been  th e  possible observation  of b e ta  delayed p ro to n  em ission  from  69a s  levels p o pu la ted  in  th e  decay of 69Se from  levels w ith  deform atio n  va lues of th e  o rder of B ~ 0 .5 . H-12 T here a re  also som e s tru c tu re s  m  th is  m assregion  w hose p roperties a re  of in te re s t to  a s U o p h y s i c i s t s o ^ n g t o ^p a th  of nuc lides p roduced  in  explosive hydrogen b u rn in g  also called th e  rp  p rocess.Not only a re  spectroscopy  m ea su re m e n ts  possible, b u t  it  is  a lso  possib le  to  envisioi on  l i n e  n u S ? 5 e n l i t i o n  for som e nuc lides along th e  drip  line. One se t of un iq u e  m ea su re m e n ts  w ould be  to  be  able to  m easu re  ac tu a l an g u la r d is tr ib u tio n s  for w hatever ^ S e d  o S i c l e s  m igh t be  observed. S uch  m easu rem en ts  could  be perform ed for d irect o n ^ fo r  b e to  delayed charged  partic le  decay. B ecause  m ost of th ese  isom ers mvolye ^ u S e d ^ o t o r i T e v e n  tiiose  h i even-Z nuclides are  likely to  have sizab  e m o m en ts  an d  hence  considerab le  o rien tation . It is  c lear th a t  relaxation  will be  a  p rob lem  m  th is  m ass  region b u t  in  th e  ra re  e a r th  region w here th e  fields in  Fe are  very  large, even a  few m icroseconds will p e rm it re laxation  an d  em ission  from  aligned s ta te s . O ne su c h  exam pis  th e  L u-154 decay sy s te m .14O ne of th e  n ecessa ry  fea tu res  o f a  facility for acceleration  of rad ioactive b e am s will h e  t h e  s te o s  o u rsu e d  S S e v e  isobaric  purity- of th e  accelerated  b eam  It could  be  useft to  a sce rta in  w h e th e r th is  m ig h t be  accom plished  by  th e  u se  of a  2 -step  la s e r  p rocess, t h i f  n r ^ s s  a  first la se r w ould be  u se d  to  excite a  narrow  reso n an ce  m  th e  im p u n ty  isobaric  singly charged  ion to  a  s ta te  w hich  could  subsequen tly  be  ionized b y  a  secondS r t o a S a e r f + 2  Then, at the next bend, the isobaric hnpuniy, now w ith  a t +2charge  coulcfbe readily  se p a ra ted  from  th e  b eam  of in te re s t w hose ions still a re  a t  only a  +1 charge.T h is w ork  is su p p o rted  b y  th e  U. S. D ept, of E nergy  u n d e r G ra n t D E -FG 02-94E R 408341 K. Heyde, Symposium. American Chemical Society Meeting in Chicago, August 1993.2 B. Fogelberg et aL, Nucl. Phys. A429, 205 (1984).3 K. Sistemich et al., Z. Phys. A 283, 305 (1978).4 C. A. Stone and W. B. Walters. Z. Phys. A 328, 257 U987).5 c. A. Stone and W. B. Walters, Hyperflne Interactions 22, 375 U985).6 K. Ogawa, et o l .  Proc. NFFS, Bemkastel-Kues, Inst. Phys. Conf. Ser. 132, 533 (7 L. Peker et aL, Phys. Lett. 36B, 547 (1971).3 K. Ogawa, Phys. Rev. C 28, 953 (1983).9 S. Hofman et a l ,  Z. Phys. A, 333, 107 (1989).10 M. A. C. Hotchkis et aL, Phys. Rev. C 35, 315 (1987).11 Ph. Dessagne et aL, Phys. Rev. C 37, 2687 (1987).12 P. G. Hansen, B. Johson, and A  Richter, Nucl. Phys Al 5 1 1 8 , U99 )•13 a. Champagne and M. Wiescher, Ann. Rev. Nucl. Sci. 42, 39 (1992).14 K. S. Vierinen et al., Phys. Rev. C 38, 1509 (1988).115STU D Y  O F  T R A N SU R A N IU M  N U C LID ES W ITH  R A D IO A C T IV E  N U C L E A R  BEA M S FR O M  ISACW. LOVELAND  Dept, o f  Chemistry Oregon State University,Corvallis, OR 97331, USAABSTRACTThe availability of significant intensities of neutron-rich radioactive beams at the proposed ISAC facility will allow important studies of fusion with neutron-rich projectiles. Several new heavy nuclei can be synthesized using fusion and multinucleon transfer reactions. Such nuclei will be important in chemistry and atomic physics as well as in nuclear physics. In addition, possible opportunities for study of new aspects of the fission process are pointed out.1. IntroductionThe use o f  radioactive nuclear beams to produce new transuranium nuclei or larger quantities o f  existing nuclei has been suggested as a motivation for radioactive beam facilities. The desire to use radioactive beams in the synthesis o f  heavy nuclei, particularly those o f  n-rich nuclei, is quite understandable. In general, the known isotopes o f  the heaviest elements tend to be n-deficient (relative to P-stability). I f  one could produce more n-rich isotopes o f  a given element, one would expect increased stability. This increase in stability could amount to one or more orders o f  magnitude which, given the short half-lives, could be very important for studies o f  the chemical and atom ic properties o f  these elements.The heavy elements can serve as an important testing ground for our understanding o f  fusion and multinucleon transfer reactions. Because o f  the large Coulomb forces present in heavy elem ent fusion and transfer reactions, one can have an approximate cancellation o f  the nuclear forces that drive the fusion and transfer reactions. Thus, the study o f  these reactions in the heavy elem ents is sensitive to details o f  the dynamics o f  these reactions that are difficult to discern in studies with lighter elements. For example, the dynamic hindrances to fusion in neutron-deficient system s (the "extra-push" phenomena), the role o f  "cold fusion" in synthesizing new nuclei, and the enhancement o f  fusion for "magic" projectiles were primarily discovered and studied using the heavy elements. The availability o f  exotic neutron-rich projectile nuclei can be important in understanding several unexplained aspects o f  fusion and transfer reaction phenom enology.The availability o f  heavy neutron-rich nuclei can allow us to study "inverse" fission. The comparison o f  this reaction with normal fission can lead to new  insight into the dynam ics o f  both processes.In the material that follow s, I have considered these three possible research areas assum ing that a radioactive beam facility such as that described in the ISL proposal[l] was available. I have assumed that the ISAC facility w ill produce beams that are similar in intensity to those quoted in the ISL proposal and I have tried to indicate the opportunities available from a first stage ISAC (beam s with A  < 60) and a second stage ISAC (beams o f  all A ).2. Production o f  N ew  H eavy N ucleiI have evaluated quantitatively the possibilities for synthesis o f  heavy nuclei with radioactive nuclear beams. Because I chose to employ a brute force approach, considering every possible combination o f  stable or readily available radioactive target nuclei and all proposed radioactive beamnuclei, I settled on using a set o f  semi-empirical formulas for cross section calculations, along with appropriate choices o f  nuclear masses and semi-empirical prescription o f  nuclear de-excitation (r„/Tf values). (A  more fundamental approach[2], using m odels for com plete fission and the statistical de­excitation o f  the product nuclei would have been prohibitive from the point o f  view  o f  computer time.) To validate this simplistic approach, I considered a number o f  heavy element synthesis reactions including known cases involving radioactive beams and compared predictions o f  the semi-empirical formalism with measurements. I used this formalism to evaluate the production rates o f  heavy nuclei expected in radioactive beam facilities. I restrict attention in my calculations to synthesis using com plete fusion reactions. A  fuller account in which multinucleon transfer reactions, deep inelastic transfer, etc. are considered has been prepared[3].I chose to represent the com plete fusion cross section using a formalism developed by Armbruster[4]. The cross section for s-w ave fusion at the Bass barrier V B, is given aswhereandP^ o (V b) = 0.5 e x p [ - 7 1 (x mean- x j ]( 1)(2)with* *  = 0-71xmean = 2 x  (k 2 + k + k + k"2) 1/2(3)x  -  ((Z, + Z 2)2/(A j + A ,))/(Z  VA)^,1250.883 1 -  1.7826'(N , + N 2 -  Zj -  Z2) (N1+N2+z1+z2)•k =A21/3(4)This represents a parameterization o f  the concept o f  a dynamical hindrance o f  fusion developed  by Swiatecki, et al[5]. Estimates o f  the fusion cross sections made using equations 1-4 might be considered lower limits since higher partial waves are neglected and possible fusion enhancements with n-rich projectiles are neglected. On the other hand, this one dimensional fusion barrier approach has been shown[6] to overestimate expected fusion cross sections for symmetric reactions involved  deformed species such as the n-rich fission fragments. Evidence w ill be presented for canceling errors in this approximation.Once formed, the fusion products can de-excite by particle em ission or fission. The excitation  energy o f  the fusion products was calculated assuming the reaction took place at the Bass barrier[7] with Q values determined using the latest mass values from M oller and N ix[8]. (Although the masses o f  Liran and Zeldes[9] g ive a superior fit to the known heavy elem ent masses, the physics behind the M oller-N ix tables was thought to be superior and thus more appropriate for extrapolation into regions o f  unknown nuclei. A  parallel set o f  calculations using the Liran-Zeldes masses has been done and117the results o f  that calculation do not differ significantly from that reported here.) U sing the same rationale as used for the fusion calculations, an abbreviated calculation o f  the effect o f  de-excitation  was made. Specifically,fusrnr \(5)where |Tn/rfJ is assumed to be energy independent. The mean values o f  |i n/l  fJ were taken as arithmetic averages o f  the rn/Tf prescriptions o f  Sikkeland, et a lU Ol, and Cherepanov, et a l[l 1]. The probability o f  evaporating x  neutrons, Px, was taken from the Jackson model [12].To test this crude m odel for fusion cross sections, w e compare (Figure 1) the measured and calculated production cross sections for the reactions used to synthesize elements 101-109. The general agreement (within a factor o f  10) between the calculated and observed cross sections for m ost o f  these xn reactions seems acceptable in view  o f  the approximations in the calculations and uncertainties in the measurements. This agreement is also consistent with previous approaches to predict heavy element xn cross sections. For som e nuclei, the calculated and observed values o f  the cross sections differ by 2-3 orders o f  magnitude. This can be taken as a cautionary note regarding the formalism  used herein.Some years ago, Unik, et al[13], measured the cross sections for producing actinide nuclei in the U  beam stops o f  a high energy proton accelerator. This experiment can be thought o f  as a crude prototype o f  ISAC. The heaviest actinide found was 248C f with an abundance o f  1.2 x  104 atoms. Using the formalism described above, along with measured values[14] for the spectrum and yield  o f  14C and dE/dx values for 14C in 238U, one calculates an expected yield o f  1.8 x  105 atoms o f  248C f from the 238U  (14C, 4n) reaction. Given the uncertainties in describing the production o f  these nuclides in a very thick target, this agreement seem s satisfactory.Using the formalism described above, I have evaluated the production o f  heavy nuclei (Z > 100) in an ISL-type[l] facility, such as ISAC. For this facility, I have assumed that all beams w hose half- lives exceed 10s would be available at the design intensities. I have considered all stable nuclei and all available heavy nuclei as target materials with target thicknesses o f  1 mg/cm2 except for the heaviest elements, where smaller, realistic thicknesses were assumed. I calculated the heavy nuclei production rates for all possible target-projectile combinations.The results are shown in Figure 2  and Table 1. I note that m y estimated heavy elem ent production rate for 264104 o f  ~ 27  atoms/day is consistent with that estimated, using a very different approach, in the ISL proposal (o f  22 atoms/day). Synthesis o f  new  n-rich isotopes o f  elem ents 104 (10-100 atoms/day) and elem ent 105 (5-20 atoms/day) seems feasible. The production rates for new  isotopes o f  elements 106 and 107 seem  marginal (~ l-5  atoms/day and 0.5-1 atom/day, respectively). Typical best production reactions involve asymmetric reactions such as 246Cm (200 ,4 n ) ,  249Bk (200 ,4 n ) ,  252C f (20O, 4n). For elements 108 and above, the predicted production rates decrease from 0.1 atoms/day (108) to 0.02 atoms/day (112 and above), i.e., 1 atom every 2 weeks to 2 months. In the fusion model used in the calculations, the best synthesis reactions are symmetric radiative capture reactions, such as 138Ba (142Ba, y). The predicted fusion cross section is very low  (0 .7  x  10'36 cm2) but the products are produced "cold."The isotopes o f  elements 104, 105, and 106 produced in these reactions (using readily available heavy element target materials) are m ostly unknown nuclei whose half-lives are predicted to be substantially longer than the known isotopes o f  these elements. A s such, their synthesis would present a significant expansion o f  the possibilities o f  chem ical and atomic physics research with these(5)(5)117transactinide elements.Table 1. "Best Case" Reactions -  ISL-type facility such as ISAC.246Cm (20O, 4n) 262104 11 atoms/day252C f ( 14C, 3n) 263104 180 atoms/day248Cm (20O, 4n) 263104 27 atoms/day242Pu (24Na, 4n) 262105 3 atoms/day242Pu (25Na, 4n) 263105 4 atoms/day253Es ( I4C, 3n) 264105 59 atoms/day249Bk (20O, 4n) 265105 6 atoms/day252C f (20O, 4n) 268106 0.4 atoms/day252C f ( 190 ,  4n) 267 1 06 0.3 atoms/day138Ba (142Ba, y) 2801 12 0.006 atoms/dayM agda[15] has made similar calculations showing that significant enhancements in the production cross sections o f  heavy nuclei with multinucleon transfer reactions w ill occur with neutron-rich projectiles (compared with the use o f  stable projectiles). For example, she has predicted the possibility  o f  forming new, extremely long-lived isotopes o f  lawrencium using neutron-rich projectiles such as 16C reacting with 254Es. These isotopes are predicted to have half-lives o f  105 - 107 s, which would  allow  detailed studies o f  the electron configuration and chemical behavior o f  this elem ent not possible now . (The relevant scientific issues concerning lawrencium involve questions o f  w hy the Dirac equation predictions for the ground state electron configuration (and its chemical consequences) have not been seen experimentally. This study o f  "relativity in a test-tube" has several long-range implications for chemistry and our understanding o f  the periodic table.) Using the ISAC facility, one should be able to make about 20  atoms/day o f  these unusually stable species. Similarly, one should be able to use the reaction o f  20O with 254Es to make about 10 atoms/day o f  272N s, w hose half-life is predicted to be about 5 s. Considering that the only known isotopes o f  nielsbohrium have half-lives in the m s region, this development could enable chem ical studies that are simply im possible today.A ll o f  these estimates are conservative because they do not assume any enhancements o f  the cross sections for fusion and transfer reactions with the neutron-rich projectiles. Various authors[16- 18] have suggested that there w ill be significant enhancements to the ftision cross sections for neutron- rich projectiles due to the lowering o f  the fusion barrier and the excitation o f  the soft dipole m ode. It has been pointed out[ 19-21] however, that the breakup o f  the neutron-rich projectile w ill substantially decrease any expected fusion enhancements. Nonetheless, some enhancement o f  the fusion cross section is expected, and such an enhancement will only increase the predicted production rates o f  these new  heavy nuclei.A lso  note that all o f  these opportunities are available from the first stage o f  a proposed ISAC  facility.119The synthesis o f  elements 107-109 was made possible by the use o f  "cold fusion" reactions involving target nuclei in the lead-bismuth region. The use o f  these reactions allowed the formation o f  com posite nuclei with very low excitation energies because o f  the special stability o f  the target nuclei. This is one example o f  a set o f  relatively well documented examples o f  enhanced fusion cross sections due to nuclear structure effects. Unlike the above example, most o f  the these effects are poorly understood. For example, Lazarev and Oganessian[22] have focussed attention on the extensive studies o f  the fusion o f  near-magic zirconium and tin nuclei. In Table 2, w e show the measured shifts in the fusion threshold (A B^, i.e. the "extra-push" energy) for a series o f  reactions. A lso  shown are the fusion scaling parameters x effi, and (Z2/A )eff. It is clear that the observed changes in the fusion  threshold are exactly opposite from what one would expect from the extra-push model for the fusion  o f  very heavy ions. Yet this model is known[23] to be our best representation o f  the fusion process. One suspects that one is seeing the effects o f  nuclear structure upon the fusion process since the lowest values o f  the fusion threshold shifts occur for the m agic projectile 90Zr(N=5 0). There is an obvious extension o f  these studies that could be performed using the types o f  radioactive beams that would be available from a second stage o f  ISAC. This would involve the use o f  the doubly m agic I32Sn nucleus which is estimated to be available from an ISL facility with an intensity o f  108-109 ions/s. One could  study the fusion o f  I32Sn with the pair o f  ^Zr and %Zr. Comparison o f  the results o f  measurement o f  the fusion excitation functions in these reactions with the data in Table 2 would clarify the role o f  shell structure upon the fusion process.3. Fusion StudiesTable 2. Nuclear structure effects in the fusion o f  Zr and Sn nuclei.Reaction (Z2/A )^ X effE L (B ass)M eVABM eVV. . . xn^Zr +  %Zr 34.4 0.701 33 3.7±0.3MZr +  92Zr 35.2 0.710 26 3.7±0.2 - 1  mb^Zr +  ^Zr 35.6 0.714 23 3.7±0.296Zr +  124Sn 36.6 0.765 26 26.7±0.594Zr +  I24Sn 37.0 0.769 27 22.9±0.7 ~1 p.b92Zr +  124Sn 37.4 0.773 30 21.1±3.0^Zr +  l24Sn 37.8 0.778 30 20.3+3.0 ~%Zr +  132Sn ^Zr +  132Sn35.536.60.7570.7692332expectedxn0.1 to 1 mbA s shown in Table 1, one predicts the possibility o f  cold radiative fusion with radioactive projectiles corresponding to the heavy mass peak o f  the fission distribution. While the expected rates o f  such reactions leading to superheavy nuclei are too low  to be useful, the possibility exists o f  studying such processes in reactions with lighter target nuclei. Such radiative fusion reactions are also useful for probing fusion hindrances. For example, the reaction o f  the radioactive beam nucleus 132Sn with 130Te at the Bass barrier is expected to form 262N o with just the em ission o f  gamma-rays. The fusion o f  these two nuclei is expected to be highly hindered (a fc= 6x1 O'34 cm2), but the expected  intensity from ISAC o f  t32Sn is large enough to allow study o f  this reaction because the evaporation residues are formed cold.4. Inverse FissionLazarev and Oganessian[22] have suggested the possibility o f  studying inverse fission reactions using neutron-rich projectiles reacting with neutron-rich target nuclei. They point out that studying the properties o f  the fission o f  a heavy nucleus into two fragments and the complementary study o f  the fusion o f  two fission fragments to form the fissioning nucleus can give important information about the dynam ics o f  both processes, fusion and fission. A  prototype case that could be studied at ISAC  would involve the comparison o f  the fission o f  210Po (formed by the reaction o f  209Bi with protons) and the fusion o f  the neutron-rich target nucleus 130Te with the neutron-rich projectile fragment 78Ge to form neutron-rich Po nuclei similar to the fissioning system  2I0Po. The projectile fragment nucleus 78Ge w ill be produced with sufficient intensities (~109 -1 0 10 ions/s) to allow the measurement o f  the fusion cross section, given the possibility o f  detecting the alpha-particle emitting Po evaporation  residues along with fission fragments.References[1] The Isospin Laboratory, LALP-91-51.[2] W. R eisdorf and M. Schadel, Z. Phys. A 343. 47  (1992).[3] W. Loveland, Phys. Rev. C (submitted).[4] P. Armbruster, Ann. Rev. Nucl. Part. Sci. 35, 135 (1985).[5] S. Bjom holm  and W.J. Swiatecki, Nucl. Phys. A 391. 471 (1982).[6] J.R. N ix  and A.J. Sierk, Phys. Rev. C15, 2072  (1977).[7] R. Bass, Nuclear Reactions with H eavy Ions. (Springer, Berlin, 1980).[8] P. M oller and J.R. N ix, At. Data and Nucl. Data Tables (to be published).[9] S. Liran and N. Zeldes, At. Data and Nucl. Data Tables 17, 431 (1976).[10] T. Sikkeland, A. Ghiorso and M. Nurmia, Phys. Rev. 172. 1232 (1968).[11] E.A . Cherepanov, A .S. Iljinov, and M .V. M ebel, J. Phys. G. 9, 931 (1983).[12] J.D. Jackson, Can. J. Phys. 34. 767 (1956).[13] J.P. Unik, et al-, Nucl. Phys. A 191. 233 (1972).[14] A .M . Poskanzer, G.W. Butler, and E.K. Hyde, Phys. Rev. C3, 882 (1971).[15] M . Magda, private communication.[16] N . Takigawa and H. Sagawa, Phys. Lett. B 265. 23 (1991).[17] M .S. Hussein, Phys. Rev. C44. 446(1991); N ucl. Phys. A 531. 192 (1991).[18] C.H. Dasso and R. Donangelo, Phys. Lett. B 276. 1 (1992).[19] C.E. Aguiar, V.C. Barbosa, C.H. Dasso and R. Donangelo, Phys. Rev. C46.R45 (1992)'.[20] C.A. Bertulani, L.F. Canto, and M. S. Hussein, Phys. Rep. 226. 281 (1993).[21] N . Takigawa, M. Kuratani, and H. Sagawa, Phys. Rev. C47. R2470 (1993).[22] Y . Lazarev and Y. Oganessian, J. de Phys. (to be published).[23] C.E. Aguiar, V.C. Barbosa, S.R. Souza and E.C. de Oliveira, Phys. Rev C47, 2396 (1993).1211°g(°'calc(cm2))Figure 1. Comparison o f  observed and calculated xn  cross sections for the production o f  isotopes o f  elements 101-109.Heavy E lem ent S yn thesis  Using ISLFigure 2. Heavy nuclide production rates using ISL beams.Coulomb Excitation Studies with Radioactive NiiHear ReamsR.F. Casten, Brookhaven National Laboratory, Upton, NY, USA 11973Recently, some rem arkable correlations betw een nuclear observables have been d iscovered  w hich, be ing  u tte rly  sim ple and  nearly  un iversal, h in t a t deep underly ing  origins. They have a tw ofold relation to radioactive nuclear beam  (RNB) studies. As examples, consider the figure below.The top shows a plot which reveals that the data for over 150 nuclei lie on a nearly perfect straight line satisfying the equationE(4j) = 2.0 E(2|) + ewhere e = 156(10) keV is a constant. This equation is that of an anharm onic vibrator (AHV). This correlation is rem arkable: the nuclei involved span an enorm ous range of structures from  nearly harm onic vibrators (upper right) to a y-soft rotors (near E(2|) ~ 350 keV) to soft transitional nuclei (near E (2 |) -150-200 keV) w ith a variety of interm ediate cases. Yet all satisfy the same AHV m odel w ith  constant anharm onicity  e. This is totally unexpected and not understood. A key question relating directly to RNB research is this: is this correlation a feature only of those nuclei w e already have available for'S tudy—and, if so, w hat is special about these nuclei—or is it a generic property  of all nuclei—and, if so, w hat are the general properties of nuclear shell structure and interactions that give rise to it? This can only be tested by extensive RNB m easurements of new nuclei far off stability. It m ay well be that the first evidence for radically new  types of structure that m ay exist far off stability will be deviations from universal plots such as these.An ubiquitous feature of RNB science will be the constant struggle to glean inform ation about nuclear structure from orders of m agnitude less data than we are accustom ed to. It w ould  therefore be extrem ely useful and propitious if it w ere possible to develop new , and m uch sim pler, signatures of structure. Consider an example. Nuclear transition regions typically fall into the two classes of spherical vibrator to rotor and y-soft to rotor. Examples are the Sm, Gd, and A=100 regions on the one hand, and the Os, Pt region on the other. Yet, it is extremely difficult to distinguish these: extensive data, extending to m any states and m any nuclei, have been required . N ow  consider the bottom  panel of the figure, w hich show s B (E2:0 |->2|)/A  (in W .u.) p lo tted  against R4 / 2  = E(4^)/E(2j'). R em arkably, this123correlation splits into two tracks. Still more remarkably, the upper track comprises nuclei in a spherical v ibrator-> rotor transition and the lower track consists of y- soft-»rotor nuclei. Thus the m easurement of only E (2 |), E(4|), and the B(E2) in one or two nuclei now  suffice to identify the class of transition. The developm ent of new  signatures of structure such as this are fully as useful "efficiency boosters" for RNBs as advances in ion source or detector efficiency.To exploit the pow er of these correlations and to test their generality, an extensive program  of low energy Coulomb excitation experiments is proposed. The experiments w ould be designed specifically to excite only the lowest 21 and (usually) the 4 | state and to measure only the B(E2:0|—>2|) value. They w ould utilize heavy (A>80) RNBs of 1.5-2.0 M eV /A  in inverse kinematics on light targets (e.g. 12C). Thus, they w ould be quick, simple, and would avoid the awesome quantities of data traditionally needed in near-barrier Coulomb excitation experiments.The low energy assures the excitation of only the lowest state or two. Yet the cross sections are quite large. The low energy also makes such experiments feasible in early phases of new  RNB facilities. The inverse kinem atics focuses all the scattered radioactive ions forward in a narrow cone (for A=100 on C, ©max ~ 7°). Thus they can be rem oved from the experimental area, m inim izing background problem s. The de-excitation y rays can be detected even in singles, although individual events could be tagged w ith a scattered beam  detector (e.g. PPAC) downstream. Doppler effects are negligible. The y-ray spectra are utterly simple with one or two peaks. Hence efficient, compact Nal detectors can likely be used.For vibrational nuclei (T1 / 2  (2 |) < 0.1 ns) a single detector near the target suffices. For rotational nuclei (T1 / 2  (21) ~ few ns), de-excitation occurs in flight downstream  and a linear array  of 4 or 5 detectors spanning 20-30 centim eters, each well collimated, w ould be appropriate. In this case, the B(E2) is obtained from the decay curve, and it is not even necessary to monitor the beam  intensity. With beams of only 106 ions/sec, a typical nucleus can be measured in less than a day.An extensive program  of such Coulomb excitation experim ents could easily m ap out the structure of broad regions of new nuclei, and test the universality of correlations of collective observables such as those shown in the figure.W ork supported under contract No. DE-AC02-76CH00016 with the United States Departm ent of Energy.E2+ (keV)3+ot+02( \fv SR 4 /2Figure Caption: The top shows a nearly universal correlation of E(4j) with E(2j) for all collective non-rotational even nuclei from  Z=38-82. The bottom  shows the 2- tracked evolution of B(E2:2j—>0j) values against R4 / 2  -  E (4 |)/E (2 |) for nuclei w ith Z=50-82.125.Beta-Delayed Alpha Emission from Neutron-Rich Ught-Mass NuclidesP. L. Reeder, Y. Kim. W. K. Hensley, H. S. Miley, R. A. Warner Pacific Northwest Laboratory Richland. WA 99352D. J. Vieira. J. M. Wouters, Z. Y. Zhou, and H. L. Siefert Los Alamos National Laboratory Los Alamos, NM 87545ABSTRACTThe Time-of-Flight Isochronous spectrometer at the LAMPF accelerator at Los Alamos has b6en used to measure the energy spectra of known and potential beta-d§layed alpha emitting nuclides in the neutron-rich light-mass regioh.INTRODUCTIONBeta-delayed alpha emission is energetically allowed if the beta decay energy (Qp) is greater than the binding energy (Ba ) of the alpha particle in the beta decay daughter nuclide. Among the light-mass neutron-rich nuclides, there are several known examples of beta-delayed alpha emission such as 8*9 Li, 11>lzBe, 12B, and 16,17,18^. However, ifwe examine the energetics. we note that all the B isotopes of mass’ 12-15, 17 and all the isotopes of N of mass 16-21 have Qp greater than Ba and are potential candidates for beta-delayed alpha emission.In the course of measuring beta half-lives and delayed-neutron emission probabilities for neutron-rich light-mass nuclides by use of the Time- of-Fl ight Isochronous (TOFI) spectrometer at the LAMPF accelerator, we simultaneously did a survey to look for new examples of beta-delayed alpha emission among the B and N isotopes. The TOFI spectrometeridentifies the Z, A. and Q of each ion produced by fragmentationreactions from proton bombardment of a 232Th target. However, there is no physical separation of different nuclides and all ions are’brought to a common focus a t t h e  exit of the spectrometer. These ions are implanted in a thin Si detector which measures the energy of each ion as it arrives. Because the LAMPF accelerator is pulsed (120 pps), there is time between each beam pulse to measure the energy of any emitted charged particle! from ions implanted in the Si detector. Bycorrelating thes§ decay pulses with specific ions, we can determine theenergy spectrum Of charged particles associated with decay of specific nuclides. Many df the techniques employed in this work could be applied to studies at the proposed ISAC facility.EXPERIMENTThe ion beams from the TOFI spectrometer are deposited in the thin Si detector as showri in Figure 1. The thick Si detector behind the thin detector is used to reject any very light ions 2H, and 4He) thatmight penetrate the thin detector. Between beam pulses. We use the thick detector to detect beta particles from decay of ions implanted in the thin detector. In addition, a thin, cylindrical plasticscintillator surrounds both Si detectors and is also used to detect betaparticles. Together, the thick Si and plastic scintillator detectorshave about 60% efficiency for detecting betas from the thin Si detector. Each time an event between beam pulses is detected in any of the thin Si. thick Si. or plastic scintillator detectors, the data acquisition system records the pulse height spectrum of all three detectors, which detector(s) fired, and the absolute time of the event.Plastic Scintil ator (2 mm thick)Thick Si fhin Si*ig- tnr?ut of Si and P1astic scintillator detectors at final focus of TOFI spectrometer.The energy spectfUm in the thin Si detector for all events in coincidence With a pulse in either the thick Si or plastic scintillator is shown in Figurfe 2. The TOFI spectrometer was tuned to optimize the detection of very neutron-rich nuclides in the region from Li to F Wehv^alnh^nArf v i  beta- del ay ed charged particle spectrum to be dominated by alpha part ic le from decay of «Li and 9 Li - both of which have very large branching fatios for alpha emission (Pa ). In confirmation, we include in FigurS 2 the known alpha spectrum from H i  as measured by Wilkinson and Alburger. Beta decay of 8 L1 leads to an excited state of °Be which immediately breaks up into two alpha particles. The experiment of Wilkinson and Alburger measured the energy of one of the alpha particles. In our experiment, the 8 Li ions are implanted within the Si detector aftd both alpha particles are detected together. The energies of the literature data plotted in Figure 2 have been multiplied by a factor of two to account for the detection of both alphas. The agreement of the Li data with the higher energy portion of our datad-ecay^f^Li t P°rtion of our spectrum is totally dominated by127Energy (MeV)Fig. 2. Charged particle pulse height spectrum for all events.Literature data for H Li alpha spectrum is also shown (see text).The decay of 9 Li is to 9Be. However, 50% of the decays go to excited states which emit neutrons and lead to 8Be. In this case, the 8Be is produced in low excitation states and not as much energy is available for the breakug into two alpha particles. The alpha particle spectrum from decay of y Li has been measured previously and is consistent with the lower energy (jortion of our data shown in Figure 2.2 Because this decay involves a three body breakup (n + 2a), there is not a simple correlation betwdfen the energy spectrum measured for a single alpha and the total energy as measured in our experiment.Our goal is to obtain beta-delayed alpha spectra for each nuclide irieluded in our data set from Li to F. To do this, We need to correlate the charged particle spectra with specific nuclides (Z, A) and to correct for accidental coincidences. This was done in a manner similar to the Multiple Time Analysis (MTA) technique described in our paper on delayed neutron half-lives.3 In this work, we constructed time interval histograms between a specific nuclide and all subsequent beta particles within a time range of 10 half-lives. This is essentially a delayed coincidence procedure in which all true correlations are included and the accidental correlations produce a flat background component in the time interval histogram. Beta events consisted of events having a pulsein the thin Si detector plus a pulse in either of the external beta detectors. We then created two pulse height spectra - one spectrum corresponding to events occurring between 0 and 3 half-lives and the second spectrum corresponding to events between 6 and 9 half-lives The second spectrum was corrected for the 1.37% of the correlated events still remaining and then subtracted from the first spectrum. The netspecific nuclide^PeSentS the char9ed Particle spectrum correlated to a RESULTSThe net spectrum correlated to 8 Li is plotted in figure 3 and comparedto the literature data (corrected for the fact that we see both alphas)For this nuclide; every beta decay is accompanied by an alpha signal, sowe do not expect to see a distinct region of low energy pulses from betaparticles leaving the thin detector. All other nuclides should show such pulses.Energy (MeV)Fig. 3. Total alpha energy spectrum from decay of implanted 8 Li. The energy scale of the literature data for the single alpha spectrum has b§en multiplied by a factor of two.129A more typical situation is illustrated in Figure 4. This shows the early time and late time pulse height spectra for the nuclide 18N. Note that both spectra are dominated by accidental coincidences with 8 Li and 9 Li alphas. However, we do see some peaks which are present in the early time spectrum that are not present in the late time spectrum. The difference of these two spectra is shown in Figure 5. The known spectrum consists of two narrow peaks at 1.40 and 1.82 MeV and a broad peak at 2.8 MeV.* All three features are present in our data although the statistics are marginal for the broad peak. The counts at low energy with large error bars are presumably due to beta particles which deposited a small amount of energy before leaving the thin detector.The distribution of such beta events should be unfolded from our alpha spectra to obtaifi accurate alpha energies.Energy (MeV)Fig. 4. Early- and late-time charged particle spectra from 18N.0 1 2 3 4 5 6Energy (M eV)Fig. 5. Beta-delayed alpha spectrunv from implanted 18N. Arrows show expected location of known peaks.The above procedures were applied to 18 nuclides in the neutron-rich light-mass region. Our sensitivity for observing beta-delayed alphas ranged from Pa s of 10 4 to 10'3 depending on the half-life of the particular nuclidfe An example of a possible new beta-delayed alpha emitter ( B) is shown in Figure 6. In this case, the net spectrum from B has been nortnaiized to the same number of ions as 14B and subtracted from the net B Spectrum. This was an attempt to eliminate the low energy betas frorfi the B spectrum since 13B does not have appreciable beta-delayed alpha branches. Although the statistics are marginal, there are hints of two peaks in the 24B spectrum which are close to the location of possible peaks from the known level scheme for 14C If these peaks are teal. the Pa for *4B would be of the order of 10'4 .Energy (MeV)Fig. 6. Spectrum of beta-delayed alphas from 14B. Arrows point to location of possible peaks based on 14C level scheme (see text).The techniques used in this work might also be applied to studying the energy spectra of delayed neutrons from very neutron-rich nuclides. In most cases, delayed neutron emission occurs from levels lying lower in excitation than the levels responsible for delayed alpha emission. Thus delayed neutron Emission is far more probable in these nuclides. In the particular case 6f 15B decay, delayed neutrons are emitted in almost 100% of the beta decays. The energy spectrum of these neutrons has been measured and consists of several well-defined peaks.5 We thus expect to see peaks in the 15B charged particle spectrum due to the recoil of the nucleus following neutron emission. The energies of these peaks will be in the range of 120-350 keV. In our experiment, there are very large uncertainties for the data in this energy region due to the large contribution frolii accidental coincidences with 9 Li alphas. Future experiments Which minimize the 9 Li contribution may possibly see these peaks due to nuclear recoil.As mentioned above, the results reported here are based on an experiment designed to emphasize the lowest Z nuclides. The actual Z distribution for that experiment is shown in Figure 7. In 1993, we performed a similar experiment in which the Z region from F to Cl was emphasized.Iue+Z4.u’str!^ut’2n. for ttiat experiment is also shown in Figure 7. Note f° °I N nuc1]!jes has changed by a factor of about 500?ono ? fexpenments. Although we have not begun to analyze the1993 experiment, we can see immediately from the raw pulse height spectrum for all charged particle events that the 8 -9 Li yields are greatly reduced relative to the *8N yield. As shown in Figure 8 the two sharp peaks from 18N are clearly visible in the 1993 raw data. The spectrum shown iri Figure 8 represents about 5% of the total data in that experiment so we expect to have excellent statistics. We have high hopes of observing beta-delayed alpha emission from 19.20.21N in the 1993 data provided the Pa is greater than about 10‘5 .107106105CO 104c3ootoO102101O O0 100 200 300 400 500 600 700 800 900 1000Channel Number^ 9 ;  1■ Distribution of nuclear charge (Z) for ions observed in 1992 and 1993 experiments.CONCLUSIONSThe implantation of radioactive nuclides into Si detectors is a very sensitive method for detecting beta-delayed charged particle emission. As discussed elsewhere in this Workshop, the use of double-sided Si strip detectors is even more sensitive for this application in that the levels 00 preceding beta decays can be reduced to negligible133Channel NumberFig. 8. Charged particle pulse height spectrum for all events from 1993 experiment, (see text).REFERENCES1D. H. Wilkinson and D. E. Alburger, Phys. Rev. Lett. Z&. 1127 (1971) as reported by E. K. Warburton, Phys. Rev. C H ,  303 (1986).2M. Langevin, C. Detraz, D. Guillemaud, F. Naulin, M. Epherre. R. Klapisch, S. K. T. Mark. M. de Saint Simon. C. Thibault. and F.Touchard, Nucl. Phys. A33fi. 449 (1981).3P. L. Reeder. R. A. Warner, W. K. Hensley. D. J. Vieira, and J. M. Wouters, Phys. Rev. C 44. 1435 (1991).4Z. Zhao. M. Gai, B. J. Lund, S. L. Rugari, D. Mikolas. B. A. Brown. J.A. Nolen, Jr.. and M. Samuel. Phys. Rev. C 12, 1985 (1989).5R. Harkewicz. D. J. Morrissey. B. A. Brown, J. A. Nolen, N. A. Orr, B.M. Sherrill, J. S. Winfield, and J. A. Winger. Phys. Rev. C 44 2365(1991).P h ysics w ith  R adioactive Ion B eam  FacilitiesSam M. AustinTRIUM F, 4004 Wesbrook Mall, Vancouver, B.C. Canada V6T 2A3andNational Superconducting Cyclotron Laboratory, Michigan State University East Lansing Michigan, 48824 U.S.A.AbstractThis talk consists of two parts. First a description of the present and proposed ra­dioactive beam facilities at MSU/NSCL, and some of the detection apparatus that may also be applicable to experiments at ISAC-1. And second, a description of some experi­ments that could provide information about the neutron skins and halos of neutron rich nuclei.1. PHYSICS AND FACILITIES AT MSU/NSCL1.1. In tro d u ctio nPhysics with radioactive beams is a m ajor part of the research program at the National Superconducting Cyclotron Laboratory (NSCL), accounting for over half of the proposals and a similar fraction of the beam time. This program is concentrated on experiments at energies above 25 MeV/nucleon, reflecting the use of fragmentation in the formation of the radioactive ion beams (RIB’s). The NSCL is proposing to upgrade the present facility with two goals in mind: operation at significantly higher intensity, up to 1 particle microampere for 200 MeV/nucleon light ions, and higher energy, 100 MeV/nucleon for the heaviest ions. As a result, typical RIB intensities will increase by factors of 100 to 1000.1.2. P resen t F acilityThe present NSCL facility is based on the K1200 superconducting cyclotron as a driver. The K1200 is highly reliable and reproducible, but is limited in intensity to about 60 particle namperes for lighter ions, primarily because of the high charge states Q w Z  required from the ECR ion source. The resulting intensities are 107/sec for some ions near the valley of stability, but are often in the range of 100-2000/sec. This has been sufficient to perform a wide variety of experiments, including studies of charge exchange, elastic scattering, reaction cross sections, and breakup momentum distributions. It has also been possible to study nucleus-nucleus collisions with an extended range of isospin in the incident systems. To make such experiments possible with limited beam intensities, large-solid-angle detectors have been necessary, and it is clear tha t any radioactive beam facility will need such devices. Even if the intensity for favorable ions is in the 1010 to 1011 range, much of the interesting physics will lie toward the drip lines, where the intensities will be small. This presents many opportunities for users to contribute to the facility through the construction of efficient detection apparatus.Figure 1 shows the present layout of the experimental areas and the principal pieces of experimental apparatus. These include the RPMS, which incorporates a Wien filter; the135SUPERBALL, a neutron multiplicity filter incorporating 14 tons of Gd loaded scintillator; the S800, a large-solid-angle spectrograph to be completed in 1995; a 7 Tesla solenoid which serves as a large-solid-angle collector for RIB studies; a general purpose 235 cm diameter scattering chamber; and two highly granular 4 pi detectors for nucleus-nucleus collisions studies. I anticipate tha t all of these detectors will be used for experiments with RIB’s.1.3. Proposed. N S C L  UpgradeIn the proposed configuration, the presently inactive K500 cyclotron will be used to inject the K1200, resulting in greatly improved facility performance. The proposed upgrade is cost effective because the accelerators already exist and because most of the experimental apparatus is already in place.During the upgrade, the K500 cyclotron will be improved to the technical standards of the K1200, a new injection line will be built from the K500 to the K1200, the K1200 will be adapted for median plane injection, additional shielding will be installed, and the A1200 fragment separator will be replaced. The new separator shown in Fig. 2 will have a larger bending power (K =  1900) and acceptance (10 msr).Intensities for light ion beams will be 1000 pna at 200 MeV/nucleon, and energies for the heaviest ions (238U) will be near 100 MeV/nucleon. As a result typical RIB intensities will increase by factors of 100 to 1000. Both the program of nuclear structure with stable and radioactive beams, and the program of nucleus-nucleus collisions will benefit from this upgrade.2. D E T E C T IO N  SY STEM SHere I ’ll describe two types of detectors that could be useful for experiments at ISAC-1.2.1. Large So lid  A ng le  N eu tron  detectorThe large solid angle detector at MSU/NSCL1 is suitable for studies of ^-delayed neu­tron emission, and of reactions such as (d, n) in inverse kinematics. A diagram of the device is shown in Fig. 3. It consists of 16 plastic (BC412) scintillators, each with dimensions 157x7.6x2.54 cm3 and viewed on both ends by photomultiplier tubes. Their radius of cur­vature is 100 cm and the subtended solid angle is 1.9 sr.For measurements of /3-delayed neutron emission, the ion of interest is stopped in a foil or detector located at the centre if the device, and the time of flight of the neutron to detectors is measured and summed. The results of the first measurement with this device1 are shown in Fig. 4.The detector will also be used in a study of the d (10,11’12Be, n '12'13B)n reactions at M SU/NSCL .2 The experimental setup is shown in Fig. 5. The beam enters the detector through a gap between the scintillator strips, and strikes a 2H target placed at the center of curvature. For a beam energy of 30 MeV/nucleon, the neutron energies of interest are in the 6-12 MeV range, suitable for this detector. Neutron energies are measured by time of flight (overall resolution will be about 1 MeV) and the heavy product is detected in down-stream silicon detectors. The experiments with the n ’12Be beams will each take a day or two at beam intensities of about 106/sec.2.2. Time Projection Chamber (T P C )The general principle of a TPC is that three-dimensional information on particle tracks is obtained from the simultaneous measurement of particle drift time and position (in two di­mensions). A schematic view of a simple prototype device used in experiments at MSU/NSCL and GANIL3 is shown in Fig. 6. In this device the radioactive beam enters from the left and then stops in the detector gas. Electrons drift vertically to the anode wires. Their drift time and the position are then obtained from the anode wires and cathode strips; this information is sufficient to allow reconstruction of the particle track. The energy is also obtained from the wire planes. The experiments referred to above, were attem pts3 to study the beta-delayed two-proton decay of 22Al, with the goal of observing simultaneous two proton decays. Such decays would be distinguished from sequential processes by observing the angular correlation of the events-it is hoped tha t the detailed information provided by this device will yield an unambiguous interpretation.This device should also be useful for the observation of reactions with radioactive beams of low intensity, where very large solid angle is required. One looks for the point where the incident beam particle disappears and other particles with different ionization or angle appear. Similar approaches using multiple sampling ionization chambers have been applied in studies of the 8Li(a, n) reaction4 and of sub-barrier fusion reactions induced by the (halo?) nucleus 6He (Ref.5).3. FUSION OF HALO AND SKIN NUCLEIThere is strong evidence tha t lightly bound neutron rich nuclei such as 11Li and n Be have an extended neutron distribution or halo. For example 11 Li is well described as a 9Li core surrounded by two valence neutrons lying far from the core on average. This has a variety of consequences, among them a prediction tha t a halo nucleus has strong low-lying E l strength. As a  result, halo nuclei are easily polarizable in heavy ion collisions.This phenomenon is expected to increase the probability of fusion of such a halo nucleus with a target in a  nuclear collision at energies below the Coulomb barrier. The halo nucleus deforms in the electric field of the target nucleus; its core, which contains charge, is repelled by the target charge. As a result the neutrons in the halo “contact” the target while the charged core is still relatively far away (see Fig. 7), fusion begins at energies below the normal Coulomb barrier. The results of a calculation for 11 Li + Pb by Takigawa, et al.6 are shown in Fig. 8. This process could be studied at ISAC-1, but with lighter targets, so the entire sub-barrier region would be accessible at 1.5 MeV/nucleon.A related phenomenon is the possible occurrence of neutron skins for nuclei far from stability. For stable nuclei the rms radii for neutrons and protons are nearly identical, differing by less than about 0.1 fm, even for Pb for which N  — Z  =  44. In contrast, for nuclei far from stability neutron and proton radii may differ. This behavior is related to the difference in Fermi energies for neutrons and protons for lightly bound neutron rich nuclei. Predictions for a variety of nuclei and for the Na isotopes in particular7 are shown in Figs. 9 and 10. Although detailed calculations have not been done, other elements have chains of isotopes with similar Fermi energy differences. One could study the relative values of the cross sections for a chain of isotopes at a single center of mass energy to see the effects of the growing neutron skin. These nuclei may provide an ideal situation to study the distribution of barriers in137fusion, perhaps caused by the neutron flow phenomenon postulated by Stelson8,9 to  explain the fusion of medium mass nuclei.These fusion studies may greatly enhance our understanding of fusion reactions. It is perhaps surprising tha t nuclear fusion is still not understood quantitatively. Fusion cross sections in the sub-barrier region do not have an exponential shape and are greatly enhanced, by factors of 100 or more, over potential model predictions. This enhancement will probably be greater for the neutron skin/halo nuclei. These enhancements and an understanding of their cause might point the way to the production of super heavy nuclei in a future generation of radioactive beam facilities.4. A C K N O W L E D G M E N T SThis research is supported in part by the National Science Foundation.R E F E R E N C E S1. R. Harkewicz, D.J. Morrissey, B.A. Brown, J.A. Nolen, Jr., N.A. Orr, B.M. Sherrill, J.S. Winfield, and J.A. Winger, Phys. Rev. C 44, 2365 (1991).2. J. Brown, (private communication).3. D. Bazin, (private communication).4. R.N. Boyd, et al., Phys. Rev. Lett. 68, 1283 (1992).5. J.D. Hinnefeld, J.J. Kolata, M. Belbot, K. Lamkin, M. Zahar, P. Santi, and J.Kugi, Bull. Am. Phys. Soc. 38, 1841 (1993).6. N. Takigawa, M. Kuratani, and H. Sagawa, Phys. Rev. C 47, R2470 (1993).7. D. Hirata, H. Toki, I. Tanihata, K. Sumiyoshi, Y. Sugahara, and R. Brockmann, Pro­ceedings of the Third International Conference on Radioactive Nuclear Beams, Michigan State University, May 1993, Ed. D.J. Morrissey, (Editions Frontieres, Gif-sur-Yvette, France, 1993) p. 247.8. P.H. Stelson, Phys. Lett. 205B, 190 (1988).9. P.H. Stelson, H.J. Kim, M. Beckerman, D. Shapira, and R.L. Robinson, Phys. Rev. C 41 , 1584 (1990).Fig. 1. NSCL experimental areas.139AC-500 lo K-1200 COUPLING LINEFig. 3. Schematic view of the NSCL large solid angle neutron-detector array, as set up for studies of /3-delayed neutron emission with a beam from a fragment separator. From Ref.1Fig. 4. Neutron spectrum1 from the /3-delayed neutron decay of 15B, using the instrument shown in Fig. 3.141Top ViewSecondary beam from A1200Targetw/ / Position Sensitive Silicon DetectorsFig. 5. Experimental set up for study of d(10,11,12Be, n,i2,i3g^n reactions. For orientation, the beam enters from the top of the detector as shown in Fig. 3.30 cm HV drift planeAnode wires (12] Shielding (50iun)zAMultiplication zoneyFig. 6. Diagram of a Time-Projection Chamber,3 showing the decay, into two products, of a nuclide which has stopped in the device.Target1 Li TargetFig. 7. Polarization of a halo nucleus. The upper diagram shows the projectile far from the target-the core and neutron halo are symmetric. In the lower diagram, the core has been displaced relative to the core when the collision begins. Since the charge resides in the core the Coulomb repulsion will be reduced and fusion can occur at lower bombarding energies.Fig. 8. Isotope and energy dependence of the fusion cross section.6 Note the strong enhancement of the cross section for 11 Li compared to that for the other isotopes of Li.[MeV]Fig. 9. Dependence of the neutron skin thickness on the difference of Fermi energies for neutrons and protons.1433.53.02.58  12 16 20 NFig. 10. Dependence of the rms radii of neutrons and protons on neutron number for the Na isotopes.7 The calculations are in reasonable agreement with the experimental values of the proton radii and of the binding energies (not shown).Na (Deformed)****o data*- °  0 0 . 0 ** * * « , » ! *\  o ° R p*1 1 iNUCLEAR ASTROPHYSICS WITH RADIOACTIVE BEAMS ATOAK RIDGENICHOLAS P. T. BATEMAN Wright Nuclear Structue Laboratory, Yale University, 272 Whitney Ave., New Haven, CT 06520, USAABSTRACTThis paper provides an overview of the planned nuclear astrophysics program at Oak Ridge. The emphasis of the program will be on explosive hydrogen burning, and primary instrument used for the studies will be therecoil mass seperator obtained from Daresbury. A description of the firstplanned experiment is given.1. In trod uctio nI'd like to begin this paper by mentioning that I am writing it as the representative of a large (for nuclear physics) collaboration of nuclear astrophysicists. This RIBENS (Radioactive Ion Beams for Explosive Nucleosynthesis Studies) collaboration will be conducting nuclear astrophysics experiments at the Oak Ridge Holifield Radioactive Beam Facility (HRIBF). The person largely responsible for assembling and coordinating the collaboration is Dr. Michael Smith at ORNL. As the collaboration name implies our emphasis willbe on explosive nucleosynthesis, initially focussing on hydrogenburning and the rp-process. This paper is a brief description of the planned program.2. The FacilityThe Holifield Heavy Ion Research Facility at Oak Ridge National Laboratory consisted of a vertical tandem with a maximum energy of 25 MV and a cyclotron used as a booster. With two accelerators in place the lab was an ideal location to build a radioactive beam facility of the ISOL variety, in which radioisotopes are produced by one accelerator and extracted into a second ion source for acceleration by a second accelerator. The conversion of the heavy ion facility into an ISOL facility began ion 1992. To date the HRIBF is on schedule and beams should be available by July 1995 (Ga93).The ORIC cyclotron will produce the primary beam at Oak Ridge. This beam will be either protons, deuterons, or alphas, depending on the secondary beam to be produced. Ultimately the145maximum power available should be 2 kW. The primary target will be chosen to maximize the production of the desired radioactive ion. To improve the extraction efficiency the target will be heated. From the target, the radioactive ions will diffuse a short distance to the secondary ion source.The secondary ion source deck will be at 300 kV and willinclude (on the deck) a 152° angle magnet for first stage massseparation to one part in two hundred. Because of the high neutron flux near the production target, the power supplies and controls for the source and deck will be in a separate room, shielded from the target. The deck and these two rooms have been completed, and the hardware is currently being installed.The first of the two planned secondary ion sources, a FEBIAD ion source (a positive ion source) is being tested on site at Oak Ridge. When this source is in use a charge exchange chamber filled with cesium will be used to produce the negative ions needed for acceleration in a tandem. This chamber will be on the source deck,after the first mass separation magnet. After the radioactive beamcomes off the deck it will be bent through a 110° magnet before being injected into the tandem. This should give an ultimate mass separation of one part in 20 000. From the tandem the beam can be delivered to several experimental areas.3. Initial Beams and the rp-processFigure 1 shows the lower end of the table of isotopes. The planned initial beams at Oak Ridge are shown shaded. Also shown is a calculated path for the rp-process. The rp (rapid proton capture) -process involves proton capture reactions on nuclei on time scales that are short compared to the appropriate half-lives. The sequence of (p,y) reactions may lead to much higher energy generation and to a substantial production of iron-peak (or even higher mass) nuclei. Such nucleosynthesis would have to take place in a hot dense explosive environment where hydrogen is the dominant element. Possible sites for such explosions include the hottest novae, X-ray bursts, and the shock waves of supernovae (Ch92). To understand such events quantitatively it is essential to know the relevant nuclear cross sections. As shown in figure 1, there is a substantial overlap between the planned beams at Oak Ridge and the beams of interest for studies of explosive nucleosynthesis, making the Oak Ridge facility very useful for such studies.I  CALCULATED PATH OF RP-PROCESS 1 NUCLEOSYNTHESIS§§j INITIAL BEAMS■  STABLE NUCLEIii*»■ iiijjjiiii ■ ■ ■■pp ii bh ■ ■ ■s  ■ ■ ■ ■ ■■ a r n i i ^ B  ■11G*0Rff» UHE -v.e i■■ a i«8» ■■Figure 1: The table of isotopes showing the first beams top be developped at HRIBF and a possible path for the rp-process.4. Experim ental ConsiderationsMeasurements made with radioactive beams are complicated by the additional high background environment created by the beta decay of the beam. Hence studying proton capture cross sections by simple gamma detection is very difficult. However, the fact that one must work in inverse kinematics provides a means to get around this problem. In inverse kinematics the recoils are strongly forward focused, so it is possible to have a 4n detector with a reasonably small aperture in the lab. To use such a detector in our program we need a spectrometer with several important characteristics. First, it must have extremely good recoil-projectile separation, since proton capture cross sections of astrophysical interest are very small. Second, it needs to have a large enough aperture to include all the recoils from most of the reactions to be studied. Third, since the projectiles and the recoils have essentially the same momentum, a separator based on a velocity filter would be best (Sm91). The recoil mass separator at Daresbury in the UK fills these criteria very well, and it became available when the Daresbury laboratory was closed in 1993. Recently it was decided to relocate the DRS to Oak Ridge.Quidrupok Triplet Magnets 50 degree Sector Magnet Water-cooled Velocity Sliu Scxtupole Magnet*Qmdiupcdc Tripto Magnrn Velocity FilterVelocity R h aQnedrupoteTriplet Mtgneu Target ChamberFigure 2: The Daresbury Recoil Separator (DRS)5. The Daresbury Recoil Separator (DRS)The DRS (Ja88) is shown in figure 2. It consists of two velocity filters, a 50° sector magnet, three quadrupole triplets, and two sextupole singlets. At Daresbury the DRS was used to separate the recoils from fusion-evapouration reactions for nuclear structure studies. The first triplet brought the recoils along parallel trajectories through the velocity filters, while the second triplet brought them to a focus at the velocity slits. Because of the nature of the reactions being studied the slits were generally left open to maximize transmission. After the slits the sector magnet dispersed the recoils in A/q along the focal plane, and the last triplet brought each of these groups to a focus.Table I: Parameters of the Daresbury recoil separator as measured at Daresbury.From James et al. (Ja88)Table I shows some of the operational parameters of theseparator (as measured at Daresbury). Most important for ourpurposes is the primary beam suppression of >108. Because this was not essential for the research at Daresbury, the DRS was notoptimized for beam suppression. We are studying the optics to try to obtain a suppression factor of 1012 - 1013. One possibility is to tune the device differently and insert a second set of slits between the two velocity filters.6. Schedule for the DRSAt present the DRS is still in the UK, and the legal agreement for its transfer to ORNL is nearly complete. The schedule calls for the DRS to be disassembled by March, to arrive in Oak Ridge in June1994, and to be installed by January 1995. A year from now we will begin to commision the DRS using stable beams from thetandem. Finally, when radioactive beams are scheduled to be available in July 1995 the DRS and the rest of the nuclear astrophysics endstation should be ready for operation.7. The 17F (p ,y )18Ne ReactionAs an example of the kinds of issues we will face at Oak Ridge I will discuss our first planned nuclear astrophysics experiment, which will study the 17F(p,y)18Ne reaction.In the Galaxy most of the matter heavier than helium is in the form of CNO (Carbon-Nitrogen-Oxygen) isotopes. In a hot dense explosive environment these isotopes can capture protons at very high rates. They are rapidly processed into 140  and 150  via the Hot CNO cycle (figure 3). The rate of energy generation is then limitedAngular Acceptance ±45 mrad (horizontal) ±45 mrad (vertical) ±5 mm (horizontal) ±3.5 mm (vertical) ±1.5 mm (horizontal) ±10 mm (vertical) ±2%±5%> 1081/300Beam SpotImage sizeVelocity Acceptance at full aperture Energy Range Primary Beam Rejection A/q resolution (FWHM)149by the beta decay half lives of these isotopes, which are one minute and two minutes, respectively.The two oxygen isotopes cannot capture protons because the resulting nuclei would be particle unstable. However, they can undergo nuclear reactions with alpha particles, though the coulomb barriers are higher. In the case of 140  this leads to the sequence 140(a,p)17F(p,y)18Ne(P+v)18F(p,a)l50, which is also shown in figure 3. These reactions can dramatically increase energy generation since the half life of 18Ne is only two seconds. To understand the evolution of such an explosion one must understand the rate of energy generation, and to understand the rate of energy generation we need to know the cross sections for these processes. Our measurement of the 17F(p,y)18Ne reaction cross section with a radioactive beam will address this issue.NeFONCFigure 3: The HCNO cycle and the bi-cycle. The bi-cycle consists of the sequence that follows the (a,y) reaction on 140. When the bi-cycle is initiated energy generation increases dramatically.At the relevant temperatures and densities the 17F(p,y)18N e the most important resonance appears to be an s-wave resonance that has been observed to have a centre-of-mass energy of about 640 keV. This state in 18Ne was observed in a time of flightspectrum of neutrons from the 160 (8He,n)i8Ne reaction at one angle (Ga91), and it has been assigned = 3+because there is only one possible analog state in the well-studied mirror nucleus 180. Since there has only been one observation of the presumed state, all cross section calculations use the properties of the mirror state to determine the properties of the 3+ resonance. Hence this reaction rate is not known as well as is needed to accurately model explosions. Measuring the cross section directly would improve this situation.8. The ExperimentWe will bombard a polyethylene foil with a 17F beam to measure the cross section of p(17F,18Ne)y. Beam energies will be chosen to correspond to the s-wave resonance mentioned above, and to two other resonances which are at very similar energies, and which may have large cross sections. The recoiling 18Ne ions will be sent through the DRS to the focal plane where they will be implanted into a tape. There will, of course, be some 17F projectiles that also reach the focal plane, and are implanted into the tape. To distinguish between the two species we will take advantage of their very different half lives, 60 s for 17F and 2 s for 18Ne. At two second intervals the tape will be moved sequentially through a series of ten counting stations, each consisting of two 3" x  3" Nal(TI) crystals. The rate of back to back 511 keV gamma rays in each station will map out the decay curve for the mixture of 18Ne and 17F; this curve gives the number of 18Ne ions implanted, and thus the cross section.Finally, I show in table II a list of some of the reactions that we hope to study at Oak Ridge. They are listed by decreasing priority, with the reactions grouped together having roughly the same priority.151Table II:ReactionPossible nuclear astrophysics program for radioactive beams at Oak Ridge.Importance ExperimentalTechniqueHCNO Delayed ActivityHCNO breakout Delayed ActivityHCNO Particle/Recoilcoincidencerp bottleneck Recoil Detectionrp-process Recoil-y Coincidencerp-process Recoil-y Coincidencep-process Recoil-y Coincidencerp bottleneck Recoil Detectionrp-process Recoil Detectionrp-process Recoil DetectionHCNO breakout Delayed ActivityHCNO breakout Particle-RecoilCoincidencerp termination Recoil Detectionextended rp Delayed Activityrp-process Recoil Detectionrp-process Recoil Detection17F(p,y)18N e 18F(p,y)19N e 18F(p,a)15031S(p,y)32CI 33CI(p,y)34A r 34CI(p,y)35A r 73As(p,y)74Se2 7 S i(p,y)2 8 P26Si(p,y)2 7 P30S(p,y)31 CI150 (a ,y )19N e 140 (a ,p )17F56Ni(p,y)57Co 64Ge(p,y)65As 21 Na(p,y)22M g 22Mg(p,y)23AI9. References[Ch921A.E. Champagne & M. Wiescher, Annu. Rev. Nucl. Part. Sci. 42 (1992) 39[Ga91] A. Garcia et al., Phys. Rev., C44 (1991) 2012 [Ga93] J.D. Garrett et al., Nucl. Phys., A557 (1993) 701 [Ja88] A.N. James et al. Nucl. Inst. & Methods, A267 (1988) 144 [Sm91] M.S. Smith et al. Nucl. Inst. & Methods, A306 (1991) 233Contribution for the ISAC Workshop, Lake Louise, Alberta, February 1994A STORAGE RING FOR RADIOACTIVE BEAMS D.M. MoltzNuclear Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720ABSTRACT:Preliminary ideas are presented for the scientific justification o f a storage ring for radioactive beams. This storage ring would be suitable for many nuclear and atomic physics experiments. Ideally, it would be constructed and tested at an existing low-energy heavy-ion facility before relocation to a major radioactive beam facility.There have been significant discussions regarding the scientific justification for a major North American radioactive beam facility. These discussions led to the Isospin Laboratory (ISL) conceptualization put forth in a rather extensive document [1]. Because there exists several facilities which either have, or will have, radioactive beams available produced via projectile fragmentation, it has long been assumed that ISL would be an ISOL (isotope separator on-line) based facility. This assumption leads to many technical problems associated with high radiation fields, large target power densities, and the post-acceleration of very low-energy beams with large m/q ratios. Although these problems all deserve significant efforts, final beam intensities vary drastically over the entire nuclidic chart, necessitating the development of detector techniques which can be useful even at the lowest intensities. The low beam intensities would therefore require detectors w hich  approach the 4 ir theoretical so lid  an gle lim it w ith  large  granularity. Detector arrays composed of germanium, silicon, phoswich, or hybrid systems are generally quite expensive yet necessary for radioactive beam experiments. For example, it would be difficult to justify measuring even something as simple as elastic scattering by moving a single detector from angle to angle to obtain an angular distribution. With a 4x detector array, one could obtain the entire angular distribution concurrently assuming the granularity is sufficient to yield a suitable number of data points in the relevant angular range.There exists a second important problem with radioactive beams incident upon stable targets, namely the intense background in the detector systems from the radioactive beam itself. Even if the post-accelerated beam was transported with 99.9% efficiency through collimators, targets, etc., there would still be count rates in the detector array vicinity of up to 1 0 10/second; this decay rate would be unacceptable for all detector arrays. In this paper I present some preliminary ideas for a detection system which could increase the available beam intensities for more weakly produced radioactive species, eliminate most of the in situ background due to the beam decay, and possibly provide a technique to accurately measure all ground state atomic masses; this system is a storage ring.153The idea for using a storage ring as a  detector is not unique. T hey have been used as accum ulators, accelerating structures, and decelerating structures; they have been  used at low  and high en ergies, w ith  electrons, protons, antiprotons and heavy ions; they have been used for solid  state, a tom ic, nuclear and particle physics experim ents; they form  the basic entity for light sources from  ultraviolet to hard x-ray frequencies. T he idea o f  using a storage ring as part o f  a detection system  has arisen prim arily in nuclear and atom ic p h ysics. A  proposal for building  a storage ring prim arily for atom ic physics w as developed  at Oak R id ge [2]. O ne particularly  interesting proposal for the u se o f  a storage ring w as that d eveloped  for the G SI E SR  ring [3]. T he basic idea o f  this proposal is to operate the ring at the transition point w h ich  is inherently a d ispersive so lution . A lthough the solution  is d ispersive, the fligh t path length is independent o f  the exact m om entum , thereby m aking the system  essentially  isochronous for all atom s o f  the sam e isotope. Thus i f  one can  ach ieve a large num ber o f  orbits b efore losing  the circulating  particle, one obtains an equ ivalently  long flight path. T he p ossib ility  thus ex ists to m easure the atom ic m ass o f  very  ex o tic  n uclei on e atom  at a tim e. There ex ists no fundam ental reason w hy, how ever, that this u se o f  a storage ring need be confined  to energies on  the order o f  10 T esla-m . Several years ago  I proposed to build a storage ring prim arily to  m easure m asses at ~  10 M eV /nu cleon . Such a storage ring w ould have b een  m oderately co stly . In recent years, how ever, the nuclear sc ien ce  on e could en vision  perform ing w ith  a storage ring has been  increased dram atically by  the advent o f  several n ew  radioactive beam  facilities in North  Am erica.R adioactive beam s present a myriad o f  new  possib ilities and have been covered  extensively  in m any con feren ces and publications. A  subset o f  these experim ents such as elastic  scattering, reaction dynam ics, in-beam  gam m a rays, giant resonances, inverse kinem atic reactions, low -en ergy  fusion  cross sections, and nucleus-nucleus Brem strahlung w ould be particularly suited for study w ith a storage ring. There a lso  ex ists  m any atom ic physics experim ents com in g  under the general headings o f  low -energy  electron-ion  interactions, ion-atom  co llision s, and atom ic spectroscopy, which could  b e enhanced w ith  the u se o f  a storage ring. It is important to  note that although it has been  sh ow n  [4] that on e can  never ach ieve the thick  target y ields in a  nuclear process w ith  a circulating beam  and thin target, all o f  the experim ents suggested above cannot tolerate these thick targets because they can stop sign ificant amounts o f  the radioactive beam s, thereby creating an intolerable background. There has been  no attempt to fu lly  justify  the u se o f  a storage ring as part o f  a detection  system . Instead I have attempted  to g iv e  a b rief o v erv iew  o f  the general utility o f  such a device.It is a lso  w orthw hile to g iv e  som e operating parameters for a storage ring envisioned  for use w ith a rad ioactive beam  fac ility . O ne design  possib ility  is show n in F ig . 1. T he operating characteristics g iv en  in T able 1 w ere conservatively  chosen  based upon the d esign  energy goals for the ISL post-accelerator structure. The design  pressure should g iv e  su fficien tly  long coast tim es for m ost experim ents.Table 1: Design parameters for a storage ring suitable for use with post-acceleratorradioactive beams.6  dipoles - 1.6T8 - 1 2  quadrupoles2  kicker magnets (2 - 1 0 0  /xs)Bp —2.0 TmPring =  10-11 - 10-12  Torr Electron beam cooling, -6 0  kV, 5ACircumference - 35.5 mq/A Max Energy/nucleon0.5 46.30.4 30.00.25 1 1 .8Of course it is important to build such a complicated system in parallel with any major radioactive beam facility (such as ISL, but not limited to ISL). This development should occur not only where significant accelerator expertise resides, but also where an accelerator exists which could provide stable heavy ion beams in the relevant energy range. Such conditions exist at Lawrence Berkeley Laboratory because of the wide range of available beams at the 8 8 -Inch Cyclotron. Geographically, such a storage ring could easily be placed at the rear of the existing experimental hall. This is depicted in Fig. 2.It would be imperative to not only test such a device, but also perform some significant physics measurements. These could include some atomic physics, many proton-rich mass measurements very far from stability, and some simple radioactive beam measurements such as mirror elastic scattering. Based upon the cost estimate put together for the Oak Ridge proposal[2 ], it is estimated that a storage ring as outlined here would cost in the $15-20M range, but this investment would be very worthwhile considering the scientific potential.This work was supported by Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Division of Nuclear Physics, U.S. Department of Energy under Contract DE- AC03-76SF00098.155R E FER EN C ES[1] P roceedings o f  the W orkshop on  the Science o f  Intense R adioactive Ion B eam s, April 10- 12, 1990, L os A lam os, N M , Report L A -11964-C .[2] H IST R A P, proposal for "A H eavy Ion Storage R ing for A tom ic P hysics" , Oak RidgeN ational Laboratory, N ovem ber, 1988.[3] H . W olln ik , private com m unication .[4] L . Buchm ann, J .S . V incent, J .M . D ’Auria, J. K ing and R .E . A zum a, in proceed ing o fR adioactive N uclear B eam s: T he First International C onference, 16-18  O ctober 1989,B erkeley, C A , (W orld S cien tific), p .46 .Beam OutDipoleMagnetElectron "  CoolinqKickerMagnetD etector RegionBeam In2 T-m  Storage RingFig. 1 One design possibility for a storage ring suitable for use at a radioactive beam facility.157to'toT3Or—fCD 3  «—*5*oo«—*■o'33 "<Dino>1pTOfo3TOQ-O-ao'rtoo.K-5xQ§3!?to'>g<—*■a>rCOr00OO13*OSTn*■<o_oo3M. Hass, Weizmann Institute, Department of Physics, Rehovat, Israel 76100 IntroductionSeveral methods exist for the creation of nuclear polarization of long-lived (ground states) nuclear levels. Among the most extensively used are low temperature nuclear orientation and laser techniques. These techniques are limited, however, to specific cases. For example, low temperature nuclear orientation is limited to implanted nuclei in thermal equilibrium with the surrounding lattice. An alternative method could be the method of tilted foil polarisation in which the nuclei are oriented in the hyperfine field resulting from the surrounding electrons which are oriented in flight by a tilted foil. Even though the expected polarisation is small, it is a "universal" and easy method.So far the method has been applied to quadrupole moment measurements of excited states with PAD type measurements,[hss9I] parity violation experiments11’™901 and ground state g-factor measurements with beta NMR technique. lrcs861 Examination of the magnitude of polarisation can be analyzed to provide spectroscopic information such as the ratio of Gamow-Teller to Fermi matrix elements in beta decay. Beam polarisation for production of high energy polarised radioactive beams could be explored.In particular, the measurement of g-factors of ground states in pairs of mirror nuclei and the construction of sums and differences of such moments can provide direct information on the isovector and isoscalar parts of the nuclear current. The combination of a radioactive beam facility with the tilted multi-foil method allows the production of such nuclei, their subsequent polarisation and then measurement of the g-factor using the /3-NMR technique.At present, a Weizmann-Hahn-Meitner-Manchester-CERN collaboration is involved in an ongoing experiment at ISOLDE (CERN), utilizing a 300 kV platform to boost the 60 keV ISOLDE beam to energies around 500 keV allowing passage of A =23 ions through several carbon foils. The present proposal for TRIUMF will make use of the early stage of acceleration, with energies around 100-200 keV/amu, allowing similar measurements for the hitherto unexplored N =Z nuclei in the f shell.M eth o dBeam foil spectroscopy1*1" 821 was developed in the late 60s at accelerators as a method for the study of fluorescence from ions passing through thin carbon foils. The processes undergone by fast ions passing through a solid are complex and are still the focus of active research. Ions travelling through the bulk will experience an interaction which has cylindrical symmetry with respect to the axis defined by the direction of linear momentum and as a result the ions will emerge from the crystal with a cylindrical electron cloud or, to use the correct term, the ions will be aligned.The symmetry at the surface can be broken by tilting the foil. The new symmetry can be described by two vectors, the outgoing surface normal, n, and the direction of the ion velocity, v, (see figure 1). The electron cloud will feel a gradually decreasing field on "the foil side" as it leaves the foil, which will result in a polarisation of the electrons or in other words an atomic orientation Pj. Qualitatively the process can be understood as enhanced probability of electron pick up at "the foils side" where the electrons are accessible for a longer time and the relative velocity is lower. On the basis of this simple description the atomic polarisation is expected toTilted Foil Polarisation and Magnetic Moments in Mirror Nuclei159increase with tilt angle and this is a lso  confirm ed by experim ental results. Several more quantitative descriptions ex ist w hich  all account for the observed increase o f  polarisation with  tilt angle.In turn, this atom ic polarisation can be transferred to the nucleus v ia  the hyperfine interaction. H ow ever, the net nuclear polarisation after a sing le foil is very m oderate, typ ically  less than a few  percent, so  to increase the polarisation arrays o f  fo ils  are used . T he polarisation o f  theto precess. I f  the system  is left to precess a large number o f  tim es, cot» 1, the presence o f  many different hyperfine frequencies w ill eventually  result in that the average <  I >  w ill point in the direction o f  F . In a m u lti-fo il stack the atom ic polarisation w ill be destroyed at the entry o f  the next fo il, w h ile  the nuclear polarisation-rem ains unaffected. On leaving the foil the atom icbetw een the average < I >  and the average < J >  (see figure 2 ). E ven  though the increase o f  the nuclear polarisation P, is not as pronounced com pared to P; for low  spin (ground states) as com pared to high nuclear sp in , a m ulti-fo il arrangem ent can still be very advantageous due to charge state distributions and the various "active" atom ic configuration in a g iven  charge state (see  b elow ). Polarisation o f  Pj =  0 .0 2 -0 .0 7  can be expected .AXFigure 1The tilted-foil geometry. The induced polarisation can qualitatively be attributed enhanced probability for electron pick up on die side where the relative velocity is lower.atom ic system  w ill result in a hyperfine field  at the nucleus in w hich  the nuclear spin w ill startpolarisation is restored and the process is repeated resulting in a gradual decrease o f  the angle2. 3 4Figure 2The atomic polarisation will be transferred to the nuclear system through hyperfine interactions. In a multi-foil stack where «t»l, the atomic polarisation will be reset at the entry of each foil while the nuclear polarisation remains virtually untouched and will increase with the number of foils until saturation is reached.After passing through the last foil the ion is stopped in a catcher lattice of cubic symmetry free of internal fields cooled to a low temperature and placed in a "holding" magnetic field Bext (~0.1T), in the direction of Pj, in order to preserve the polarization along a well defined axis during the nuclear decay. The combination of catcher temperature and holding field is chosen to make the depolarisation time long compared with the nuclear lifetime. /?± radiation from the (g.s) level under study is detected at 0 ° and 180° to the direction of Pj and the 0°-180° asymmetry is determined.The implanted nuclei in a cubic environment experience a simple Zeeman splitting associated with the holding field Bext. Under the influence of an applied RF field at right angles to Bext resonant absorption at frequency v «  will result in destruction of the beta asymmetry.The frequency of the NMR resonance which can be measured to high precision is a measure of the nuclear g-factor. Thus the magnetic moment of the isotope under study can be extracted without detailed interpretation of the observed asymmetry.In a non-cubic environment the interaction with the electric field gradient will lead to a splitting of the NMR resonance line. Its measurement allows the determination of the nuclear quadrupole moment, if the electric field gradient is known.The obvious advantages of using post-accelerated beams for tilted foild polarisation is the extension of the method up to heavy elements and the potential use of large foil arrays for higher degree of polarisation. A 60 keV beam is not energetic enough to penetrate even a single foil with acceptable transmission and angular speed. At ISOLDE, a high voltage platform is used for acceleration of the beam to 520 keV. This is sufficient for the study of elements with A < 50 using up to a maximum of three foils for the lighter elements. Results from a recent experiment at ISOLDE demonstrate a nuclear polarisation of P! ~  0.002 for 520 keV 23/zg ions implanted into Pt at 4°K, using 3 carbon foils at 70°. One initial interest at higher beam energies would be to study the polarisation enhancement with use of multiple foil. Theoretical considerations show that under certain conditions the polarisation enhancements with increasing number of foils can be considerable giving great hope for creating sizeable polarisation in the transmitted beam.As stated above, the main advantage foreseen by using 100-200 keV/amu energies from this facility are in the prospects of increased polarisation by using more foils and in the possibility to explore the N =Z  region in the f-shell. Both objectives are not possible at present RIB facilities.S u m m a ryWe propose a research program for measuring magnetic moments of ground states of mirror nuclei using the tilted foil nuclear polarisation (TFP) method at ISOLDE. The TFP method has the potential to measure magnetic moments of nuclear ground states for all non-zero values of1. It is particularly suitable for cases where low temperature nuclear orientation is not applicable because of the need for spin-lattice relaxation to take place within the nuclear lifetime and also since it has certain implant/host combination limitations imposed by the requirement of large hyperfine fields. Thus the TFP method has the potential to fill an important gap. The main goals of the present proposal are to investigate the unexplored regions of T=3/2 nuclei in the s-d shell and t=  1 /2  nuclei in the f  shell by measuring magnetic moments of mirror nuclei.161\Additionally, examination of the magnitude of the polarisation can be analysed to provide information regarding /3-decay matrix elements, for example, the ratios of Gamow-Teller to Fermi matrix elements.R eferen cest ^ M .  Hass et al., Phys. Rev. C 43 (1991) 2140. t ^ C .  Broude et al., Z. Phys. A 336 (1990) 133. irog86]w_F. Rogers et al., Phys. Lett. B 177 (1986) 293.[ber82]H_G. Berry and M. Hass, Ann. Rev. Nucl. Part. Sci. 32 (1982) 1.DETAILS OF THE PROPOSED MAJOR ISAC FACILITYJ. BeveridgeTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3 J.M. D’AuriaDepartment of Chemistry, Simon Fraser University, Burnaby, B.C. V5A 1S6AbstractThis report will present details of the proposed TRIUMF ISAC Facility as developed by TRIUMF staff and others. This report formed the basis of a presentation at an open meeting at the Workshop to solicit comments from the prospective users community.OVERVIEWThe proposed ISAC facility at TRIUMF consists of a thick target on-line isotope separator (ISOL) coupled to a linear post-accelerator. The intermediate energy (500 MeV), high intensity (<  100/iA) proton beam for the TRIUMF cyclotron will be used as the production system and a new beam line would be required north of the present experimenter hall. A general description of the facility can be found in an earlier submission to the Workshop. In this report more details of the ISOL facility are presented.163THE ISOL SYSTEM1. IntroductionThe ISOL system consists of the primary production beam, the target/ion source, the mass separators and the separated beam transport system. These systems together act as the source of radioactive beams to be provided to the accelerators or the low enery experimental areas. A similar system has been developed at ISOLDE at CERN over the last 25 years; first at the SC [ALL87, RAV89] using 600 MeV protons and now, using 1 GeV protons from the PS Booster [KUG92, KUG93a]. The beam current at the CERN accelerators is, however, limited to about 3 f i A .  At TRIUMF we intend to utilize the intense (<100 /jlA )  proton beam available from the 500 MeV cyclotron to produce a significant increase in radioactive beam intensities over those available at present.The effective operation of the ISOL system is crucial to the overall ISAC facility performance. It is therefore essential that the many technical issues inherent to operation with high proton beam current be resolved. Flexibility and expansion capability will be required to meet present and future demands of the experimental program and must be incorporated into the facility design.The proposed ISOL facility layout is shown in Fig. 1. Protons of variable energy (200-500 MeV) with intensities up to 100 /xA are extracted from the TRIUMF cyclotron from the 2A extraction port. These protons are transported in a tunnel directed north of the present facility and delivered to one of three target stations. Each target station has a production target and ion source, a beam dump, proton beam monitoring and the first section of the ion beam transport. A beam dump is provided at the end of the proton beam line to allow the beam to be removed from a production target without changing the extraction conditions in the cyclotron. Space has been provided in the beam tunnel to implement a second proton beam by i m a g in g  a split stripping foil in the cyclotron at a septum magnet position. This would allow two target stations to operate simultaneously with different proton intensities if this is required in the future.Two target/ion source locations are considered to be a minimum requirement for the efficient operation of a large radioactive beam facility. Production targets are subject to failure and must be conditioned in situ before operation. The second target station will therefore provide much needed flexibility in target change operations. A third target station is provided for in the present proposal to allow for future expansion and improvement possibilities. Although it is not anticipated that this source location will be required in the first years of operation, the building space must be provided in the initial construction.TRIUMF has many years of experience in handling the operational radiation fields and radioactive components associated with high current meson production targets. The production of radioactive beams from thick heavy targets has the additional problem of the creation of large inventories of radioactivity, some of which is mobile, in the target/ion source area. The handling of this activity and the target area components will be critical to the facility operation. A strategy has been adopted in which the target stations are contained in a heavily shielded building which is directly connected to a hot cell facility. This approach is based on theFig. 1 ISOL Facility165successfu l experience at T R IU M F  o f  vertically  serv ic ing  m odular com ponents em bedded in a  c lo se  packed radiation shield coupled  w ith  the requirem ent for quick access to the production  target and o f  containm ent o f  any m obile activ ity . Careful design  o f  both the m odular com ponents and the rem ote handling system s w ill be required to ensure the operational viability  o f  this system .T he target/ion source m odule w ill be the key com ponent o f  the ISO L system . This m odule must be serviced , m odified  and exchanged on a regular basis to satisfy the varying dem ands o f  the physics program . Its design  w ill have to address m any o f  the m ost d ifficu lt problem s associated  w ith the production o f  radioactive beam s. T hese include high voltage serv ices, containm ent o f  radioactivity, accom m odation  o f  various target/ion  source com binations, radiation-hard  com ponents and ease o f  rem ote handling. A  prototype m odule has been  designed  w hich  resolves  the m ajority o f  these problem s, h ow ever, this w ill have to be fabricated and tested before a final design  is established.The target/ion  source com bination represents the m ost technically  d ifficu lt aspect o f  a high  intensity radioactive beam  facility . Present designs should accom m odate 10 /xA w hich  w ould  increase available intensities o f  m any radionuclides beyond those available today. H ow ever, the design  o f  production targets capable o f  w ithstanding the full proton beam  intensity w ithout com prom ising its y ield  o f  radioactive isotopes w ill be a challenge. Several approaches to  the dissipation  o f  the pow er deposited in these targets by the proton beam  have been  investigated  and a realistic so lution  for d ie rem oval o f  heat from  the target container seem s very probable. The heat transfer w ithin  the target material itse lf  h ow ever is h igh ly  target dependent and it is clear that 100 /xA operation w ill not be p ossib le for all target m aterials. T he developm ent o f  high intensity targets w ill be the subject o f  on goin g research and developm ent at d ie ISA C  facility  and there is great confidence that innovative ideas w ill be forthcom ing w hich  w ill provide  significant im provem ents to the present target system s.E xperience at operational on -line separators clearly  sh ow s that there is not a universal target/ion  source com bination  for the production o f  all required radioactive sp ecies. Several different types o f  ex isting  ion sources have been  review ed  and w ill be accom m odated w ith in  the ISO L design . In addition, new  ion source developm ents must be anticipated and flex ib ility  m ust be provided  in the system  design  to a llow  a reasonable p ossib ility  o f  their successfu l im plem entation. T he  initial ISA C  operation anticipates the use o f  s in g ly  charged ions for w hich  effic ien t sources have  already been  proven . The effic ien t production o f  m ultiple charged ions w ould  significantly  enhance the beam  capabilities o f  ISA C  and therefore the developm ent o f  such sources w ill be  pursued v igorou sly .T he ion beam s from  m ost sources contain  many sp ecies w hich  are not o f  interest to a particular experim ent and in fact w ill provide intolerable backgrounds if  not rem oved  from  the beam . In m any cases, these unwanted beam s are several orders o f  m agnitude m ore intense than the beam  o f  interest. A  m ass separator is therefore an essential requirem ent in the beam  transport betw een  the ion source and the experim ent. T he quality o f  m ass separation required w ill be h ighly  dependent on  the particular experim ent and the production source. In som e cases, high mass resolutions w ill b e  required to separate contam inations in the beam , w h ile , in others, high  acceptance w ill be o f  m ore im portance. T w o  separators are therefore included in the ISOLdesign. A medium resolution separator will be attached to the first source location to be constructed and a high resolution separator to the second. The mass separators have been placed at the proton beam level and separation is done in the horizontal plane. The separated beams are transported by an electrostatic system to either the low energy experimental area or the accelerators.2. Beam L in e  2AProton beams to be delivered to the ISAC production targets will be extracted from the beam line 2A port of the TRIUMF cyclotron. This can be done independently of other extracted beams by inserting a stripping foil at the appropriate location in die machine. The extraction of beams from this port was considered many years ago [STI81, STI80A, STI80B] and beam properties for 400-500 MeV were determined at that time. The beam properties for 200-500 MeV extraction have been calculated recently [LEE93]. Variable energy beam extraction requires a combination magnet to be placed at the exit port which will direct the different energy beams into a common transport beam line. A combination magnet is presently available which is capable of handling beams of 400-500 MeV. A longer combination magnet will be required for lower energy beams.The section of beam line 2A transport line within the cyclotron vault (Fig. 2) accepts the beam from the combination magnet and produces a doubly achromatic double waist after a 60° bend. At this location the beam direction is to the north and parallel to the cyclotron vault wall. This double waist is reproduced outside the vault wall by means of two quadrupole doublets which act as a unit section. Beyond this point the beam is transported in a tunnel by identical unit sections. This transport design provides a great deal of flexibility in the positioning of the target stations.Fig. 2 Beam Line 2A - Vault Section167T he beam  m ay be directed to a target station by p lacing a d ipole at a w aist location  or at the sym m etry location  betw een  the unit section  doublet pairs. In the present layout, beam  is provided to three target stations by 2 0 °  bends p laced at w aist points. The length o f  the unit section  has b een  ch osen  to g iv e  a  distance betw een  target stations o f  about 14 m . A  quadrupole doublet p laced  after the bend produces a  beam spot at the target w h ich  is variable in diameter from  0 .2 - 2 .0  cm .The conduction  o f  heat, generated by  target-beam  interactions, from  the centre o f  som e  production targets m akes the delivery o f  an annular beam  a h igh ly  desirable feature. This can  be done, in p rincip le, by focusing a sm all beam  on  the target and rotating this beam  with  steering elem ents p laced in the transport line. C alculations indicate that a  set o f  steering  elem ents p laced before the first target d ipole w ill a llow  a 1 cm  diam eter annular beam  to be  producted at any o f  the three target stations w ith on ly  m odest fie ld  requirem ents.The sim ultaneous operation o f  tw o target stations w ill require som e splitting or sharing o f  the proton beam . A  m ethod o f  splitting extracted beam s from  T R IU M F  by utilizing a sp lit stripping fo il in the cyclotron  w as investigated earlier by K ost [K O S82]. W ith this m ethod, the split fo il is im aged at a m agnetic septum  w here the tw o beam s are separated. In principle, the split ratio can be varied by  changing the fo il design  and loss-free  splitting is ach ieved . Space has been  provided in the beam  line 2 A  transport tunnel to accom m odate a second parallel proton beam  line so  that this sp litting m ethod could be im plem ented, i f  required in the future. Other m ethods such as tim e sharing o f  the beam  w ith  pulsed  bending m agnets are p ossib le but have not been  considered in any detail here.3. The Target Area3.1 GeneralIsotope generating targets proposed for ISA C  m ust withstand bom bardm ent by 10 -100  /j.A  o f  50 0  M eV  protons. T his proton bom bardm ent w ill generate very  high operating and residual radiation  fields. A  v ia b le  approach m ust be provided to handle and serv ice  the h igh ly  radioactive  com ponents near the production targets. A lso , the facility  m ust incorporate radiation shields to reduce operating fields outside the containm ent build ing to b io lo g ica lly  acceptable leve ls  for the m axim um  expected  beam  currents.The ISA C  rem ote handling concept and ISA C  Target F acility  has been  based on  fifteen  years o f  experience at the three operating m eson  factories — PSI, L A M P F  and T R IU M F . T he m eson  production target and beam  stop areas o f  these facilities have pow er d issipation and radiation  levels  sim ilar to , or greater than, those expected  at ISA C . M eson factory experience show s that the correct approach to handling com ponents in high current, thick target areas is to p lace them  in tightly sh ielded  canyons w ith a large target sh ield . Both PSI and T R IU M F  access the com ponents vertica lly  and do m ost repairs in dedicated hot ce lls .The target facility  design  [B E V 93, B E V 94] m ust address three important com plications that are not encountered in the m eson  factory target areas. T hese are the containm ent o f  large amountso f  m obile radioactivity, the high voltage required for beam  extraction , and quick routine replacem ent o f  short lived  target system s. In the present design  these issues are resolved  by  placing the target in a sealed  self-contained m odule. T his m odule can be transferred directly to  a hot cell facility  for m aintenance operations.3.2 Target FacilityFigure 3 sh ow s a plan v iew  o f  the proposed target facility . T his facility  is m ade up o f  three target stations, a target m aintenance centre, and a target serv ice  tunnel. T he target stations are located in a  long sealed  building serviced  by an overhead crane. T hey are separated by 14 m  and offse t from  the proton beam  tunnel to a llow  personnel access to this tunnel w hen the beam  is o ff . T he sh ield ing betw een  the target stations provides so m e independence o f  operation. T his  independence w ill not be com plete and access to the target areas w ill be restricted during proton  beam operation. T he target m aintenance centre is at one end o f  the target b uild ing. It contains a hot ce ll, warm ce ll, decontam ination facilities and a radioactive storage area needed to keep  the target area com ponents operational. T he target serv ice  tunnel parallels the target building  and contains h igh v o ltage , vacuum , water and other serv ice  system s. It is separated from  the target area to a llow  personnel access during all beam operations.Fig. 3 ISAC Target Facility - Plan View169A  plan v iew  o f  a target station is show n in F ig . 4 . Beam  line elem ents near the target are installed inside a large Tee-shaped vacuum  cham ber surrounded by closed  packed iron shielding. This general design  elim inates the air activation problem  associated  w ith high current target areas by rem oving all the air from the surrounding area. T he front-end design  breaks naturally into four m odules:•  an entrance m odule containing beam  diagnostics, an entrance collim ator and pump port.•  a beam  dump m odule containing beam  diagnostics, pre-dum p collim ator, and a beam  dum p.•  a  target m odule containing the target, ion source, and extraction electrode.•  an ion beam  line front-end m odule containing the front-end elem ents o f  the in beam  line.FeetI I I I 1 I I IMetersB eam  Entrance ModuleTorqet. Ion Source. i i  Extraction Module Bcom Dump ModuleIon Beam  Line Front End Module Iron ShieldingCroneTrovelLimitsFig. 4 Target Station - Plan ViewT he vacuum  design  seeks to elim inate the need for radiation-hard vacuum  connections at beam  height by using a sing le vesse l approach. T he front-end com ponents, w ith their integral sh ields, are inserted vertically  into a sing le large vacuum  v esse l. M ost vacuum  connections are situated  w here elastom er seals m ay be used and on ly  tw o beam -height connections ex ist - the proton  entrance and the ion beam exit. T hey could be w elded connections, m oving the c losest flanged  jo in t outside the target hall.Figure 5 is a section  through the target facility , show ing the target stations are housed w ithin  a h eavily  sh ielded  containm ent building. Steel sh ielding is p laced  c lo se  to the target and this is surrounded by concrete sh ielding. W hen target station m odules are rem oved , the sh ielding  form s a  canyon . T he target is placed deep in the canyon to low er both the operating and residual radiation fields at the canyon top. The operating lev e ls  are reduced to a llow  long term  use o f  nonradiation-hard m aterials. T he residual fields are reduced to a llow  personnel access  for m aintenance. A ll com ponents installed in the canyon are m ounted on the bottom  o f  sh ielding  plugs that extend to the canyon top. S erv ices, such as pow er and w ater, connect to the com ponents at the canyon top. B elow  this level, jo in ts and connections are m inim ized and radiation hard. H igh residual radiation fields w ill m ake m anual in situ  m aintenance o f  com ponent parts b e low  the canyon top im possib le. Such operations are carried out in a hot ce ll.Fig. 5 ISAC Target Hall - Cross Section at TargetA large v o id  is left in the sh ielding im m ediately ab ove the target stations. T he target hall crane operates in this space, w hich  connects to the m aintenance fac ilities . S u ffic ien t height is provided  to a llow  front-end m odules to be lifted free o f  the shield canyon  and transported to the hot ce ll. One m eter o f  boron loaded concrete, m ounted on  rollers, can  b e positioned  ab ove each target station. T his easily  rem oved shield reduces the thickness and the cost o f  the target h a ll’s ro o f  and reduces operational fields in the vo id  area. T his build ing design  p laces the target stations in a h eav ily  sh ielded  bunker that is d irectly connected to the hot cell facilities and has many advantages w hen  considering speed o f  serv ic ing  or access and contam ination control.1713.3 Remote HandlingThe ISAC targets are expected to be less robust than TRIUMF’s high current targets due to the extreme operating conditions required. Also, different target/ion source combinations will be used to produce the required radioactive beams. Therefore, an effective remote handling and servicing system will be required to bring about the quick and frequent target charges needed. All modules in the target area will have high levels of residual activity and will be potentially contaminated with mobile activity. Both aspects are considered in the handling design.The general remote handling approach has been built into the target facility design described above. Target area components will be designed as discreet modules with integral shields. The component’s services are brought up to the top of a shield canyon where connections need not be radiation-hard. Component maintenance involves disconnecting services and craning the module to the hot cell. Rolling aside a concrete shield gives the overhead crane access to the modules. Flaskless transport is permitted by the thick building walls provided personnel are excluded from the area. Target module transfers to the hot cell must therefore be done completely remotely.The connection and disconnection of the target module services can be done manually or remotely. The manual option is proposed for at least the initial years of ISAC operation as this will be less costly than a completely remote system. However, lengthy cool-down times may be required after extended operation at 100 fiA. Residual fields and the requirement to turn off the proton beam will make speed and simplicity of this operation mandatory. Connection designs will allow conversion to a completely automated system with only a minimum of redesign and refitting.The mobile contamination produced in the target area is normally contained within the target module. Contamination of the interior of the target building is considered possible since the modules will be transported without a containing flask. This building must therefore be considered as an extension of the hot cell complex and all entrances must be controlled and provided with appropriate contamination control. The air within the building must be controlled and HEPA filtered. The interior surfaces will be designed to be easily decontaminated. All fluid drains will go to sump tanks for monitoring before disposal.The layout of the maintenance centre includes the hot cell, warm cell, and module storage area as shown in Fig. 6 . This area is intended to provide all the maintenance, storage and commissioning requirements of the elements in the target area. Two cranes are provided to ease handling operations. The target hall crane covers the three target stations, the module storage area and the hot cell. The maintenance centre crane covers the hot and warm cells, most of the storage area and the loading bay.The module storage area is located between the hot cell and the first target station. It provides a place to store, trouble shoot, test and condition front-end modules. An array of pumped and shielded storage silos are provided to store modules under vacuum. One silo is provided with the necessary services for the testing and preconditioning of target modules before installation. This area is accessible during beam operation; however an interlocked gate prevents personnelfrom entering the shielding maze leading to the target areas. Service and testing of modules will therefore be possible during beam production. The hot cell provides facilities to remotely maintain, replace, decontaminate, or inspect the highly radioactive components removed from the target area. It is a conventional design with concrete shielding walls, lead glass viewing windows, and sealable roof ports to allow crane access to the cell. Personnel access to the top of the cell, if required, is possible when a target module is in the cell. Two hot cell bays are provided, each with direct actuated master slave manipulators and transfer ports to the connecting low level warm cell. The mechanical bay includes remote viewing, service equipment, and an elevating turntable to support and position the component being serviced. A floor pit allows the removal of containment vessels from the target modules. The decontamination bay is separated from the mechanical bay by a removable wall. It is provided with decontamination equipment that is used to remove mobile activity from components when this is required. The hot cell is kept under negative pressure by its own HEPA filtered air handling system.Fig. 6 Target Maintenance CenterA multi-purpose warm cell allows handling of components with low radiation levels and limited removable contamination. A pair of heavy duty master-slave manipulators operate over the shielding wall. Several large radiation shielding windows provide viewing into the warm cell and crane access is provided by the maintenance centre crane. Personnel access to the warm cell is through a locked-gate shielding maze where contamination monitoring is provided. Much of the work in the warm cell will be hands-on. Typical jobs include sealing shipping casks and temporary storage pigs, transferring new components into the hot cell, and the manual repair of equipment having low radiation levels. The warm cell also serves as the hot cell maintenance access bay.173A support annex houses the remote handling control room, offices, personnel change rooms, radiation safety monitoring equipment, and target hall entry air-locks. The equipment needed to control the remotely operated cranes, viewing systems and other devices is in the control room. Cameras are mounted in strategic locations throughout the building and on the cranes. A larger air-lock is provided for truck transport of equipment into the target hall.3.4 The Target/Ion Source ModuleIn the present target area design, the target, ion source and extraction system are all contained within a single remotely handleable module. The design of this module will be critical to the operation of the ISAC facility as the difficult problems of target/ion source servicing, ion beam extraction and high voltage isolation must be resolved in a highly radioactive environment. This module will contain the majority of the radioactivity produced by the proton beam and containment of this activity will have to a major design feature. The targets and ion sources are expected to be changed frequently during ISAC operations. Therefore, handling of the module will have to be a relatively simple and quick procedure.A section through the target/ion source module as presently conceived is shown in Fig. 7. The module is completely contained in a surrounding vacuum enclosure with a single exit hole to allow the extraction of the ion beam. It is inserted into an external vacuum enclosure. Windows will be provided for the entry and exit of the proton beam and a special radiation-hard valve will seal the ion beam exit hole when required. Service connections to the module, vacuum pumps and the high voltage insulator are all shielded from the production target by 2  m of steel.k r l  M j n f i  Fig. 7 Target/Ion Source Module SectionsConventional nonradiation-hard components can therefore be used at this location. The target, ion source and extraction system are mounted on a central shield plug which carries the required high voltage services. This plug is isolated from the grounded vacuum enclosure by a 5 cm gap. This gap provides both high voltage isolation and the required pumping channel to the ion source region. Vacuum pumps will be integral to the module to avoid the use of large vacuum valves and to maximize pumping speed.The target, ion source and extraction system are envisaged to be modular components which can be connected to and disconnected from the shield plug by a manipulator in the hot cell. All such connections will have to be radiation-hard. The alignment of the component modules will be critical to the source operation and will be performed in the hot cell. Sufficient space has been provided at the base of the shield plug to accommodate the largest envisioned ion source and extraction system.The repair of the target/ion source module is possible by removing the module from the surrounding vacuum tank with an overhead crane and transporting it to the hot cell. Any mobile activity in the target area will therefore be contained during transport. Disconnection of the services to the top of the module will have to be quick and compatible with remote operation in the event that this becomes necessary. At the hot cell the outer vacuum vessel will be removed to allow required operations to the target area components. The outer vessel can then be replaced and the module returned to the target station or stored in the module storage and testing area.3.5 The High Voltage SystemThe high voltage systems which service the target stations will be housed in Faraday cages located in a service tunnel running parallel to the target areas. Radiation levels in this service tunnel will be sufficiently low to allow access during beam operations. High voltage services from the Faraday cage will be routed to the target area through an electrically shielded service maze. Devices required for the operation of the target and ion source will be located within the cage and powered through an isolation transformer. Control of these devices will be via computer optical links. Closed-circuit water systems and any required pumping systems will be located at ground potential and fed through insulating sections to the Faraday cage.The high voltage system will be designed for 100 kV operation although 60 kV is the present operational voltage specified for ISAC. This will give some flexibility in responding to future needs and provide a design margin for the entire high voltage system. The extraction voltage must be very stable for operation with the high resolution mass separator and the high voltage power supply will have to maintain ±1 V in 60,000. This stability is well within the limits of present technology.4. Production Targets4.1 General Target ConsiderationsOne of the obvious crucial challenges for an intense radioactive beams facility resides in the175technology associated with the production target and the ability of the target to tolerate the harsh thermal and radiation environment associated with an intense production beam.Targets for the production of a wide range of radioactive species through interactions by energetic protons have been developed principally at facilities using "thick" targets, notably ISOLDE [IS086] and TISOL [0X087]. In considering targets for the production of intense beams of radioactive species, the basic concepts follow the same approach, but with much more intense proton beams (up to 1 0 0  times as intense).To achieve optimum production of short-lived species, the length of the target (and thus the production rate) must be balanced with the requirement for fast release of the produced activities. In practice, targets meeting these dual requirements have thicknesses of approximately one interaction length. For such targets, even modest production cross sections give high yields, essentially taking advantage of the long range uniquely associated with energetic protons. Other light ion projectiles can be used, but intense production beams of energetic protons (up to 1 0 0  nA) are more readily available than other projectiles, such as 3He.The requirement for fast release of the produced activities leads to consideration of a number of target operating parameters, most notably elevated temperatures. Even relatively volatile activities are released more quantitatively at elevated target operating temperatures that sometimes approach target material stability limits. Other target material characteristics that play a key role to good release are diffusion rates (always higher for elevated temperatures), desorption rates (element specific), and physical (vapor pressure limitations), or sintering limits (for powder targets) or chemical (decomposition) stability. Other target release properties, such as the possibility for chemical selectivity either by release, or transport to an ion source or ionization, are addressed naturally for systems based on those already in use.When the production beam intensity is increased significantly above the presently available experience level as proposed for the ISA C  facility, a new set of target-specific problems is introduced. At high production beam intensities, heating of the target material through beam energy losses requires the use of forced target cooling, even for targets that normally operate at high tem peratures. Such cooling should be applied keep ing in m ind the desired target operating temperature and the temperature distribution in the target material. Thus, for an intense radioactive beams production target, transport of heat within the target material and subsequent heat extraction is of concern to the design of any thick target.4.2 Existing Target SystemsThick target systems presently in use form the basis for considering targets to be used with intense production beams. These include generically, solid, liquid (molten metal), and powder targets.In general, solid targets consist of refractory metallic foils operated at high temperature (around 2 0 0 0 °C ) and are preferred for systems where the release of desired activities demands high temperature operation or is governed by surface desorption. Liquid targets are suitable for release of activities more volatile than the target material, and are usually limited to operatingtemperatures not much higher than the target melting points. Both metal (solid) and liquid targets are relatively dense, and beam heating from intense production beams can pose serious heat removal problems, while at the same time maintaining the required operating temperatures.Powder targets take on many forms, mainly of refractory chemical compounds. These targets present the most varied ranges of operating temperatures, depending on the chemical or physical stability of the material employed. They are also usually of low density, therefore heating from intense production beams is reduced compared to dense targets. However, the transport of induced heat from the interior of the target is inefficient due to inherently low thermal conductivity, and can result in large thermal gradients existing within the target material itself. Indeed, this aspect of powder targets may impose constraints on the tolerable maximum production beam current.4.3 Target Analysis ToolsTwo types of analysis are needed to evaluate the suitability of candidate target systems; an analysis of beam-induced heating within the target, and analysis of the thermal characteristics of the target, including the efficacy of cooling schemes.For die beam heating analyses, energetic beam interaction codes based on Monte Carlo techniques have been developed which can provide beam energy loss profiles within the target systems. The two codes extensively used in the literature are LAHET [PRA89] and FLUKA [ARN8 6 ], and succeeding versions. LAHET can be linked to the MCNP code [GR081], and succeeding versions, to provide tracking of interactions of primary and secondary particles down to thermal energies, and includes energetic fission interactions in high-Z target materials. The FLUKA code, used at CERN, is comparable in application. Both codes can quickly predict beam energy deposition profiles to statistical levels of a few percent.Thermal analysis is provided  by the fin ite-elem ent approach o f  the A N S Y S  cod e [A N S 9 3 ], which  can be used both to evaluate the m echanical strains o f  a system , and to determ ine the thermal profiles generated from  the beam  energy losses by  m eans o f  heat transport netw orks.Both of these analytical approaches have been used in the proposal for the target concept presented here.4.4 Proposed Target SystemThe target system proposed for ISAC is based on a credible extension of previous studies reported [EAT87, TAL92], which evaluated existing ISOLDE target systems with regard to maximum production beam currents tolerated for radiative, conductive, and heat-pipe cooling approaches for heat removal. In these studies, the transfer of beam-induced heat out of the target systems was addressed, and the way pointed toward target concepts that might accommodate the large heat deposition rates resulting from the interaction of 1 0 0  fiA production beams with thick, dense targets.The proposed target concept is presented in more detail in a TRIUMF Design Note [TAL94].177Highlights of the approach and a summary of the features for the concept will be presented here.In recognition of the requirement of many targets to operate at elevated temperatures, to be cooled with proven water-cooling techniques, the suggestion of a helium-filled radial gap [TAL92] has been embodied in the proposed concept in a modified form. The helium-filled gap represented a thermal impedance to the transport of heat from the target, allowing the high- temperature target material to be coupled in a controlled manner to the low-temperature water cooling jacket.In the concept proposed for ISAC, shown in Fig. 8 , the thermal impedance takes the form of imperfectly-fitting joints between a graphite sleeve and the (tantalum) target chamber and (copper) cooling jacket. These joints exhibit a "contact thermal resistance" that has been treated extensively in the literature for application to space vehicle cooling problems, or for electronic circuit cooling. The application of this approach to high-temperature radioactive nuclei production targets, while a natural extension, is challenging because of the need for quantitative description of the thermal and mechanical properties of the joints between these specific target chamber materials.ra d iu s3.0 cm2.0 cm1.0 cmBEAMCopper Coollnq'JacketGraphite SIC coal ing  on graphi le^ outer pap ^ ^—  Inner popTantalum case. ___________ Tantalum target C/Li— '0 cm•>. ax ia l2 cm 4 cm 6 cm distanceFig. 8 Conceptual Target ModelIf the interfaces between the target components are treated generally, then it is a natural consequence of the analysis to replace the joints by regions of porous metal, ceramic, or woven wire mesh having lower (and predictable) thermal conductivity than the other target components, but design-adjustable to provide for a number of target operating temperature ranges. In addition, the interfaces can be modified to provide a uniform axial target temperature distribution at the target chamber.An analysis of the proposed concept has been made, including the thermal response to deposition of heat in a tantalum foil target by a 100 /uA, 500 MeV proton beam (from TRIUMF). Asample thermal distribution is shown in Fig. 9 where the effect of the thermal contact joints is readily apparent. Variations of the analysis have revealed that other selected low-conductivity interfaces can be used for selection of a design value for the operating target temperature, and that the axial temperature distribution at the target chamber wall can be made uniform through axial modification of the interface conductivity (such as employing graduated grooves to reduce the thermal interface contact).2500u  2000b 1500 ■*-> oL.!  1 0 0 0  ffiH5000  — l — * ■ 1 *— *— »  * 1— «  i  i— i  i  i  i .  .  . . . . . .  _0-0 0.5 1.0 1.5 2.0 2.5 3.0Radius, cmFig. 9 Temperature Distributions Two-Gap ModelThus, the target concept proposed has the following attractive features: it is mechanicallysimple; provides for selection of operating temperature range; can be designed to provide uniform axial temperatures; uses simple water-cooling techniques; and is expected to be relatively inexpensive to construct.While the proposed target concept has many apparent promising features, an experimental development program is called for that can address the important issues of characterizing interface material thermal conductivities, including contact joint conductivities for the materials suggested. Simulation tests are underway for validating the thermal predictions with contact joints, and an apparatus is under development to measure thermal conductivities of candidate interface materials.Another concern arises in the temperature gradients existing in the target materials themselves. For solid or liquid targets, heat transport from the target material to the target chamber can be treated using known material thermal conductivities. However, for powder targets the thermal gradients within the target may prove to be unacceptably large, but cannot be determined without measured thermal conductivities for the target materials. An experimental program to measure these conductivities, using the same apparatus referred to above, will address this concern.« '  n  T " 7— I " " '  V . . . 7 h  :  T - r Ifz=0 cm -  z-3 cm -*z=6 cm- - - - - - - - - - - - - - ‘•*- ■- U - I  ■ *  >  ,179Two significant target development efforts are being pursued elsewhere, one at the Rutherford Appleton Laboratory, and the other at the Lawrence Berkeley Laboratory. At the Rutherford Appleton Laboratory, a tantalum target is under design for testing in 1995 in a 800 MeV proton beam of up to 100 fj.A intensity [BEN93]. This target design employs forced helium cooling, and contains a total of 166 g/cm2 tantulum foils. The test is intended to demonstrate the scaling of yields of radioactive products with production beam current, and operational characteristics at temperatures of about 2300°C. The Lawrence Berkeley Laboratory effort [NIT94] involves production beam heating emulation by electron heating of a tantalum cylinder, to evaluate various cooling schemes, including forced helium gas cooling and application of water-cooled fins.4.5 Concurrent Efforts5. Ion Sources5.1 General ConsiderationsThe target and ion source of an ISOL are the initial determining elements that define the type and quality of that ISOL’s radioactive beam. While no single type of ion source can produce optimal beams of all elements, certain characteristics are common to all ion sources operating on-line. Unlike industrial or accelerator ion sources, ISOL sources must be highly efficient. The initial amount of the ionized species is determined not by a flow rate from a supply bottle, but by the production cross-sections for that species in the ISOL target. Maximizing the yield of a desired product is achieved by minimizing the losses incurred from its initial production to its extraction as a radioactive beam. Clearly, an ISOL ion source must be closely coupled to the production target to minimize losses from decay of short-lived products during their diffusion from target to source. Such close proximity further requires the ion source be thermally rugged and radiation-hard. Sources must be constructed of materials with high melting points and low vapour pressures, limiting the choice to the refractory metals and ceramics. The hostile environment also dictates that the ion sources be both simple and small. Volumes and surfaces must be kept to a minimum both for the sake of economy and to minimize delaying surface interactions of the short-lived products. Simplicity of construction ensures that a radioactive source can be either easily repaired by remote manipulation or economically thrown away after its inevitable breakdown. Fortunately, past experience at ISOL facilities has produced several types of sources that meet the required criteria.5.2 Surface Ionization and Thermal Cavity Ion SourcesThe simplest and most efficient of the ISOL ion sources are the surface ionization sources. In surface ionization, an electron is transferred when an atom (or molecule) contacts a hot surface. The atom subsequently leaves the surface as a positive or negative ion. For positive ionization, the surface must have a thermoionic work function greater than the atom’s ionization potential; for negative ionization, a low surface work function and a high atomic (molecular) electron affinity are required. The degree of ionization is dependent on the work function and ionization potential (electron affinity) difference and also scales with surface temperature. For positiveionization, refractory metal surfaces such as Ta, Re, W or Ir, at temperatures >2000°C provide ionization efficiencies of up to 95% for the alkali elements. Elements with ionization potentials < 7  eV, such as the alkali earths, rare earths, Al, Ga, In and Ra are ionized to varying degrees depending on the surface and temperature employed. For negative ionization, a LaB6 surface (work function =  2.7 eV) achieves ionization efficiencies of about 40% for Cl, Br, I and At. In theory, it may also be possible to ionize molecules with high electron affinity.Somewhat higher positive ionization efficiencies can be obtained with high temperature cavity ion sources. In these sources, ionization is enhanced beyond the predictions of simple surface ionization due to the formation of an electron plasma near the walls of a hot W cavity. This can allow ionization of elements with work functions of up to 8 eV in W (Re) cavities heated to 2700°C (2400°C). The theory of thermoionic sources has been extensively surveyed [KIR90]. The chief advantages of surface sources are their simplicity and high efficiency. In practice, a source consists simply of a refractory metal tube connected to the production target and resistance heated to the desired temperature. Some chemical selectivity is possible by the appropriate choice of ionizing surface and temperature. Surface sources produce the best quality beams for purposes of mass resolution; since ions leave the surface with thermal energies, the beam emittance and energy spread of the resulting beam are the lowest of any ISOL ion source. The disadvantage of surface sources is their limitation to elements with ionization potentials < 7 eV.5.3 Plasma Ion SourcesTo ionize elements with ionization potentials > 7  eV most ISOL facilities use sources employing a plasma generated by an arc discharge in a support gas. Though many different designs of plasma source have been used [NIE57, CHA67], the forced electron beam induced arc discharge (FEBIAD) source such as employed at GSI [KIR81] and ISOLDE [SUN92] is currently the most popular. In general, electrons from a heated cathode are accelerated in a gas-filled chamber where a magnetic field guides them to an anode grid, thus generating a plasma. Products from the production target are ionized by electron impact as they diffuse into the plasma chamber. Unlike earlier sources, FEBIAD sources have the advantage of operating with support gas pressures on the order of 10-5 mbar. This reduced operating pressure allows the use of calibrated test leaks as a source of support gas and also reduces the current density (and emittance) of the extracted beam. Unlike surface sources, plasma sources are not chemically selective; any element diffusing into the plasma chamber will be ionized. Chemical selectivity is achieved by the proper choice of target material or by controlling the temperature of the transfer line from the target to select elements of a particular volatility. Plasma sources are more complex than surface sources; a support gas, a magnetic field, cathode heating and an electron accelerating potential are required. Ionization efficiency is generally determined from the ionization of the support gas. Efficiencies of 23% (Kr) and 48% (Xe) have been reported for the GSI FEBIAD source as well as a 76% efficiency for Hg ionization [KIR81], Efficiencies are expected to show a mass dependence and scale as oim Am, where oion is the cross section for electron impact ionization. However, yields of light elements such as C, N and O have been less than expected. Arc discharge sources operate at high temperatures and are constructed of refractory materials. Since the carbides, nitrides and oxides of the refractory elements are usually themselves refractory, direct chemical reactions with the heated cathode or plasma181chamber walls significantly reduce the light element yields.5.4 Electron Cyclotron Resonance (ECR) Ion SourcesIn order to efficiently ionize the light elements, some ISOL facilities have successfully used a different type of plasma ion source. Recently, ECR ion sources have started to be used on-line, first at the TISOL facility at TRIUMF [DOM90] and subsequently at the Belgian Radioactive Ion Beam Facility [DEC91]. The ECR sources show good efficiencies for producing C,N and O ions as well as molecular species such as N2+, NO+, CO+ and C 02+.As with arc discharge sources, ECR sources consist of a plasma chamber coupled to the target. However, the plasma is generated by microwave (in the GHz range) introduced through a wave guide. A magnetic field matched to the rf frequency keeps the electrons in cyclotron resonance while a second solenoidal field can be used to confine the plasma. Due to the plasma confinement and a lower operating temperature, wall interactions are minimized. Ionization efficiencies of 10% (C), 27% (N), 55% (O), 31% (Ne) and 65% (Xe) have been measured [BEC8 6 ]. As with arc discharge sources, ECR sources are not chemically selective. In theory, any element introduced into the ECR plasma chamber would be ionized. However, in practice, only the volatile gaseous elements have (as yet) been produced on-line. An advantage of ECR sources is their ability to produce multiply charged ions. Due to higher electron density, low operating pressures and magnetic plasma confinement, ECR sources can efficiently produce ions with charge states up to 8 + . This feature can enhance the options available for post-acceleration of ion beams but can also be a disadvantage if low yield products are distributed over many charge states. The chief disadvantage of ECR sources is their relatively high beam emittance and energy spread. While highly efficient, ECR sources may not be the first choice for high resolution mass separation.5.5 Laser Ion SourcesVery recently, the ISOLDE group has tested on-line an ion source that promises both good ionization efficiency and the highest degree of chemical selectivity yet observed [MIS93]. The ISOLDE laser ion source is essentially a surface ionization source (consisting of a 1 mm diameter heated cavity coupled to the production target) into which three tunable dye laser beams are directed simultaneously. The laser frequencies are chosen such that two lasers first excite the desired product element via atomic states allowing the third laser to ionize it. Since each element has its own distinctive energy level structure, the tuned laser source offers an exceptionally high degree of chemical selectivity. In the study of Sn, Tm, Yb and Li laser ionization, a thousand-fold selectivity increase in Tm ionization and a 30% enhancement of Sn ionization efficiencies was observed. Laser sources are still in the development stage and their chief disadvantage is the low laser duty cycle.5.6 Multi-Cusp Ion SourcceIt has been suggested [SCH87] that a plasma source using a multi-cusp magnetic field could efficiently produce multiple-charged ions. Currently, a TRIUMF-LBL collaboration is designing such a source for on-line testing. The source will employ permanent magnets for the multi-cuspfield and a heated cathode for plasma generation. It is expected that good beam quality and selective multiple-charged ion extraction can be achieved with good beam quality. If successful, this source would offer a practical alternative to the ECR sources.6. Ion BeamsThe design of the radioactive beam system requires that a number of fundamental issues be addressed and resolved. These issues include the following:• extraction from the ion source• matching of the source to the mass separator• beam emittances and currents to be transported• multiple beam requirements from the mass separator• required mass resolutions• transport from the separator to the userThere is no solution which simultaneously satisfies all possible requirements so that critical choices will have to be made.Extraction of the beam from the ion source will be dependent on the source used, the conditions expected in the source during operation and the philosophy taken by the designer. The environment in which the source and extraction system must operate will have an influence on the design as considerations such as radiation hardness and radiation heating will have to be taken into account. The extraction system used at ISOLDE is a single independently mounted electrode. The alignment of this electrode to the source is critical to the beam extraction and five independent degrees of motion are required to achieve this. The design of a reliable mechanism for providing such alignments in the remote environment of the proposed target system will be a difficult task. Other options, with less elaborate mechanical requirements, will therefore have to be investigated.Beam emittances and currents will be dependent on the particular source and target system being used. These beam qualities will greatly influence the overall design of the beam facilities. Large emittance beams will require a high acceptance beam transport system and will not be amenable to high mass resolution. High current beams may be subject to space charge effects which may introduce aberrations and cause beam losses in the transport system. Typical ion sources such as surface or plasma sources will give beam emittances of 2 - 2 0  7r.mm.mr at 60 kV. ECR sources have much larger emittances (80 x.mm.mr) due to the magnetic field in the extraction region.Transport and matching optics must be provided between the ion source and the mass analyzer to tailor the beam for the separator magnets and to compensate for changes in the beam extraction. The requirement of multiple beams from a single mass separator brings with it the183implication that this transport system be electrostatic as magnetic systems will transport only a limited mass range. This in turn implies that the ion beams will not be space charge compensated due to the removal of space charge compensating electrons by the focusing elements and the space charge effects at the entrance of the mass separator where the beam must be small will be maximized. The transport of high current beams from the source to the separator may therefore be compromised. Magnetic systems maintain the space charge compensation in the beam and defocus off mass components and are therefore more appropriate for high current beams.The angular divergence of the beam to be transported will be limited by the position and size of the first lens. For typical systems this limit will be of order 30 mr and to emittance limits of order 50 7r.mm.mr. To increase this acceptance of the transport system will require a special optical design with large elements close to the ion source. If large phase space beams are to be transported, care will have to be taken to avoid or compensate aberrations which will lead to beam loss at the entrance to the separator.High mass resolution will obviously have implications on the separator design but will also require high precision and stability in the transport line from the ion source. This must be taken into account when designing this system. For example, focusing fluctuations in an electrostatic system due to variations in the ion beam current would have an effect on the separator performance. To avoid such complications and the expense of high quality optical elements, the injection line to the mass separator should be kept as short as possible.For the purposes of this design report it was decided to make the optical system between the source and the separator all magnetic. This will allow transportation of high current ion beams and seems the most likely direction to take for a high resolution system. With this choice, we will restrict our aspirations to only one beam from a mass separator. Electrostatic transport and multiple beam options could be considered if the physics program indicates that this would be a favorable option but these will not be described here.The present design of the source and beam layout is shown in Fig. 10. A medium resolution mass separator is attached to the first source location and will be the initial facility installed. A high resolution separator with 220° of total bend is attached to the second source location. The choice of a medium resolution system as opposed to two high resolution systems was made primarily on the basis of cost. The separation is done in the horizontal plane to keep associated radioactivity at beam level and to facilitate construction of and access to the separators. Vertical separators have been considered and provide some advantage in cost but access to the focal plane instrumentation is difficult to achieve conveniently. The specifications of the two mass separators are given in Table 1. These represent the optical performances with low emittance beams. Actual performance will be independent on precision and stability of the elements and the ion beam emittance. Both separators will be capable of transmitting a mass range up to 240 of singly charged ions with energies of 60 keV.Extraction and initial beam transport will be identical for each of the source locations. Extraction from the ion source will be highly source dependent and may consist of single or multiple electrode system. Space has been allowed in the target module design to incorporatethis extraction and some matching elements such as steering devices or other lens systems. A quadrupole doublet is placed as close to the ion source as practically possible to maximize the transport acceptance and a second doublet used to transport the ion beam from the high radiation area. These four quadrupoles form the matching section between the ion source and the mass separators.Fig. 10 Ion Beam LayoutThe medium resolution mass separator consists of two antisymmetric QQD systems with a total bend angle of 110° and a nominal mass resolving power of 5000. Weak sextupoles are required before and after each bending magnet to correct aberrations. A source defining aperture will be placed at the separator entrance and a mass selection slit at the focal plane.The high resolution mass separator consists of two antisymmetric QQDDQQ systems each with a total bend angle of 220°. It will also have a source defining entrance aperture. Aberrations are corrected by four weak sextupole magnets placed before and after the second (first) dipole magnets in each system. This separator will have a dispersion at the focal plane of ?? cm/20,000. A movable slit system will be placed on the focal plane to select the mass to be185transmitted. The second bend section could be maintained at an elevated potential to provide additional beam purification [WOL92] if this is required, however, this possibility has not been incorporated in the present design.Table 1 MASS SEPARATORSMedium Resolution High ResolutionBend Radius 2.0 m 2.0 mTotal Bend Angle0•ni0II'w'0o2 x 110 = 220°Magnet Gap 10 cm full gap 10 cm full gapEntrance Angle 27.5 27.5Exit Angle 27.5 27.5Maximum Field Strength 2.65 Kg 2.65 kgFocal Plane Dispersion 4.23 cm/% 20.08/cm %Focal Plane Angle 4.5° 24.06°• Mass Resolving Power ( ) 2,000 10,000Correction Elements 2 sextupoles 4 sextupoles + 4 octupoles2 octupolesTransport of the separated beams will be done entirely with electrostatic elements as the beam currents will be low and tuning will be dependent only on the beam acceleration voltage. A switchyard arrangement will transport the beams from either of the mass separators to a low energy experimental area situated at the separator level or to the accelerators. A 90° achromatic vertical bend will allow transport of the beam to grade level where a similar 90° bend will direct it into a transport and matching section preceeding the initial accelerating stages. A system of five quadrupoles provides appropriate matching to the RFQ.6.1 ISOL Beam DiagnosticsPreliminary considerations for the beam diagnostics for the ISOL/ISAC facility have been given in a TRIUMF design note [REI93].The purpose of the ISOL beam diagnostics is to give the necessary information for adjusting the beam lines from the ion sources to the separators, selecting the appropriate ion species in the two stages of the separators, adjusting the beam line from the separators to the RFQ, and adjusting the beam lines to the low-energy experiments. The ISAC facility wil operate with beams of a very wide range of intensity, from 102 to 1011 ions per second [IS092], It isimpossible to cover this intensity range with the same beam diagnostic devices; therefore either multiple diagnostic elements must be used or a more indirect approach taken for low intensity beams.The sensitivity of the beam diagnostic elements used at ISOLDE covers the range from 106 to 1011 ions/s (0.2 pA-20 nA) [RAV93]. When less intense beams are used the system is set up with a more intense pilot beam of the same or near mass. The same procedures will be employed at the ISOL/ISAC facility and the sensitivity of the beam diagnostic elements used at ISOLDE will be sufficient here.The basic beam observation unit for the ISOL system will be a standardized unit similar to those at ISOLDE [KUG93b] which combine a wire scanner and a Faraday cup. The wire scanners are mounted at 45° and use V-shaped wires with a 90° angle. Thus, the horizontal and vertical beam profiles are measured with the same mechanism. The spatial resolution is mainly determined by the diameter of the wire to 0.5 mm. These standardized scanner and Faraday cup units will be used in the beam transport systems from the separators to the RFQ, between the different linear accelerators, and in the beam lines to the low and high energy experiments.The standard wire scanners cannot be made radiation-hard, because of a pre-amplifier, which must be placed directly on the moving wire. Therefore, in the high radiation areas, before the separators, the beam profile will be measured by moveable slits, placed in front of a special, radiation-hard Faraday cup. The same slits will also be used to trim the beam emittance from the ion sources before the entry to the separators. Other radiation-hard Faraday cups will be placed immediately after the ion sources, to make it possible to verify at any time that they are working.The diagnostics at the separator focal planes are crucial parts of the system. These areas become radioactive and therefore must be designed and built for very high reliability. Whenever possible, there should be backup systems installed, which can be used in case of failure of the primary system. Horizontal slits will be used to select the appropriate ions. The two jaws of the slits move together along the focal plane. The separation between the jaws will be adjustable to form a variable slit. Faraday cups will be placed behind the slits, to analyze the different ion species in the beam. Horizontal wire scanners, with enough stroke to cover a mass range of ± 6 %, will be placed just before the slits and will be used for scanning the neighbouring mass range of the ion beam, and to monitor a reference beam while running low intensity beams. A removable vertical wire grid will be used to measure the vertical beam profile.In order to obtain a good understanding of the ISOL/ISAC beam optics, it is essential to know the emittance of the beam from the ion sources. Therefore, an emittance measuring apparatus will be placed before the separators, after the emittance defining slits. Another system will be placed just before the RFQ. Each emittance measuring apparatus will consist of two moveable slit plates and a Faraday cup. The slit plates will contain horizontal and vertical slits, and will be moved at 45° with respect to those planes. The slit width and the distance between the slit plates will be chosen to achieve a resolution in the emittance measurement of about 0 . 2  x mm X mrad at 60 kV.187To commission and calibrate the isotope separators and the beam transport system, and for trouble-shooting these after commissioning, it will be useful to have a reference beam from the ion sources. This reference beam will also be useful for testing the beam diagnostics equipment when bench tests are not possible, e.g., tests of beam profile measurements. Several of the ion sources will therefore be equipped with a very small controlled leak, through which a known gas, such as xenon, can be introduced to produce a reference beam.7. Vacuum SystemThe vacuum requirements of the ISOL system are very demanding. High pumping speeds are required in the target/ion source region to remove background gases. This is difficult to achieve if pumps must be placed in a non radioactive environment and separated from the target by close packed shielding. In addition, many of the gases to be pumped will be radioactive and precautions must be taken to contain this radioactivity. The transport lines for the mass separated beams are very long and a good vacuum must be maintained over the entire length to avoid beam losses due to interactions with the residual gas.Turbomolecular pumps are presently proposed to provide the requirement of high pumping speeds for all gases. In the present design of the target module, these pumps are located at the top of the module and pump the target/ion source region through the 5 cm high voltage gap which has a conductance of about 1000 f/sec. The turbo backing pumps are sealed pumps which are exhausted via a compressor to an evacuated waste volume. The gases collected in this waste volume may be stored for a period of time to allow decay of the shorter lived species before controlled release to the atmosphere. The pipe length between the turbo pump and the backing pump is made especially long to provide a delay time in which short lived species may decay, thereby reducing the radiation dose to the backing pump. Radioactive species will collect in the oil of the backing pumps and therefore these pumps and the waste volumes will be located in shielded regions of the service tunnel and will be monitored with radiation detectors. Separate pumps will be used to evacuate the system to a rough vacuum. These pumps will be vented through the target area HEPA filter system since only low levels of radioactivity are expeccted during this operation.The amount of radioactive gas pumped in the mass separator region is expected to be much lower than in the target region but still significant. The vacuum systems for this region will therefore be similar to the target area systems, however all pumps and waste volumes will be located in the separator enclosures. Beyond the mass separators the amount of radioactive gas to be pumped will be small and pumps will be exhausted directly to the atmosphere through one of the HEPA filtered air handling systems.There will be 70-100 m of beam transport between the production target and the accelerators or the low energy experimental area. To minimize the loss of beam particles due to scattering in the residual gas, a very good (1 0 ~ 7 mbar) vacuum will have to be maintained over this entire distance. This implies the use of high vacuum components and procedures and the ability to mildly bake (100°C) the system after assembly. For practical purposes, the system will be divided into a series of individual vacuum sections separated by inline gate valves. Each section will have a dedicated high vacuum pumping system and gauges. Hot cathode gauges will beused for high vacuum measurements and convectron gauges for rough vacuum. Roughing will be provided by a centralized roughing system except for the production and experimental target areas. Control and interlocking will be done with programmable logic controllers (PLC) which will communicate with the central control system. A central dry nitrogen venting system will be provided and integrated with the vacuum controls.8. O ff-L ine  SourceThere is a need for an off-line-ion-source (OLIS) to be installed at the front end of the ISAC first-accelerator. To tune the linac we will need an analog beam (beam having the same charge to mass ratio as the RIB coming from the on-line isotopic separator). Some experiments will require beams of stable elements in order to set up experimental apparatus and electronics; and for research using beams of stable elements.This OLIS system should produce many elements; the best candidate is a very hot plasma ion source similar to the electron cyclotron resonance ion source (ECRIS). This ion source should be placed in front of the RFQ in order to inject the analyzed beam in the matching section located downstream of the RFQ.The OLIS bench will include an ECRIS on its high voltage platform (60 kV), an analyzing magnet with a resolving power of 2 0 0 , and its own diagnostics elements for beam intensity and emittance measurements.189References[ALL87][ANS93][ARN8 6 ][BEC8 6 ][BEN93][BEV93][BEV94][CHA67][DEC91][DOM90][EAT87][GR081][IS086][IS092] [KIR81] [KIR90] [KOS82]B. Allardyce and H. Ravn, Nucl. Instr. & Meth. B26, 112 (1987).ANSYS Users’ Guide for Revision 5.0, Swanson Analysis Systems, Inc., Houston, PA (1993).A.A. Arnio, A. Fasso, H.J. Moering, J. Ranft and G.R. Stevenson, "FLUKA8 6  User Guide", CERN report TIS-RP7168 (1986).V. Bechtold, H. Dohrmann and S.A. Sheikh, Proceedings of the 7th Workshop on ECR Ion Sources, Julich, 1986 p.248.Private Communication, J.R.J. Bennett (1993).J.L. Beveridge and G.S. Clark, Conceptual design o f a high current radioactive beam target facility (TISOL 2) TRIUMF Design Note TRI-DN-93-15, May 1993.J.L. Beveridge, G.S. Clark and C. Mark, Conceptual design of a high current radioactive beam target facility (ISAC) TRIUMF Design Note TRI-DN-94-2, to be published.I. Chavet and R. Bernas, Nucl. Instr. & Meth. 51, 77 (1967).P. Decrock, M. Huyse, P. Van Duppen, F. Baeten, C. Dorn and Y. Jongen, Nucl. Instr. & Meth. B58, 252 (1991).M. Dombsky, J.M. D’Auria, L. Buchmann, H. Sprenger, J. Vincent, P. McNeely and G. Roy, Nucl. Instr. & Meth. A295, 291 (1990).T.W. Eaton, H.L. Ravn and the ISOLDE Collaboration, Beam heating o f thick targets for on-line mass separators, Nucl. Instr. & Meth. B26, 190 (1987).Group X-6 , MCNP - A general Monte Carlo code for neutron and photon transport, Los Alamos National Laboratory report LA-7396-M Revised (April 1981).ISOLDE Users’ Guide, ed. H.J. Kluge, CERN report 86-03 (1986). Specifications for the IsoSpin Laboratory, October 1992.R. Kirchner, Nucl. Instr. & Meth. 186, 275 (1981).R. Kirchner, Nucl. Instr. & Meth. A292, 203 (1990).C. Kost, TRIUMF Report TRI-DN-82-17, TRIUMF, 1982[KUG92][KUG93a][KUG93b][LEE93][MIS93][NIE57][NIT94][0X087][PRA89][RAV89][RAV93][REI93][SCH87][STI80A][STI80B][STI81][SUN92][TAL92]E. Kugler, Nucl. Instr. & Meth. B70, 41 (1992).E. Kugler, Nucl. Instr. & Meth. B79, 322 (1993).E. Kugler, G.-J. Focker, Private Communication (November 1993).R. Lee, Private Communication, TRIUMF, August 1993.V.I. Mishin, V.N. Fedoseyev, H.-J. Kluge, V.S. Letokhov, H.L. Ravn, F.Scheerer, Y. Shirakabe, S. Sundell, O. Tengblad and the ISOLDE Collaboration,Nucl. Instr. & Meth. B73, 550 (1993).K.O. Nielsen, Nucl. Instr. & Meth. 1, 289 (1957).Private Communication, J.M. Nitschke (1993).K. Oxorn, J.E. Crawford, H. Dautet, J.K.P. Lee, R.B. Moore, L. Nikkinen, L. Buchmann, J.M. D’Auria, R. Kokke, A.J. Otter, H. Sprenger and J. Vincent, The installation of a prototype on-line isotope separator at TRIUMF (TISOL), Nucl. Instr. Meth. B26, 143 (1987).R.E. Prael and H. Lichtenstein, User Guide to LCS: The LAHET Code System, Los Alamos National Laboratory report LA-UR-89-3014 (1989).H.L. Ravn and B. Allardyce, Treatise on Heavy Ion Science, ed. by D. Allan Bromley, (Plenum Press, New York, 1989) Vol. 8 , p .363.H. Ravn, Private Communication (August 1993).D. Reistad, Y. Yin, Preliminary comments about instrumentation fo r  ISOL/ISAC, TRIUMF Design Note TRI-DN-93-29 (November 1993).M.R. Schubaly, Nucl. Instr. & Meth. B26, 195 (1987).G.M. Stinson, TRIUMF Report TRI-DNA-80-3, TRIUMF, 1980.G.M. Stinson, TRIUMF Report TRI-DNA-80-7, TRIUMF, 1980.G.M. Stinson, TRIUMF Report TRI-DNA-81-3, TRIUMF, 1981.S. Sundell, H. Ravn and the ISOLDE Collaboration, Nucl. Instr. & Meth. B70 160 (1992).W.L. Talbert, H.-H. Hsu and F.C. Prenger, Beam heating and cooling of thick targets for on-line production o f exotic nuclei, Nucl. Instr. & Meth. B70, 175(1992).191[TAL94][WOL92]W.L. Talbert and T.A. Hodges, Conceptual design of targets for production of intense radioactive beams, TRIUMF Design Note TRI-DN-94-xxx (1994).H. Wollnik, Proceeding of the Oak Ridge Workshop, Oak Ridge, TN, October 1992.F U N D A M E N T A L  S Y M M E T R Y  T E S T S  W I T H  T R A P P E D  N E U T R A L  A T O M S0 . HAUSSERSimon Fraser University, Burnaby, B.C., Canada, V5A 1S6andTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada, V6T 2A3A b str a c tThe TISOL/ISAC facility will provide one of the brightest sources of radioactive iso­topes in the world. The isotopically pure atoms can be viewed a s  mini-laboratories in which to perform tests of the standard model (SM) of the electroweak and strong inter­actions. These low-energy tests in complex nuclei are complementary to those pursued at the high-energy colliders. The TISOL/ISAC experiments exploit the large yields of mass separated atoms, and the recent rapid progress in laser trapping and cooling of atoms. Trapped atoms can be produced at a high rate (up to 1010 atoms/s), are confined in space to a few mm3, and are nearly 100% polarizable. W ith trapped neutral atoms one should be able to attain much higher luminosity than with traditional atomic beams.We propose to investigate the weak charge in complex, heavy nuclei by observing the parity-nonconserving component of the 7S-8S transition in isotopes of Francium. The weak charge is extraordinarily sensitive to certain types of new physics, such as extra Z bosons, leptoquarks, and e g  contact terms associated with compositeness. Compared to existing measurements for 133Cs the PNC effects in Fr are an order of magnitude larger. Furthermore, measurements on several isotopes should provide an independent test of the theoretical matrix elements needed to extract the weak charge from the measured PNC effect. In addition, and independently, we propose to perform symmetry tests of the SM by studying beta decay of laser-trapped radioactive nuclei. Experiments with well local­ized, carrier-free, and nearly 100% polarized beta emitters might afford the possibility of improving or confirming existing limits on physics outside the SM. For example, measure­ments of the asymmetry A and the longitudinal polarization ratio Ra for positrons from 37K could place stringent limits on the mass of a predominantly righthanded W boson postulated by manifestly left-right symmetric models.1. P a r tic le  p h y s ic s  a t lo w  en erg iesIt is a fashionable belief held by many, especially particle physicists, th a t, although semileptonic decay data in complex nuclei have played an im portant role in the historic development of the electroweak interaction, all the im portant “action” has now shifted to the high-energy colliders. In the following we propose tests of the standard model (SM) in complex nuclei produced by the upgraded TISOL/ISAC facility which are complementary to and, in specific areas, competitive with those performed at higher energies. The low-energy experiments can be carried out by small groups at relatively low cost, and they benefit from high statistical accuracy and the multiplicity of atomic and nuclear quantum states.The SM, SU(3)c x SU(2)l x U (l), is now well established and compatible with all known experimental data obtained so far. It assumes tha t the fundamental constituents of m atter are three generations of fermions: three lepton doublets, and three quark doublets193having three distinct color quantum numbers. The electroweak force between fermions is mediated by the exchange of four gauge bosons, W *, Z° and the photon, whereas the strong force between the quarks is mediated by the exchange of eight colored gluons. The masses of gauge bosons and fermions are acquired by coupling to a hypothetical Higgs scalar. The SM contains parameters such as fermion masses and interfermion couplings which are not accounted for by the model. Several of the SM assumptions appear to  be adhoc and unnatural: i) neutrinos are assumed to be massless; ii) W * gauge bosons occur only with lefthanded helicity couplings; iii) CP violation is allowed but not explained.Experiments at the current high-energy frontier are concerned with confirming the observation of the top quark, and with finding the Higgs scalar and other new particles which would show the way to higher unification theories which would contain the SM as a lower-order theory.Extensions of the SM might show up at low energies if experiments of sufficiently high accuracy (typically a  few 10~3 for non-vanishing observables) can be carried out. For some searches it may be an advantage to be at low energy since the interference amplitudes for established (W, Z°) physics and new physics are both real; this is not the case for experiments near the Z° pole. Furthermore, the multiplicity of well-defined nuclear quantum states may allow one to select a suitable beta decay transition which allows the most sensitive test of a specific extension of the SM. Nuclear uncertainties which might obscure the new physics of interest can often be controlled to sufficient accuracy. In beta decay one can select fast transitions to keep the influence of nuclear matrix elements of recoil order at a negligible level V All the beta decay experiments contemplated for TISOL/ISAC are thus of the allowed or super-allowed type. At an accuracy level below 1%, anticipated in the future for studies of parity-nonconservation (PNC) in atoms, knowledge of the distributions of u- and d-quarks (or protons and neutrons) over the nuclear volume becomes im portant 2. These nuclear effects can in principle be tested independently by performing PNC experiments with a series of isotopes.In the following we discuss the unique capabilities of an upgraded TISOL/ISAC facil­ity for performing symmetry tests of the SM. Most of the experiments will exploit recent progress in developing laser traps for neutral atoms. Initial funding for a TRIUMF Neutral Atom Trap (TRINAT) facility has been obtained from NSERC and TRIUMF. The princi­ple and properties of neutral atom traps coupled to TISOL/ISAC are discussed in the next section. We then consider a specific experiment to study atomic PNC in Francium. Finally, we examine precision measurements of semileptonic decays of radioactive nuclei trapped by TISOL/TRINAT and their value in comparison with traditional beta decay studies.2. T h e  p o te n t ia l  o f  n e u tr a l a to m  tra p s lo a d ed  b y  T IS O L /IS A CThe upgraded isotope separator proposed for the ISAC facility at TRIUMF is poten­tially the world’s brightest source of mass-separated alkali at