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Proceedings of the DASS/SASP (Dual Arm Spectrometer System/ Second Arm Spectrometer) Workshop, Vancouver,… Walden, P. L.; Iqbal, M. J. 1986

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TRIUMFPROCEEDINGSOF THEDASS/SASP (DUAL ARM SPECTROMETER SYSTEM/ SECOND ARM SPECTROMETER) WORKSHOPVANCOUVER MARCH 17-18, 1986Editors: P.L. Walden, TRIUMF M.J. Iqbal, University of AlbertaMESON FACILITY OF:UNIVERSITY OF ALBERTA SIMON FRASER UNIVERSITY UNIVERSITY OF VICTORIA UNIVERSITY OF BRITISH COLUMBIAOPERATED UNDER A CONTRIBUTION FROM THE NATIONAL RESEARCH COUNCIL OF CANADA TRI-86-1'TRI-86-1PROCEEDINGSOF THEDASS/SASP (DUAL ARM SPECTROMETER SYSTEM/ SECOND ARM SPECTROMETER) WORKSHOPVANCOUVER MARCH 17-18, 1986Editors:P.L. Walden, TRIUMFM.J. Iqbal, University of AlbertaOrganized by: Sponsored by:P.L. Walden, TRIUMFNatural Sciences and Engineering Research Council of Canada TRIUMF TUECPostal address: TRIUMF4004 Wesbrook Mall Vancouver, B.C. Canada V6T 2A3 December 1986PREFACEThe DASS/SASP Workshop was held from March 17-18, 1986 at TRIUMF in Vancouver, British Columbia, Canada. The workshop was organized primari­ly by P.L. Walden (TRIUMF) with support from other members of the DASS/ SASP task force namely E.G. Auld (University of British Columbia) and C.A. Miller and S. Yen (TRIUMF). Funding for the workshop came from TRIUMF, TUEC, and NSERC (Canada).Approximately 67 physicists and engineers attended this two day event which consisted of fourteen talks. The twelve main talks are pre­sented in this volume. The objective of the workshop was to discuss the physics that could be done with a dual arm spectrometer system (DASS), which is a proposed nuclear physics facility for TRIUMF. As the TRIUMF physics community is well aware, one arm of this system is already in hand, the MRS. The main effort then of bringing the DASS facility on line is to design and manufacture a second arm spectrometer (SASP).The hoped-for result of this workshop was to be a series of pro­posals submitted to the July 1986 session of the Experiments Evaluation Committee which specifically request use of the DASS/SASP facility. This goal was met with 3 proposals being submitted and accepted, all with high priority. The title and spokesman of each accepted proposal is given in Table I. Copies of these proposals are available from either TRIUMF or the spokesman.Table I. Accepted Proposals DASS/SASP •EEC No. Title Spokesman416 Neutron knockout with SASP C.A. Miller TRIUMF417 Survey of the (p,ir+) reaction in the A resonance regionP.L. Walden TRIUMF418 Nucleon effective polarization in Ca, Zr and PbW.J. McDonald U. of AlbertaA secondary goal of the workshop was to target members of the physics community who might be interested in using the DASS/SASP facility once it is finished. To accomplish this a questionnaire was handed out to workshop participants and also by post to a list of experimenters whose background made them likely candidates as users. At present there have been forty-three responses. It is hoped that this list will grow as SASP becomes more of a reality. The questionnaire was worded so as to commit people to expressing a desire to submit proposals if the DASS/SASP facility existed at the present moment. The distribution of interests is shown below in Table II. A complete list of the potential users is given at the end of these proceedings.iiiTable II. Potential Manpower Commitments to DASS/SASP Programs1. (p,2p) 272. (p,p'n) 203. (n,p)/(p,n) 114. (p,it) 175. (p.irx) 23x=p,n,d, etc.6. other 3total manpower 43The organizers wish to thank all who contributed to the success of the workshop. Special thanks are due to Michael LaBrooy and Krish Thiruchittampalam of the Information Office who assisted in the registra­tion and the video recording of the workshop. Special thanks are also due to Pat Stewart and Maureen White for their secretarial assistance. Finally acknowledgements and thanks must go to Ada Strathdee and Denise Mason for preparing these proceedings.ivCONTENTSPageOpening remarksP. Kitching ...................................................  1ir PRODUCTION PHYSICSPion physics using coincidence experimentsG. Walker ...................................  4Prospects for (p,ir) physics in the A region using the SASP spectrometerP. Walden .....................................................  1 3(p,irx) experimentsW. Falk .......................................................  28EXPERIMENTAL FACILITIESThe IUCF/University of Maryland dual spectrometer facilityP. Roos .......................................................  4 1DASS/SASP report of the conceptual designE.G. Auld .....................................................  52The DASS/SASP data acquisition systemG. Ludgate ....................................................  60NUCLEON PHYSICSNuclear reactions with intermediate energy protonsR. Dymarz .....................................................  64(p,2p) scattering in nucleiW.J. McDonald .................................................. 84Neutron knockout measurements with DASSC.A. Miller .................................................... 91(n,p) and (p,n) reactions at TRIUMFs* Yen ........................................................  105Nuclear medium effects in quasi-free (p,2p)M.J. Iqbal .............................................    1 1 7DASS/SASP users list ............................................. 123vOPENING REMARKSP. KitchingTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3Let me begin by welcoming you all to TRIUMF and to the DASS/SASP workshop. I hope that you will find the workshop a useful and productive one, and that those of you who have not visited TRIUMF before have the opportunity to look around the lab and see something of Vancouver.The major purpose of this workshop for us is to stimulate ideas and proposals for the proposed second arm spectrometer which we are hoping to build here (Fig. 1). Let me now say something about how major projects like this are funded at TRIUMF.The total annual operating budget is around 30 million Canadian dollars. The major part of that comes as a contribution from the National Research Council of Canada ($25 million) which covers the opera­tion of the basic TRIUMF facility. Included within this are some funds for building new facilities such as an DASS/SASP project, and I will say a little more about that later. Approximately $2.5-3 million comes from the individual users, university researchers mainly, who apply to NSERC (the Canadian funding agency for university research) to do experiments at TRIUMF. This represents the money they obtain to do particular experiments at TRIUMF. Foreign users bring in around $1 million per year from their countries of origin. There is also about $1 million a year which comes from the four TRIUMF universities, particularly UBC.So that is essentially the source of the funds. Let me now show you how long-term budget planning is developed. We have a rolling Five-Year Plan which is updated every year but which basically goes five years into the future. This identifies how we plan to spend the money. There are three components to that Plan; the first is the basic facility support and is determined essentially by the National Research Council. It amounts to about two-thirds of the $25 million; its purpose is to keep the basic facility running. The other two components of the budget areFig. 1. Overall elevation view of the SASP (left) and MRS (right) spectrometers.2F2VE Y E A R  PJLAS3MAJOR PROJECTSIncrease proton beam intensity factor 3 (unpolarised beam) soo^ a factor 10 (polarised beam)Rebuild secondary channelsincreased meson fluxes (factor 2 -3 ) H - extraction capability necessary for injection into any post accelerator Superconducting muon channel joint project with Japanese Longitudinal polarisationcompletion of present proton facility Second magnetic spectrometer exploits continuous Triumf beam  New detectorpion spectrometer ?? iASSFig. 3. Current list of projects on the Five-Year Plan.experimental support (fixed amount, allowing for inflation) which is to provide the support for experiments, e.g. cryogenic targets, electronics, computing and so on, and facility development support. This latter is the money which is to be spent on building new facilities at TRIUMF, such as new beam lines and spectrometers. This fund is also held fixed at just over $4 million per year.The Five-Year Plan which indicates how money is to be spent is approved by the Treasury Board upon advice first from the National Research Council, which in turn is advised by the Director. Advice to the Director comes from his advisory committees, the Long-Range Planning Committee and Operating Committee, and by his administrators. Figure 2 shows the structure. The NRC Advisory Board on TRIUMF (ABOT) and the TRIUMF Board of Management must approve the Five-Year Plan but do not usually take an active part in generating it. Figure 3 shows new and continuing projects on the current Five-Year Plan. As you can see the second arm spectrometer is on there but with only a small amount of money to be spent in the coming fiscal year (April 1, 1986-April 1 1987). When we sought the advice of the Long-Range Planning Committee last summer, they gave the second arm spectrometer their endorsement and support, but said that it should have lower priority than completing the upgrade of the present MRS spectrometer. We have taken their advice and that is why the longitudinal polarization project, the last piece of the upgrade of the MRS facility, is intended to be completed before the start of major work on the second arm spectrometer. The TRIUMF Operating Committee con­sidered the Five-Year Plan last fall. It was their feeling thatTRIUfFUSERS'Fig. 2. Input structure of the Five-Year Plan.3management should urge the proponents of the second arm spectrometer to present experimental proposals which would use the facility to the next meeting of our Experiments Evaluation Committee (EEC). This procedure would enable a realistic evaluation of the scientific case for the facility to be made. A similar procedure was followed for a number of recent proposals for new facilities, most notably for the ISOL project and the charge exchange facility.This, then, brings us back to the main purpose of this workshop. There is, of course, severe competition for funds at TRIUMF. Without a strong scientific case being made to the EEC the second arm spectrometer project is likely to languish. We need a number of good proposals for the new facility which will be essential. We hope that this workshop will play a major role in stimulating such proposals. The next meeting of the EEC takes place on July 9-11 and the deadline for submitting proposals is June 2.4PION PHYSICS USING COINCIDENCE EXPERIMENTSG.E. Walker*Nuclear Theory Center and Physics Department Indiana University, Bloomington, Indiana 47405ABSTRACTWe briefly review selected inclusive (p,it) experimental results. A two-nucleon model of the (p,it) reaction is discussed. Recent applica­tions of the model indicate it may be useful for interpreting future ex­clusive (p ,p^tt) experiments. We discuss the advantages of exclusive (p.P-*11) studies for investigating the role of the A isobar in intermedi­ate nuclear reactions. Connections between (p,p'ir) studies and other exclusive reactions such as (e,e'ir), (iTjir^ N), and (tt,2tt) are emphasized.I. INTRODUCTIONIn this discussion we stress the utility of combined studies of ex­clusive reactions such as (e,e"ir), (it , tt^ N) and ( tt , 2tt ) along with (PjP^tt), on which we concentrate, for elucidating the role of the A isobar in intermediate energy nuclear reactions. One of the major opportunities af­forded by intense-beam-current facilities dedicated to intermediate ener­gy nuclear physics is to study the modifications of the A resonance in the nuclear many-body environment. While significant progress has been made, for example due to the development and application of isobar- nuclear models, there is still much to learn. Thus while many of the re­actions to be discussed may certainly have implications for quark-nuclear studies or the applications of relativistic quantum field theories, we concentrate on the possible role of the isobar in the reaction descrip­tion. In the next section we review the basic characteristics of the (p,tt) reaction as well as giving an overview of selected (p ,tt) experimen­tal data near threshold. We also briefly review a microscopic two-nucleon model of the (p,ir) reaction. This model includes an intermediate, propa­gating, interacting A and has given reasonable agreement with a recent (p,ir) experiment at TRIUMF. Finally in this section we preview some of the possibilities afforded by exclusive (p,p'ir) studies near 500 MeV. In Sec. Illwe discuss other exclusive pion production reactions such as (e,e'ir), ('p) and (ir,2p) data and discuss, in each case, how (p.p'iv) data can be very helpful in further testing selected isobar mechanisms suggested as being important for each reaction. In the final section we review our main points regarding isobar studies involving theoretical and experimental work associated with exclusive electron, pion, and proton- induced pion production. Some of the material contained in the discussion below is also discussed in Ref. 1.II. REVIEW OF PROTON-INDUCED PION PRODUCTION ON COMPLEX NUCLEIThere is considerable experimental data on the (pjir*) reaction leading to bound or quasi—bound nuclear states in the proton projectile energy region 150 <, T & 800 MeV.2 Both analyzing power and cross section angular distribution *data are available. Proton-induced pion production*Work supported in part by the U.S. National Science Foundation.5results in a large momentum transfer q £ 2kf (~ 550 MeV/c) to the nucleus. The reaction allows study of a process (pion emission) that plays an important role in binding the nucleus. It also has the possibility of providing wave function and/or reaction mechanism information of interest for other high-momentum transfer medium energy reactions such as (p,y) and (e,e^ ir). Presently there does not exist a theoretical approach that has been shown to yield quantitative agreement with the wide range of high quality data available. However, the two-nucleon model of Iqbal and Walker3 has recently been shown to give good agreement with experiment for a stretched transition in 12C(p,ir)13Cg/2+  at T^ 3 *5 = 354 MeV. 11 The situation is complicated by such effects as a) the importance of multi- step processes because of the large momentum transfer to the nucleus — (this may include an important two-nucleon mechanism involving a propa­gating intermediate A with medium corrections), b) relativistic correc­tions, and c) nuclear structure, distorting potential, and vertex form factor uncertainties at high momentum transfer.Experimentally, there has been emphasis on energies below ~250 MeV and on targets of 90Zr or lighter where the density of states is less and distortion effects are relatively reduced. The experimental results to date seem to have considerable lack of systematics (i.e., we have not yet recognized a pattern). In Figs. 1-4 we show some representative data. 5 - 8  In Figs. 1 and 2 a typical excitation function and angular distributions are shown for Tp <> 200 MeV, 1 2C(p,ir+)1 3C. The excitation spectrum is ap­parently composed of "single particle" states [g.s. (lp^), 3.09 MeV (2si/2), and 3.85 MeV C^ 5/2 )  ^ an<* two-particle one-hole (2p-lh) states [such as the 6 . 8 6 MeV 5/2"^  and 9.5 MeV 9/2+ states]. Figure 1 illustrates that 2p-lh states can be as strongly excited as single particle states (depending on the nucleus). In the A resonance region, the reaction may be more selective, emphasizing 2p-lh stretched states. 11 Figure 2 includes several angular distributions exhibiting dips near 90° (perhaps associ­ated with a p wave, cos 0 dependence). The angular distributions for some single particle and 2p-lh states have the dip structure. The angu­lar distribution associated with the analyzing power sometimes exhibits considerable energy dependence, as shown in Fig. 3.The (p,ir) reaction mechanism may, in fact, be several competing single nucleon and two-nucleon mechanisms. The (p,ir-) reaction is be­lieved to result essentially from a two-nucleon mechanism. Arguments based on a two-nucleon model of the (p,ir-) reaction have resulted in correct predictions of the j dependence of the relative sign of the analyzing power. 8 Recent studies of the (p,w-) reaction indicate a selectivity presumably associated with the excitation of high spin 2p-lh states (see Fig. 4) . 9 This is consistent with a two-nucleon mechanism for a large momentum transfer process. This selectivity has been used to tentatively identify states reached via (p,ir-) in heavier nuclei. 10The first attempts to model the (p ,tt ) reaction involved a DWBA single nucleon stripping model. 2 The fact that there is only one active nucleon in the model and the process requires evaluation of the final bound nucleon wave function at very high momentum transfer means there is considerable sensitivity to wave function and optical potential param­eters. The observed strong excitation of 2p-lh states in the (p.ir1) reaction and an understanding of the (p,ir-) data are not naturally in­cluded in this model because such processes naturally involve two active nucleons. In addition it is known that single pion rescattering with an6Fig. 1. An excitation spectrum for the 12C(p>ir"*’)1^C reaction with 200 MeV incident protons showing the strong excitation both of states assumed to be of a single particle and two- particle hole nature. Figure taken from Ref. 5.Fig. 2. Selected angular distributions for the 12C(p,ir+)13C reaction for several incident proton energies. Figure taken from Ref. 6.70.20- 0.2-0.4- 0.6- 0.8-1.0■s1.>s< 0 - 0.2 -0.4 - 0.6 - 0.8 0.8 0.6 0.40.2 0 - 0.2 -0.4 - 0.6 - 0.8 -1.0Tp * 200 MeVSTp« 225 MeV| Tp*250 MeV-J  » I  i 10* 40* 80* 120* 160*°TT LABFig. 3. Thefor the reaction ) '■'g.200-250 MeV incident protons. Figure taken from Ref. 7.Excitation Energy (MeV)Fig. 4. An example of (p ,tt+) and (p.x-) showing the greater selec­tivity of the (p,ir“) reaction for the relative excitation of diffe­rent states in the same final nucleus. Figure taken from Ref. dependence of Ay(0) : 12C(p,TT+ 13Cg s> forintermediate A formation is an important ingredient of theories that fit the two-body NN *■ dir+ reaction. Some of this effect can be included in an average way in the many-body environment via pion distortions. One more recent example of a single nucleon mechanism is the relativistic calculation of Cooper and Sherif.11 It may be useful to supplement the two-nucleon mechanism discussed below with a plane wave (to avoid pos­sible double counting) single nucleon mechanism. Two-nucleon mechanism models incorporating Dirac phenomenology seem an especially attractive avenue for future theoretical research.In the following we briefly summarize the approach we have taken in developing a microscopic two-nucleon model (TNM).3 We show in Fig. 58*•A(Z,N)(A )<■>ICIID)JOCb |< 3T3 hOFig. 5. Some diagrams appearing in a two-nucleon mechanism of pion pro­duction. Diagram A (B) is referred to as a target (projectile) emis­sion contribution resulting in in­termediate A production. Diagrams C and D are nonresonant contributions.Fig. 6. The differential cross- section contribution arising from projectile and target emission diagrams assuming harmonic oscil­lator (HO) or Saxon-Woods (SW) single nucleon bound orbitals. Full proton and pion distortions have been included in this TNM calculation. Figure taken from Ref. 3.some typical (resonant and nonresonant) diagrams associated with the TNM under discussion. We utilize a TNM incorporating an intermediate propa­gating and interacting delta, virtual pion and rho exchange, and includ­ing the effects of realistic external proton and pion distortions. The details of the model, formulae, and calculational procedure are given in Ref. 3. The results for a study of the 12C(p,ir+)13C(g.s.) transition (Tp = 250 MeV) indicate that shapes of angular distributions are not qualitatively changed by modest variations in the A-nucleus optical potential, proton and pion optical potentials, and the choice of single particle orbitals (harmonic oscillator or Saxon-Woods). This relative in­sensitivity means that one can now carry out studies hopefully leading to a better understanding of the reaction mechanism(s) involved in pion pro­duction. Calculations to date have shown that it is important to allow for the propagation of the intermediate A and to include the energy transfer component in the intermediate meson propagator.3 The projectile emission piece (for the transition studied, see Fig. 6) dominates over the target emission term for the inclusive reaction. The exclusive (PjP^11) reaction, where the momentum transfer to the target can be on the order of the Fermi momentum, may have kinematic regions where the targetl2C (p,7r+ ) lsC(g.s.)1. PROJECTILE EMISSION (HO)2. PROJECTILE EMISSION (WS)3. TARGET EMISSION (HO)4. TARGET EMISSION (WS)Tp = 2 5 0  MeVPION DW PARAMETERS~ b0= ( - 0 .4 3 ,0 .5 2 ) -  .= (7 .11 ,3 .32 ) -9emission diagram is relatively important. Studies of the predicted relative importance of the target and projectile emission diagrams for various geometries is an important future area of theoretical investiga­tion for the exclusive (p,p'ir) reaction. This is of significance for comparative studies of (e,e^ ir) and (p,p'ir) because it is the projectile emission "isobar" term in (p,p"ir) that is similar to important intermedi­ate A contributions to (e.e'ir), (tt , ir^ p) and ( it , 2 tt) .The exclusive (p.p'tf) reaction allows an exciting extension of the earlier (p,tt) studies. By varying the energy and angle of detection of the final coincident particles, a broad range of momentum transfer to the nucleus can occur (from |q| ~ Fermi momentum to the GeV/c region). By carrying out broad continuum studies one can minimize the effect of nuc­lear structure uncertainties. By utilizing Dalitz plots one may be able to isolate the important isobar physics in the exclusive reaction. In this connection experimental studies in kinematic regions where isobar diagrams are predicted to dominate in (p,p'ir) and (p,2ir) along with iso­bar contributions in (e.e'-rr), ('p) and (tt,2tt) should provide useful insights into modifications of the isobar in the many-body environment (as well as the validity or fertility of reaction mechanism models which focus on isobar-hole diagrams).In the next section we discuss other reactions closely related to the (p,p'ir) reaction with a focus on using intermediate energy nuclear physics to study isobar formation and propagation in the nuclear environment.III. RELATION OF (p.p'ir) TO OTHER MESON PRODUCTION COINCIDENCE REACTIONSPion production using electromagnetic probes should involve an im­portant contribution from an intermediate isobar term when the energy transferred to the hadronic system is approximately M^ - Mjg. Thus inter­mediate energy (y ,tt) or (e,e'if) reactions are potentially important sources of information on A properties and propagation in nuclei. Tiator and Drechsel12 have made a theoretical study of (e,e^ ir) including the isobar diagram shown in Fig. 7 in addition to the Born terms. They findFig. 7. An isobar contribution to the (e,e'ir) reaction.that those response structure function contributions to the cross section containing a transverse piece can be appreciably altered by the A "ex­change current” term. For example they find typical enhancements of fac­tors of two in the (e.C'tr) cross section for 200 < toe < 400 MeV when the isobar terms are included. Although there is insufficient experimental data to test the predictions it is encouraging that there is satisfactory10agreement between the predictions in Ref. 12 and experiment for the reac­tion 3He(e,3H)e"i7+. It is possible to arrange the experimental geometry so that the same three-momentum magnitude and energy is carried by the virtual boson [meson for (p,p'i7), photon for (e,e"i7)] leading to inter­mediate isobar formation for the projectile emission term in (p,p"i7) and the isobar term in (e,e"i7). An example is Te ~ T„ ~ 500 MeV with projec­tile energy loss of ~200 MeV and ©scat „ 1 / 5  gsc t ^ io° . Detailed com­parative studies of the (e,e"i7) and (p,p"i7) reactions in the continuum region where nuclear details are less important should be useful for A formation and propagation studies.The (i7,i7"p) reaction has already been useful in detailed studies of nuclear isobars. The simple application of the distorted wave impulse approximation for studies of quasi-elastic (17,17") has already been shown to be inadequate. There exists considerable theoretical research on the changes in the uN interaction due to the nuclear medium. Of special im­portance is the development of the isobar-hole model.13 Thies11* has shown, using the isobar-hole model, that below the resonance medium effects significantly reduce the i7N interaction in the medium compared to the free space rN t matrix. Above the resonance the situation is re­versed. The isobar-hole model, including these effects, yields much better agreement with quasi-elastic scattering data15 than prediction using the free space i7N interaction.The exclusive (i7+,i7+p) and (i7-,i7-p) reactions have been useful in demonstrating the limits of the first-order isobar-models-15 *17 Large deviations from the predicted ratio of (i7+,i7"*’p) are observed in the smallangle cross-section data for p-shell nucleon removal in 160(17,17"p)15N.Significant enhancements (reductions) are required to fit the ratios near (away) from the maximum of the 17"^ cross-section data. There may be an additional term that interferes with the diagram included in the lowest-order isobar-hole calculation.17’13 One possible term, see Fig. 8(a,b), involves a second-order process where the intermediate Ainteracts with another nucleon resulting in that nucleon's ejection from the nuc­leus. Note that for a contribution suchv /as Fig. 8 (a) the final coincidence 17 and p do not both come the intermediate A . In addition to studying the contribution of this term in (i7±,i7±"p), we suggest that in reactions such as (p,i7-n) it might be a dominant contributor [note one- and two-nucleon mechanisms cannot contribute to the (p,i7-n) reaction (see Fig. 8(c))].It would be useful both for isobar stud­ies and to study the general importance of three-nucleon mechanisms to investi­gate theoretically and experimentally the (p,i7_n) interaction.P(N.)Fig. 8. (a) A second-order contributionto (17,17 *N); (b) A lowest-order contribu­tion to (i7,i7"N); ( c )  A similar contribu­tion to (a) that should be important in( p , i 7 ~ n )  .11s^ l *2,'The exclusive (tt , 2tt ) reaction has already been the subject of sev­eral theoretical investigations.19-23 One can expect experimental infor- mation on this reaction to substantially increase in the near future. An early DWIA investigation20 stressed the sensitivity of (ir,2ir) cross sec­tions to the nuclear spin-isospin Migdal parameter %'. A more detailed treatment21 of the elementary ttN irirN amplitude results in a substantial increase (~4 times) for the total (tt,2tt) cross section on free protons compared to that found in Ref. 20. For our purposes it is important to note that there exists an additional contribution to the (tt ,2tt ) amplitude in nuclei. This contribution involves pion absorption on one nucleon leading to an intermediate A which by a sub- a)sequent AN + AA interaction leads to double A formation.22 The two observed pions in (tt , 2tt ) could be produced in this mechanism via a 2A2N 4N2tt or 2A -2N2tt mechanism. It has been predic­ted23 that the ratio R = (o(ir-,ir+ir-)/ a(ir-,ir-TT-)) is decreased by more than a factor of two, in the energy region T^  ~ 300-400 MeV, by the inclusion of the AA mechanism. It appears that this reaction is sensitive to a variety of interesting effects associated with pions29 and virtual intermediate A's propagating in the nuclear medium. From our perspective an interesting feature is the connection between, as an exam­ple, the (tt ,2tt ) contribution shown in Fig. 9(a) and the (p ,2tt) diagram given in Fig. 9(c). Note that for the (p,ir-ir-) reaction Fig. 9(b) cannot contribute.Note also that there is an interesting connection between (p,2x) and (e,2ir).Fig. 9. (a) A double isobar con­tributor to ( tt , 2tt ) . (b),(c) Con­tributions to (p,2n).IV. SUMMARYThe proton-induced meson production coincidence experiments (p,p'ir) and (p,2ir) have a close connection with coincidence experiments utilizing electron and pion projectiles such as (e,e'ir), (e,2Tr), (it,tt'N), and(tt ,2tt). In this talk we have drawn selected diagrams to make this con­nection manifest. We have stressed the utility of combined coincidence studies in studying the role of isobars in intermediate nuclear reac­tions. There is the possibility that using the Dalitz plot technique one can determine whether a final coincidence nucleon and pion originated from a virtual isobar. Ths first job for experiment and theory is to determine whether, in fact, an isobar dominated two-nucleon mechanism is an adequate representation of the (p,p'fr) process. In this talk we brief­ly reviewed a microscopic two-nucleon model that has had some recent suc­cess in describing the inclusive (p,ir) reaction near the resonance region. It is important to use models of this type supplemented with existing information from the isobar-hole model to theoretically study the (p,p'*Tr) reaction so that some guidance can be provided for experimentalists12planning concidence experiments. Our theoretical group at Indiana is in the process of developing the tools to make such initial theoretical (PjP'tt) studies.REFERENCES1. G.E. Walker, "Nuclear Physics Investigations using Medium Energy Coincidence Experiments". Lectures given at the NATO Advanced Study Institute on New Vistas in Electro-Nuclear Physics, Aug. 22-Sept. 4, 1985, Banff, Alberta, Canada (Institute Proceedings to be published by Plenum Press).2. Some recent reviews of the experimental and theoretical situation in­clude D.F. Measday and G.A. Miller, Annu. Rev. Nucl. Part. Sci. 29^,121 (1979); B. Hoistad, Adv. Nucl. Phys. 1^ , 135 (1979); H.W. Fearing, Prog. Part. Nucl. Phys. 113 (1981), Pion Production and Absorption in Nuclei - 1981, AIP Conf. Proc. No. 79, edited by R.D. Bent (AIP,New York, 1981); G.E. Walker, Comments Nucl. Part. Phys. All. 155 (1983).3. M.J. Iqbal and G.E. Walker, Phys. Rev. C 32_, 556 (1985).4. G.M. Huber et al., submitted to Phys. Rev. Lett., March 1986.5. F. Soga et al., Phys. Rev. C 22^, 1348 (1980).6. F. Soga £t al., Phys. Rev. C 24, 570 (1981).7. G.J. Lolos £t^  al., Phys. Rev. C 25, 1086 (1982).8. W.W. Jacobs et al., Phys. Rev. Lett. 49, 855 (1982).9. S.E. Vigdor jst^ al., Nucl. Phys. A396, 61c (1983).10. M.C. Green ££ al •» Phys. Rev. Lett. 53, 1893 (1984).11. E.D. Cooper and H.S. Sherif, Phys. Rev. C 26^, 3024 (1982).12. L. Tiator and D. Drechsel, Nucl. Phys. A360, 208 (1981).13. M. Hirata, F. Lenz and K. Yazaki, Ann. Phys. 108, 116 (1977);M. Hirata et ail., Ann. Phys. 120, 205 (1979);Y. Horikawa, M. Thies and F. Lenz, Nucl. Phys. A345, 386 (1980)F. Lenz, Spin Excitations in Nuclei, eds. F. Petrovitch et al.(Plenum, New York, 1984), p. 267.14. M. Thies, Nucl. Phys. A382, 434 (1982).15. M. Baumgartner et al., Phys. Lett. 112B, 35 (1982);C.H.Q. Ingram et al., Phys. Rev. C 27, 1578 (1983).16. E. Piasetzky et al., Phys. Rev. C 25_, 2687 (1982).17. G.S. Kyle £t_ al., Phys. Rev. Lett. 52, 974 (1984).18. M. Hirata, F. Lenz, and M. Thies, Phys. Rev. C 28^, 785 (1983).19. R. Rockmore, Phys. Rev. C 1A, 1953 (1975).20. J. Cohen and J.M. Eisenberg, Nucl. Phys. A395, 389 (1983).21. R.S. Bhalerao et al., Phys. Rev. C 30, 224 (1984).22. G.E. Brown, Phys. Lett. 118B, 39 (1982);B. Schwesinger et al., Phys. Lett. 132B, 269 (1983).23. L.C. Liu (private communication).24. E. Oset and M.J. Vicente-Vacas, Univ. of Illinois preprint 170(1985).13PROSPECTS FOR (p ,tt) PHYSICS IN THE A REGION USING THE SASP SPECTROMETERP.L. WaldenTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3ABSTRACTThe present experimental status of the (p ,it) reaction is reviewed and interpreted within the framework of a NN + NNir process. Reasons are given as to why (p,ff) measurements in the A region (200 MeV < Tp < 500 MeV) would yield new insights into the reaction mechanism. Severalinitial (p,ir) experiments using the SASP spectrometer are suggested.1. PRESENT STATUS OF ( p , 0One of the initial experiments run on the QQSP spectrometer at IUCFwas a survey of the (p,ir_) reaction with various nuclear targets at 200 MeV. The results of this survey revealed the previously unsuspected systematic of the final state nucleus preferring to be in a high-spin two-particle one-hole excitation with respect to the initial state nuc­leus.1 Partial results of this survey are shown in Fig. 1. The feature to note is the concentration of the (p,ir~) strength into one or a few discrete levels near 3-7 MeV excitation. This concentration appears to be strongest for targets where a neutron subshell has just been filled and the corresponding p subshell is empty. The clue that these states are stretched two-particle one-hole high-spin states came from the0 (p.TT- ) N# E„*205MeVExcitation Energy (MeV)Fig. 1. Energy spectra from the (p,ir_) reaction on several targets show­ing the strong selectivity described in the text. The spectra were taken on the QQSP at IUCF at 9lab ~ 30°.14Fig. 2. A graphic illustration of an A(p,i:)A+l process in which the subprocess NN NNir dominates. The directions of the initial nucleon i and the final state nucleons f are shown for minimum momentum trans­fer. The it is produced on the near side of the nucleus due to absorp­tion.180(p,ir-)1 Ne reaction where the resultant state at 4.6 MeV was identi­fied with a 2p-lh level of Jp = (13/2)—.This survey illustrates two basic merits of doing such experiments. This is especially true of surveys which probe new physics with new instruments. First, there is not a great need to justify the experiment beforehand with detailed predictions from sophisticated theories. The predictions would probably be wrong anyway. Second, the discoveries re­vealed in the survey are usually unexpected and usually shed light towards the correct formulation of the theory.In the case of the QQSP survey, the light pointed to a possible NN -»• NNtt dominance being the driving mechanism behind the (p,n) reaction. This dominance would explain in a natural way the preference for (p,ir~) to go into 2p-lh final states, and the large momentum transfers associ­ated with the (p ,tt) reaction would also explain a preference for final high-spin configurations.This process was best described by Vigdor2 using an illustration similar to the one shown in Fig. 2. It is assumed that the pion is pro­duced on the near side of the nucleus with respect to a pion detector because of the preferential absorption on it's produced on the far side of the nucleus. This means that pion production is a peripheral process associated with an impact parameter "b”. In order to share the momentum transfer equally between the three nucleons involved the initial state nucleon must be headed towards the beam proton with momentum "?i", and the two final state nucleons must be headed along the direction of the beam proton with momentum ”?f". With this directional constraint all three nucleons can have momenta which are close to the fermi level. Notice that for the (p,ir) exclusive reaction the it has the highest possible energy which then constrains the relative momentum between the final state nucleons to be minimal. The angular momentum transfer is given by the equation:AJ =  | b x ( p f - p ± )| +  |Sf - ^ ± | ( 1 )where "S^" and "Sf" are the initial and final total spin projections of the reacting nucleons normal to the reaction plane. Note that for maximal angular momentum transfer "5^" should be aligned with the initial orbital angular momentum "^xP^" (i.e., a reaction via j> subshell is preferred).Thus it should be well noted that the kinematics of the (p,u) reaction, if NN ->■ NNtt dominance is envoked, severely constrains the directionality15of the Interacting nucleons, and peripherality along with large momentum transfer impart a large angular momentum kick to the final state nucleus.It is proposed that NN •* NNtt dominance also plays a major role in(p»ir+)» but this process is masked by the many reaction routes availablefor (p ,tt+) to single-particle excitations, so that the dominance of the 2p-lh high-spin states over other transitions is not apparent. This is easily seen by the fact that (p ,tt+) can proceed via pp ■* pnir+, or pn > nmr+, both of which can have the initial nucleon hole filled by a final state nucleon. Thus the initial nucleon orbital is not unique, and all orbitals can contribute to a single particle excitation. On the other hand 2p-lh transitions because of the creation of a hole can have only one initial state orbital contributing. It is to be noted that (p,ir-) is forced to create a hole because this reaction can only proceed via pn + ppir" in which the final state protons cannot fill up the neutron hole. If NN NNtt dominance is the reaction mechanism for (p,ir), then (p ,tt-) has some clear advantages over (p ,tt+) in gaining understanding about the reaction mechanism itself.There are several substantiating pieces of evidence to back up this picture of NN NNtt dominance, all of which come from the results usingthe QQSP. The first piece of evidencecomes from a theoretical model of Brown et al.3 They essentially predicted the relative strengths of the final state transitions from (p,ir~) on (vlf?/2) subshell targets. The model used is basically simple, a plane wave zero-range reaction model combining 2-proton stripping with 1-neutron pickup (the basic ingredients of the NN NNtt process) . The success of this model is shown in Fig. 3, where the relative strengths of predicted final states are shown against the experimental evidence. The cases shown410Theory d2o-/dftdE Relative Cross SectionExperiment d <r/dfldE in units of nb/sr-M eVV  =r - A r i , jCq(p, 7T " )  TiC a (p ,7 r_ ) Ti303030305 4 330302 1 0  5 4 3 2Excitation Energy (MeV)have varying amounts of (iTlf7/2) and (vlf7/2 ) subshell fullness. For the results desired, the model succeeds remarkably well.Fig. 3. Comparison of the experimental ( p , i T _ ) cross sections with the calcu­lated relative cross sections of Brown et al. for Tp = 206 MeV and 0lab = 30°.A simple-minded corollary to Brown et al. would have the relative dominant 2p-lh strengths on different nuclear targets with the same interacting nuclear shell to be in the ratio of16[Fv(1) • Eir(l)]:[Fv(2) • (2)] , (2)where Fv is the number of neutrons in the subshell, E^ is the number ofproton holes in the subshell, and 1 and 2 refer to the two different tar­gets. For example in Fig. 1, the relative strength of 1+2Ca(p,iT-)lt3Ti vs.4 C^aCpjir-)1*9!! from (2) should be 1:4, and of 89Y(p,ir-) 98Nb vs.90Zr(p,ir-)91Mo should be 1:1. This is very close to being quantitativelycorrect.The second piece of evidence comes from the study of the analyzing powers of the 12 ’13 * 14C(p ,ir“)13 ’14 50(g. s. ) reactions.1* Here the finaltwo protons from pn ■* ppir- are stuck into and fill the (irlp1/2) subshell for all of the reactions. Thus it can be argued on the basis of fermionstatistics that the final two protons are in a relative ^  state. Asimple analysis of pn -*■ ppir" with this final state restriction requiresthe initial pn state to be in a spin triplet. The neutron subshellsaffected in these reactions are the lp3/2 subshell in the case of 12C andthe I-P1 / 2 subshell in the case of 13C and 1LfC. Referring^back to Fig. 2, a lp3/2 target would require to be pointing out of the _page whereas a lp1/2 target would require to be pointing into the page. In otherwords the nuclear target is effectivelypolarized. Coupling this fact with the requirement of an initial NN triplet state would predict that the analyzing powers between the 12C and 13>1I+C targets be equal and opposite. This simple prediction is dra­matically substantiated by the experimental result shown in Fig. 4.Fig. 4. The analysing powers and cross sec­tions for the 12 ’13 ,11+C(p jir-)13 *llf 50 reac­tions. Note the polarity reversal mentioned in the text.Vigdor2 has argued that the preceding result can only be understood in the light of a NN + NNir hypothesis. However, even though a polarity reversal is predicted, a strict application of the visual argument using Fig. 2 would require a reversal of the observed analyzing power signs shown in Fig. 4 (i.e. Ano for 12C ( p , O 130 should be positive and viceversa). This minor problem appears to be more substantial than a break­down of a simple visual model. A sophisticated two-nucleon code for (p,it ) reactions by Bent, Dillig, and Conte5 "ORCHID" predicts analyzing powers with the wrong signs as well. This failure to get the signs right is disconcerting especially in the light of other substantiating NN NNir evidence. It would be gratifying to have an explanation.This failure is all the more puzzling because another visual model by Toki and Rubo,6 which is a third piece of evidence supporting a17Fig. 5. Analyzing powers for (p.u-) into stretched 2p-lh high- spin states. The solid line is a prediction from Ref. 6 based on a pn + pp^Sy)^- subprocess.NN ■> NNir hypothesis, gets the sign correct. Analyzing powers from (p,ir~) to stretched 2p-lh high-spin states7 shown in Fig. 5 show a generally positive sign pattern which is remarkably insensitive to the target mass. Toki and Kubo have argued, as was pointed out previously in describing Fig. 2, that the kinematics of (p ,tt) and the NN NNtt hypothesis leave very little relative kinetic energy between the final state nucleons.This would favour the final state nucleons being in a relative 1 SQ state. Notice that this is the exact same requirement used to predict theAno siSn change for (p ,ir —) on C targets going to 0 ground states.The analysis is therefore identical. For the example shown in Fig. 5, all targets involve a neutron from an initial j> subshell which requires in Fig. 2 to be aligned with the orbital angular momentum, (i.e.,S^  points out of the page). The requirement of an initial triplet NN state requires the spin of the beam particle to be aligned with as well. This gives a positive Aq q . Toki and Kudo's result is plotted as the solid line in Fig. 5. The curve is generated simply by the pref­erential absorption of it's from different sides of the nucleus as a func­tion of angle. The result is qualitatively correct. However, thissuccess has been tempered somewhat by Vigdor8 who pointed out the sign reversal problem described above using the same model.II. PRESENT STATUS OF (p,ir+)In spite of a small problem of a sign reversal in one particular case, the qualitative evidence supporting NN -»• NNn dominance in (p,ir-) reactions is nevertheless impressive. However, a successful sophisti­cated theory of the (p,ir) reaction does not yet exist. There are two new18explicit 2-nucleon codes9’10 and a revamped 1-nucleon code using a delta-hole model11 now in existence. None of these codes have yet been adequately tested as they all require the simplification of delta excita­tion to produce it's which should be the dominating tt production process in the 200 MeV < Tp < 500 MeV energy range. Unfortunately not much data presently exist in this energy regime. It turns out that when data from this regime are finally forthcoming in sufficient quantity it will probably be better to compare the theories to (p,ir+) data in spite of the advantages inherent in the (p,ir-) reaction.The disadvantages of using (p ,tt+) to test reaction mechanisms com­pared to (p ,tt“) has perhaps been overstated. A look at an excitation spectrum of 12C(p,ir+)13C at 354 MeV (Fig. 6a)12 reveals a dominant stretched 2p-lh high-spin 9/2+ state at 9.5 MeV. Similar pictures exist for two other (p ,tt"*") reactions; 13C(p ,ir~*~) 19C at 354 MeV (Fig. 6b)13 reveals a 5~ state at 14.87 MeV,19 and 160(p ,tt"*") 170 at 800 MeV13 reveals a 11/2” state at 7.75 MeV. Figures 6a and 6b are data from TRIUMF's MRS spectrometer and Fig. 6 is from the HRS at LAMPF. The point to notice about these figures is that the appearance of the spectra is similar to those of (p,r-) at lower energies.It seems that as the energy increases for fixed angle or equivalent­ly as the momentum transfer increases, the (p ,tt+) spectra take on theappearance of the (p,ir_) spectra observed at 200 MeV. In fact ananalysis16 of the (p ,tt+) reaction in the region of 200 MeV < Tp < 260 MeV using the "Resolution" spectrometer data16 from TRIUMF reveals a global tendency for the differential cross section to have a simplestatistical 2J+1 weighting for all states. This means that the reaction mechanism drives the final state into the state of highest possible angular momentum. These states just happen to be the stretched 2p-lh high-spin states mentioned previously.The similarity of the (p,ir+) and (p,ir~) spectra warrants closer examination. The creation of a 2p-lh state via (p,ir+) can uniquely determine the initial nucleon subshell just as well as (p,ir ). The prob­lem with (p,ir+) is that it can go via two NN +■ NNtt processes to (p,ir-)'s one (pp pmr+ and pn -*■ pmr+ to pn pprr-). However, if one assumes that the NN -*■ NNtt process is dominated by intermediate & formation [NN AN NNtt ] for Tp > 200 MeV, the number of reactions can be effectivelyreduced to one, the pp pmr+ reaction. In fact this assumption further requires the final pn state to be in a relative T=0 3S1 state. This state has the same quantum numbers as the deuteron. Thus the (p,ir+) reaction can be just as specific as the (p ,tt_) reaction, and furthermore it has a fundamental pp dir+ reaction which is easy to measure experimentally.To demonstrate this above claim we must again use the fact that theA(p,ir)A+l reaction and NN -*■ NNtt dominance leave very little relativekinetic energy between the final state nuclear nucleons. Thus it is most probable that the final state nucleons are in a relative S-state, and this is what will be assumed. Next we consider the simplest intermediate AN state that can be created from an initial NN state, that of a AN in a relative S-wave. This state can have T=2, or 1 with Jp = 2"*", or 1"*”. It is impossible to reach T=2 from NN, so that the intermediate A amplitudes must have T=l. A Jp = 1+ state can only be constructed from 3S1 whichis disallowed for pp by the Pauli principle. Furthermore the extendedPauli principle requires a 3S1 NN state to have T=0 which cannot couple to an intermediate AN. Hence a AN S state can only couple to a T=1,19EXCITATION ENERGY (MeV)Fig. 6. Samples of A(p,ir+)A+1 spectra showing the dominance of 2p-lh high-spin states. (a) 12C(p,tt+)13C (TRIUMF); (b) 13C(p,ir+)lltC (TRIUMF); (c) 160(p,ir+)170 (LAMPF).20JP = 2+ initial NN amplitude. Note that this amplitude has natural parity. If the final state nucleons are in a T=l, state, only theunnatural parity amplitudes of 0-, 1+, 2“, ... are accessible to thefinal tt(N N )  state because of the negative intrinsic parity of the tt. Thus a T=1 final NN state cannot couple to a 2+ AN S state. The only NN ■* NN tt transition allowed under the above assumptions is a pp + pmr+ reaction with the initial pp state in a ^ 2  singlet and the final pn in a T=0 3S1 triplet.The analysis of the preceding paragraph is a restatement of the well-known fact that the 1D2[p-p] + 3P j [ ir-pn( 3S j) ] amplitude is by far the dominant amplitude for pp -*■ dir"*" in the delta region. ^  It should be noted here that not only does the AN S-state amplitude, the onlycouple to pp pn(3S1)ir+ but also the dominant AN p-state amplitude, the 3F3, only couple to pp pn(3S1)ir+ as well. The 3F3 also has natural spin parity and thus cannot couple to T=1 final NN states. Hence the strongest A effects to be found in ( p , tt)  above Tp = 200 MeV should be found in ( p , tt+ ) ,  not (*Fig. 7. Cross sections and analysing powers of the pp -*■ pnir+ reaction at 450 MeV as a function of the pion kinetic energy. All quantities are in the laboratory frame. Vertical arrows indicate the pion threshold ener­gies for the pp -*■ pmr+ reaction. The solid curves are the analysing powers for the pp -*■ dir+ reaction at the equivalent free energy Ep and 4-momentum transfer t. Experimental values of the pp -*■ dir+ reaction at the corresponding angles are indicated by filled squares.There is some experimental evidence to back up the above analysis.Figure 7 (Ref. 18) shows the analysing power "Ano" of the pp -► pmr+ reaction. It is interesting to note that the Ano predicted from pp ■* dir+ data19*20 (solid line) matches that of the 3-body final state upto, in one case, 40 MeV excitation of the pn system. This would suggestthat the final state pn has a strong preference to be in a 3S j state. This result is in agreement with the assumption that a relative S state dominates the final NN pair in ( p , tt) .Another piece of evidence to support this S state dominance comes from measuring the Ano of the 12C(p ,tt+)X reaction.21 The results are shown in Fig. 8. The predictions (solid line) are based on a pp >21Fig. 8. Cross sections and analysing powers of the 12C(p,ir+)X reaction at 450 MeV. All quantities given are in the laboratory frame. The solid curves are the prediction from a plane wave 12C(p, ir+pn( 3S j))1*B model with a pp + tt+(3S1) subprocess. The calculated cross sections were multiplied by the factor indicated on each curve prior to knock-out model for the reaction mechanism using measured pp -»■ dir"1" data19*20 as input. The model is very naive and even uses plane waves. At small tt angles where the use of plane waves should have the least effect the predicted Ano's are in excellent agreement with the data. The shape of the cross section seems to be correctly calculated as well. At larger it angles the data fall below the prediction, but this could be the effect of using plane waves. Distorted waves would reduce the effective it angle and hence predict a more negative Ano. The results in Fig. 10 not only support the conjecture of S state dominance but also of NN NNtt dominance of (p,ir) as well.A similar experimental result to Ref. 21 has been gathered from 13C(p ,tt+)1‘»C data22 at Tp = 200 MeV. The Ano has been plotted(Fig. 9) for 1‘*C(Ex~20 MeV) which is in the continuum. The data show a strong similarity to the pp -»• dtr+ Ano at an equivalent energy.Figure 9 also shows the Ano for 13C(p, tt-) ll+0(Ex~20 MeV) as well. The conjecture, based on the (p,tt+) example, is that this Ano may reflect the Ano for the underlying pn + pp(1Sq)tt_ process.There is one more piece of supportive evidence which will bring us back to Fig. 2 and stretched 2p-lh high-spin states again. For the Ano of 12C(p,ir+) 13C(9.5 MeV,9/2^ ") the data23 show between 216 MeV < Tp < 250 MeV the behaviour plotted in Fig. 10. For comparison the pp dTT+ Ano19>20 at an equivalent energy and identical 4-momentum transfer"t" are plotted as well. Note that the position of the peaks, dips andcross-overs between the fundamental NN -*■ NNtt process and the nuclear reaction are identical, but in the nuclear case the peaks and dips areenhanced. This feature could possibly have a simple explanation similar to the one given by Toki and Kubo6 and in Ref. 4. The 13C(9.5 MeV,9/2+) has a stretched configuration of [irp3/2)~ ^ irp j/2) 1 ] (2+) ( vd5/2) 1 so that220.2-0.4-0.6°c {?>-) “o Tp *200 MeV o <E,> -19 MeV-6^ j5> jrT l3C (? ,r * )“ c Tp . 20<^MeVXX  • <E_> “  20 MevX »T ■"I i . ‘I ■ J pp-*^ V+30“ 60° 90° 120° 150°Fig. 9. Continuum analysing power distributions for 13C(p,ir^ ) and 13C(p,ir-). The Ajjq distribution for near-threshold pp-*-dir+, after tranformation to the nucleon- nucleus frame is shown for com­parison.oz<1.0 0.6 0.2 - 0.2 - 0.6 -  1.0 -  0.6 -  0.2 -_ l  , j ,----------------216 MeV pp— d ir*  401 MeVEF = 52 MeV•  250 MeV  pp— 6 tt* 4 7 0  MeVEF = 83 MeV- 0.2- 0.690  180®cmFig. 10. The analysing power of 12C(p,ir+) 13C to the stretched 2p-lh state at 9.5 MeV and = 9/2”*". The changes in Ano between beam energies 216 and 250 MeV are shown. The pp dir+ An0 at the same equivalent free energy Ep and identical 4-momentum transfer are shown for comparison.the target proton comes from the P3/2 shell which requires to point out of the page (see Fig. 2). Since the intermediate A amplitude goes mostly via 1D2, a singlet, the incoming beam particle is required to have its spin pointing in the opposite direction which is into the page. The predicted Ano can then be as much as -1 which is close to what isobserved at Tp < 216 MeV. Above this energy the 3F3 and other partial amplitudes start to play a significant role in pp -*■ dir+ so that the pre­dicted result is harder to ascertain without requiring a detailed calcu­lation. However, the general features of the pp -► dir+ Ano are stillapparent at 250 MeV and it would be interesting to follow the energy behaviour of Ano to higher energies to see if this trend is followed.If the above analysis holds correct, then the enhancement observed in the An0 of (p,n) is a detection of a spin-orbit coupling effect. Such an effect has been detected before in (p,2p) reactions21* where thetwo final protons have been kinematically selected just to produce akinematical configuration similar to the one shown in Fig. 2. However, the CP,7T) reaction produces such a configuration naturally.In summary, then, the (p,T+) reaction can prove to be just as effec­tive a probe for studying the reaction mechanism as the (p,T-) reaction.23III. FUTURE PROSPECTS FOR (p, it)It should be clear by now that in the light of the NN NNtt hypothe­sis the (p, tt) is a strongly directional and spin sensitive reaction. Hence the analysing power, Ano, can play a significant role in the interpretation of the data. For future progress in this field, spin sen­sitive measurements like An0 should always be measured, if possible, in addition to the usual cross section.Also progress in this field will come where results can be compared to theory. As it was stated in Sec. II, these theories9*1°»11 are expected to be most valid in the delta region 200 MeV < Tp < 500 MeV. There is a need for such data as not much currently exists and for reasons discussed in Sec. II, it should be (p,ir+) data. Figure 11 show­ing preliminary data from TRIUMF13 indicates as it was conjectured that (p,TT-) will be weak in the delta region. The (p ,tt+) reactions for q = 600 MeV/c momentum transfer show clearly a peak at the expected delta resonance, ~350 MeV, whereas the (p,r-) reactions show a falling cross section. What little data exist at 350 MeV12 compares favourably to the theory of Iqbal and Walker.9 Figure 12 shows the cross section for theTp (MeV)Fig. 11. Preliminary results from E234 at TRIUMF. qcm ~ 572 MeV/c. The 200 MeV data is from IUCF. For (p,Tr+), the reactions are o 1 3C(p,Tr'*") ltfC(6.73; 3“) , and • 13C(p ,tt+)11+C(11.67;4-). For (p.O, the reaction is 13C(p,ir ) lltO(6.27;3_).0 c m  ( d e g )Fig. 12. The 12C(p, tt+ ) 1 3C reaction at 354 MeV to the 2p-lh high-spin stretched at 9.5 MeV, J?=9/2+. The prediction of the TNM model of Iqbal and Walker (solid line) is shown against the experimental data.2412C(p,ir+) 13C reaction to the 9/2+ 2p-lh state at 9.5 MeV against the theoretical result. In view of this result, it is at present unfortunate that there are no analyzing power results either from experiment or from theory.A plea should be made here for a continuing analogy of (p,ir) with (p,2p) with regards to spin-orbit effects. The (p,2p) predictions use the fundamental pp elastic reaction partial wave analysis as input into a DWIA model.21* The (p,tt) theories, in contrast, are all built up from first principles. Seeing the difficulties "first principles" have with just pp + dir+ (Ref. 17), it would seem a herculean task for theory to get A(p,r)A+l even partially correct. Since pp pn(3S1)ir+ could be the dominant fundamental (p,tt+) process in the delta region, as previously shown, it seems reasonable that a DWIA model using pp + d-rf*" as input could succeed. This is all the more true as an adequate partial wave analysis of pp ->■ dif*’ now exists.25It now comes to the time to discuss what (p,r) experiments in the energy region of 200 MeV < Tp < 500 MeV should be run on a spectrometer with a large solid angle such as the proposed SASP instrument. To return to the example of the QQSP at IUCF, the first experiment should be a survey, but this time with the (p,ir+) reaction. Although it is necessary to use some guidance from theory, experience should teach us to expect surprises with the (p,tt) reaction, and hence some sort of blanket cover­age of the field should be undertaken. The value of the SASP instrument would be that because of the large solid angle a reasonable survey could be completed in a reasonable time.Thus for the first experiment, complete angular distributions of da/dfi and Ano should be measured on many nuclear targets as a func­tion of energy between 200 MeV < Tp < 500 MeV. The first targets chosen should be nuclei that have just filled a proton subshell such as:1. 12C I P , »2 . i6o iP;;2 j< ,3. 28Si lD5/2 j> ,4. 32S 2S1/2 , and5. **°Ca lD3/2 j< •The targets have been chosen to exploit possible spin-orbit coupling ef­fect between the j> and the j< subshells. The 32S target should be interesting because the shell should exhibit no spin-orbit effects.As an indication of such spin-orbit effects to be seen, there are some old data26 on 160(p,ir+)170 which show a striking difference in Ano between the 170(g.s.) and the 0.87 l/2+ state (Fig. 13). One can conjec­ture that in the former case the reaction route is predominately from the lp3/2 shell which enhances the normal pp pn(3S1)ir+ Ano whereas in the latter case the reaction route is predominately from the lp1/2 shell which de-enhances the normal Ano giving the flat Ano.Another feature that should be looked for in this survey is the ex­citation of very narrow highly excited states (Ex > 20 MeV) that haveso far been seen in (p,Tt+) on ^2C and 13c.l3»l*t 13^  example isclearly shown at 23.6 MeV in Fig. 6b.13 At 200 MeV both known states have been shown to have almost identical cross sections27 and Ano27 (Fig. 14). The amazing feature of the Ano is that they are zero, a surprising contrast to the negative Ano in the continuum around it25400 -e.c .m .7TFig. 13. Cross sections and analysing powers for 1 60(p ,tt+) 170 reaction at near-threshold energies. Data for the ground state (5/2^) and the first excited (0.87 MeV, l/2+) are shown.20 0 -CscsI100-80-60-40-20-.4 I2C (p ,ir+ ) 'sC *(2 l.5  MeV) PRELIM. RESULTS o ISC (p,Tr+)'4C* (23.20 MeV)'Tp* 200  MeVH 1--1--h H H -t 1-0.6 0.4 0.2 ,  0 - 0.2 -0.4 - 0.6 -— I 1---------1---------1_____ I______I_____ I_____ I_____ I___ f30* 60* 90* 120* ISO*e.Fig. 14. Cross sections and analysing powers for12C(P,ir+)13C and13C(p,TT+)1‘tC reactions to the very narrow high excitation (Ex>20 MeV) states seen in both reac­tions. For the 13C spectrum see Fig. 6(b).(Fig. 9.). There has been speculation11* that these states represent AT=3/2 transitions of (p,ir), and that the states seen are T=3/2 and T=2 states of 13C and 14C, respectively. Such a speculation has some diffi­culties as the dominant pp pn(5S1)ir+ process cannot generate AT=3/2 excitations, as Ref. 14 pointed out. Furthermore this speculation would predict that (p,"-) to mirror nuclei, such as 13C(p,ir-) 11+0, would have the same cross section as (p,ir+) to these states. This is not observed.These states represent somewhat of a mystery then, and the proposedsurvey would explore the extensiveness of this phenomenon and its energy behaviour.As a second experiment the large solid angle of SASP should be ex­ploited to further look at the small cross section (p,ir~) reaction. Although the analysis in Sec. II stated that A effects in ( p . O  should be weak, they should nevertheless be looked for as the (p,ir) reaction has been in the past a source of surprises. Preliminary data28 on180(p>7r )17Ne (Fig. 15) indicate that the cross section to the 17Ne(4.6,13/2~) stretched state rises with energy in contrast to the trend in Fig.13. Since the statistics are very poor, such evidence cannot be conclu­sive. However, such statistics would justify an experiment using SASP, which would gather 10 times the statistics on this same reaction.2610.oc10bX>Tp(M«V)•  200 (IUCF)▼ 250 (E234) i /  . ■ 350 (E234) j  /T// ' V/KC(p.»+>0C<9.5)- q -605 MeV/cfla0 (p ,ir-) '5Ne(4.6) q - 608 MeV/c :Tp(MeV)0 200 (IUCF)* 250 (E234)A 350 (E234)10 20 30Fig. 15. Preliminary results from Expt. 234. Very low statistics re­sults on 180(p,tr")19Ne (4.6, 13/2+). Indications are that the cross sec­tion is rising with energy similarto (p ,tt+). n = Pc.m / mTTFinally in the interest of further testing spin-orbit coupling effects a (p,n_) experiment should be done on a subshell target to see if the Anr, to a stretched state is reversed to the observed trend in Fig. 5.Q lx   q cOne candidate would be the 0HS(p,ir )J3Ar reaction looking at the j<ld3/2 subshell. If such a reversal of polarity would be seen, it would be analogous to the effect seen in Ref. 4 for (p,ir~) to the ground states of 13,14,150<REFERENCES1. S.E. Vigdor et al., Phys. Rev. Lett. 49, 1314 (1982).2. S.E. Vigdor, Invited talk presented at the International Symposium onNuclear Spectroscopy and Nuclear Interactions, Osaka, Japan, March 21 to 24, 1984.3. B.A. Brown et al., Phys. Rev. Lett. _5L> 1952 (1983).4. W.W. Jacobs et al., Phys. Rev. Lett. 49, 855 (1982).5. R.D. Bent, private communication.6. H. Toki and K.-I. Kubo, Phys. Rev. Lett. 54, 1203 (1985).7. M.C. Green et al., Phys. Rev. Lett. 53, 1893 (1984).8. S.E. Vigdor, Phys. Rev. Lett. 54, 1204 (1985).9. M.J. Iqbal and G.E. Walker, Phys. Rev. C 32^ , 556 (1985).10. M. Dillig et al., IUCF 1985 Scientific and Technical Report.11. E.D. Cooper and A. Matsuyama, Nucl. Phys. A460, 699 (1986).12. G.M. Huber et al, submitted to Phys. Rev. Lett.13. G.M. Huber et al., to be published.14. E. Korkmaz et al., IUCF 1984 Scientific and Technical Report.15. B. Hoistad, AIP Conf. Proc. 79^, 105 (1981), ed. R.D. Bent.16. W. Ziegler, Ph.D. thesis, University of British Columbia (1985).2717. J. Niskanen, Nucl. Phys. A298, 417 (1978).18. W. Falk et al., Phys. Rev. C 32, 1972 (1985).19. G. Giles et al., Phys. Rev. C 28, 2551 (1983).20. G. Giles, Ph.D. thesis, University of British Columbia (1985).21. W. Falk et al., Phys. Rev. C 33, 988 (1986).22. E. Korkmaz et al., IUCF Newsletter 37, 1 (1985).23. G.J. Lolos et al., Phys. Rev. C 30, 574 (1984).24. W.J. McDonald, these proceedings.25. D.V. Bugg, J. Phys. G 717 (1984);H. Kamo and W. Watari, Prog. Theor. Phys. 62, 1035 (1979);G. Jones, private communication.26. T.P. Sjoreen et al., Phys. Rev. C 24^, 1135 (1981).27. S.E. Vigdor, private communication.28. R. Bent, Expt. 234 in TRIUMF 1983 Annual Report on Scientific Activities.28(p, it x) EXPERIMENTS*W. R. FalkUniversity of Manitoba, Winnipeg, Man., Canada, R3T 2N2ABSTRACTThe importance of coincidence experiments that investigate reactions of the type A ( p ,t t x ) C ,  where x  is a nucleon or a deuteron, is discussed. Such studies provide the opportunity to understand how the fundamental pion production processes are modified by the nuclear medium and by momentum considerations in the initial and final states. New exper­imental facilities in the nature of large solid angle, large momentum bite spectrometers, are essential for such investigations.INTRODUCTIONIn the exclusive A(p,iT)B reactions discussed by the previous speaker the lack of freedom to vary a given dynamical parameter, without also varying other parameters at the same time, is severely restrictive. Reactions with three-particle final states provide much greater freedom to select dynamical parameters as, for example, the momentum transfer, the momentum sharing, or the relative energy between any two of the three final particles. Thus the possibility of examining different aspects of the pion production process and their sensitivities to other details in the interaction is provided. It must also be said at the outset that very few theoretical calculations of such reactions exist at the present time that provide specific predictions that can be checked experi­mentally. However, theoretical developments incorporated in current nuclear pion production models could be extended fairly easily to these reactions. Given the present situation this paper explores in rather broad and general terms what avenues of investigation may be most f ruitful.At proton bombarding energies in the energy range from 200-500 MeV many experiments point to the dominant role played by the intermediate formation of the A(1232 MeV). Indeed, it has been recognized for many years that this might be so. Current models of nuclear pion production all stress the centrality of intermediate A formation (see refs. 2 and 3, for example). The intra-nuclear cascade model (INC) used to describe inclusive N-nucleus and nucleus-nucleus pion production has, as one of its main ingredients, the intermediate formation of the A (ref. 4). Figure 1 illustrates the potential role of the NN->NA process leading to three different final states. The first, Fig. la, represents the exclusive reaction, Fig. lb, the emission of a nucleon along with the pion, and Fig. lc, the emission of a deuteron (or two nucleons) together with a pion. Kinematically complete experiments for three-body final states of the kind represented by Figs. lb and lc have not been performed, except in few-nucleon experiments.Work supported in part by the Natural Sciences and Engineering Research Council of Canada.Fig. 1. Pion production via 400 600 800 1000intermediate A formation. Tp (M eV )Fig. 2. NN-*NNtt isospin cross sections (ref. 6).INCLUSIVE REACTIONS A(p,ir)XSome indication of the importance and feasibility of studying the three-body final state reactions of Fig. 1 can be obtained from examining the inclusive A(p ,tt)x reaction. Only a small fraction of the total pion production yield in the interaction^p+A is to be found in the exclusive reaction A(p,7T)B. DiGiacomo et al. ^ ve measured inclusive pion yields from proton interactions with C+and U at energies^f 330,400 and 500 MeV. At 400 MeV the total tt cross section from C is 7.6 mb. An interesting comparison of this result can be made by summing the free pN->NNTT isospin cross sections at this energy. A plot of these isospin cross sections, as given by Ver West and Arndt , is shown in Fig. 2. Summing the+appr<j>priate terms (cr is negligible at this energy)+yields a pp->diT +NNtt cross section of 1.43 mb and a pn—>NNtt cross section of 0.04 mb. The total cross section for C should thus be about 8.8 mb, in reasonable agreement with the experimental value of 7.6 mb. Thus the isospin cross sections seem to give a good account of the observations. This however, must be too simplistic, since the Fermi momentum of the struck nucleons, nuclear absorption, and other_ef^ects have been ignored. Indeed, in order to understand tj^ghigh tt /tt yield ratio, which is about 1/8 for C and 1/2 for U, one is led to conclude that A rescattering, charge exchange and pion absorption play a major role in these processes .A second important aspect of the inclusive reactions is the information provided by analyzing power measurements. We have recently reported such measurements on C at 400 and 450 MeV bombarding energie^. An unsophisticated model based on the elementary pp->d7r"*" and pN->NN7T pion production processes was employed, where the incident proton interacts with a target nucleon, distributed in momentum according to the predictions from (e,e'p) experiments. Figure 3 illustrates this301 ^ 4\GU)T argetMt.E tbCM*o0.20- 0.2-0.4td ne io b  - 4 6 ‘Km ,,E ,G URecoil ^  e r"VE„Fig. 3. Model of the quasifree pion production mechanism in nuclei.i  r r — i— »— •----------i— »— i— i— i— i— i— i— r~■pp -»pmr*, 450 MeV56° 64° 74°•■'Ij i t / r  ^I• ••* iii• • t i__LIy / 1 I------ 1— I-----1 I I . / / - I -----1— I-----1— ■ ' I / /  I » . I.80 90 100 MCI 60 70 80 90 60 70 80 40 50 60 40 50(MeV)Fig. 4. Cross sections and analyzing powers of the p p - > - p m T  reaction.Fig. 5. Cross sections and analyzing powers for the C(p,ir )X reaction.31model. Complete kinematic calculations were carried out for the collision of an incident proton with a target nucleon having energies E^ and E„ and momenta p^  and p^ respectively. The c.m. pion angle and momentum resulting from this collision were taken as the appropriate kinemati^al and dynamical parameters for calculating the corresponding pp->dTT differential cross sections for the two spin orientations. The angular and spin dependence+of the pN->NN7T channels was taken to be the same as that of the pp->dTT reaction.Justification |or this is obtained from the measured analyzing powers of the pp->pn7T reaction at low relative np energies, and their successful interpretation in terms of the pp->dTT analysing powers. An example of these latter data and the calculated pp->pnir analyzing powers is shown in Fig. 4. 12 +Cross sections and analyzing powers for the C(p,TT )X reaction at 450 MeV are shown in Fig. 5 together with the predictions of the above model (solid lines). The magnitude and energy dependence of the analyzing powers at forward angles is quite well predicted, while at the larger angles the magnitude of the experimental analyzing powers is considerably larger than predicted by this model. The cross section calculations in most cases exhibit the general shape of an inverted parabola, consistent with the trend of the data. The maxima of these parabola occur at pion energies close to the pion energy in the free pp->dfT reaction, which in turn coincide quite well with the peak of the experimental differential cross sections.Result^ of the model calculations al^o indicate that only 40-45 % of the total tt yield arises from the pp->dTT reaction, and the balance from the other pN->NNTT channels. This can be understood from the effect of the Fermi motion of the target nucleons in shifting upward the effective interaction energies, since the cross sections for the latter channels increase rapidly with increasing energy.COINCIDENCE A(p,dTT+)C REACTIONSThe above discussion suggests that important insights might be gained by the kinematically complete studies of the reaction A(p,dTT+)C. The predicted strength of this channel - about 50% of the total pion production, barring extensive deuteron breakup - makes it particularly attractive for investigation. In the first instance the kinematic regime discussed above - the quasifree region, where the struck nucleon momentum is modest - would provide useful confirmation of the role of the basic processes considered. A measurement of the fraction of the pion events associated with the production f^ a free deuteron_|_could be obtained.DWIA models have been developed f(j>r nuclear (p,tt ) reactions that incorporate the fundamental p p - > d T T  reaction as the basic pion production process. Such models could easily be extended to the case of a free deuteron in the final state.An indication of the technical challenge of such experiments is illustrated in Fig. 6 where the deuteron intensity distribution (from the previousl^descrijjreij ^ model calculations) has been plotted for the reaction C(p,d7T ) B, at 400 MeV, with the pion detected at 46° with respect to the beam direction. The deuteron distribution is strongly forward peaked, as expected, with most deuterons emitted at angles of less than 20 . Two spectrometers, one for the detection of the pion and the other for the detection of the deuteron, would provide32Fig. 6. Contour plot of the relative deuteron inten­sity distribution for a pion detected at 46° in the x-y plane.excellent missing mass resolution in such experiments. On the other hand, the angular range to be investigated, together with the large momentum range of the detected particles at fixed angles (the deuteron kinetic energies range from 60-240 MeV in this case) make detailed investigations of this kind very tedious. Instead, initial survey measurements might most profitably be carried out using the large second arm spectrometer for detecting the pions, and an array of Ae-E counters, providing particle identification and energy measurement, for detecting protons and deuterons. Indeed, one might also add neutron detection capability to these counters to provide a very versatile detection system. Such a detection system would greatly facilitate not only the A(p,d7T )C investigations, and extend the capability to detect neutrons and protons from deuteron breakup, but would also be applicable to the other experiments to be discussed presently. In fact, data for several different experiments could likely be collected simultaneously.Simulation of some of the dynamical conditions pertinen^ to the exclusive A(p,ir)B reaction can be reproduced with the A(p,d7T )C reaction as one moves away from the quasifree region. As a reminder we note the c.m. energies, in Fig. 7, that can be attained in the head-on collision of two nucleons, where the target nucleon has a range of momenta and an assigned total energy of 920 MeV. Thus the collision of a 350 MeV proton with a target nucleon of momentum 200 MeV/c brings the c.m. energy into the lower domain of the NA system. Probing such momentum cgm^on^nts in the nuclear wave functions results in momentum transfers q=p -p^ which+span a considerable range between the momentum transfers in the pp->d7T and A(p ,tt )B reactions. Figure 8 defines the relevant parameters in the collision and Fig. 9 shows the pionCM Energy ( MeV)332200 ■21002000Fig. 7. Center of mass energies attained in the head-on collision of two nucleons.= 9 2 0  MeV100 200 300 400F>2 (M e V / c )/b ;c 9»Jo  1 Pr s R e l a t i v e  m om en tu m  [ / pr/ r o f d i n BFig. 8. Definition of parameters in the nuclear NN pion production collision.Ex. En. (MeV)34—  t^C ( p,T+)^Cg.s. Tp = 350 MeV  - -  ,2C ( p ,d ^ ) " B ,  P2= 150—  ,2C ( P»d ®'*)11 B* P2 = 3 0 0  pp-^w+do>®2Fig. 9. Pion momentum,and momentum transferas a function of 0 .TTAll quantities are eval­uated in the lab frame.a>2Fig. 10. Effective excitation energy of nucleus B, and the relative deuteron momentum.35momentum and the momentum transfer q a| a fun^ion of the pion scattering angle for 350 MeV grotons incident on H and C. Interestingly, at a pion angle of 80 the momentum transfer is about 870 MeV/c in all cases. The kinematics of the reaction also define the relative momentum of the deuteron and the residual nucleus C, as well as the internal energy of d+C (i.e. the equivalent excitation energy of B). Plots of these two quantities as a function^f the pion scattering angle are shown in Fig. 10 for 350 MeV protons on C, and struck nucleon momenta of 150 and 30^MeV/c. The higher momentum target nucleon Results in effective C excitation energies of 33-114 IjleV and d- B relative momenta from 220-550 MeV/c. Thus the A(p,dTr )C reaction is able to investigate much of+the kinematical and dynamical regime of interest to the exclusive A^p , tt )B reaction.The A(p,dir )C reaction is expected to exhibit a strong spin dependence, given the results discussed earlier. Such experiments should thus be performed using polarized protons since valuable signatures will be contained in the analyzing power information.Kinematics for this reaction for essentially all cases of interest, indicate that the deuteron is emitted strongly in the forward direction (generally at 0-20 with respect to the beam direction). Providing for the situation where the deuteron breaks up in the field of the nucleus, or where it is produced as an unbound np pair with some internalexcitation, still has as consequence the forward emission of one or bothnucleons in most cases. The detection of a deuteron or nucleon in the forward direction is thus a requirement in all situations.As an example of the count rates expected in a coincidence experiment we consider 400 MeV protons incident on C, using a beam intensity of 1 nA and a target of 100 mg/cm . The second arm spectrometer with a solid angle of 10 msr is used as the pion detector, and an array of counters, each subtending a solid angle of 6 msr, placed in the forward direction for the detection of deuterons and nucleons.For an inclusive pion production cross section of 10 jjb/sr.MeV the singles pion count rate is about 30/s in a 10 MeV energy bin. The estimated coincidence count rate (see Fig. 6) is then about 1/s.However, the singles count rate in the forward-positioned counters would be in excess of 60,000/s, assuming an elastic cross section of 300 mb/sr at 10 . Random coincidence background will thus be the limiting factor in the data collection rates in these experiments. Four to five magnetic field settings of the pion spectrometer would be required to cover the range of pion momenta. An array of 5 to 10 counters would cover a large fraction of the angular range of the emitted deuterons and nucleons.A PRODUCTIONGiven the dominant role that the intermediate A is believed to playin all pion production at these energies it would seem important to lookfor direct signature of its pres^gce. In NN experiments this dominant role of the A has been confirmed in the pp->pn tt reaction at 800 MeV as shown in the cross section measurements in Fig. 11. Only recently has such gvidenc  ^| bgcome available for p-nucleus co^isions where the reaction Li(p,A ) He was investigated at 1040 MeV g The experimental arrangement is shown in Fig. 12 where a He counter telescope was used to look directly for the 'two-body' signature of the final state. A scintillation counter hodoscope was used to provide36| |Fig. 11. Excitation of the A in the pp + pmT reaction.redundant information and reduce background contamination. An unambiguous signal for the reaction was seen which permitted the extraction of the differential cross section shown in Fig. 13. The solid curve in this fig^e represents a DWBA calculation for this reaction, performed by Jain and Hasan and Jain^. Both the magnitude and shape of the differential cross section are reproduced remarkably well in this essentially parameter-free calculation. They conclude that this determines for the first time, in a direct way, that the effective spin-isospin coupling potential, V(NN->NA), can be correctly ^scribed by the one-pion and one-rho exchange interaction. Because the A is produced in the free state such studies provide the opportunity to learn about the A-nucleus interaction potential and A propagation in the nuclear medium. The great importance of understanding this reaction in a comprehensive way has led the above authors to perform calculations for the Li(p,A++) He reactions at a number of energies in the range from 400 to 1250 MeV. Calculated angular distributions and total cross sections are shown in Figs. 14 and 15. At 500 MeV the differential cross section is about 4 yb/sr at 0 , and the total cross section has dropped by an order of magnitude from its value at 1000 MeV. Nevertheless, such experiments should be feasible usj.yg the detection system previously described. The proton from the A decay is emitted strongly in the forward direction, while the pion is not so constrained. Other garget nuclei on which the (p,A ) reaction could be investigated are B and C, although the presence of several final nuclear states will complicate the interpretation, unless these states can be resolved.Nuclear pion production via intermediate A formation in any two— or37l l lU G e V lc f lFig. 12. Experimental setup for the Fig. 13.6 .. »++\6„Li(p,A ) He reaction.Experimental resultsand theoreticalprediction for the6 ., A+ + \ 6 Li(p,A ) He reaction.Fig. 14. Calculated angular distributions.T ( M e V )Fig. 15. Calculated total cross section.38three-body final state reactions should be characterized by a strong spin-dependence exhibiting different analyzing powers for interactions with j=l+l/2 ^nd j=l-l/2 target nucleons. This follows from the fact that the T=1 initial NN state, in which the interacting nucleons are in the spin singlet state, dominates the interaction . The situation is depicted in Fig. 16 which, when coupled with arguments about pion absorption, suggests opposite analyzing powers for j=l+l/ 2  and j=1—1/2 states. Similar predictions for the A(p,tt )B reaction at lower energies by Vigdor have indeed been borne out by experiment. Unfortunately, however, the observed analyzing power dependence is opposite to what this simple model predicts. Thus caution must be exercised in making specific predictions from such simple pictures.NON-A REACTIONS WITH THREE-BODY FINAL STATESThe (p,nTT+) reaction on T=0 target nuclei, proceeding to the low-lying final states of the same nucleus (T=0), occupies a rather special role in this general class of three-body final state reactions, in that this reaction^gannot proceed via the intermediate formation of the A. Sherif et al. have suggested that this reaction may be used to answer certain specific questions regarding the NNtt vertex function. They have performed calculations o^the dif|e£gntial cross sections for the reactions He(p,nTT ) He and Ca(p,niT ) Ca for specific neutron angles as a function of the detected pion angle. Figure 17 shows their results for the latter reaction at 500 MeV. While these c^oss sections are quite small - in the neighborhood of 1 Ub/sr'.MeV - they should be amenable to experimental investigation using the detection system discussed previously.SUMMARYThis survey of potential ( p , N T r ’ )  and ( p , d T T + )  reactions has attempted to show that a rich field of experimentation is available that can shed much light on pion production processes in nuclei. All these studies would benefit from the use of polarized incident protons since the inherent spin-dependence of the NN->NNtt reactions provides valuable signatures of the subprocesses involved. Specific results of these investigations would include a clearer picture of how the fundamental NN->NNttreactions are modified in the presence of the nuclear medium, an understanding of the role played by the intermediate formation of the A, and a better understanding of A-nucleus dynamics.A fortunate aspect of most of the reactions discussed is that all have rather similar experimental characteristics in terms of the detection systems required and the typical angular and dynamic (momentum) ranges of the particles to be detected. Indeed, the combination of a large solid angle pion spectrometer together with an array of broad range nucleon detectors would provide an excellent tool for collecting data on a number of these reactions simultaneously. Very specific questions, where much better missing mass resolution is required, could then be addressed using the MRS and SASP in the dual arm configuration.A clear need also exists for calculations to be carried out for the reactions discussed, that are firmly grounded theoretically, if the interpretation of these studies is to provide new insights into pion production.39Fig. 16. Spin-dependence in A-dominated interactions.Fig. 17. Calculated cross section for ^Ca(p,nTT+)^Ca.40REFERENCES1. W.O. Lock and D.F. Measday, Intermediate Energy Nuclear Physics (Methuen, London, 1970), p.224.2. B.D. Keister and L.S. Kisslinger, Nucl. Phys. A412, 301 (1984).3. M.J. Iqbal and G.E. Walker, Phys. Rev. C32, 556 (1985).4. Z. Fraenkel et al., Phys. Rev. C26, 1618 (1982), and references therein.5. N.J. DiGiacomo et al., Phys. Rev. C31, 292 (1985).6 . B.J. VerWest and R.A. Arndt, Phys. Rev. C25, 1979 (1982).7. W.R. Falk et al., Phys. Rev. C33, 988 (1986).8 . W.R. Falk et al., Phys. Rev. C32, 1972 (1985).9. H.W. Fearing, Phys. Rev. C16, 313 (1977).10. J. Hudomalj et al., Phys. Rev. C18, 2666 (1978).11. T. Hennino et al., Phys. Rev. Lett. 48, 997 (1982).12. B.K. Jain, Phys. Rev. Lett. 50, 815 (1983).13. H. Hasan and B.K. Jain, Phys. Rev. C33, 1020 (1986).14. D.V. Bugg, Nucl. Phys. A437, 534 (1985).15. S.E. Vigdor, International Symposium on Nuclear Spectroscopy and Nuclear Interactions, Osaka, Japan, 1984.16. H.S. Sherif et al., Phys. Lett. 83B, 293 (1979).41THE IUCF/UNIVERSITY OF MARYLAND DUAL SPECTROMETER FACILITY*P.G. RoosUniversity of Maryland, College Park, MD 20742ABSTRACTThis paper discusses the IUCF/University of Maryland Dual Spectrom­eter Facility —  the components of the spectrometers, their design parameters, the focal plane system and electronics, and the front end. Also discussed are a few initial experiments planned for this facility and other possible avenues of research.INTRODUCTIONIn this talk I will discuss not only the facility itself, magnets, focal plane system, etc., but also briefly discuss some of the physics planned when completed. Before beginning this discussion I would like to spend a couple of minutes discussing the history of this project, which provides some insight as to the eventual choice of the properties of the two spectrometers.In 1979 the NSF held a review of all NSF supported university facilities. At this presentation we proposed the construction of a dual spectrometer facility for particle-particle correlation studies at the Maryland Cyclotron. These spectrometers were to have large acceptance (solid angle and momentum bite), but rather modest resolution. At about this same time IUCF was lobbying for a high resolution modern spectrom­eter, primarily for high resolution spectroscopy; e.g., inelastic scat­tering and transfer reactions.In 1980 the Maryland Cyclotron ceased to exist (RIP). At that time we held discussions with IUCF and together with them submitted a joint proposal for a dual spectrometer facility to be operated at IUCF. To cover both the UM and IUCF interest, one spectrometer was to be capable of very high resolution with reasonable acceptance, while the other was to have large acceptance with modest resolution.About one year later this proposal was funded by the NSF at a cost of approximately $1.5 M. This cost represents essentially the hardware costs of the spectrometers. The power supplies and some beam transport magnets were donated by the University of Maryland Cyclotron; the beam line bender for dispersion matching is the IUCF QDDM spectrometer. Funds for the focal plane array were awarded in a separate proposal, and a new proposal for a dual sliding seal scattering chamber will soon be submitted. In addition, most of the costs for design and installation are being borne by the IUCF operating budget as one of their major equipment projects.SPECTROMETERSFigure 1 shows the design of the two spectrometers (designed by R. Pollock of IUCF). Their detailed properties are listed in Table I.*Work supported in part by the U.S. National Science Foundation.42Table I. Parameters of the IUCF Dual Spectrometer SystemK=600 (3 dispersion modes) K=300low normal highMaximum momentum (MeV/c) 860 1080 1005 760Maximum proton energy (MeV) 334 493 437 269Maximum magnetic rigidity 3.00 3.60 3.50 2.55(T-m)Maximum dipole fields, 1.23/1.64 1.64/1.64 1.64/1.23 1.70/1.70D1/D2 (T)Nominal bend radius (m) 2 . 1 1.3Nominal bend angle 115° 70°Maximum solid angle 6 . 0 14A0A<j> (msr)Maximum radial acceptance ±44 ± 6 6A0 (mrad)Maximum axial acceptance ±44 ± 6 8A<|> (mrad)Momentum range Pm,x/pmin 1.131 1.097 1.063 1.357Resolving power p/6p -30,000 -2 , 0 0 0Momentum dispersion (cm/%) 6 . 2 8 . 1 9.8 2.4Energy dispersion (keV/mm) 65 50 40 170(for 200 MeV protons)Mininum scattering angle 0 4.5° (at 1 msr) 2 0°with external beam stop 1 2° (at 6 msr)Fig. 1. Basic designs of the K-300 and K-600 spectrometers.43The K-600 consists of two dipoles and an entrance quadrupole-hexapole combination. An aberation correction coil (H) and a kinematiccorrection coil (K) are included in the dipoles. The solid angle of the spectrometer is a quite respectable 6 msr. The K-600 design has some very special features. By operating the two dipoles independently,three focal plane positions with varying dispersion can be used. (Theproperties of the three dispersion modes are listed in Table I.) As aresult one can use the high dispersion mode (~6% momentum bite) for high resolution spectroscopy, or use the low dispersion mode (~13% momentum bite) for coincidence work where generally coverage of phase space is a more important consideration. A second feature is that the K-600 spec­trometer can be operated at angles as small as 4.5° with an externalFaraday cup using a special septum magnet. This small angle mode, alongwith the normal mode, are pictured in Fig. 2.Normal Mode Small Angle ModeFig. 2. Normal and small angle modes of the K-600 spectrometer.The K-300 spectrometer is a much simpler magnet with an entrance quadrupole and single dipole. However, the pole piece of the dipole is split to provide additional focusing and aberation corrections. Also the quadrupole, identical in basic design to that of the K-600, containsK  COO'P 0 L 6H £ * A T o L E44higher multipoles for aberation corrections. From Table I we see that this spectrometer is optimized for solid angle (14 msr) and momentum bite (~35%) at a cost of momentum resolution (p/6p ~ 2000). The K-300 spectrometer is primarily intended for coincidence measurements, but also would probably be the spectrometer of choice for some inclusive reaction studies.To maximize the range of angle pairs which can be covered in coin­cidence measurements, the K-300 will be mounted vertically. This has the undersirable feature that dispersion matching to the beam (horizon­tal dispersion) cannot be done. In addition, definition of the inplane scattering angle is more difficult. However, considering the high quality of the beam at IUCF, the intrinsic resolution of the K-300 spectrometer, and the requirements of presently conceived experiments, we believe that the flexibility in angle is the overriding considera­tion.Presently, the installation of the spectrometers is proceeding at the north end of the original IUCF building. This location, in addition to the rest of the facility, is shown in Fig. 3. Eventually, when the IUCF Cooler Ring is operation, the spectrometers will be moved to the site on the ring shown in Fig. 3. The angular range covered by each of the spectrometers at these sites are the following:1. Control Computer *  Consol*2 . Dot* A cqu is it io n  Computers3 . 800 fcV Ion Source Term inal4 . 800 kV loo  Soarc* Term inal3. Low Energy In a c t io n  A *«o  U na 8 . S trlp p ar Loop Storm** “ » *7. In je c to r  (k -1 5 ) C yc lo tron8. In ter-M ach ia* k**a U a *9. Halo C yc lo tro *10. Slgh-Cnergy Sana U n *  3 and Ian* S p l i t t in g  Syetaa11. H lgh -In tan a lty  Arna12. Low -Ia tana lty  S ta tioncm S ca tte r in g  Chamher14. P loa  gp octroaa ta r (QQ8 P)15. M agnetic Spoctromatar (QB0M)IS . P o la rlan d  Meatran F a c i l i t y  (PUP) 17. Neutron Kkparlnnata1 f a c i l i t y  IS .  Maw Dual Spectrom eter K m  (DSP)19. Maw Soon U n *  ( 9 )  t *  Cooler20. Maw S torage S lag  with E lectronCoo lin g21. Maw C ooler S u lId la g  Add ition22. Equipment Setup Area23. M echanicals AreaC y c lo t ro *  P loo r  Plan 13. 182Fig. 3. IUCF floor plan showing the two location of the dual spectrom­eter facility.45K-600 K-300 Minimum SeparationNorth End of Bldg 105°L 29°R75°L > 135°R ;> 3o°Cooler Ring 14°L -*• 102°L 14°R 156°R^ 30°The focal plane detector systems for both spectrometers will con­sists of two sets of wire chambers to measure position and angle and two to three plastic scintillators. To achieve the K-600 design goal of p/Ap = 3x10“*, position and angle must be measured very accurately (Ax <0.2 mm, A0 < 3 mrad). This dictates the use of vertical drift chambers1 for the 2 x-planes. The y-direction is less critical, the information primarily being utilized for background suppression. Therefore, drift chambers of the Los Alamos type2 will be used for the y-planes. In addition, a diagonal drift chamber (U chamber) will be included to aid in the identification of multiple-hit events. The much more modest requirments of the K-300 (Ax < 0.75 mm, A0 < 15 mrad, Ay < 2 mm, A<t> < 50 mrad) allow the use of Los Alamos type drift chambers for all planes.Behind the wire chamber a stack of two or three plastic scintilla­tors will be used for timing, particle identification, and generating event triggers. Information from phototubes on each end of the scintil­lators will provide additional consistency checks. We plan to have a selection of scintillator thicknesses to allow optimization for the various experiments. An example of the focal plane system for the K-600 is shown in Fig. 4.7— Scint 2 — Scint I”> -R * o r  X - Y  Detector-F r o n t  X -Y  Detector+ 6%Fig. 4. Focal plane detectors for the K-600 spectrometer.46Before turning to a discussion of the electronics, let me mention that considering the recent importance of the measurements of spin- transfer information, a focal plane polarimeter will be available in the early stages of operation. A design which has an efficiency of ~1.5% and an analyzing power of ~0.5 for 200 MeV protons has been proposed.To take maximum advantage of the dual spectrometer facility and the scarce beam time available at IUCF, it was deemed essential that the focal plane electronics and readout be capable of handling a singles rate of at least 105 cts/sec and an event rate of 5x103—10*+ cts/sec. This requires individual wire readout and TDC's and ADC’s with conver­sion times in the few microsecond range. Furthermore, a smart front end with parallel processors capable of some preprocessing of events is essential.The adopted scheme is shown in Fig. 5. To keep the costs at a reasonable level, 20 to 25 wires from each plane (separated sufficiently so that they are not triggered by the same hit) are or'd by means of a multiplexor (MUX) to the individual TDC's. Each event is then passed through a buffer to a set of 10 parallel processors where partial analy­sis of the event can proceed and bad events rejected. After preprocess­ing the events are passed on to a VAX where they can be analyzed in a sample mode.Diagona1-Wire ChamberandK-600 SdMTHXATKM READOUTSPflEAMPirKRS/OtSCttMMATORSK-600 FOCAL PLA*CRAM FIFO DMA TO•ltfer mlx ti-rvK-300 FOCAL FLAREHMfa x-mS.fBACtt Y-PIAHE IK -3 0 0  S C M T LL A T K M  READOUTSandDiagonal-Wire ChamberF ig . 5. Schematic block diagram of the proposed readout and event processing system.As probably most of you know, th is pro ject has su ffered  a number of delays, and continues to proceed slow ly due to the pressures of the Cooler Ring construction . However, events are proceeding: the beamtransport system is  e s sen t ia lly  complete; the K-600 spectrometer has been shimmed and mapped and is  being in s ta lle d ; the K-300 spectrometer has been mapped once and shims are being fab rica ted ; the fo ca l plane electron ics and front end are designed and la rg e ly  completed; and the47wire chambers and plastic scintillators are in various stages of con­struction. As of now (3/14/86) the schedule calls for tests of the beam line in April and initial beam tests of the K-600 (with complete focal plane) in early August, We would, therefore, expect the K-600 to be available this fall for singles experiments.The schedule for the K-300 is less well defined and is at the mercy of the Cooler Ring, IUCF's top priority project, as well as the normal operation and maintenance of the Cyclotron. We are hopeful that the K-300 will be installed and available for testing in the summer of 1987. However, this schedule will depend on all other laboratory pro­jects.Eventually, both spectrometers will be moved to the site on the cooler ring. With respect to the schedule for completion of the ring and the movement of the spectrometers, I would not even hazard a guess.A SAMPLE OF PROPOSED AND POSSIBLE EXPERIMENTSThere are a variety of experiments possible with the K-600 alone utilizing the high resolution. For example, there is already an ap­proved experiment3 to study inelastic proton scattering from calcium isotopes. Others will use a focal plane polarimeter to measure spin- transfer properties. Rather than discuss such experiments, I will concentrate on some coincidence experiments. For coincidence measure­ments, the dual spectrometer facility is capable of providing data of a quality three orders of magnitude better (in terms of resolution, count rate, and background) than previously available. Many previously impos­sible studies become possible with this facility.A(p ,2p)BAn approved experiment4 will measure the (p,2p) reaction at 200 MeV on 12C, 40Ca, and 90Zr obtaining high statistics for transitions to low- lying states in the residual nucleus. The experiment will measure thecross sections and analyzing power (AA < 0.02) for a number of angle pairs, both symmetric and asymetric.This experiment utilizes the predicted5 and confirmed6 feature that the struck target nucleon is effectively polarized for situations in which the final state protons have either unequal angles or energies due primarily to different attenuations. A primary motivation of this experiment is an attempt to study the nucleon-nucleon interaction in the nuclear medium under conditions of differing density and distance off- shell. To clarify the motivation, we write the factorized DWIA cross section, 7 neglecting spin-orbit terms, for an incident beam with polari­zation Pq asdBidn^Ej (Pq) “ dfl (O)[1+(P0+^eff)‘A + ^0*pef f cnn ] I 4>dw| 2where <j>py is the distorted momentum distribution of the struck nucle- on> ?eff is the polarization of the struck nucleon caused by distortioneffects, and I§(°)’ and Cnn are the two-body p-p unpolarized crosssection, analyzing power, and spin correlation coefficient.48Assuming this expression to be correct, measurements of ($,2p) cross section and analyzing powers allow the extraction of the p-pscattering observables (da/df2,A,Cnn) in the nuclear medium. By appro­priate choice of angles and residual states one can isolate the variousterms in a manner which is almost independent of the distorted waveparametrization. In particular, some of the obvious choices and consid­erations are as follows:(a) For knockout of an S-state (1=0) Peff=0, and one focuses on A;(b) For 110 and symmetric angles (01=— 02) we find Spp~90° so that A=0 and one focuses on Cnn. However, in this case Cnn appears only as a product with Peff so one must use the DWIA to extract Cnn;(c) For the knockout of spin-orbit partners, there is an approxi­mate relationship between Peff which is well satisfied, namelyPeff (J«4* V2 ) -  -  Pef f  = 1'2  y >(d) Measurements of spin-orbit partners at asymmetric angle pairs where both A and Cnn contribute should allow the separation of these various terms.Although my discussion, and thinking about the experiment, has clearly been largely based on the equation presented above, we are cognizant of the uncertainties and impact introduced by reaction mechan­ism and nuclear structure. In the proposed experiment the choice of targets, nuclear levels, angles and energies will allow for interrelated studies of reaction mechanism (factorization, distorted wave treatment), nuclear structure, and the nucleon-nucleon quantities discussed above.This experiment promises to produce very interesting results. Work here at TRIUMF already indicated difficulties with the analysis of (j?,2p) data with conventional DWIA calculations. For a review of the present status of (p,2p) studies, as well as a more detailed discussion, see Ref. 8 . The most exciting possible explanation is that the spin dependence of the nucleon-nucleon interaction is strongly modified in the nuclear interior. However, it may well be that the reaction model is too simple. In either case the extensive data provided by the pro­posed experiment should indicate the source of difficulty.Before continuing to the next experiment I would like to point out that the Dual Spectrometer Facility will provide count rates such that the error in analyzing power in a 5 MeV bin for 1+0Ca(p,2p) 39K(3/2 ,g.s.) will be < 0.02 in a one-hour run. This rate is comparable to many sin­gles measurements. In addition, the rate is sufficient to allow reason­able measurements of the polarization of one of the outgoing protons. These measurements will not be considered until after the initial measurements of cross section and analyzing power, to determine if there is any reason to make this more difficult measurement.Studies of the (p,2p) ContinuumA second approved proposal for the dual spectrometer facility is that of Segal et al. 9 This work is oriented toward a study of reaction dynamics by detailed measurements of the continuum (> four-body phase space) produced in (p,2p) reactions. The choice of experimental config­uration is based on the hypothesis that one high energy proton is pro­duced directly in an initial nucleon-nucleon collision. A second proton is then produced by the multiple scattering of the target (or projec-49tile) nucleon in the nucleus. In a simplistic treatment the data then allow extraction of the mean free path of the proton in the nucleus. A more formal treatment10 attributes most of the continuum yield toinitial p-p interactions with valence nucleons followed by multiple scattering, described by the experimental (p,p') data. These calcula­tions provide a good description of 58Ni(p,2p) at 100 and 200 MeV.The experiment will measure (p,2p) cross sections on a light, medi­um, and heavy nucleus at a variety of angle pairs. These data will be used to test reaction models, such as that of Ref. 10. If the original hypothesis is correct, one will be able to extract quantities related to the classical mean-free-path. An interesting, and utilitarian, side­light is that confirmation of the model would then allow estimates of the continuum yield due to valence particles, and place limits on theyields from deeper-lying hole states. Based on the scant results todate, we doubt that the (p,2p) reaction will be a useful tool in thestudy of deep-hole states.Cluster Knockout ReactionsWith the expected improvement in count rate (~102) and energy resolution ( ~1 /5—1/2 0) new or greatly improved studies of cluster knock­out reactions become possible. For example, (p,pa) cross sections can be measured with a factor of 10 improvement in the statistical error compared to presently available data. To date most experiments have concentrated on ground-state transitions, which are about a factor of ten stronger than excited states. In addition, analyzing power data with excellent precision is possible. With these new measurements both ground state and excited state data will provide precision tests of the DWIA treatment of the reaction, and assuming the applicability of the DWIA should better define the bound cluster wave function, and thereby the spectroscopic factor. In addition, one will be able to look for transitions forbidden by the simplest cluster knockout DWIA treatment, such as the excitation of unnatural parity states. Observation of these states will place limits on the importance of multistep processes and/or the presence of excited states of the clusters in the target nucleus.Similar experiments can be envisaged for other cluster knockout reactions. In any event the facility opens many new avenues of research into the cluster structure of nuclei.Pion ProductionThe experiment which I personally find most interesting is the study of pion production using the (p,pir) and (p,dir) reactions. These reactions were discussed at this meeting by W. Falk. Unfortunately for me, work with the dual spectrometer facility at IUCF will have to await the completion of the cooler ring, which will allow the beam energy to be ramped to energies higher than 200 MeV. Several year ago we attempt­ed a measurement of ^(p.px*) at 205 MeV and found the cross sections to be too small to obtain an acceptable real to accidental ratio. That experiment gave R/A < 1/10. However, that situation will improve rapid­ly with increasing energy.The experiment I would like to do is to measure all allowed reac­tions of (pjpir*) and (p.dn*) on a series of targets 1,2H, J,4He, and 12C starting at T 0 = 500 MeV and working down toward threshold. This series50provides the following:(1) A measurement of the fundamental processes for production of ir+ and tt- using 1H(p,pir+)n and 2H(p,p7r-)2p;(2) A measurement of the (p,pir) production as the number ofnucleons, density, distance off the energy shell, and Fermi motion change dramatically;(3) A measurement of the (p,dir+) reaction (e.g., ‘+He(p,diT+)t)isolates, to a large extent, the effect of Fermi motion on the produc­tion mechanism; and(4) Measurements of (p,dir-) should define the importance of final state interaction charge exchange.There are a variety of other aspects to these studies and I believe that such a series of experiments has tremendous potential in terms of improving our understanding of pion production, particularly near threshold where to produce the pion one not only needs Fermi motion, but also an additional interaction to put the reaction on-shell.Having thought about this experiment several years ago (in suffic­ient detail that it reached the level of a draft proposal to TRIUMF), I carried out some DWIA calculations of the 2^C(p,dir+) ^ B(g.s.) reaction by factorizing the p+p-»d+TT+ vertex. The predicted cross sections are shown in Fig. 6 for 3 energies. Clearly the count rates will be veryTq = 300 MeV ed = 25°Tq = 400 MeV 0d = 25°T0 = 500 MeV 0d = 25°Fig. 6 . Energy sharing cross sections for 12C(p,dir+) 1 ^ (g.s.) predicted by a factorized DWIA calculation. Calculations are shown for three bombarding energies (Tp = 300,400, and 500 MeV), a fixed outgoing deu­teron angle 25° and two pion angles (-25° and -50°).good with a dual spectrometer facility.51SUMMARYI have reviewed the design and progress of the IUCF/UM Dual Spec­trometer Facility and indicated several coincidence measurements that will be carried out when construction is complete. We are confidentthat the tremendous enhancement in the quality of data provided by this facility will lead to new and exciting physics discoveries, and look forward to its completion.I would like to acknowledge the efforts and dedication of Prof. Peter Schwandt in his role as project manager. I would also like to acknowledge the technical staff at IUCF who have done an excellent job on this project in spite of the numerous competing demands for theirtime and expertise.REFERENCES1. W. Bertozzi et al., Nucl. Instrum. Methods 141, 457 (1977).2. e.g., C.L. Morris, Nucl. Instrum. Methods 196, 263 (1982), and references therein.3. J. Kelly and A. Saha, spokesmen, "Microscopic Structure of the Calcium Isotopes" (6/85), approved experiment at IUCF.4. H.L. Chen, N.S. Chant, and P.G. Roos, spokesmen, "Measurement ofthe (i?,2p) Reaction at 200 MeV with the Dual Spectrometer Facility"(12/85), approved experiment at IUCF.5. V.E. Herscovitz, Th.A.J. Marris, and M.R. Teodoro, Phys. Lett. 69B, 33 (1977); Th.A.J. Maris, M.R. Teodoro, and C.A.Z. Vasconcellos, Nucl. Phys. A322, 461 (1979).6 . e.g., P. Kitching et al., Phys. Rev. Lett. 37, 1600 (1976); P. Kitching et al., Nucl. Phys. A340, 423 (1980); see also Ref. 8 .7. e.g., N.S. Chant, "Knockout Reactions with Hadronic Probes," IUCF Workshop on Nuclear Structure at High Spin, Excitation, and Momen­tum Transfer, Bloomington, IN (1985).8 . P. Kitching, W.J. McDonald, Th.A.J. Maris, and C.A.Z. Vasconcellos, Advances in Nuclear Physics, edited by J.W. Negele and E. Voyt (Plenum Publishing Corp., 1985), Vol. 15, p. 43.9. R. Segal, spokesman, "Coincidence Study of Quasifree (p,2p) Reac­tions" (12/85), approved experiment at IUCF.10. G. Ciangaru et al., Phys. Rev. C 29, 1289 (1984).52DASS/SASP REPORT OF THE CONCEPTUAL DESIGN E.G. AuldDepartment of Physics, University of British Columbia Vancouver, B.C. V6T 2A6SASP is a Second Arm SPectrometer designed for use in single arm high resolution experiments like (p,x) and for use in a Dual Arm Spectrometer System (DASS) in conjunction with the MRS Spectrometer for experiments like (p,2p). The present funding schedule calls for its completion at TRIUMF to be in late 1989.SECOND ARM SPECTROMETER FEASIBILITY STUDYThe feasibility study of the Second Arm Spectrometer was completed in the 1985/86 fiscal year. A detailed report of the study was presented to the TRIUMF Long Range Planning Committee (LRPC) in July 1985. This submission described the spectrometer details and some of the interesting physics that could be done with SASP or with SASP and the MRS in a dual arm spectrometer arrangement (DASS).This paper describes the basic characteristics of the SASP as pre­sented to the LRPC and indicates the progress made to the design since then.The SASP is intended for use both as a large solid angle device for low cross-section single arm experiments, such as (p,it) and (n,p) and as a second arm in conjunction with the existing MRS spectrometer for coincidence experiments such as (p,2p) and (p,irx). The configuration of the optical elements consists of two multipoles followed by a vertical bend clam-shell dipole. The preliminary design sketches of the system are shown in Figs. 2, 3 and 4.The expected performance characteristics are shown in the following table:Table I. SASP Specifications.Central momentum....................660 MeV/cMomentum bite....................... ±10%Solid Angle, .at 594 MeV/c........... 9.3 627 MeV/c.......... 11.3 660..MeV/c.......... 11.5 693..MeV/c.......... 11.5 726..MeV/c.......... 11.5 msrResolution.(with 2 mr multiple)...(scattering at focal plane)......0 .0 2%D/M................................ 4.56 cm/%Flight 660 MeV/c 6.70 mAngular acceptance.(bend plane) ±85 mr... (non-bend plane) ±43 mrFocal plane tilt.................... 45°Total bend angle.................... 90°Angular range...................... 20-150°Angular resolution.(1 mm beam spot).......(with no front end chamber) 2 mr53The first order transport calculations produced the following trans­fer coefficients:R(ll)= -0.6149 R(12)= 0.0000 R(16)=2.8047R(21)= +4.1438 R(22)=-l.6307 R(26)=5.8482R(33)= -4.0609 R(34)=-0.2000R(43)=-12.7623 R(44)=-0.8743The details of the intrinsic resolution as a function of the momen­tum is shown in Fig. 1. Folded into this calculation is 2 mr of multiple scattering in the focal plane proportional chambers. This type of resolution implies that for the pion production experiments, as an example, the energy resolution on the detected pion would vary from 46 keV to 165 keV for the (p,tr) reaction on 2 8Si. The better resolution occurring at 200 MeV incident proton energy and the other for 500 MeV incident energy.The preliminary design of the spectrometer indicates no insurmount­able engineering tasks. What we want to build requires standard engineering solutions to all the design problems. Figure 2 shows the general layout and dimensions of the spectrometer, including an initial idea for the support frame, the detector array support and the shielding.The general features to point out are the fact that the 90° bend and the low position of the focal plane will make it quite easy to shield the direct target background radiation. The focal plane detector array will consist of vertical drift chambers with both x and y sensitivity (as per the MRS), segmented plastic scintillators and a Cherenkov counter for pion coincidence (and to reject the protons of the same momentum). The "beam dump” location for operating the spectrometer at zero degrees is sketched in at its approximate location. The first quadrupole will be removable to allow the insertion of the (n,p) target for recoil measurements.3.oSPT 20(.0-")1.0o ------- 1------- 1-------1______ I- i o  - s  o  s ioAP *7PFig. 1. SASP intrinsic resolution, with 2 mr multiple scattering included in the focal plane chambers.54Fig. 2. Overall layout of the Second Arm Spectrometer (SASP).There are three magnetic elements: a multipole doublet followed by a clam shell dipole. The maximum central momentum for the high resolution design will be 660 MeV/c; however, all the coils and power supplies for the dipole and the multipoles will be designed to be capable of producing 10% more magnetic field than is necessary for the 660 MeV/c. The overall assembly of the SASP system will allow easy transfer of the spectrometer to other experimental areas within the present and future areas of TRIUMF.The multipole element nearest to the target chamber and hence the one that will come nearest to the beam line will have to be specially designed to allow as close an approach to the beam line as possible. If a design similar to that of the MRS open sided quadrupole (shown in Fig. 3) is adopted then a minimum angle of 20° seems possible. Theclosest angle of approach between the MRS and the SASP must be less than60°; therefore,care must be taken in making sure the first MRS quadrupole does not require the same space that either of the SASP quadrupoles might require. The present layout indicates that an approach angle of 40° is feasible.The characteristics of the multipole nearest the target chamber are the following: The maximum quadrupole strength at the pole will be 8.50kG and the sextupole will be 1.01 kG. Its effective length will be 30 cmand its aperture will be 20 cm. The minimum angle of approach it willhave to the beam pipe will be 2 0°.The second multipole will have a maximum quadrupole strength of 8.5 kG and a sextupole field of 0.10 kG. Its aperture will be 20 cm and55its effective length will be 38 cm. The pole pieces will be detachable and will be shaped to provide the higher multiple components.The drift lengths from the tar­get to the dipole are the following:target to first quad(FEQ)=0.70 m first quad to second quad(BEQ) = 0.32 m second quad to dipole = 1 . 2 0 mThere may be some magnetic coupling between the two elements, because of the short drift length from the first to second quadrupoles. The aperture of both quadrupoles is 2 0 cm.There will not be sufficient room between the two elements toplace a vacuum connection, hence the pipe should be inserted through both quads during assembly. Both elements will be separately mounted on a rail system that will allow them to bemoved radially, with extra positional adjustments both vertically andtransversely. The vacuum system will be designed in such a way that either quadrupole can be removed from the assembly to allow the insertion of special targets or chambers.The clamshell dipole has a tapered magnet gap ranging from 10 cm at the inside radius (with a maximum field of 1.6 T) to 15 cm at the outside radius (with a field of 1.07 T). Figure 4 shows a copy of a sketch thatH. Enge prepared for this design. The dipole will weigh 90 tons, and the power supply will require 150 kW. The design will allow the coil power to be increased so as to produce fields 1 0% higher than those mentioned above. This would cause the magnet to go into saturation where the gap is approximately 1 0 cm, but would still provide a useful albeit lower resolution operation for momenta beyond 720 MeV/c. How far we go is obviously going to be determined by cost considerations. The bottom of the dipole comes within 2 0 cm of the floor, thus placing some special constraints on the support frame design.The coils of the dipole will have to be saddle shaped at the entrance and exit to allow the insertion of field clamps to ensure that the magnetic field conforms to the concave shape of the pole edges.The spectrometer should be able to operate at zero degrees in the laboratory, but at lower beam intensities. There is enough room to insert some shielding to shadow the beam spot from the focal plane detector.The vacuum chamber will not be self-contained in that the magnet poles will form part of the chamber wall, hence putting the pole pieces inside the vacuum. This has the advantage that the full magnet gap can be utilized, with no dead space due to vacuum chamber walls. The connec­tions between the spectrometer vacuum and the scattering chamber vacuumFig. 3. Sketch of the MRS open sided quadrupole. This could be a first order design for the SASP front end quadrupole.56S*£cr/»*/ AA.Fig. 4. Sketch of the Clam dipole, prepared by H. to be via a sliding seal. Provision will be made for the insertion of a front end low pressure wire chamber if certain special experiments require it. A couple of access ports for magnetic field probes for the dipole are also required.Figures 5 and 6 show views of the MRS and SASP in combination with a conceptual design of the SASP frame. A free standing frame is preferable but the vertical dimensions of the dipole may not allow this. The con­cept of having the dipole built into the frame structure is acceptable. The centre post connection and drive wheels will be attached directly to the magnet. The drive mechanism and angular readout will be controllable remotely. The support frame and drive must match with the centre post on the MRS and allow a mutual approach angle between the MRS and SASP of less than 60° for the (p,2p) requirements. The present design allows this angle to be as small as 40°. The drive mechanism will be designed to minimize positional hysteresis and to avoid correlated errors between57Fig. 5. Overall layout of the MRS and the SASP. Elevation view.Fig. 6 . Overall layout of the MRS and the SASP. Plan view.components of the motion (i.e., vertical motion from air pads should not cause a shift in theta).The 90° bend of the dipole allows us to employ a compact support structure for the shielding and the focal plane detector array, which will not impinge on a large area of the proton hall. The design can be made in such a way that different experimental detectors can be58interchanged easily and accurately, a philosophy that was successfully applied with the Resolution Spectrometer that the ( p , tt)  group used on beamline IB.Good personnel access is essential. The basic detector support should be easily removable from the area to allow preassembly of spe­cialized detectors. The cable access will be designed in such a way as to allow full rotation of the system without fouling the lines.The detector array will consist of three types of counters: vertical drift chambers, for trajectory information; plastic scintillators for dE/Dx, TOF and trigger information; and a Cerenkov counter for fast pion identification and rejection of the high momentum but low velocity pro­tons. The focal plane angle is about 45°, with the lower end nearer the target.About 20 cm thickness of iron shielding will be used to shadow the focal plane detectors from particles emanating directly from the target. Experience with the MRS has shown that the remaining particles hitting the focal plane detectors do not originate directly from the target, but are neutral particles in the proton hall. Shielding from this background would require enclosing the detectors in a shielded box, an expensive proposition which we do not consider at the present time. We would rely instead on a stringent event trigger to eliminate unwanted background, hence the need for the multiple layers of focal plane detectors.The data acquisition system must be capable of operating in a single channel mode with the SASP spectrometer on its own or in a two channel mode with the SASP and MRS in coincidence. Both the MRS and SASP must be operated independently at the same time and the acquisition system must allow such a mode with ease and transparency. Given the present CAMAC hardware constraints, this would seem to imply a separate crate control­ler system for each spectrometer in which the fast trigger logic of each spectrometer would both define an event. If these events were from single arm experiments, then there would be no further logic. If these events had to be in coincidence, then one more level of logic would be required in order to set both CAMAC systems into the transfer mode to the computer.There should be some effort to make the SASP system from the latest state of the art electronics. There is every indication that the data and singles rates in the focal plane detector could be quite high.The software support for the SASP should consist of the following types of programs:a) A basic introduction to the system. This program would be a menu driven program, which would provide the new or inexperienced user with all the information he would need to know in order to prepare for, per­form, and analyze a SASP or SASP/DASP type of experiment. This program would also help him initiate and operate the data acquisition and any local analysis he might want to do.b) The data acquisition program. This program should be callable from the "Introduction" program or directly callable by a simple terminal command. It must perform several functions as well as being user friend­ly (i.e., jargon free, easy command structure, changeable on line). Its two main subfunctions would be:i) diagnostics: real time analysis of a portion of thedata. This process must be changeable easily during a shift.59ii) data taking and storage: high speed, tape or diskstorage with a format structure compatible for all major computer systems on which the final analysis may be done. The technical details should not have to be dealt with by the "average experimenter" (i.e., the default choices must be well chosen), but the technical details should be well described so that the average experimenter can understand.In order to do the coincidence experiments between the SASP and the MRS, two major items of the present MRS system will have to be changed.The scattering chamber: To allow reaction products into both spec­trometers, a window subtending 160° on both sides of the beam will be required, as will independent angular adjustments of the spectrometers. This probably means the construction of a completely new scattering chamber.The MRS support frame: In its present form, the MRS support framewill not allow the two spectrometers to be set any closer than 90° apart in angle. A preliminary design has shown that the front end of the MRS frame could be modified to allow a much closer angle of approach for the two spectrometers. However, the work represents a considerable amount of reconstruction at the front end of the present frame.In summary, the objectives of the feasibility study have been fully met. A Second Arm Spectrometer can be built which in combination with the MRS will provide TRIUMF with a world class dual spectrometer facility for studying proton-nuclear interactions.The present funding schedule for the project calls for the major items to be delivered in the fiscal years 87/88 and 88/89. This schedule is contingent upon getting strong support from the TRIUMF EEC meeting to be held in July of 1986.60THE DASS/SASP DATA ACQUISITION SYSTEM G. LudgateTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3INTRODUCTIONWhen discussing the data acquisition and analysis system for DASS/ SASP one can afford to be a little speculative as time is on our side and a few months can see dramatic changes in price/performance for hardware. The SASP is basically another spectrometer, and TRIUMF already has one successful example of a large spectrometer and its data acquisition system, the MRS. But let us continue as though we were starting afresh.USER REQUIREMENTSThe basic requirement of all data acquisition systems in nuclear and particle physics is to gather events of physics interest, with a minimum of "background”, as fast as possible. Do we have anything to learn from the large particle physics experiments in exploiting the natural concur­rency of this problem and can it be applied to help DASS/SASP? We will return to the question later.DASS/SASP can, as the full name implies, operate in conjunction with the MRS or stand alone. The required operating modes are therefore:• MRS independently, alone• SASP independently, alone• Subdetectors of MRS or SASP independently and concurrently• MRS and SASP independently and concurrently• MRS and SASP dependently and concurrently as DASSThese modes are easily justified. The MRS (SASP) may be required for an experiment which precludes the operation of the SASP (MRS) due to experimental conditions or physical space requirements. It must be possible to switch quickly between these two independent modes.Likewise it is not hard to envisage that during a period of settingup equipment physicists may wish to trigger one or more subdetectors(called partitions) of the MRS and/or SASP independently and at the same time. Event trigger hardware and data acquisition software must both accommodate rapid changes into and out of this mode. The triggers from each partition must not interfere with each other and the event-data must find its way back to the correct analysis or logging process on the controlling host computer.There may be times when both spectrometers wish to run independently but at the same time. Essentially one would then need to duplicate the present MRS system. The most interesting requirement is when MRS and SASP are to be run in concert as DASS.The complexity of these modes coupled with the "inner working" ofeach mode led to the requirement that a computed-aided instruction (CAI) program be produced for those users who were just starting to use the system for the first time or else were infrequent users. Regular users would expect to interact more directly with the system in a more traditional manner.61TRIUMF DA PLANSThe "standard" data acquisition system at TRIUMF has been, for several years, a DEC PDP-11 running a modified version of MULTI/DA. Other data acquisition systems are in use on DEC and Data General computers but they all suffer from the same limitation, namely:• The 16-bit vertical address space limits the size of programs and data structures.• The computers are based on a single bus architecture connecting memory, the CPU and all peripherals, which has a bandwidth closely matched to the normal requirements of the CPU. Any activity on the bus therefore leads to a slowing of the CPU and thus the ability to analyse data.The next generation system will be one that supports both data acquisition and data analysis concurrently. It will be based on the DEC VAX-11 family of computers, and each system will come equipped with a 6250 bpi tapedrive, a disc of the order 450 Mb, to provide a firm base for both of the primary functions. The new systems will initially be connected to CAMAC data acquisition hardware. The System Crate architec­ture has been selected for systems carrying out event-by-event experi­ments. Its advantages are:• supports up to 7 CAMAC branches each with up to 7 crates.• supports a multiprocessor architecture - a CES 2180 STARBURST willbe located in the System Crate to perform the real-time task of acquiring data from CAMAC modules and buffering it for transfer to the VAX.• supports a DMA device that can autonomously perform CAMAC commands and transfer the results directly into VAX memory.Other data acquisition hardware systems, such as VME or FASTBUS, could also be easily interfaced.On the software side the intention is to provide support for easilyprogramming data acquisition and real-time event rejection in theSTARBURST and to provide a framework wherein these data are transferred to the VAX transparently. Once on the VAX, analysis programs having an interest in the data must be able to obtain the data quickly and with a minimum of users programming and, similarly, output from these programs should be made available to a further set of analysis programs, etc. One or more of these programs would likely write the data to tape, as conven­tion dictates.Of increasing interest in the data acquisition world is the use of networks, both local and continental in scope. The use of these networks will be promoted.In summary TRIUMF intends to support a number of software tools for which a data acquisition system could be built and tailored to the needs of a facility or a particular experiment, and to document these tools on paper or, more importantly, on line.The progress to date can be quickly listed as:• the purchase of a VAX-11/750, 6250 bpi tapedrive and 400 Mb disc, targetted for MRS62• the purchase of a MicroVAX II• the purchase of a System Crate CAMAC system complete with STARBURST and DMA controller• the completion of the software architecture and about 50% of its implementation• the start of documents, in earnestDASS/SASP HARDWAREIf you consider data acquisition to be mainly a communication prob­lem then the discussion of the hardware breaks down into:• the micro-area network, the traditional data acquisition computer and front-end electronics• the local area network• the wide area networkThe micro-area network for the MRS is CAMAC and NIM, but it may make good sense for some or all of DASS/SASP to be in FASTBUS. This is eco­nomical if the fast readout of a large number of channels is required, and after all it is the total cost that matters for the detector, itsoutput electronics and the digitizing electronics. A detector with more channels read by FASTBUS may be cheaper than one with fewer, perhaps more sophisticated, channels read by CAMAC.For the MRS and SASP to run concurrently as DASS one host VAXcomputer with two tapedrives may not be adequate if the users require sophisticated reconstruction algorithms for a good percentage of the acquired events. A MicroVAX II may well be required to handle thecomputation load (probably not the data transfer load). In fact it would be too difficult to incorporate the MicroVAX II into the System Crate as an analysis—only computer and allow another computer to record the data onto tape. In general the MRS and SASP data acquisition systems should be characterized by incorporating multiple microprocessors into their design to enrich the final datastream being written to tape.The word 'tape' used in the previous paragraphs can be regarded as standing for 'tapedrives' or 'as yet to be purchased data-recordingdevices’ such as optical discs, now available in 1, 2 and 4 gigabytewrite-once varieties.The local area network at TRIUMF is based on DECNET carried overEthernet. This network allows VAX users such features as remote log-in, file transfer, remote printing, messaging, etc. Users of the DASS/SASPsystem should not, therefore, feel restricted to the host computer for that system. It would be just as easy to perform real-time analysis ofdata on the 8600 cluster as on the host itself. As long as the eventanalysis time is longer than the event copying time (over the network) a factor of 4 in speed could be expected by using the 8600.For the wide area network several choices exist and more may be on the horizon. It is possible today to obtain 9600 bits-per-second network links into and out of TRIUMF over BITNET, DECNET, HEPNET, DATAPAC and COLOURED BOOK. The software packages that 'drive' these networks allow you some of the functionality available in the local area network. This picture will change in the near future due to the computer protocol standardization work of the CCITT and ISO bodies. Full functionality between any two vendors' computers is nearly here. The new technology63and software developed for the ISDN telephone exchanges will make 65 kb per second a standard speed for data (and voice) communications. Higher speeds of 1.5 Mb per second will be more easily available as all switch­ing and transmission move to become fully digital. One may be able to ship data home - in real time.DASS/SASP SOFTWAREA computer-aided instruction program will be made available to help new users and remind infrequent users of the operating principles of the DASS/SASP data acquisition and analysis system. The program could be driven by menu commands, light pen, touch screen, mouse, track ball or by voice command. Colour terminals will be heavily in use. The program will lead users through trails of their interest showing how to configure programs for analysis and start the system acquiring data. The tutorial program would invoke a command-driven control program for users to test their knowledge before continuing.The command-driven program would be for more experienced users and would be the means by which the front-end electronics are initialized and a run started. The aim of the software system is to both acquire data and to allow its easy analysis before being stored. Multiple micropro­cessors could be employed in the front end to examine raw events in real time and filter out those events that would be normally rejected by off­line analysis. This enriching of the datastream reduces the processing load of re-examining the data off line. Well planned experiments may have the software systems complete before an experiment begins and be able to record useful physical parameters of interest rather than theresults of digitization as is often the rule today.SUMMARYThe recent speed increases in both acquisition hardware and analysis computers have opened up the possibility of significantly reducing the amount of data stored for an experiment by converting it to information in real time (information is processed and summarized data). Thisreduces the off-line workload and leaves more time for extracting physics from the information and for planning future experiments. The step required to embrace this is to ensure software systems for experiments are ready and tested before the experiment begins, as are the hardwaresystems. This is not always possible for every experiment, but it shouldbe possible for most facilities constructed at TRIUMF and for a series of experiments of a similar nature.64NT ITT .EAR REACTIONS WITH INTERMEDIATE ENERGY PROTONS*R. DymarzTheoretical Physics Institute, University of Alberta, Edmonton, Alta.andTRIUMF, Vancouver, B.C.ABSTRACTNuclear reactions initiated by the intermediate energy protons in which only nucleons are identified in the exit channel are addressed. First the effective nucleon-nucleon interaction and the optical model potential is defined and next exclusive and inclusive (p,N) and (p,p'N) reactions are discussed.1. INTRODUCTIONThe nucleon is one of the most powerful probes to study a nuclear structure and nuclear forces. The interaction of nucleons with nucleus should be, in principle, treated within many-body theory of finite sys­tems. At present such treatment is possible only with several drastic approximations and in practical calculations we rely on simple models which, fortunately, become more and more "microscopic". The ocrrmon feature of these models is the separation of reaction mechanism frcm the nuclear structure effects and frcm the problem of nucleon-nucleon (NN) interaction in the nuclear environment. In my talk I will aover the first and third aspect of the problem. The arrangement of my talk is following: In section 2 I will describe briefly both the construc­tion of the effective NN potential frcm free NN scattering matrix (t^) and in the nuclear matter and the construction of the microscopic opti­cal potential. In section 3 I will ocrment on elastic scattering, inelastic scattering and on charge exchange reactions (p,n) leading to the discrete, excited states in the final nucleus. In section 4 I will talk about reactions (p,p'N) in the quasi-free region vhich also lead to well defined states in the residual nucleus. The reactions with the state of final nucleus well defined are generally called exclusive reactions. In contrast in section 5 I will discuss inclusive (p,p‘) and (p,p'N) reactions where states of the final nucleus is not identi­fied. Conclusions are presented in section 6.2. THE EFFECTIVE NN INTERACTION The NN potential can be written in a general formVe£flr) = Vc<r) + VUS(r)&*§ + V r)S12 (1)where S is a total spin, L - angular momentum of relative motion and S^2 is "the tensor operator. The central part of the interaction can be*Work supported by the Natural Sciences and Engineering Research Cbuncil of Canada.65decomposed in the spin (o)/isospin (x) space as followsV (r)=V (r) + V (r)(o.*a0) + V (r)x, *x0 + V (r)(a, •o„)(x, *x„). (2) c ~ o ~ a ~ ~1 ~2 x ~ ~1 ~2 ox ~ ~l ~2 ~1 ~2The spin-orbit part VLg(r) and tensor part can also be decomposed into isoscalar/isovector components. The above representation is in the form of the transferred quanta and is convenient in analysis of inelas­tic scattering. The relations with other representations is given in Ref. 1.Ihe local representation of the effective NN interaction is a drastic approximation to the realistic situation and is motivated rather by practical reasons (simplicity of using it in the nuclear reaction calculations) than on jhysical ground2. This local form of the Veff is usually related to the experimental free NN t-matrix (tj^ ) when used in the impulse approximation calculations or to the nuclear matter g-matrix vdien medium effects are expected to be important.The procedure of deriving Veff from t ^  is, unfortunately, rot unique. Ihe most popular one seems to be the one developed by Love and Franey3 . In Ref. 3 the tNN(E,q) is expressed by a local coordinate space interaction with antisyrrmetrization included explicitly in NN system (and in N- nucleus system when is used in nuclear reac­tion calculations):t^E.q) = /d3r e’^'S Veff[l + ( - D V l e ^ 11 . (3)Here P* is the space exchange operator and (-1)L ensures antisynmetri- zation. For computational simplicity VQ (r) and VLS(r) in Bq. (1) are taken to be a sum of the Yukawa (Y(r)) functions and VT(r) is taken to be Y(r)*r2. Ihe strengths of the potentials and ranges of the Yukawa function are searched such that the right hand side of Eq. (3) repro­duces experimental t ^  on the left hand side of Eq. (3). The parame­ters are tabulated for proton laboratory energies 50-1000 MaV.2 There are many uncertainties in these parameters. They come, for example, from errors connected with experimental t^, from representing Veff by- Yukawa functions, or from procedure of searching for "best" parameters. The errors introduced to N-nucleus calculations can be significant especially at higher momentum transfer.The medium effects (like Pauli blocking or Fermi motion) are incorporated into the nuclear matter g-matrix which satisfy the Bethe- Goldstone equationg(co,kF ) = V + VGQ(a),kF)g(a),kF) , (4 )vhere V is an MSI bare potential, u> is the total energy of the interact­ing nucleons and kp is the Fermi momentum. The Green's function G g U , k F ) obeys the outgoing boundary conditions and contains medium effects vhich arise from the requirement of propagation in unoccupied intermediate states under the influence of the average potential in the nuclear medium. There are several calculations along this line repor- ted1*-6 but in analyses at intermediate energies the most frequently used is the so called "Hamburg potential"6. The Hamburg potential is the g-matrix calculated with Paris NN-potential and approximated by a66sum of Yukawa functions as was described earlier for the Love and Franey potential3. The parameters of this potential are tabulated for several kp and for several proton laboratory energies between 60 and 400 MeV.6The Brueckner g-matrix is oonmonly interpreted as an effective interaction and used in analysis of elastic and inelastic scattering. The advantage of using g-matrix instead of derived from t ^  wasdemonstrated in some cases. However, while using the g-matrix in the microscopic construction of the optical potential has been justified by Hufner and Mahaux7 within the hole-line expansion theory, no derivation or justification of the use of the Brueckner g-matrix for inelastic transitions exists. Rather— as was shown recently8— this assumption is not quite correct when density dependence of g-matrix is strong. In addition to this basic problem there are uncertainties in g-matrix connected with the approximations adopted in numerical calculations. These uncertainties can already be seen in elastic scattering, where g-matrix enters through the optical potential only.The optical potential for finite nuclei is usually calculated in the folding model which can be interpreted as a first-order (single scattering) approximation within multiple scattering formalism. The optical potential can be written in this approximation as a sum of the local direct (D) and ron-local exchange (EX) terms which both are ener­gy dependent (see Ref. 9 for discussion and further references)UNL(£1,£]_»E) = 6(£1-£i)/d£2p(^ 2)vD(~l'~2'E)+p(El'~2)vEX(~l'E2'E)' (5)where £i(£o^ ;i-s a coordinate of incident (bound) nucleon. One can define local equivalent optical potentialU(rx ,E) = JuNL(r1 ,r‘,E)i|;(rpdr' , (51)where <Jj is the scattering wave function of the incident nucleon. Cer­tainly, the approximation of nonlocal potential by local one may well be a source of significant errors. The next approximation is to replace nonlocal interaction v°(EX' by the corresponding local and energy- (and density- in the case of g-matrix) dependent effective interactionVD E^K^(ri,r2,E) - gD,EX(|r1-r2|,p,E) .Tb eliminate ^(r1 ) from Eq. (51) usually a local momentum approximation is used giving a final expression for the potentialD EXU(r,E) = /p(r2)gc(s,p(R),E)dr2+ J p t r ^ r ^  (s, p(R) ,E) jQ(k|s| )dr2 ,where s = and B = (£i+E?)/2 and C refers to central part. Thenuclear matter density is p(r) and p(r^,r2) is the mixed density ma­trix, while ju is the Bessel function of zero order. The evaluation of mixed density involves further approximations and not being unique is a source of aiditional uncertainties. Comparing calculations with the experimental data it should be remembered that all these approximations67are made for the purpose of simplifying calculations and may not be correct in a particular reaction.In the relativistic approach a parallel development of models follows. However, for example, the optical potential is usually calcu­lated10 in the so-called "tp" approximation with t being the experimen­tal free t ^  matrix. In this approximation exchange is taken into account implicitly and only recently the relativistic analogue of the Love and Franey3 potential (exchange treated explicitly) has been cons­tructed and used in p>-nucleus calculations11.Due to the approximate treatment of exchange the "tp" is not quite correct in elastic scattering (see discussion in section 2) and is erroneous in inelastic scattering where sometimes the contribution from knock-out exchange term is of the same order of magnitude as a direct term. (See Ref. 12 for detailed discussion of the relativistic approach to inelastic scattering and further references).The relativistic Brueckner-Hartree-Fock calculations in nuclear matter were initiated by the Brooklyn group13 and have been reported by other authors11+. However, the resulting relativistic g-matrix was not used systematically in constructing the optical potential or in the inelastic scattering calculations.3. THE ELASTIC AND INELASTIC (p,N) SCATTERING TO BOUND STATES3.1 Elastic scatteringIn the context of the problems discussed in the preceding section a few observations can be made:(a) A differential cross section is not very sensitive either to the medium effects or to the relativistic effects. However, it is essent­ial in both the nonrelativistic and relativistic approach that exchange is taken explicitly in N-nucleus scattering (see Ref. 15). In Fig. 1 we show the differential cross section as a function of the momentum transfer squared. In part (a) of the figure the experimental data are compared with the relativistic impulse approximation calculations in the "tp" approximation and in part (b) comparison is made with the nonrelativistic folding model calculations in which nuclear matter g- matrix was used. One can see that energy dependence of the cross sec­tion at relatively small momentum transfer is not reproduced well in relativistic "tp" model while it is well reproduced in the nonrelati­vistic model where exchange is taken explicitly into account.(b) A large momentum transfer cross section can be reproduced neither within relativistic approach nor nonrelativistic models with nuclear matter g-matrix. The diffraction pattern of the calculated cross sec­tion is shifted towards smaller q in comparison with the experimental one. The mechanisms other than those included in the models discussed probably are important at large q.16(c) Even with nuclear matter g-matrix the experimental cross section in scattering on light nuclei is reproduced only qualitatively (see Fig. 2).(d) Ihe total reaction (oR) and total (orp=oR+ cross sections are too large when calculated within nonrelativistic model (IA or with g-matrix) (see Fig. 3). The relativistic models work well but proba­bly medium effects have to be taken into account at low energies (see Ref. 16).68- see next page -o n QFig. 1. The differential cross section for p- Pb elastic scattering. The solid lines represent the results of (a) relativistic impulse approximation calculations and (b) nonrelativistic folding optical model with medium modified effective NN interaction. The upper momentum transfer squared scale is for the upper curves and the bottom one is for the lower curves.O0 c.m. ( deg )Fig. 2. The elastic scattering cross section of proton on 160 at Ep=135 MeV. The solid line (G) and dashed line (G0) represent the results of the calculation with the full g-matrix and with the g-matrix at P=0, respectively.69CVJOJCTFig. 1.(GeV/c):70El(MA0Fig. 3. The total reaction (aR) and total (aT) cross sections. The curves RIA and RM4 represent relativistic impulse and relativistic medium modified calculations respectively. The curves NRIA and NFMM represent nonrelativistic inpulse approximation and medium modified (with nuclear matter g-matrix) calculations. See Ref. 15 for details and references to experimental data.713.2 Inelastic and charge exchange scatteringThe inelastic and the charge exchange scattering of protons probes different components of the Ve££. Below are shown the transitions with transferred spin (AS) and isospin (AT), the corresponding components of the central part of the effective NN interaction and the states (or resonances) excited:Interaction: V V Vo T 0 0T(AS,AT): (0,0) (0,1) (1,0) (1,1)State or GQR GDR 2",3+ MlResonance: first 2+, 3 ... IAR GTwhere GQR (GDR) refers to the giant quadrupole (dipole) resonance, IAR is the isobaric analogue resonance and GT and Ml refer to Gamow-Teller and magnetic dipole resonances respectively. The spin-orbit and tensor part of the Ve£f contribute also to some of the transiton indicated above but this contribution is significant at large momentum transfer.Ihe excitation with (0,0) transition is strongest; about 10 times stronger than excitations with other terms of interaction involved. Because of this the transitions to the natural parity states (n=(-)^ ) and GQR have large cross section. Usually these transitions are ana­lysed within collective model with transition potential proportional to the derivative of the optical potential which although contains also other pieces of NN interaction is dominated by V0 component.The analysis of experimental data on (p,n) reaction suggests that all the Fermi strength (VT) is collected in the IAR. In contrast to to this in the Gamow-Teller transition (AS=AT=1), mediated by spin- isospin component of the force (V ), only about half of the strength was found17. It was suggested that this missing strength is hidden in the background even at high excitation energies or the transition is quenched by the intermediate A formation19 (see also Ref. 20 for dis­cussion of quenching). Details will be discussed in the talk by S. Yen.A similar quenching to that measured in the GT transitions was found in Ml transitions in many nuclei both with AT=1 and ATO aiH it gave impetus for searching this missing strength in background as ATX transition cannot be quenched by virtual A excitation.A quenching is defined as a ratio of experimental cross section to the calculated one. Usually the cross section is evaluated in the DWIA or with g-matrix as a transition potential. It is obvious that both the transition potential and the distorting optical potential should be calculated correctly before any firm conclusion can be drawn about correct value of quenching.4. EXCLUSIVE QUASI-FREE PROTON SCATTERINGIhe quasi-free reactions (p,p'N) and (e,e'p) were from the beginn­ing designed to study cne-particle aspects of nuclear structure. Ihe mechanism of quasi-free scattering was supposed to be simple: incomingparticle knocks out nucleon in the nucleus which ranains in a cne-hole state. Ihe experimental data are analysed usually in the impulse72approximation (IA) with the formula exhibiting this simplicity (see Ref. 21 for references)® ^ r - « A i | T - " | 2 .(. , . (6 )1 2  1 Mvhere K represents kinematical and phase-space factor C2S is the spec­troscopic factor of the struck nucleon and o (9dN) is the off-shell differential pN cross section. The scattering matrix T has the form1““  ~ (7)where x 's a ie the scattering wave functions of the incoming (o) and outgoing (1 ,2) nucleons and 4> is overlap of the initial and final nucleus wave functions. In the distorted wave impulse approximation (DWIA) x's are calculated in the optical potential in the proper chan­nel and o(9dN) is taken on the energy shell. The function <}> is calcu­lated usually as a single particle bound state wave function in a Woods-Saxon potential.The Bqn. (6 ) is derived from the full lowest order distorted- wave Bom approximation formula for the amplitude22(8 )where V(jr,£' ) is NN effective interaction, g is the centre of mass coordinate of the interacting nucleons and r and r' are their relative coordinate in the initial and final states. This derivation is based on many approximations and simplifications and recently with more precise experimental data available, it became obvious that some cor­rections to the factorized formula (6 ) are necessary22,23. In particu­lar the measurements of the analysing power required that spin orbit distorting potential be taken into account and the cross-section- factorized formula be replaced by the amplitude-factorized one2 4. Such replacement, unfortunately, does not assure the correct description of measured Ay for large and/or asymmetric scattering angles25.In Fig. 4 we compare our DWIA calculations with experimental data for the quasi-free scattering of protons on 160 at an energy of 200 MeV. The shared energy cross-section and analysing power ware measured for the reaction 160(p,2p)15N at angles 6i= 02= 47° (TRIUMF) and forthe 160(p,p'n)150 reactions at 61=62= 45° (Indiana); both the transi­tions to the ground (j=l/2) and excited state (j=3/2) ware identified.The DWIA calculations were performed as described in Ref. 26 and thecross sections in the figure are plotted with the spectroscopic factors as indicated (i.e. half of the values of simple shell model predic­tion) . The following observations concerning the conparisons presented in Fig. 4 can be made.(a) The experimental cross section is reproduced only qualitatively within DWIA model.(b) The spectroscopic factors are too snail in comparison with the results of (e,e'p) (see Ref.26 for discussion and further references).= / d- X v(]3R d3r d3r'x!")*(k ,R + \ r)x  ^}*(k ,~ ~' ~ 1 1 ~ 1 ~ 2 1  2 ~ 2£'£' )x0(]S0»S ~ 2 £' H(g + 2 E') >73Fig. 4. Energy sharing cross section and analysing power for the quasi- free reactions (p,p'N) on 160 at proton laboratory energy of 200 MeV. The curves are DWIA calculations (see text for details).Fig. 5. The ratio of the (p,2p)/(p,p'n) reactions on 160 with 500 MeV protons. Hie solid (dashed) curve represents full (with distorting spin-orbit potential neglected) DWIA calculations. The dotted curve is the free scattering (p,p)/(p,n) cross section ratio (taken from Ref. 26).74(c) The analysing power is not reproduced even qualitatively within CWIA model.In Ref. 26 we have analysed the same reactions as those discussed above but for an incident proton energy of 500 NfeV and for asymmetricangles (0i= 21.5° and 02= 35°-75°). The overall conclusions were simi­lar to those mentioned in comment (a) and (b) (A^  was not measured).In Ref. 26 we also analysed in details the ratio of the (p,2p)/(p,p'n)cross sections and we have found that this ratio is reproduced neither by free scattering cross section (p,p)/(p,n) ratio nor by the full CWIA calculations (see Fig. 5). I have to mention that in Ref. 26 we used amplitude-factorized formula with the distorting optical potentials (central and spin-orbit) calculated microscopically with the nuclear matter g-matrix as an effective NN interaction. The failure of our attempt to reproduce experimental data satisfactorily convinced us that more precise evaluation of the amplitude (8) is needed. Unfortunately, numerical evaluation of Eq. (8) is difficult to perform and only quali­tative estimates27 have been made.The (p,2p) and (p,p'n) will be discussed in details by W.J. McDonald and C.A. Miller in this workshop. ISbw I would like to discuss some problems connected with (p,p'N) reactions on 3He and deuterium. As we mentioned earlier the scattering wave functions in Bq. (7) are calculated in the optical model potential. This approach still works for the reaction ttHe(p,2p)3He28 but the (p,pN) reactions on 3He29,30 and cxi deuterium31-33 are usually analysed in the plane wave approxima­tion (PWA) and the final state interaction is taken into account only occasionally and in an approximate way. In the EWA the expression for T (Bq. (7)) simplifies and the cross section becomes proportional to the momentum distribution of the struck nucleonFbr the deuteron, for example, |<j>|2 = u2(k)+w2(k), where u(k) and w(k) are Fourier transforms of the S and D wave components. It was known for a long time31,32 that the Bq. (9) breaks down for a deuteron momen­ta k>200 MeV/c. The measured cross section is an order of magnitude larger than that given by Bq. (9) already at k=300 MeV/c. In the recent experiment33 momentum k=650 MeV/c has been reached and measured cross section was found to be almost constant at large k (see Fig. 6).The enormously large cross section at large k (corresponding large scattering angles ±0p) was explained by production of virtual A. How­ever, the role of A's seams to diminish at large k31+ and the experi­ments at large 0 could determine if mechanisms other than production of virtual A is important at large k. A sudden breakdown of formula (9) is in apparent contradiction with the results of (e,e'p) experi­ments35 where contributions from A, meson exchange currents (MEC) and final state interaction are moderate and they are only corrections to the formula (9) (see Fig. 7). The other mechanisms which are missing at large k are multiple scattering effects. Although the multiple scattering corrections when calculated36 were found to be small it seems that the method of Ref. 36 was not accurate at energies where it was applied. The multiple scattering effects are suggested to bedOjd^dEdo U (k)!2o(0pN) (9)750  5 0  0 0  2 0 0  3 00  4 0 0  5 0 0  6 0 0  650T  ©IN'ftIMs .b *0• i  " " r rP W I» [P » R IS ( ^ \«V?-----— ft ft ~‘ft>|n j (M « V / c )0 p (°)Fig. 6. The differential cross section for d(p,2p)n reaction at T_= 509 NfeV. The dashed curve is the PWIA prediction with the Paris potential. The solid curve is the cross section for virtual A-excitation (Ref. 34) and the dotted curve is the in­coherent sum of the two (figure taken fran Ref. 33).Fig. 7. Differential cross section for d(e,e'p) reaction. The calcula­tions (with Reid soft aore poten­tial) are as follows: BA - withnucleon plane wave functions; N = BA+FSI (final state interaction); IC - isobar configurations; MEI meson exchange current (figure taken from Ref. 35).important in explaining the experimental results (particularly A ) in the inclusive reactions (see section 5).The role of the A in the reactions like (p,p‘N) or (e,e'p) depends strongly on the amount of energy transferred (u>) to the system by the projectile, i.e. cn the kinematical conditions of reaction. The large momentum transfer and small energy transfer in the inclusive electron scattering (e,e‘) is weakly dependent on the A excitation and MEC and (see discussion in Refs. 37 and 38) unlike the inclusive (p,p') scat­tering (see section 5), it is free of multiple scattering effects. As such it is an ideal process to probe large momentum components of the nuclear wave function through formula similar to Bq. (9). It was shewn39 that the cross section for inclusive electron scattering under the above mentioned conditions (large q, small u>) scales (i.e. depends not on q and u> separately but on some contained variable) in variabley^'Vlqlo(a),cr)dw = (Za + (A-Z)a )F(y)dv , ep en *where °ep( en) are elementary e-N cross sections and F(y) measures the76probability to find a nucleon with momentum k(|=y. The scaling hypothe­sis has proved to be correct for both 3He(e,e')37 and 2H(e,e')38 reac­tion. In both cases the momentum distribution was tested up to k=800 MeV/c and in both cases the substantial underestimation of experimental values of |<j>(k)|2 at large k was obtained when the "most reliable" wave functions for 3He and deuteron were used.Recently we have obtained a deuteron wave function with AA compo­nents4 0. The presence of AA components in the deuteron wave function has enormous impact on large momentum behaviour of the deuteron wave function 4>(k). As a consequence we are able to correctly reproduce the scaling function F(y) ip to y corresponding to k«800 MaV. As we see here and as we will discuss later (section 5) the study of the large momentum components in the nucleus seems to be of great importance. It appears that the cross section in the inclusive scattering of nucleons scales also in a variable related to y and reconciliation of electron and nucleon data appears to be a serious challenge for future experi­mental and theoretical studies.5. INCLUSIVE REACTIONS WITH PROTONSBy inclusive reactions we understand all reactions in which the fined, state of the nucleus is not identified. Those can be the reac­tions with one (p,x) or more (p;x,y,z) particles in final state detected: the secondary particle can be any particle or light ion(x,y,z=n,N,d,3He...12C). We wall be interested here only with nucleons as secondary particles. The detection of particles in coincidence is experimentally difficult and because of this most inclusive experiments reported are wath only detection of one particle in the final state. However, the coincidence experiments are crucial for understanding the mechanism of production of particles in N-nucleus and nucleus-nucleus collisions. (See for a recent review, Ref. 41).5.1 Inclusive A(p,N) reactionsThe shape of the spectrum of the secondary nucleons in inclusive A(p,N) reaction depends strongly on the energy of incident protons and on the scattering angle. Generally at lower energies a broad peak at forward angles is observed as can be seen in Fig. 8. At large angles the peak disappears and eventually at backward angles cross sections fall exponentially as a function of energy (see Fig. 8). At higher energy the peak is rot so broad and is followed by a broader one con­nected with virtual production of A isobar (see Fig. 9). As can be seen in Fig. 9 the angular dependence of the cross section at higher energies is very similar to that at lower energies.(a) Snail angle scatteringThe broad peak corresponding to not very large energy losses observed at forward angles exhibits a kinematical behaviour expected for the scattering of the incident proton by a bound target nucleon. The energy at the maximum is ~q2/2m where q is the momentum transfer and m is nucleon mass. The detailed study of this quasi-free (QF) peak is crucial for understanding the mechanism of the early stage of the nuclear cascade developing in the nucleus in N-nucleus reactions. For77K I N E T I C  ENERGY,  T ( M e V )Fig. 8. Comparisons of neutron and proton spectra (solid circles) from the bombardment of an 27A1 target by 90 MeV protons with the predic­tions (solid lines) of a PWIA cal­culation for quasi-free scattering (taken frcm Ref. 42).Fig. 9. Single proton inclusive spectra for 800 MeV protons. Solid curves are drawn for guid­ing the eye. Arrows indicate the proton momenta for proton- nucleon quasi-elastic scatter­ing (taken from Ref. 43).P ( G e V / C )Fig. 10. Giant resonance spectra at 8° and 16° for a 208Pb target. The heavy arrows indicate the maxima in the broad continuum peaks (taken frcm Ref. 44).Fig. 11. Plot of Ay(0) versus scattering angle for the region near the centroid of the proposed quasi-free peak and for regions of excitation 10 and 15 MeV above that for the quasi-free peak. The curves shew the phase shift pre­dictions for pp and np scattering (taken frcm Ref. 44).QUASIFREEQUASIFREE ♦ tO  M«VQUASIFREE ♦ 15 M «V78heavier targets where the QF peak composes a great amount of background for the giant resonances (see Fig. 10) a precise knowledge of QF peak would allow a calculation of this background unambiguously.Although it seems now to be vrell established that the QF peak ori­ginates from single NN collision, a detailed comparison with experiment suggests that other mechanisms (multiple scattering, higher multiple resonances) can contribute to the cross section in this region. The additional information about the reaction mechanisms contributing to the cross section in QF region can be obtained from measurements of spin observables. A few such measurements were reported for analysing power A *♦**—'♦ 6 , Spin flip S ^ 45 ,47 ,1+8 and other spin transfer coef­ficients^9. Although there are sane discrepancies between different experiments46,47 the general conclusion is that the measured spin observables are in better agreement with free pN values in QF region than off this region. (See Fig. 11). The existing discrepancies can be attributed to the distortion effects or to sane relativistic effects. The Ay in the A region is well reproduced within the nuclear cascade model with intermediate production of A50.(b) Large angle scatteringThe inclusive scattering at large and backward angles (with large momentum transfer) is even more interesting than the region of forward angles. The interest in this type of reaction started with the experi­ment by Frankel et al.51 where inclusive cross sections of p,d and t production were measured at 0=180° in reactions with intermediate energy protons (600 and 800 MeV) on several targets from beryllium to lead. The measured cross section for secondary particles was fitted by simple expression a(p) ~ exp(-ap2) (see Fig. 12). The explanation for production of fast protons at backward direction was first offered by Amado and Wbloshyn52 in the so called direct knock-out model (DK). The argument of the model is based on the observation that because of large energies and short time involved this should be direct reaction and not statistical. The simplest direct mechanism in the reaction discussed is single pN scattering. Because protons observed at angles 0 >90° are in the region kinematically forbidden for free pN scattering, the pro­posed mechanism requires that the struck nucleons be moving backward with high virtual momentum before the collision. If the model were correct the type~of measurenents reported in Ref. 51 would be useful in studies of high momentum components of the wave function. The proposed model flourished in several papers and in the so called "quasi-two-body scaling" hypothesis53 which states that in the reactions of the general type A(x,y) where x and y can be N,d or light ions and A is any nucleus the backward cross section is governed by quasi-two-body kinematics with the scaling variable l<^n being the minimal momentum of residual recoil nucleus. Later543 it was shown for the (p,p‘) reaction that this scaling can be interpreted quite differently with the two-body kinematics corresponding to the on-shell scattering before and after collision. The interpretation of experimental data within this simple single-scattering model requires a great amount of the large momentum components in the one-nucleon wave function in apparent contradiction with the results of inclusive scattering of electrons and calculations79Fig. 12. Differential cross section for 180° production of protons. The fits are made with Bp exp(-ctpP2/2nu) (taken frcm Ref. 51).Fig. 13. Analysing pcwer in d(p,p')x reaction at 120° is compared with the free pp scattering and with the values of Ay reported for Li and Ta in Ref. 61 (taken frcm Ref.64).Kirin ©«V/c}• ♦“NP .K C t - *P l  • P2 ♦ X ep=i£ eB=n0’d4o~Fig. 14. The integrated one- nucleon momentum distribution for inclusive scattering of 200 protons, 0.6-1 GeV protons (high energy data fit), elec­trons and alpha particles (180 MeV/nuclean) (taken frcm Ref. 54b).Fig. 15(a) The observed experi­mental coincidence cross sec­tion between the forward- and backward-going protons (Ref.69). The numbers on the curves denote the contour line of the cross section, (b) The calcula­ted coincidence cross section using the deuteron-like cluster model, (c) The calculated coin­cidence cross section using single scattering mechanism (taken frcm Ref. 70).80with standard models55. The differences between the proton and elec­tron data seen to reduce substantially for low incident proton energy5‘♦b . in Fig. 14 the so called integrated one-nucleon momentumdistributionG(ktnin> - J  n(k>k **minis shown for inclusive scattering of 200 MeV protons and compared with high energy proton data fit51+3 , inclusive scattering of electrons and alpha particles. While agreement with electron data is quite satisfac­tory, the large differences between low and high proton energy data are surprising. It is argued5*b that the final state interaction of nuclear fragments is responsible for these differences as at higher energies the multiple scattering effects are much larger than at low energies. The unphysical amount of the large momentum components is necessary to mock up this effect within single scattering model.There were many other models proposed which fitted the experimen­tal data of Ref. 51 equally well as DK model. Those models ranged from the so called multi-nucleon transfer model56 through correlated cluster model57 to the equilibrium models58,59. Tb falsify at least seme of the models new kind of experiments are needed. One of the possibility is to measure "less inclusive" reactions like A(p,p’N) and this subject will be addressed shortly in the next section. The other possibility is to measure some spin observables. Analysing power A measurements has been reported for few targets60-61*. The results of all experiments generally are compatible swith nonzero Ay. This suggests that the mechansim of reaction is not statistical a m  then all models based on statistical arguments should be ruled out from further consideration (and also model or Ref. 56). However, the measured analyzing power also differs from the Ay in free piSI scattering even at scattering on deuteron61* (see Fig. 13), suggesting that model like IK is also not quite correct. The multiple scattering effects play probably a major role in determining the Ay65 but unlike for forward angles no reliable calculations of Ay. at backward scattering angle exist up to now.5.2 Inclusive A(p,p‘N) reactionsIn the inclusive (p,p'N) reaction at least one of the outgoing nucleons is far removed frcm the kinematic region accessible in the quasi-elastic (p,p’N) reaction discussed earlier, where excitation energy and recoil momentum of the final nucleon are measured. Contrary then to quasi-free (p,p'N) reaction, where both the outgoing nucleons are detected at forward angles (Qf), in inclusive (p,p'N) reaction one of them is detected at large angle (e >90°). The experiments were per­formed with the light targets to minimize the rescattering effects of the outgoing nucleons. At energy of 800 MeV reaction (p,2p) was stu­died on 6Li66, at 640 MeV on 12C67, at 300 MeV on 9Be68.Recently the results of the most extensive study of the inclusive (p,p'x) experiment have been reported69. In this experiment the 800 MeV protons were scattered c h i C, KCL and Fb targets. The forward (0f= 15°) protons or deuterons were detected in coincidence with the backward protons (0 = 118°) in both the in plane (in pi.) and out of81plane (out pi.) configurations. In the subtracted (oin °o t 1  ^cross section a strong correlation of the maximum of the §ross°sec?i6n in both the channels (p,2p) and (p,p'd) was observed. This region of momenta of the two outgoing particles, where the correlation has been seen is well separated from the region of momenta, where the single scattering mechanism in the p-p scattering is expected to dominate and it gives evidence that the production of backward protons comes partly from scattering on deuteron-like clusters. The calculations70 within the correlated cluster model reproduce the experimental data quite well (see Fig. 15). It seems that more coincidence experiments along the line of the experiment reported in Ref. 70 are necessary for under­standing the mechanism of production of backward protons in inclusive scattering.6. CONCLUSIONSThe aim of try talk was to give a brief review of topics connected with the reactions induced by the intermediate energy protons and to underline some problems encountered when interpreting the experimental data within the currently acceptable models of nuclear reactions and nuclear forces. The models I discussed were nonrelativistic and I only listed those developed in relativistic approach. It is obvious that in constructing the models the approximations are made for the purpose of simplifying calculations and not on physical grounds. As was pointed out in Ref. 2 the local representation of effective NN interaction is one of the examples of this procedure. The obvious problems with des­cribing data in quasi-elastic (p,pN) reactions as discussed in section 4 seam to support the arguments developed in Ref. 2.In connection with the discussion of the d(p,2p)n reaction it seems obvious that the relative importance of multiple scattering effects and more exotic mechanisms like the intermediate-A formation has yet to be established in reactions in off-quasi-free scattering region. Ihe inclusive scattering of protons in quasi-free scattering region can be qualitatively reproduced in the single scattering appro­ximation. However, understanding the details of the cross section, the spin observables and, in particular, the backward angle scattering require more complicated mechanism to be considered.I would like to thank Drs. F.C. Khanna and D.M. Sheppard for reading the manuscript.REFERENCES1. G. Bertsch et al., Nucl. Ehys. A 284, 399 (1977).2. E.F. Redish, in: Antinucleon- and nucleon-nucleus interactions, ed. G.E. Vfelker, Ch.D. Goodman and C. 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Rev. C 24, 2684 (1981).67. V.I. Komarov et al., Bays. Lett. 80B, 30 (1978);V.I. Komarov et al., Nucl. Bays. A 326, 297 (1979).68. R.E.L. Green et al., Nucl. Bays. A 405, 463 (1983).69. Y. Miake et al., Phys. Rev. C 31, 2168 (1985).70. V. Haneishi and T. Fujita, Bays. Rev. C 33, 260 (1986).84(p,2p) Scattering in Nuclei W.J. McDonaldUniversity of Alberta, Edmonton, Alberta, Canada T6G2E9ABSTRACTThe potential of the (p,2p) reaction for studying properties of the NN interaction and states of bound nucleons in nuclei has been long recognized and many pioneering experiments have been done. However, the several conditions required for systematic study of this process are only now on the threshold of being realized. The necessary ingredients include:- a) proton accelerators with good energy resolution, high duty factor, and polarized beam capability in the energy range of 200 to 600 MeV, and b) dual arm spectrometer systems capable of isolating discrete nuclear states and covering the kinematical regions of interest both in and out of the reaction plane.INTRODUCTIONThe concept of quasi-free scattering of protons by nucleons in a nucleus came in 1952 at the Berkeley 350 MeV Cyclotron when pairs of protons were observed emerging from a Lithium target bombarded by protons 1,2. The proton pairs were correlated in angle with a spread that was later shown to be consistent with the Fermi momentum distribution of nucleons confined to a nuclear volume 3. It appeared that an incoming proton was being scattered from an individual nucleon moving in the target and that nucleons other than the struck one were not strongly affected. This was an exciting result because physicists now had a way to observe the momentum states of nucleons within the nucleus and examine their momentum distributions. The parallel between this story and the recent discovery of quark-gluon'jets emerging from collisions of high energy protons is interesting. Both discoveries represent extensions of Rutherford scattering, one to see the substructure of nuclei and the other to see the substructure of the nucleon constituents.In subsequent studies of (p,2p) processes summed energy spectra of the proton pairs were used to determine binding energies of the ejected protons. The results provided direct evidence of shell structure in nuclei by resolving the individual shells predicted by Mayer  ^ and Jensen et al and even demonstrated the expected spin orbit splitting of nuclear orbits 6,7. In the period since these pioneering experiments much experimental and theoretical work has been done and is summarized in several reviews (see for example reference 8 for a complete listing).PRESENT STATUSTo illustrate the present status of (p,2p) data and interpretation in terms of model calculations I will use some of the results from TRIUMF. Fig. 1 shows somerepresentative missing energy spectra for the 1 8 0 (p,2 p)85reaction.200incZJoOSeparation Energy (MeV)1 6 ,fig 1.Separation energy spectrum for 505 MeV 160(p,2p)These data were obtained for a 500 MeV proton beam with a time of flight spectrometer operating in coincidence with the TRIUMF MRS spectrometer. The resolution is not adequate to separate final states although it does allow a rough separation of p1 / 2 and p3 / 2 groups. The inability to resolveindividual states is a serious limitation when one attempts to do spectroscopy, but some studies of the NN interaction in the nuclear medium are possible. Better resolution can be obtained at lower energies but it is highly preferable to do experiments over the energy range where the nucleus is most transparent, ie ~ 200 to 600 MeV.In fig. 2 cross section and asymmetry data are shown for ’0(p,2p). These data were obtained at 200 MeV with scintillation counters but the resolution was not significantly better than that shown in fig. 1. They demonstrate strong spin-orbit dependence and show that asymmetries of the outgoing protons are roughly consistent with expectations based on the the Shell Model and the Distorted Wave Impulse Approximation (DWIA). In this model, it turns out that the struck proton is, in general, polarized and depending on the kinematics, the degree of polarization can be quite large in agreement with experiment. It comes about as a result of distortion of the outgoing channel waves by the optical potential in combination with spin-orbit coupling in the target nucleus and the sensitivity of the pp interaction to the relative spin directions of the colliding protons 8.The ability of the DWIA Model to predict the cross sections and asymmetries is not outstanding, as shown in fig. 2 but the main features of the spin dependence are86fig 2.1 60(p,2p) cross sections and asymmetries for 200 MeVincident protonsreproduced. It is possible to obtain the effective polarization of the struck nucleon from measured asymmetries and thus test to see if we get the same result for p 1 / 2 and P 3 / 2 states as one would expect. This kind of comparisontest should be reasonably independent of the details of the DWIA calculation. As shown in ref. 9 and fig. 3, the results are fairly good for symmetric kinematical situations but not for all geometries. It has been noted 9 that agreement is greatly improved if one sets the pp scatteringpolarization, P (0) equal to zero (fig. 4). Similar resultshave been obtained for the Id states in ^8Ca (ref. 18 and figs. 5 and 6 ). Several possible explanations for this phenomena have been proposed, including medium effects on the NN vertex 8 and changes in the effective pp scattering angle in the nucleus11,FUTURE POSSIBILITIESBefore attempting to discuss future experiments it is useful to consider again the original reasons for optimism about the potential of (p,2p) measurements. The "window into the nucleus" that the Berkeley results suggested has provided some important confirmations of our ideas about nuclear structure and reactions. However, progress toward the870.0- r i------- - r  r▲  A  i '/2  4 .^ %  4  p e f f  - 4  %  A 3 /2A * - 2 p c f f1------A-*e . - 3 0 ’’ 3 0 “ '}  8,-10° f  0 2 = 3 5 °  A* T♦  ♦♦  •. i  A A A  a-i q i l  a A A  af 1  9 , =  3 0 “6 r  4 0 °  A^ * 1 a a  a *  . ,°  # r 3 0 ° l l -  . 0 2 = 4 5 °i  +*1 i r£ 2 = 3 5 0 'I i i i i iT *- 0 , : 4 0 °  V0 2 = 4 0 °  1  i f  i12040 80 120 40 80T, (MeV)fig 3. peff from 1 60(p,2p) datadevelopment of an effective "microscope" which would let us examine the specimen closely has been slow. The nucleus is not very transparent to hadron probes and the lack of a solid theoretical footing for hadron interactions is a serious problem. For both of these reasons the (e,e'p) reaction has proven to be more effective for studies of nucleon separation energies and momentum distributions. Nevertheless, the (p,2p) reaction has some advantages which can be exploited using present technology.What are these advantages and how could we make use of them? Perhaps the most important feature of the hadron probe is the opportunity to control the spin and isospin parameters on the NN interaction. By using polarized proton beams we can control the spin, and by detecting coincident pn or pp particle pairs in the exit channels we have a handle on the isospin as well. C. A. Miller will discuss the (p,pn) measurement possibilities later in this workshop. A second important advantage is the large cross section. Next to elastic scattering, quasi-free scattering is the most probable way for a proton to interact with a nucleus and, given a good detection system, a variety of exclusive experiments are possible. Finally, (p,2p) shares with (e,e'p) the considerable advantage of kinematic control. The momentum of the struck nucleon (which equals minus the recoil momentum) can be controlled independently of the NN interaction kinematics and this makes it possible in principle to separate nuclear structure from reaction88mechanism effects.With these ideas in mind it is possible to examine some experiments which would be possible with a dual arm spectrometer system and the TRIUMF polarized proton beam. With a pair of good spectrometers it would be possible to do some detailed spectroscopic studies of the nucleon orbitals as a function of atomic mass, A. For example, measurements °f Peff could provide a sensitive test of nuclear structure models and give useful information about the structure of filled shells 8. Of course more work is necessary on the theoretical side too so that the reasons for the present lack of agreement between predicted asymmetries and experiment are understood.0 .50.0- 0 . 50 .50.0-0 .50 .50.0- 0 . 5T --------i------- 1--------1--------t-------- r -\A^  eff^  4 (: 2 P ««3/2)<0 0♦° 6 . 0 +  • 6 , *3 0 *  ♦ *e 2-3 cr‘ l  ^T <•B . - 3 0 *0 2* 3 5 *  ♦ T♦ ♦ H . i i WI ' i "  *- r1 e .-  3 0 *  ' l L 4 0 *  t  tIT T v1 1 T f *  ," e . - 3 0 *  J ^ F  e , - 4 5 *  J t. l u iI f , { t t ' . v  t f1 ^  1e  *3 5 *  I e '* 3 5 *1 1 1 . ! -------1--------1-- L_1* \ (ft>e .« 4 o *  t  e , * 4 o *i z j_____ i_____ i- - 1-- 1—40 B0 120 40  80  120T, ( M e V )fig 4. peff from 160(p,2p) data with P(0)=OFrom the point of view of reaction mechanisms, it would be interesting to investigate the dependence of P (0 ) on the effective density of the nucleus where the reaction is localized and this could be done by controlling the reaction kinematics appropriately. This would be a nice way to begin a search for medium effects on the NN interaction. The possibility also exists to include isotopic spin dependencein the search by comparing P(0) determinations from (p,pn) and (p,2p) reactions in the same kinematic situations.fig 5. peff from 40Ca(p,2p) datafig 6 . peff from 4 0Ca(p,2p) data with P(0)90Most of the necessary ingredients already exist at TRIUMF. A polarized proton beam is available with high duty factor and covering the energy region where the nucleus is least absorptive to protons. The Indiana University Cyclotron Facility is likely to have a complementary facility which will permit studies at the lower end of the energy range of interest but the TRIUMF energy range is ideal and probably necessary. The presently available reaction models are less reliable at lower energies and in any case it will be necessary to study the energy dependence in order to establish that any reaction model can be believed. The MRS is an appropriate detection system for one of the exit channels and has < 100 keV energy resolution. It may also be suitable for neutron detection to observe (p,pn) reactions. If approved, the proposal to build a complementary second arm spectrometer would provide a very good facility for extending the TRIUMF (p,2p) program. One feature which is not part of the present proposal is a way to reach out-of-plane kinematical situations. While it is certainly true that many experiments can be done without such a capability it would be very advantageous to have the kinematic flexibility. This is particularly true when one attempts to separate the nuclear structure from the NN interaction effects. Frequently one would like to follow a kinematic locus in which the parameters of one or the other of these is kept constant. Invariably, this means going out of the reaction plane to some extent.In summary, the addition of a second arm spectrometer to the TRIUMF facility would provide a unique opportunity for finally exploiting the potential of quasi-free scattering of protons. Consideration should be given to a support structure for the second arm which would permit out-of-plane measurements.REFERENCES1. Chamberlain, 0., and Segre,E., Phys. Rev. 87, 81 (1952).2. Cladis, J. B., Hess, W. N., and Moyer, B. J., Phys. Rev.87, 425 (1952).3. Wilcox, J. M., and Moyer, B. J., Phys. Rev. 99, 875 (1955).4. Haxel, O., Jensen, J. H. D., and Suess, H. E., Phys. Rev. 75 , 1766 (1949).5. Meyer, M. G., Phys. Rev. 75, 1969 (1949).6 . Tyren, H., Maris, Th. A. J., and Hillman, P., II NuovoCimento 6 ,  1507 (1957).7. Tyren, H., Hillman, P., and Maris, Th. A. J., Nucl.Phys., 7, 10 (1958).8 . Kitching, P., McDonald, W. J., Maris, Th. A. J., andVasconcellos, C. A. Z.,in Advances in Nuclear Physics, vol 15, Plenum pub.,43 (1985)9. Veit, E., Ph. D. dissertation, Universidad Federal do RioGrande do Sul, Brazil (1981)10. Antoniuk, L., Kitching, P., Miller, C. A., Hutcheon, D.A., McDonald, W. J., Neilson, G. C., and Olsen, W. C., Nucl. Phys., A 3 7 0 , 389 (1981).11. Miller, C. A., Nucl. Phys., A353 , 157c (1981).91NEUTRON KNOCKOUT MEASUREMENTS WITH DASSC.A. MillerTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3ABSTRACTWe propose that the Second Arm Spectrometer now being designed will be very useful as part of a facility for the study of neutron knockout from nuclei. Although the momentum distribution and separation energies of nuclear protons are now best studied at the modern (e,e'p) facilities, such is not the case for neutrons. Uncertainties in the theoretical interpretation of the (p,pn) data due to strong distortions can be reduced through the comparison of (e,e'p) and (p,2p) measurements on the same target. In this way the proton probe can be "calibrated”. We present two distinct experimental approaches with complementary capabil­ities. On the basis of count rate estimates, we anticipate modest beam time requirements for this type of experiment. An example of a possible first measurement is suggested.INTRODUCTIONKinematically complete measurements of nucleon knock-out reactions exciting nuclear hole states provide the most direct information about momentum distributions and separation energies of nucleons. The three- body final state is determined experimentally by measuring the momenta of the scattered probe particle and ejected nucleon, from which the energy and momentum of the recoiling nucleus can be inferred. If a suitable kinematic regime is chosen so that the nucleus is reasonably transparent to the probe and ejectile, the recoil momentum is closely related to the initial momentum of the struck nucleon. The quantitative extraction of nuclear momentum distributions and occupation probabilities for shell model orbitals require the introduction of the nuclear optical model to account for the interaction with the nucleus of ejectile and the probe if it is hadronic.Some decades ago, such measurements using protons as the probe provided dramatic qualitative confirmation of the shell model picture of the nucleus. Since then the application of this reaction as a quantita­tive probe of the details of nuclear structure has been retarded on the one hand by experimental difficulties, especially in the case of the electromagnetic probe and on the other by inadequacies of the reaction model especially in the case of the strongly absorbed hadronic probe. However, fundamental information about the momentum distributions and separation energies of nuclear protons has been obtained. The (e,e'p) results are reviewed in Ref. 1 and (p,2p) results in Ref. 2.We are now experiencing the beginning of what promises to be a renaissance in the field because of some recent developments. There are being commissioned dual-spectrometer facilities capable of high resolu­tion in recoil nucleus excitation. As well, there are becoming available higher duty factor electron beams and polarized proton beams at energies best suited for penetrating nuclei. Finally, the reaction model so crucial for interpreting the data from hadronic probes is being improved. We will return to this point.92$2:titinr^OFig. 1. Fragmentation of lf-hole strength in 9 0Zr. The errors in­dicated are statistical.Proton hole state spectroscopy is being led by (e,e'p) facilities such as the new dual-spectrometer system at NIKHEF-H in Amsterdam. Although the interpretation of (e,e'p) data is complicated by the one strongly-interacting particle in the final state, there is evidence that reliable spectroscopic information can be extracted. 3 For example, the fragmentation of If strength in 90Zr has been studied, leading to the distribution shown in Fig. 1. The total If strength from 0 to 20 MeV amounts to 8.9 compared with the shell model sum rule limit of 14, indicating a substantial depletion of shells just below the Fermi surface.In spite of the clear advantage of the electron probe, (p,2p) experiments still have at least two roles to play. One is the detailed investigation of the effect of the nucleon medium on the nucleon-nucleon interaction. Here spin-transfer observables which are relatively insen­sitive to nuclear structure offer a way of disentangling the different aspects of the reaction mechanism. 9 The other role is based on the demonstrated sensitivity of (p,2p) analyzing powers to the J-value of the struck nuclear proton. Spectroscopic information obtained in this way could complement that from (e,e'p) experiments if a suitable high-resolution (p,2p) facility were available in the best energy range near 400 MeV. Here both initial and final state particles have the lowest possible interaction probabilities so that two-step processes are suppressed. Such a facility will exist when the dual spectrometer facil­ity now under construction at IUCF is installed on the cooler/tripler ring now also being constructed. However, a schedule for this does not exist at present. Figure 2 shows the quality of separation energy spectra even for deep hole states that can be obtained with a hadronic probe of sufficiently high energy.For obvious instrumental reasons, neutron hole-state spectroscopy has lagged far behind that of protons. Although there is some hope that the next generation of high duty factor electron accelerators will make (e,e'n) experiments feasible, 1 it remains that the (p,pn) reaction isE.lMeV)wZMe.e'p),9Y i  =393Nidne >qw»OMi mrrgy, MeVFig. 2. Proton and neutron separation energy spectra ob­tained from knockout from lp shell nuclei by 1 GeV protons (Ref. 14).the only practical probe with direct access to the bulk of the momentum distributions of neutrons in nuclei. Other reactions such as nucleon pick-up are sensitive only to the high-momentum tail.In the foreseeable future, knockout reaction data will be inter­preted in the context of the distorted wave impulse approximation (DWIA). In this model, the transition amplitude is taken to bewhere the x's are optical model scattering wave functions describing the interaction of the probe particle a and the ejectile b with the target nucleus A or recoil nucleus C. The spin indices have been suppressed. The optical potentials are constrained by nuclear elastic scattering and total reaction cross section data and are increasingly influenced by guidance from microscopic calculations of the optical potentials based on the density-dependent nucleon-nucleon interaction. 6 ({ij^ is the initialbound state wave function of the struck nucleon which for protons is strongly constrained by electron scattering data. The matrix elements of tflN represent the two-body scattering amplitude for the94probe-ejectile interaction. Its momentum dependence within the range of momentum-smearing introduced by the distorted waves is normally ignored in the "factorization" or "zero-range" approximation. We have written the amplitude with explicit density-dependence.Will it be possible to obtain convincing spectroscopic information from a strongly absorbed proton probe? We expect the key to this problem will be in the comparison of the results from (e,e'p) and (p,2p) measure­ments of comparable quality on the same target to be studied using the (p,pn) reaction. Since the difference between neutron and proton optical potentials in a neutron-rich nucleus are expected to be modest and predictable by microscopic calculations of the optical potentials, such comparisons can be used to "calibrate" the proton probe for each target nucleus. For example, the existing medium-energy (p,2p) cross section data are fitted by the best presently available DWIA calculations with spectroscopic factor 1.5 to 2 times smaller than those obtained from (e,e'p) data. 5 - 7 There are plausible reasons for this trend such as the so-called Perey-damping effect16 of the neglected non-locality of the optical potentials. In order to apply this "normalization correction" derived from the (p,2p)/(e,e'p) comparison to the derivation of neutron spectroscopic factors, it would be necessary to show that it is indepen­dent of the subshell in a particular target nucleus from which the nucleon is ejected. Hence the (p,2p) facility which supports the (p,pn) program in this way should be capable of resolving all the states that are distinguished in the (e,e'p) measurements. A dual-arm spectrometer system is a necessity.Our confidence in information about bound neutrons derived in this way depends on our confidence in the DWIA reaction model. The applica­tion of the DWIA to nucleon knockout is subject to more critical tests than for (e,e'p) in that (p,2p) analyzing powers for knockout from known orbitals near the Fermi surface can be easily measured and compared with predictions that are relatively insensitive to details of the struck particle wave function. The J-dependence of these analyzing powers arises entirely through the distorting potentials experienced by the final state proton. Hence they are good tests of the model. Until recently, the existing data for knockout of lp and Id shell protons with known J-values were in agreement with DWIA calculations only in certain kinematic conditions. However, new calculations using the Love-Franey interaction are in fair agreement with the analyzing power data over a wide range of kinematic conditions. 7 Some examples are shown in Fig. 3. If a similar improvement is observed in the present poor agreement of DWIA calculations with the existing (p,pn) analyzing power data, 8 we will have reason to hope that this J-dependence will become a useful spectroscopic tool.Confidence in the DWIA would be firmer if the factorization approxi­mation were eliminated. Progress in the area has been slow in large part because of the practical numerical difficulties caused by the high dimen­sionality of the phase space associated with the three-body final state. However, there are some recent encouraging developments. In some calcu­lations, the factorization approximation has been weakened to the extent that the effects of nuclear-density dependence of the nucleon-nucleon interaction can be investigated. The types of density-dependence associated with the Hamburg interaction were found to have only modest effects on (p,2p) cross sections and analyzing powers. 7 The study of95i 0.5 0-0.5-1.01.010,60 : 30° -  3 0 °  •--- 1P3/2 *°Ca: 3 0 ° -  3 0 ° •--- I d ^o -1P1/2 o--f A ' »'' \ D i l i'i-rr-r*'' V .1 51 Y + - '1--- 1---1 1 --1 1.t t * *—1— 1— 1— 1— 1___ 1___160 : ACf--40° •--- ^ 3/2*0^. •--- idwo IP 1/2 o----W 3/7!6rt.1.00.50-0.50 : 3CP- 65° • ------ IP3/2  «>ca: 29°-  47° • 'o  IP 1/2 3 5/2T i— 1------------1 1 1 1 1 ___ 1________ 1 . j________ 1________ 1________ 1________40 00 00 100 120 U0 -100 -60 -20 20 00 100 *1 (M e V ) E| -  t {  (M eV)Fig. 3. (p,2p) data from Ref. 5 with DWIAcalculations from Ref. 7.other types of density-dependence is the subject of other TRIUMF pro­posals. Most exciting, however, is the work now underway to develop an unfactorized finite range calculation in the Dirac framework. 9 This will be the first self-consistent (p,2p)/(p,pn) calculation in the sense that the same covariant direct-plus-exchange model of the nucleon-nucleusinteraction is used both to generate the optical potentials and to des­cribe the primary quasi-free scattering process. Such optical potentials generated by this interaction have recently been shown to give rise to nuclear elastic scattering observables in remarkably good agreement with experiment. 10In spite of the above-mentioned possibility of using the(p>2p)/(e,e'p) comparison to "normalize" the DWIA cross sections it will be important to use the best calculations possible in interpreting the96(p,pn) data. This is because, in terms of percentages, the cross sections are more sensitive to the rms radius of the wave function of the struck nucleon than to the spectroscopic factor. 11 Hence, careful examination of the shape of the cross sections in a variety of kinematic conditions will be necessary to constrain the rms radius of the neutron wave functions since they will not be determined by electron scattering data. This also emphasizes the need for 500 MeV beam energy to ensure that the energies of the final state nucleons remain in the energy region of optimal nuclear transparency for all experimental kinematic condi­tions, including variation of their energy sharing at fixed angles. However, it is likely that the best-determined spectroscopic quantity will be the composite parameter C2S x R”mg for some power n. n has avalue near 4 for 150 MeV beam energy but it lies between 1 and 2 forenergies above 300 MeV and kinematic conditions which avoid low energy final state nucleons. This again illustrates the importance of higher energy. Studying the energy dependence should help resolve this poten­tial ambiguity btween spectroscopic factor and rms radius.THE (p,pn) EXPERIMENTAL PROBLEMWhat experimental facilities are needed to support the next genera­tion of (p,pn) experiments at intermediate energy? The obvious choice is a large solid angle, broad momentum acceptance magnetic spectrometer in coincidence with a neutron time-of-flight spectrometer with a long flight path. The proton hall at TRIUMF allows flight paths up to only 10 m. Figure 4 shows a possible experimental layout using the proposed second arm spectrometer as the proton detector. There are presently no plans at IUCF to provide for long flight paths in association with their new spec­trometers. Based on the resolution of 1 MeV in 80 achieved at IUCF with large lm neutron detectors at 19 m we expect a neutron energy resolution of 5 MeV at 200 MeV. The time reference for the time-of-f lightmeasurement will be derived fromthe proton spectrometer focal plane trigger counters with extensive corrections for flight path through the spectrometer and scintillator spatial non-uniformity using the drift chamber information. Such a resolution is useful for the investigation of relatively deep neutron hole states which are broad anyway but require a facility with large acceptance to cope with the relatively low yield spread over more phaseFig. 4. A (p,pn) configuration using SASP in coincidence with a neutron time-of-flight detector.n  * /a  w r  tyk»*.« c/7.' rToo nevX»c;4t,t  SeimD * n.H c-a.Fig. 5. Schematic layout of a (p,pn) facility consisting of the Second Arm Spectrometer in coincidence with the MRS as a neutron recoil On the other hand, the study of valence hole states requires much better resolution. This can be achieved at some considerable cost in efficiency by using the proposed TRIUMF dual-arm spectrometer system(DASS), with the neutron undergoing 0° charge exchange in an active hydrogenous converter (organic scintillator) at the entrance to one of the spectrometers. Such an arrangement is shown schematically in Fig. 5. The neutron detection system is similar to that used in the TRIUMFnucleon charge exchange facility (CHARGEX) except that instead of the primary beam being deflected, the hydrogenous converter is protected from the intense charged particle background flux from the target by anintervening compact, saturated pole tip dipole. This small magnetconstitutes the only significant cost of this (p,pn) facility beyond the construction of SASP itself. The design requirements for this dipole are very simple since no region of uniform field is required; merely the highest possible peak field along the collimated edge of the converter acceptance. Charged particles directed toward this point require the largest deflection to clear the converter and multi-wire chambers which track the recoil protons entering the spectrometer. Rather than clamp the field to prevent deflection of the primary beam by the dipole fringe field, it will be more efficient to shield the beam with a thick-walled steel pipe. The compactness of the dipole is crucial since it determines the distance to and hence the solid angle acceptance of the converter. It appears that a dipole length of 30 cm will be adequate so that the converter may be 60 cm from the target. The converter scintillator is preceded by a thin veto counter to eliminate any charged particles leaking through the dipole by scattering from the poles, for example.c MRSXI* *.r m r% * II Wt.98Experience with the existing CHARGEX facility indicates that such a recoil spectrometer has a resolution well below 1 MeV. The contribution of the beam energy spread will be less in this (p,pn) mode because the beam momentum dispersion can be matched to the proton spectrometer, obviating the need to accept the energy spread over some finite strip target width. Also, a thinner primary target combined with corres­pondingly higher beam intensity should be possible because the primary beam is transported out of the area to the well-shielded external dump. For these reasons, a neutron energy resolution of 0.5 MeV or better should be possible, probably limited by the energy resolution of the recoil scintillator. For a time-of-flight system, this would require a flight path of at least 1 0 0 m, giving rise to more accidental coincidence events from many different beam burst/velocity combinations.Of course, the central problem with the recoil spectrometer system is the small conversion efficiency of neutrons into recoil protons. In the CHARGEX facility, with a 2 cm thick scintillator of chemical composi­tion CH, the efficiency is 10-5. This can be doubled by changing to a commercially available liquid scintillator with approximate composition CH2. The thickness of 2 cm was chosen to maintain the maximum energy loss in the scintillator to be small compared to the MRS momentum accept­ance at 200 MeV so that the system acceptance would be constant over a substantial momentum range. If (p,pn) kinematics are chosen with the neutron energy in the vicinity of 300 MeV, the maximum energy loss is reduced and the MRS energy acceptance increases. Also, it is an inessen­tial convenience to have a flat momentum acceptance; it will have to be calibrated anyway using the D(p,pn)p reaction, for example, for which the cross section shape is quite well known. Hence, the converter thickness can be increased to at least 5 cm, resulting in a conversion efficiency of 5xl0-5. The thickness will be ultimately limited by the increased contribution to the neutron energy resolution by the converter energy loss resolution.The above argument illustrates one reason for choosing the MRS rather than SASP as the recoil spectrometer: the converter is moreefficient at the higher momentum accommodated by the MRS. The other reason is that the SASP optics design does not require a front end chamber in order to provide good resolution. If the MRS was the proton detector, its front end chamber would be exposed directly to the primary target and would severely limit the beam intensity. The only disadvan­tage to SASP as a proton detector is that its focal plane instrumentation must cope with the larger background fluxes transmitted by its large solid angle acceptance. However, assuming the SASP focal plane instru­mentation is similar to that presently installed on the MRS, it will accommodate fluxes in the several MHz regime; as will be seen below, this is adequate to reach beam intensities beyond which statistical uncertain­ties will not reduce significantly.The carbon content of the converter scintillator will generate back­ground from 1 2C(n,p) for residual nuclear excitation energies larger than the 12 MeV Q-value for this reaction. The relative size of this back­ground can be estimated from the CH2 (n,p) spectrum obtained with the CHARGEX facility which is shown in Fig. 6 . This relatively small con­tamination in the (p,pn) data can be subtracted by successive deconvolu­tion from lower excitation. In any case, the primary purpose of this facility is not the study of deep hole states.0 100 200  300 400  500Focol plone co o rd in a te  (1ch=0 .126  MeV)Fig. 6 . Recoil proton energy spectrum from (n,p) on CH2•We now compare the event rate capabilities of the recoil spectrom­eter versus the time-of-flight spectrometer in coincidence with the SASP as a proton detector. The values provided for the experimental param­eters are those appropriate for the recoil spectrometer. The "true"(p,pn) event rate isRt = 0tbtfipVpe (2 )where = 10 to 100 Mb/(sr2MeV) is a typical cross section for valence subshells, b is the beam flux and t is the target thickness (1 0 21 cm-2). The effective solid angle acceptances are ftp = 0.01 sr for SASP and ftn = 0.003 sr x 5X10-5 for a "CH2" converter 2 cm wide by 5.5 cm tall by 5 cm thick 60 cm from the target, and Ap is the proton energy bin chosen to be small enough not to unduly smear the recoil momentum distri­bution - say 10 MeV. e is the efficiency of the entire detector system which we take to be 0.5. The "accidental" event rate isRa = TV n b 2t2V nApAne (3)where T is the beam burst period (44x10-9s) unless the coincidenceresolving time can be less than the beam burst width (~3 ns). In calcu­lating the accidental trigger rate, we must use the beam period but in calculating the statistical error, we assume that accidentals in the same beam burst can be rejected during data reduction by applying comprehen­sive time-of-flight and scintillator response corrections to the SASP focal plane trigger time using its drift chamber data. The neutron timing signal can be derived from the recoil scintillator, corrected forspatial variation using the front end chambers of the MRS. In this casethe effective resolving time will be taken to be100where P is the beam period, w is the burst width and t' is the time resolution (2 a). ap and an are the inclusive cross sections gener­ating background fluxes of protons and neutrons, respectively. ap has been measured to be 1 mb/(sr MeV) near the quasi-free scattering peak12 and (p,n) measurements at IUCF indicate that the (p,n) continuum is similar to (p,p' ) . 13 An is the neutron energy bin width over which the yield must be integrated for each residual nuclear state and for fixed proton energy. It is essentially the resolution in missing mass. We take it to be 1 MeV.Assuming that we subtract accidental events from only one adjacent beam burst, the fractional statistical error in one proton energy bin isNt + 2NaHi “ +2N£N* Nt(5)where Nt and Na are the number of true and accidental events accumul­ated over the running time T. Hence the contribution from the acciden­tals ise 2 = = 2RaT = 2qpqnTAna " Nt 2 (RtT) 2 at2n finTA e(6)which is independent of beam flux and target thickness. For at-30 Ub/(sr2 MeV),E = 4= (seconds) 1 / 2  YT(7)It requires a 2 day run (- 2X10'“ s) to achieve Ea=.08 or 8%. It would be desirable to choose the beam intensity to make E|=l/Nt half as large as e|. This occurs when Rt=Ra* Henceb =apanTtAn(8 )Again for at = .03 mb/(sr2 MeV), b=8x1012 Hz or 1.3 PA. At this beamflux, the SASP focal plane total proton flux would be 9 MHz. At thisrate, almost half the events would have two tracks through SASP in the same beam burst. If the SASP focal plane is instrumented like that of the MRS, this will not be fatal. Each drift chamber contains two wire planes, one (X) with wires perpendicular to the bend plane and the other (U) rotated only 30°. Since the flux dispersal along the wire length is thereby kept short compared to the length of the chambers in the bendplane, only the minority of dual tracks which are close together in thedispersion coordinate will suffer from ambiguity in X/U association, requiring both of them to be discarded. Also the trigger scintillator hodoscope above the drift chambers is sufficiently granular that coinci­dent tracks are likely to hit different scintillators, allowing many of the extraneous ones to be rejected on the basis of sub-beam-burst time resolution. For these reasons, together with our experience regarding101the severity of indirect background flux in the existing MRS instrumenta­tion, we expect to be able to operate the proposed (p,pn) system withbeam intensities in the neighbourhood of 1 pA.It appears that this dual spectrometer (ppn) facility can generate good quality energy-sharing spectra for moderately deep neutron holestates at a few angle pairs for one target nucleus in approximately aweek of beam time. However, it is clear that it would be a struggle to extend this to deep hole states for which the cross sections will be smaller and spread over more final state phase space and hence much more vulnerable to accidental coincidence background. As has been pointed out previously, excellent energy resolution is not necessary for the study of relatively broad deep hole states, whereas good statistics are necessary. This leads us to propose the time-of-flight spectrometer in coincidence with the relatively large acceptance SASP as a complementary part of the TRIUMF (p,pn) facility. As in the case of the recoil spectrometer there need be little additional cost beyond the construction of the SASP since the large 1 m2 neutron detector arrays used for experiments 1 2 1 and 182 are still available. Some possible external users have similar arrays with potentially better time resolution. To estimate the event rateperformance of this system, we replace the neutron detector effective solid angle acceptance f2n with the value appropriate to the time-of-flight spectrometer with a detection efficiency of 25%:0n - 1 m2/(10 m)20.25 = 0.0025 sr .Also, the energy resolution An becomes 10 MeV and e increases somewhatto approximately 0.7. The resolving time, t ,  increases to 44 ns, onebeam burst. It is difficult to estimate the indirect background flux to which the neutron counters will be sensitive but it should be minimized by the relatively high pulse height threshold that the efficiency of 25% implies in conjunction with a total scintillator thickness of 30 cm. Finally, we take 10 pb/(sr2 MeV) as a typical deep-hole-state cross- section ot. Inserting the values into Eq. (6 ), we obtainE = (seconds) 1 / 2 .a /TOne shift of beam time will then produce statistical errors from acciden­tals per 10 MeV bin less than 4%. The beam intensity required to bring the intrinsic statistical error Et below this is then given by Eq. (8 ) to be 50 nA. However, this would yield an event trigger rate integrated over 100 MeV in both neutron and proton energy of several kHz, mostly accidentals, and proton singles fluxes into the neutron detector of 5 x1 0s which might lead to phototube stability problems because of the large energy loss in these thick scintillators. A practical choice of beam intensity might be 10 nA on the nominal target thickness of 1021 cm-2. Then in two shifts of beam time, the intrinsic statistical error will dominate at the level of 5% per 10 MeV bin. This compares very favourably with the results of past (p,2p) or (p,pn) experiments at lower energy.The general scenario we propose, then, for the study of neutron hole states in a particular target nucleus includes high-resolution102(p,2p) measurements with the dual spectrometer system in the same kinematic conditions as those planned for the (p,pn) measurements. The normalization for spectroscopic factors can be established and the J-dependence of the analyzing powers can be studied by examination of the data for a few well-resolved hole states near the Fermi surface which have been characterized by (e,e'p) measurements. Then good resolution (p,pn) measurements using the recoil spectrometer system can provide spectroscopic factors for the prominent neutron hole states at low excitation as well as confirmation that the J-dependence of the analyzing powers for states of known J is consistent with the predictions of the DWIA. Finally, (p,pn) measurements with the time-of-flight spectrometer will probe deeper hole states with good statistics with the interpretation of the missing mass spectrum being assisted by the analyzing power signatures together with the recoil momentum dependence. It seems to us that only such a coordinated approach is likely to yield the spectroscopic information about neutron single particle properties that is needed to complement the growing body of such data about protons from the (e,e'p) facilities.SAMPLE EXPERIMENTAlthough it is not possible to predict what will be the most urgent (p,pn) measurements when SASP could become available in about two years time, it is useful to offer an example of one of the first experiments if the facility were available now. An aspect of nuclear structure that can1200800>©2  4 0 0COHE 400Oo20000 10 20 30 40SE PARATIO N  ENERGY, E , (M eV )Fig. 7. Neutron separation-energy spectra (a) for the 1+0Ca (p,pn)39Ca reaction at 149.5 MeV with (9p,9n) = (44.3°,36.1°); and (b) for the lt8Ca(p,pn)1+7Ca reaction at 149.5 MeV with (0p,0n)=(47.3°,36.1°).—  • 1--. • ) 40C«(p.pld3---T------1----- 1n)38Ca Bp • 44 .3 - Bn • 3 6 .1*_2 , 1/2 ;' * l  1d5 /2■ b)46C a(p.p i■ C- L■ IP A47/.C* Bp - 4 7 .3 ' Bn ■ 3 6 .1 -/ 2 + 1 d 3 /2 >1d6 /2  11 ---- J------1----- 1-- -103be studied systematically with neutron knockout but is not accessible via proton knock-out is the effect on the energy distribution of strength in the orbitals below the Fermi surface as the one at the surface is filled. This possibility has already been exploited in the first good resolution (p,pn) experiment done at energies as high as 150 MeV. 11 By comparing knockout spectra in 1+0Ca and lt8Ca (Fig. 7) the shift in the energies of the 2s1 / 2 ancl 1-cl3 / 2 llol-e states was observed as the lf7 7 2 orbital is filled. Tentative conclusions were reached regarding the differences in neutron matter distributions in the two nuclei. In view of the uncer­tainty involving matter distributions in the calcium isotopes, 15 it is important to repeat these measurements at higher energy such as 500 MeV where the nucleus is more transparent to the final state particles. If a J-dependent analyzing power signature can be confirmed at the higher energy, it will help in confirming the interpretation of the separation energy spectra based on their momentum distributions.Data are needed from both the high resolution recoil spectrometer system to resolve the lf7/2, ^s \ / 2 and ld3 / 2 states as well as from the time-of-flight system to obtain good statistics for the broad ld5 7 2 ancl lp distributions. As an initial stage, it would be adequate to obtain one energy sharing spectrum with the recoil spectrometer system at an angle pair chosen to allow zero recoil momentum at moderate separation energy (13 MeV) and two angle pairs using the two available neutron detector arrays, one positioned to reach zero recoil momentum at 22 MeV for the dr/, state and the other for the lp region of excitation near 35 MeV.Based on the count rate estimates presented earlier, to be sure of better than 10% statistics in each of 10 MeV wide bins in the detected proton energy for each of the two targets, we need four shifts of beam time in recoil spectrometer mode and two shifts in time-of-f light mode for a total of twelve shifts. This should be polarized beam if it can be shown that there is a reliable J-dependent analyzing power signature at 500 MeV and in the mass 40 region. To test this, we would require two shifts of polarized beam in the dual spectrometer (p,2p) mode at each of two angle pairs to study the analyzing powers for knockout of protons from the 2s and Id orbitals of lf0Ca. This estimate is based on the observation that the effective acceptance of the MRS is very similar to that of the time-of-flight spectrometer.The (p,2p) measurement would be scheduled first to determine if polarized beam is useful for the (p,pn) measurements. If indications are positive, polarized beam would be used for the high resolution measurements on lt0Ca to confirm that the analyzing powers behave as expected for neutron knockout from the same orbitals. If this result also is positive, polarized beam would be indicated for all measurements.The total beam time for the entire study is estimated to be 16 shifts. Of course, this could be modified by experience during commis­sioning of the facilities.REFERENCES1. S. Frullani and J. Mougey, Adv. in Nucl. Phys. \A_ (1984).2. P. Kitching et al., Adv. in Nucl. Phys. 1J^, 43 (1985).1043. G. van der Steenhoven et al., Phys. Lett. 156B, 151 (1985);J.W.A. den Herder et al., Phys. Lett. 161B, 65 (1985);P.K.A. de Witt Huberts, Proc. Xlth Eur. Div. Conf. on Nuclear Physics with Electromagnetic Probes, 1985, Nucl. Phys. A446, 301c (1985).4. C.J. Horowitz and M.J. Iqbal, Relativistic effects on Spin Observables in Quasielastic Scattering, Phys. Rev. C 33, 2059 (1986).5. P. Kitching et al., Nucl. Phys. A340, 423 (1980).L. Antonuk et al., Nucl. Phys. A370, 389 (1981).6 . W.J. McDonald et al., Nucl. Phys. (in press).7. Y. Kudo and K. Miyazaki, Analyzing powers for (p,2p) Reactions with Effective N-N Interactions, Preprint (1986), 6th International Symposium on Polarization Phenomena in Nuclear Physics, Osaka,Japan, 1985, contributed paper 1.34.8 . "The 2H(p,pn) and 1 60(p,pn) Reactions at 200 MeV", J.W. Watson,W. Parisuwan, B.S. Flanders, P.J. Pella, R. Madey, N.S. Chant,P.G. Roos, and M. Ahmad, Bull. Am. Phys. Soc. 29, 716.9. E.D. Cooper and B. Jennings, private communication.10. C.J. Horowitz and D. Murdock, Bad Honnef Conf. preprint (1985);Phys. Lett. 168B, 31 (1986).11. M. Ahmad et al., Nucl. Phys. A424, 92 (1984).12. R.E. Segel et al., unpublished; H. Esbensen and G.F. Bertsch, Phys. Rev. C 34, 1419 (1986).13. R.E. Segel et al., Phys. Rev. C 26_, 2424 (1982);J.W. Watson et al., Phys. Rev. C 23, 2373 (1981).14. S.L. Belostotskii et al., Sov. J. Nucl. Phys. 41_, 903 (1985).15. What do we know about the radial shape of nuclei in the Ca region?Proc. Karlsruhe Int. Discussion Meeting, 1979, ed. H. Rebel,H.J. Gils and G. Schatz.16. F.G. Perey and B. Buck, Nucl. Phys. 32^, 353 (1962).105(n,p) AND (p,n) REACTIONS AT TRIUMFS. YenTRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3INTRODUCTIONThis paper deals with the nucleon charge-exchange program at TRIUMF and its possible impact on the proposed Second Arm Spectrometer (SASP). First I will present the reasons for studying (p,n) and (n,p) reactions at TRIUMF. I will then describe the present facility based on the Medium Resolution Spectrometer (MRS). I will then discuss in some detail the question of the apparent quenching of Gamow-Teller strength in (p,n) reactions and the role of the A-isobar in this quenching. Finally, I will outline a few possible avenues of future research in this area employing the SASP.WHY CHARGE-EXCHANGE AT TRIUMF?Why should we investigate nucleon charge-exchange reactions at TRIUMF? The 200 to 500 MeV energy range accessible with the TRIUMF cyclotron is the energy region where the nucleon-nucleon interaction is the weakest, so that the impulse approximation is expected to work best in this energy range. This greatly simplifies the reaction mechanism and makes for reliable comparisons between experimental data and calculations based on standard DWIA codes. In this energy range, the ratio of isovec­tor spin-flip to isovector non-spin-flip interaction potentials VaT /VT is the largest, 1 so that the TRIUMF energy range is ideal for looking at spin excitations of the nucleus. In addition, (p,n) and (n,p) reactions pick out the AT=1 excitations, so that the experimental spectra are free of the background of isoscalar excitations present in (p,p'). All this means that TRIUMF's energy range is ideal for exploring spin isovector excitations of the nucleus.The (p,n) reaction from 80 to 200 MeV has been explored for a number of years at the Indiana University Cyclotron Facility. 2 >3 Why should we be interested in (n,p) reactions? The first, and perhaps most com­pelling , reason is that it offers a way of disentangling the various factors that may contribute to the apparent quenching of the GT strength in (p,n) reactions. This will be discussed in detail later. Secondly, (n,p) has a different isospin selectivity than does (p,n). For N > Znuclei, (n,p) populates only states with final isospin Tf = TQ + l.1* This is in contrast to (p,n), which preferentially excites Tf = TQ - 1, and (p,p'), which preferentially excites Tf=Tg. Thus, (n,p) can be used to explore, for example, the Tf = TQ + 1 components of isovector giant resonances. Thirdly, in N »  Z nuclei, the Gamow-Teller excitation is strongly Pauli blocked, so that we can observe the other isovector giant resonances such as the spin isovector monopole, without interfer­ence from the strong GT strength present in (p,n) reactions. Fourthly, the (n,p) reaction results in a final nucleus of lower Coulomb energy than does the (p,n) reaction. The lower energy means that there is less spreading of the lp-lh strength due to mixing with 2p-2h configurations, so that the lp-lh strength will be more concentrated and more visible. Lastly, the (n,p) reaction can be used to give information on important106weak interaction rates, e.g. on the distribution of GT strength in 5 6Mn, important for the rate of the 5 6Fe(e,v)56Mn reaction which is the final step before the collapse of the core in supernova explosions.THE TRIUMF CHARGEX FACILITYMotivated by the considerations discussed in the last section, we began in 1983 to design a facility based on the existing Medium Resolution Spectrometer (MRS) at TRIUMF to investigate (p,n) and (n,p) reactions. W.P. Alford of the University of Western Ontario and K.P. Jackson of TRIUMF were initially the principal instigators of this project. By the summer of 1985 we had a working facility and the first experiment, llfC(p,n), was performed during that summer.Figures 1 and 2 illustrate the principle of the TRIUMF nucleon charge-exchange facility (called CHARGEX). In the (p,n) mode (Fig. 1), the primary proton beam hits the target under study. The primary beam is bent 21° by a clearing dipole magnet into a beam dump. Neutrons produced by the (p,n) reaction in the target travel forward and strike a recoil scintillator, which converts the neutrons into knockon protons via the ^(n.p) reaction. These knockon protons are then momentum analyzed in the MRS. A veto scintillator located before the recoil scintillator vetoes any charged particles hitting the recoil scintillator. The neutron to proton conversion efficiency is about 10-5. The protonTobeomdum pM R S Ouodrupole Recoil protons Wire chamberRecoil Veto( p , n ) neu trons Proton b lo cke r( p ,n  ) ta rg e t P rim ary proton beamTobeomdump?L l  ( p ,n  ) ta rg e tP rim ary proton beam( n , p )  Mo d e.M RS OuodrupoleRecoil protonsWire chamber (r\p) iarjefV e to ----------------n e a f r o n  beam  Proton b lo cke r( p , n )  lAode.Fig. 1. (p,n) mode of CHARGEX. Fig. 2. (n,p) mode of CHARGEX.107blocker prevents protons which are elastically scattered to the left in the primary target from being bent by the clearing magnet into the recoil scintillator. By measuring the energy loss in the scintillator and adding it back to the energy measured in the MRS, we can use a relatively thick ( 2 cm) scintillator without substantially degrading the overall energy resolution.In the (n,p) mode (Fig. 2), the primary proton beam hits a 7Li neutron production target. The clearing magnet again bends the primary beam into a beam dump. The 0° neutron beam from the 7Li(p,n) reaction travels straight forward, through a veto scintillator which vetoes charged particles, and into the (n,p) target stack. Protons produced by (n,p) reactions in the target stack are then momentum analyzed in the MRS.The system of detectors used in the (n,p) mode is shown in Fig. 3. First is a veto scintillator VS which vetoes events induced by charged particles in the target stack. Then follows the (n,p) target stack. This consists of 6 target layers, sandwiched between multi-wire propor­tional counter planes marked A-F. The idea is that by examining the hit patterns in the 7 MWPC planes, one can deduce which target layer the (n,p) reaction occurred in, and thereby make a correction for the energy loss of the outgoing proton in all subsequent target layers. This con­siderably improves the energy resolution of the whole system. It is also possible to run different target materials in different layers; it is usual to place a CH2 target in the last position to obtain cross sections relative to the known np cross section. The target system is called "Robert's Box" after its designer, Robert Henderson, of the University of Melbourne. Then follows a set of X and Y position-sensitive drift chambers labelled FECM, a trigger scintillator FES, and another set of drift chambers FECO. The MRS spectrometer itself consists of thequadrupole Q and dipole D, of 2.6 m bend radius and capable of bending 1500 MeV/c. The focal plane detectors consist of two sets of vertical drift chambers (XI, Ul) and (X2, U2) spaced 1 m apart, followed by anmagnet. target.TnpFig. 3. Detector system for (n,p).108I!o■l\•>*51h«-uwr  r  L i i p , * )  h9 9*1r»«*Ee • 169 l%V1m  T i  Tm  I m  T i MCUTNOM CMCMCT IM fV )i20001600in«-»cD8 1200 o2 800400-1_ |_____ I_____ I_____ LH(n.p) 7L i(p ,n )7Be =-----------------E„-198 MeVW ° °T -CH»PQC(n.p)«Bg,100 200 300 400 500Focal plane coord inate (1ch=0.126 MeV)Fig. 4. Time-of-flight spectrum of neutrons from 7 Li(p,n) reaction at 160 MeV taken at IUCF. From Ref.21.Fig. 5. Neutron spectrum from 7Li(p,n) reaction at 200 MeV, using TRIUMF CHARGEX facility.array of 1 0 plastic scintillators, followed by two large-area plastic scintillators SI and S2.I will now show some figures illustrating the performance of the CHARGEX system. Figure 4 shows the spectrum of neutron energies from the 7Li(p,n) reaction measured at 160 MeV at Indiana using a time-of-flight system. The neutron energy increases to the right. The energy distribu­tion consists of a sharp peak due to the population of the g.s. and 430 keV states in 7 Be, and a tail of lower-energy neutrons due to excita­tion of higher states in 7 Be. If we set CHARGEX up in the (p,n) mode and look at the neutrons from the 7Li(p,n) reaction at 200 MeV, we get the spectrum of Fig. 5. Here, the neutron energy increases to the left, backwards from Fig. 4. We see a large peak due to neutrons converting to protons on the hydrogen in the recoil scintillator, and a second peak about 15 MeV lower in neutron energy due to neutrons converting on the carbon. The events between the two peaks correspond to the continuum seen in Fig. 4, and are not due to instrumental background. Obviously, the events to the right of the second peak are due to (n,p) on both hydrogen and carbon, so the response function of the system to a mono- energetic neutron source is somewhat complicated at high excitation energies.Figure 6 shows a neutron spectrum from the 11+C(p,n) reaction. The FWHM is 0.7 MeV after off-line software corrections. The ratio of the areas of the 0+ 2.31 MeV Fermi transition and the 1+ 3.95 MeV Gamow- Teller transition can be used to deduce the ratio of volume integrals of the isovector spin-flip to non-spin-flip parts of the effective NN inter­action. We have done this measurement at 200, 300, 400 and 450 MeV. The results are shown in Fig. 7, where the solid curve is the ratio predictedNuiber of counts109Ep(MeV)Fig. 6 . Neutron spectrum from ll+C(p,n) reaction at 200 MeV, using TRIUMF CHARGEX facility.Fig. 7. Ratio of squared volume integrals of spin-flip to non- spin-flip parts of isovector interaction strengths, as de­rived from (p,n) experiments.500400-•fi 300-<3 200-100 -....... .......1 ....... 1. L ---- 1,2C (n .p )12B12B(g.s.) En-198 MEV 'Background '(no target)H(n.p) _- «— FWHM-1.0 M«V1Ap - uB(4.8 M«V)— J l — - _____Fig. 8 .0 100 200 300 400 500Focal plane coordinate (1ch=2mm=0.126 MeV)Proton spectrum from 1 2C(n,p) reaction, the Franey-Love NN interaction, 1 and it can be seen that the data lies considerably above the prediction.Figure 8 shows a spectrum of protons from the 1 2C(n,p)12B reaction, with CHARGEX operating in the (n,p) mode. The 12B ground state peak has a FWHM of 1.0 MeV. The peak to the left of it is due mainly to (n,p) reactions in the hydrogen in the argon-isobutane gas mixture used in "Robert’s Box". We now use an Ar-C02 mixture which eliminates this peak almost completely.Table I summarizes the performance of the TRIUMF CHARGEX system. Itis obvious that the 11 msr solid angle of the Second Arm Spectrometer will allow a 5.5-fold increase in event rate over the present 2 msr MRS, for energies up to 260 MeV.The world competition in (n,p) is as follows. The upgraded Uppsalacyclotron will have an (n,p) facility which will permit experiments inthe 80 to 185 MeV range. The design parameters call for 2.5 million110Table I. CHARGEX Facility PerformanceEnergy range 200-450 MeV with MRS 200-260 MeV with SASP9 lab 0° - 32°Resolution ~ 700 keV in (p,n) ~ 1 MeV in (n,p)Afi . . ~ 2 msr with MRSspectrometer~ 11 msr with SASPNeutron flux ~ 1 0 6 neutrons/s on a 2x4 cm2 target in (n,p) mode at 200 MeVneutrons/s on target, and a 10 msr magnetic spectrometer with 1.1 MeV resolution. First extraction from the upgraded cyclotron is planned for early 1986.Los Alamos, starting in 1988, will have the capability of studying (n,p) reactions up to 800 MeV bombarding energy. Their system will be based on a 15 msr Medium Resolution Spectrometer which is now under construction. A long flight path between the production and secondary targets will allow spin precession solenoids to provide polarized neutron beams of various orientations. Event rates are expected to be similar to those at TRIUMF.QUENCHING OF GAMOW-TELLER STRENGTHFigure 9 shows a neutron time-of-flight spectrum from the 9 0Zr(p,n) reaction, obtained at IUCF. The shaded peaks are identified from their angular distributions as 1+ strength. The dotted line shows the assumed "background" that the experimentalists subtract off.The 1+ strength in (p,n) or (n,p) is essentially a measure of thesquared matrix elements of the ot+ operators:ASg± = I ,  | <f | E  e  oy ( k ) t ± o o | i > | 2 .f k=l yFor free nucleons, Sg+=3. If nucleons in a nucleus behave like freenucleons, then we have the Gamow-Teller sum rule5 ’ 6Sg_ - Sg+ = 3(N-Z)IllCHANNELFig. 9. Neutron time-of-flight spectrum from 9 0Zr(p,n) reaction at 200 MeV, IUCF. From Ref. 21.Fraction of GT-Sum Rule Observed In tp.n)Fig. 10. Missing GT strength over the periodic table. From Ref. 21.In the absence of (n,p) experiments, Sg+ must be estimated in some way. For heavy nuclei, the Sg+ strength is largely Pauli blocked and may be approximated by zero. Alternatively, in some cases, Sg+ may be computed from a shell model calculation. In any case, 3(N-Z) provides a lower limit on Sg_. Using the background subtraction procedure described above, it was found that the GT strength measured in (p,n) is only about 60% of the sum rule. This is true for a range of nuclei spanning the periodic table (Fig. 10).Where has all the GT strength gone? There are basically 3 competing explanations. (1) Mixing of the lp-lh configurations with 2p-2h config­urations may spread and shift the lp-lh strength from the low-lying, strong peaks into regions of higher excitation energy where the strength is not observed. 7 (2) Some of the "background" under the major peaks subtracted off by the experimentalists may in fact be actually GT strength, so not all the GT strength is observed. 8 *9 Mechanisms (1) and (2) are obviously closely related. (3) In addition to the p-hdegrees of freedom, the (p,n) reaction may also excite A-hole com­ponents. 1 0 * 11 In other words, subnucleonic degrees of freedom are excited, so that the nucleons involved in the GT excitation no longer behave like free nucleons. The classical Gamow-Teller sum rule is thusviolated. Some of the excitation strength is shifted to excitation energies in the vicinity of the A-isobar. The possibility that subnuc­leonic degrees of freedom were manifesting themselves in low-energy nuclear physics phenomena such as GT transitions excited many nuclear112physicists, and this was the topic of the Telluride II conference in 1982.12Since Telluride II, belief in the role of the A has decreased. Models of the NA coupling based on ir and p exchange13 and microscopic G- matrix calculations1^  seem to indicate that the Landau-Migdal parameter ®AN’ which describes the short-range part of the A-N interaction poten­tial, is closer to 0.4 rather than to 0 .6 , as had been originally assumed on the basis of "universality" of NN and NA interactions. This would mean that the NA coupling is much weaker than had been previously assumed, so that the A would play a much smaller role in the quenching. Also, several large-basis RPA calculations1 5 * 16 allege to be able to substantially reproduce the entire 9 0Zr(p,n) spectrum without resorting to A's; these calculations indicate that substantial amounts of GT strength are located at high excitation energies away from the most prominent peaks, so that when the total GT strength is added up, there is really no quenching at all. However, these RPA calculations ignore ground-state correlations and treat 2p-2h admixtures in only a rough phenomenological fashion. A definitive experimental test is still lacking. It is our intention at TRIUMF to use (n,p) to directly measure Sg+, and together with Sg_ from the available (p,n) results, directly test the Gamow-Teller sum rule.UPCOMING (p,n) AND (n,p) EXPERIMENTSI would now like to describe a few experiments which have or will soon be taking data on the CHARGEX facility at TRIUMF.Experiment 265, a study of the llfC(p,n) reaction proposed by Parker Alford, has already been completed1 7 and was discussed earlier. The significant result, shown in Fig. 7, is that the ratio of the volume integrals of the spin-flip to non-spin-flip isovector interaction strengths |dar/JTl2 is significantly higher than predicted by the Franey- Love1 interaction, which is parametrized from free NN scattering phase shifts. The culprit is suspected to be the JT component, which may be poorly constrained by existing NN data.Experiment 266, a study of the 6Li(n,p) and 1 2C(n,p) reactions for which Peter Jackson is spokesman, is a study of the (n,p) reaction in N=Z light nuclei where the (n,p) strength should be the same as the (p,n)strength by charge symmetry. A spectrum is shown in Fig. 8 . Preliminary results indicate that the quenching factor for (n,p) is about the same as for (p,n).Experiment 267/383, a study of the 51+Fe(p,n) and 51*Fe(n,p) reactions proposed by 0. HMusser, took some data in December 1985 and will betaking more shortly. The idea is to measure both Sg+ and Sg_ in anucleus where both are non-zero and explicitly test the Gamow-Teller sum rule.Experiment 376, a study of the 3 6Zr(n,p) reaction, is a collabora­tion between the TRIUMF group, the University of Melbourne, and Tel-Aviv University. It will take data in May 1986. It aims to search for Sg-(- strength in this neutron-excess nucleus where the GT strength in the(n,p) direction is completely Pauli blocked to first order. The occur­rence of significant Sg+ strength would invalidate the conclusions of Refs. 15 and 16 that no A's are needed to explain the apparent quenching observed in (p,n). A secondary objective of this experiment is to search113: : va)nPnFig. 11. Enhancement mechanism for 1+ states in 2 0 8Pb(n,p). a) O-Km (p,n) is allowed in a neutron-excess nucleus like 2 0 8Pb; b) But 0-jta> (n,p) is blocked by the neutron excess, and can occur only via ground- state correlations; c) If the neutron can be excited to become a A, then (n,p) is no longer Pauli-blocked.for giant isovector spin resonances; the 9 0Zr(n,p) reaction is a favour­able case because the GT resonance which normally dominates the spectrum in (p,n) reactions is Pauli blocked in (n,p), providing a window through which to look for other multipolarities.Experiment 268, a study of the 2 0 8Pb(n,p) reaction, aims to measurethe cross section for exciting discrete 1+ states in 2 0 8T1. The ideaoriginated from Brown, Krewald and Speth, 18 and is illustrated in Fig. 11. These authors predict an 0° enhancement by a factor of 3.5 inthe presence of A-hole admixtures, an enhancement which would not occurif 2p-2h spreading were the cause of the quenching observed in (p,n).This experiment will also take data in May 1986.Experiment 384, "Abysmal Astrophysics", is a proposal by PeterJackson et al. to study the 5 6Fe(n,p) and 58Ni(n,p) reactions. The objective is to map out the GT strength distributions for electroncapture rates relevant in supernovae; the electron capture by 56Fe and 58Ni are principal reactions which deplete the electron gas pressure and allow the supernova to undergo collapse.Experiment 378, for which Parker Alford is spokesman, aims to studythe tf8Ti(n,p) reaction, as a test of important matrix elements relevantfor calculations of lf8Ca double beta-decay.Experiment 344, for which John Watson and his Kent State collabor­ators are principal investigators, is a study of the 2 8Si(p,n), 2 8Si(n,p), 8 8Sr(p,n), and 1 2 0Sn(n,p) reactions. The aim is to search for concentrated ljho stretched state strength in heavy nuclei; the reactions on 28Si provide a well-known benchmark to prove that things are working as they should be.Last, but certainly not least, is experiment 411, a study of lt8Ca(n,p) led by Otto Hausser and Ron Jeppeson. The doubly closed struc­ture of lt8Ca admits no GT strength to zeroth order, so a search for GT strength provides an excellent test of ground-state correlations. This experiment will be a real tour-de-force, involving over $2 million worth of ‘t8Ca metal target material on loan from Los Alamos.114FOR THE FUTUREAs the review of the present activities indicates, the nucleon charge-exchange program at TRIUMF is a vigorous and growing program,especially in the area of (n,p) studies. Apart from the initial program of selected nuclei, we will eventually want to systematically study many nuclei, analogous to what has been done in (p,n). The counting rate for (n,p) is low, since the incident neutron flux is only of order 1 0 6 per second on a 2x4 cm2 target. To obtain useful counting rates, it is necessary to use thick targets (of order 1 g/cm2), and for separated iso­topes it is often prohibitively expensive to obtain this much target material. The 5.5-fold increase in solid angle obtainable with SASPwould therefore not only mean much higher counting rates, but would makepossible some experiments which would otherwise be impossible simply because of the cost of the target material. Alternatively to obtaining a higher counting rate, one could choose to use thinner targets and enjoy better energy resolution. We will certainly need to go in this direction to improve the resolution from the present ~1 MeV.The suspected role of A's has been a major motivation for the (n,p) program at TRIUMF. Two-arm experiments such as (p,p'ir) may give better understanding of A-dynamics and the NN -»■ NA interaction in nuclei. One may, for example, pick out specific parts of the NN -*■ NA interaction by exciting specific final states of the target nucleus in a (p,A'*-1’) reac­tion. The A"1-*" may be via its decay products, p + tt+ . Only one such reaction has been studied, the ®Li(p,A++)6He reaction.The analysis by Jain20 indicates a small Landau-Migdal parameter of g ^  < 0.4 for large momentum transfers.In addition to the first-generation (p,n) and (n,p) cross-section measurements, one may look forward to difficult second-generation experi­ments which the large solid angle of the SASP would make possible. One such experiment would be to measure spin transfer coefficients in a (n,p) reaction. Assuming an incident proton beam polarization of 75%, one can achieve 25% neutron polarization with a 7Li neutron-production target, which is quite low. One can do better with a liquid deuterium target; the (p,n) reaction on deuterium at 9° would yield a neutron polarization of 67%. The poor energy resolution obtained with deuterium may not matter much for continuum measurements. With such a setup, one could search for isovector spin-flip strength in the continuum, to see if the GT strength "missing” from low excitation energies is spread to higher energies. It must be admitted that count rates will be very low, and LAMPF's (n,p) setup, with its longer flight path, will be more suitable for inserting solenoids and other devices necessary to provide neutron beams of various spin orientations.It would also be interesting to explore the decay modes of isovector giant resonances. To separate out the various multipolarities, one would measure the angular distributions from the decay protons and/or neutrons from these giant resonances, in coincidence with the SASP. The protons and neutrons would be detected in a semiconductor telescope array or liquid scintillator array which would constitute a "third arm spectro­meter". The large solid angle of the SASP would be of enormous benefit in such coincidence experiments.115To conclude, (p,n) and (n,p) are unique probes of spin isovector excitations in nuclei. As I have indicated in my presentation, there are lots of interesting things to do. The Second Arm Spectrometer would greatly enhance our capabilities in this field.ACKNOWLEDGEMENTSIt is my pleasant duty to acknowledge the many people who have contributed to the development of the TRIUMF (n,p) facility. These include: Rudi Abegg, Parker Alford, Anna Celler, Dieter Frekers, PeterGreen, Otto Hausser, Robert Henderson, Richard Helmer, Ken Hicks, Peter Jackson, Andy Miller, and Mike Vetterli. The research reported in this paper was supported in part by the Natural Sciences and Engineering Research Council of Canada.REFERENCES1. M.A. Franey and W.G. Love, Phys. Rev. C 31, 488 (1985).2. C. Gaarde, J.S. Larsen and J. Rapaport, in Spin Excitations in Nuclei, eds. F. Petrovich, G.E. Brown, G.T. Garvey, C.D. Goodman,R.A. Lindgren and W.G. Love (Plenum Press, New York, 1984), p. 65.3. C.D. Goodman, in The (p,n) Reaction and the Nucleon-Nucleon Force, eds. C.D. Goodman, S.M. Austin, S.D. Bloom, J. Rapaport and G.R. Satchler (Plenum Press, New York, 1980), p.1494. F.P. Brady, C.M. Castaneda, J. Romero, V.R. Brown and C.H. Poppe, in Ref. 2.5. C. Gaarde, J.S. Larsen, M.N. Harakeh, S.Y. Van der Weuf, M. Igarashi and A. Muller-Arnke, Nucl. Phys. A334, 248 (1980).6 . C. Gaarde, J. Rapaport, T.N. Taddeucci, C.D. Goodman, C.C. Foster,D.E. Bainum, C.A. Goulding, M.B. Greenfield, D.J. Horen,E. Sugarbaker, Nucl. Phys. A369, 258 (1981).7. G.F. Bertsch and I. Hamamoto, Phys. Rev. C 26, 1323 (1982).8 . F. Osterfeld, Phys. Rev. C 26, 762 (1982).9. F. Osterfeld and A. Schulte, Phys. Lett. 138B, 23 (1984).10. M. Ericson, A. Figureau and C. Thevenet, Phys. Lett. 45B, 19 (1973).11. E. Oset and M. Rho, Phys. Rev. Lett. 42, 47 (1979).12. Spin Excitations in Nuclei, eds. F. Petrovich, G.E. Brown,G.T. Garvey, C.D. Goodman, R.A. Lindgren and W.G. Love (Plenum Press, New York, 1984).13. A. Arima, T. Cheon, K. Shimizu, H. Hyuga and T. Suzuki, Phys. Lett. 122B, 126 (1983).14. H. Sagawa, T.-S.H. Lee and K. Ohta, Phys. Rev. C 33^, 629 (1986).15. Amir Klein, W.G. Love and N. Auerbach, Phys. Rev. C 31, 710 (1985).16. F. Osterfeld, D. Cha and J. Speth, Phys. Rev. C 31^, 372 (1985).17. W.P. Alford, R.L. Helmer, R. Abegg, A. Celler, 0. Hausser, K. Hicks, K.P. Jackson, C.A. Miller, S. Yen, R.E. Azuma, D. Frekers, R.S. Henderson, H. Baer, and C.D. Zafiratos, Phys. Lett. 179B, 20 (1986).18. V.R. Brown, S. Krewald and J. Speth, Phys. Rev. Lett. 50, 658 (1983).11619. T. Hennino, D. Bachelier, O.M. Bilaniuk, J.L. Boyard, J.C. Jourdain,M. Roy-Stephan, P. Radvanyi, M. Bedjidian, E. Descroix, P. Poessel,S. Gardien, J.Y. Grossiord, A. Guichard, M. Gusakow, R. Haroutunian,M. Jacquin, J.R. Pizzi, and A. Garin, Phys. Rev. Lett. 48, 997 (1982).20. B.K. Jain, Phys. Rev. C 29_, 1396 (1984).21. J. Rapaport, in Studying Nuclei with Medium Energy Protons:University of Alberta/TRIUMF Workshop, July 1983, ed. J.M. Greben. (TRIUMF Report TRI-83-3).117NUCLEAR MEDIUM EFFECTS IN QUASI-FREE (p,2p)M.J. IqbalTRIUMF, Vancouver, B.C., V6T 2A3, Canada, and Physics Department, University of Alberta, Edmonton, Alta., T6G 2J1Recently it has been shown1 that in a relativistic approach toquasi-elastic proton scattering, from closed shell nuclei, the mostimportant medium effects on the nucleon-nucleon (NN) interaction, due to the enhancement of the lower components of the nucleon wave function, can be characterised by assigning an average effective mass M to thenucleons. The M* dependence of the NN interaction was taken fromrelativistic impulse approximation (RIA) calculations. 2 One of theconsequences of this M* dependence of the NN interaction is a decrease inthe analysing power (Ay) compared to free NN scattering (M = M, M isfree mass of nucleon). There is a strong experimental evidence for this decrease in the analysing power in single arm (p,p') quasi-elastic experiments. 3 There are certain advantages in looking for medium effects on the NN interaction in single arm (p,p') experiments. For example, one is averaging over all the nuclear states and hence is not sensitive to the details of the nuclear structure.The advantage in considering the double arm experiments is that one can look at the observables, e.g. nucleon-nucleon analysing power, atdifferent regions of nuclear density and hence map out the density depen­dence of these observables. This will put a strong constraint on the models which try to investigate medium modifications on free nucleon- nucleon interaction. At present the (p,2p) and (p,pn) experiments which have been performed have looked at the regions of lower nuclear densities. However, the medium effects observed in quasi-free (p,p') should also be present in (p,2p) and (p,pn) on nuclei. This point has been discussed in a recent review of (p,2p) experiments by Kitching et al.1* Their observation is based upon the following analysis. The effective polarisations of two nucleons bound in orbits j = £ + 1 / 2 and j = SL - 1 / 2 respectively, satisfy, for any kinematics and distortions,pfc-l/ 2 = “ —  p£+l/ 2 •They observed, from the analysis of experimental asymmetries for 160 and 1*°Ca, that this relation can only be satisfied if one assumes that the pp analysing power is zero inside the nucleus, compared to its free value of0.3 - 0.4 at 200 MeV. This observation can be directly tested provided one considers proton scattering from those nuclear protons which have zero effective polarisations. This is the case for lS1 / 2 an  ^ £^>1 / 2 pro­tons in 1*°Ca and lS1 / 2 protons for 1 60. In this case measured experimen­tal asymmetry is equal to the NN analysing power, as can be seen from the formula, P(8 ) + PtjDnn(e) l + p4 jp(e)118where P^  is the effective polarisation of a nucleon bound in an orbit (n£j), P(0) and Dnn(9) are the usual spin observables in NN scattering and A(0) is the experimentally observed asymmetry. However, their (p,2p) experiment on 4UCa at 200 MeV, where one of the nucleons from 2S1 / 2 orbit is knocked out, gave null result. The measured analysing power was very close to the free analysing power. They correctly pointed out that this may be due to the fact that the nuclear medium effects are small when a 2Si / 2  nucleon from 1+uCa is knocked out. We will analyse their result in the M model and show where one expects to find large medium effects in (p,2p) experiments.Most of the formalism given below is described in detail in Ref. 1. However, there are some important modifications for the (p,2p) reaction. We will assume that the basic (p,2p) interaction goes through a single scattering event and that the multiple scattering effects are well repro­duced by distorted waves. This is a good approximation for light nuclei and works well in forward angle scattering. Consider the incoming proton under the influence of scalar (S) and vector (V) optical potentials. In an eikonal approximation, the wave function iswhere k is the particle momentum, Xs the Pauli spinor. The phase factor S(z) is given byS±(z) = j  — dz'{Vc(b,z') + Vso(b,z')(o*bxk - ikz')}./too kw h e r e  t h e  e f f e c t i v e  c e n t r a l  V c  a n d  s p i n - o r b i t  V SQ p o t e n t i a l s  a r eVc = S + — V + —— (S2 -V2) c M 2Mv _ 1 h<«>v cso 2Mr E+M+S-Vr = /b2+z2 .As a first approximation let us drop Vso. The effects of spin-orbit dis­tortions on (p,2p) will be discussed in a later paper. Then the trans­mission probability for going through the nucleus at an impact parameter b isI is+(b)l2 | f 4M f a> )T(b) = |e | = exp < —  J  dz ImVc(b,z)\ .We choose to define the average density for scattering from a nucleon in119a state R^j asJo bdb T3 /2 (b)|RrtJ(b) | 2 p(b) 0e££ bdb T3'2(b)|RntJ(b)|2Here p(b) Is nuclear density at an impact parameter bP ( b )JQ dz p2 (z,b)dz p(z,b)and Rn£j(b) is the effective nuclear wave function at an impact parameter bRn£j(fe) = / dz Rn£j(’/t)2+z2) •J—00In the present work we have chosen to use harmonic oscillator wave functions. The extra factor of T1 /2 (b) compared to Ref. 1 takes care of absorption of the outgoing extra proton.One can define an effective impact parameter beff at which the interaction takes place, by3ef ff:bdb (b)|Rn£j(b) | 2 T3 /2 (b)bdblR^jCb) ! 2 T3 /2 (b)G i v e n  a n  e f f e c t i v e  d e n s i t y  P e f f  f o r  ( p , 2 p )  r e a c t i o n  o n e  c a n  e s t i m a t e  a n  e f f e c t i v e  m a s s  M b yM*—  = 1 - 0.44 peff .Here Peff is measured in units of nuclear matter saturation density and 0.44 is a result of mean field theory.In Fig. 1 we have plotted harmonic oscillator IS, IP, ID and 2S wave functions for l+0Ca. We used a value of B (B = mm) parameter of .28. In Fig. 2 phenomenological scalar (S) and vector (V) optical potentials are shown for lt0Ca at 200 MeV, obtained by fits to proton elastic scattering data. 5 In Tables I and II the values for peff, M* and beff are given for IS, IP, ID and 2S states of l+0Ca and IS and IP states of 160 at 200 MeV. A comparison of Fig. 1, Fig. 2 and Table I yields some very interesting information. Consider first proton knock-out from 2Sj, 2 orbit of lt0Ca. Let us assume that the interaction takes place at z = 6 . (It turns out to be a very good approximation for (p,p') but our general1202 4  6  8R (F)10R  (F)Fig. 1. IS, IP, ID and 2S wave functions for l+0Ca. We use 8=.29 where 8=mu), m being the nucleon mass.Fig. 2. Scalar and vector optical potentials for 1+0Ca at 200 MeV. The potential strengths are in F-1.Table 1. Medium effects for (p,2p) on 1+0 Ca at 200 MeVIS IP Id 2Speff (po) .64 .50 .29 . 2 1M*/M .72 .78 .87 .91beff (F) 2.4 3.1 4.0 4.3argument is not affected by relaxing this approximation.) The beff for 2S orbit is about 4.3 F. We see that at impact parameter the S and V potentials have appreciably died down and effective density at which interaction takes place is only about 2 0% of nucleon matter density. Thus the medium effects are small. The M* is .91 in nuclear mass units. In Table III we have given analysing power as a function of scattering angle for different values of M*. We see that for M = .91 the analysing power is very close to its free value. Thus the results of Kitching et al. are expected. Let us consider now the knock-out of the nucleons ID, IP and IS orbits of 1+0Ca. As expected, as we move to inner shells the beff decreases, Peff increases and hence M decreases. The medium effects become larger. For protons knocked out from IS orbit of ^Ca we see that M* - .72. The analysing power at this value of M is about half its free value. Thus strong medium effects should be seen in (p,2p) on ^°Ca when a nucleon is knocked out from the lS1 / 2 shell. Knocking a proton out of lS1 / 2 is understandably much more difficult. However, the NN interaction at medium energies is large and high flux beams are available at TRIUMF. This is definitely an experiment one could perform121Table II. Medium effects for (p,2p) on 160 at 200 MeVIS IPPeff (Po) .4000CM•* <M / M .8200•<4-14-1<D (F) 2 . 2 2.7Table III. M dependence of analysing power at 200 MeV®cmM*- =1 . 0  MM*- =.91MM*- = . 8 8  MM*- =.82 MM*- =.72 M1 0 .14 . 1 1 . 1 0 .08 .052 0 .23 .27 .26 .23 .1530 .28 .30 .28 .25 .1640 .30 .28 .26 . 2 2 .1550 .27 .24 . 2 2 .28 .1360 . 2 2 .28 .27 .24 . 1 070 .16 .23 . 1 1 . 2 0 .0780 .08 .06 .06 .05 .03with a two arm spectrometer. In Table II we have also given beff and M* values for knocking out a proton from IS or IP orbits of 160 at 200 MeV. Again we see that the M effect is large for protons knocked out from IS orbit of 1 60. The analysing power for protons knocked out from IS orbit is about 60% of the free value. This is certainly an easier experiment compared to knocking lS1 / 2 protons out of lt0Ca. Also the spin-orbit distortion effects are smaller compared to lt0Ca. It is better to consider those (p,2p) experiments where the second proton is knocked out from S-shell orbits because in this case the effective polarisation of the struck proton is zero and hence experimentally measured asymmetries are directly related to in-medium NN analyzing power.In Table IV we have given the energy dependence of M* and beff for ID state of ^Ca. It is seen that the medium modification effects are largest at 400 MeV. Thus we suggest that the most suitable experiment to measure medium effects in (p,2p) is to measure asymmetries on 1 60 at 400 MeV when a proton is knocked out from lS1 / 2 orbit.Similar medium effects are seen in other spin observables in (p,2p) experiments. Since we have not discussed the effects of spin-orbit distortions on the spin observables in (p,2p) we will not discuss them at122Table IV. Energy dependence of medium effects for Id state of l+0CaTlab <MeV> M*/M beff <F)160 0.87 3.92 0 0 0 . 8 8 4.0300 0.89 4.1400 0.87 3.9500 0.91 4.3800 0.93 4.5this point. Asymmetries are relatively insensitive to spin-orbit distortions; hence our results discussed in this section are affected little.For other spin observables in (p,2p) a more careful analysis of spin-orbit distortions is needed. This work is in progress.Useful and stimulating discussions with Professor W.J. McDonald are greatly appreciated.REFERENCES1. C.J. Horowitz and M.J. Iqbal, Phys. Rev. C 33, 2059 (1986).2. C.J. Horowitz and B.D. Serot, Nucl. Phys. A368, 503 (1981).3. T.A. Carey et al., Phys. Rev. Lett. 53, 144 (1984).4. P. Kitching, W.J. McDonald, Th.A.J. Maris and C.A.Z. Vasconcellos,Advances in Nuclear Physics 15, 43 (1985), edited by J.W. Negele and Erich Vogt (Plenum, New York, 1985), p. 43.5. A.M. Kobos, E.D. Cooper, J.I. Johansson and H.S. Sherif, Nucl. Phys. A445, 605 (1985).123DASS/SASP USERS LISTInterestsR. ABEGG, TRIUMF, Vancouver, B.C., Canada ...............  (p,2p),(p,pn)W.P. ALFORD, University of Western Ontario,London, Ont., Canada ...............................  (p,2p)(p,n)(n,p)E.G. AULD, U.B.C., Vancouver, B.C., Canada ................  (p,ir)(p,irx)R.D. BENT, IUCF, Bloomington, IN, USA .....................  (p, ir) (p, irx)N.S. CHANT, University of Maryland ....................... (p,2p)(p, irx)College Park, MD, USA (p,pa)(p,pd)E. COOPER, TRIUMF, Vancouver, B.C., Canada ..............  Theory(p,2p)(p,pn)(p,irx)T. DRAKE, U. of Toronto, Toronto, Ont., Canada......  (p,2p)(p,pn)(p,irx)F. DUNCAN, U.B.C., Vancouver, B.C., Canada ................ (p, tt) (p, irx)R. DYMARZ, U. of Alberta, Edmonton, Alta., Canada .... Theory(p,2p)(p,pn)J. ERNST, Institut fur Strahlenund Kernphysik,Bonn, W. Germany ........................................ (p, ir)(p, irx)W. FALK, U. of Manitoba, Winnipeg, Man., Canada ........... (p, ir) (p, irx)D. FREKERS, TRIUMF, Vancouver, B.C., Canada ... (p,irx)(u,ir)(p,d2p)(p,irdp)L.G. GREENIAUS, University of Alberta....................  (p»2p)(p,p'n)Edmonton, Alta., Canada K. HICKS, TRIUMF, Vancouver, B.C., Canada .....................  (p,ir+n)G. HUBER, U. of Regina, Regina, Sask., Canada ..............  (p, ir)(p, irx)D. HUTCHEON, TRIUMF, Vancouver, B.C., Canada ...............  (p,2p)(n,p)M.J. IQBAL, U. of Alberta, Edmonton, Alta., Canada ......... Theory(p,ir)(p,irx)(p,2p)K. ITOH, U. of Saskatchewan, Saskatoon, Sask., Canada............  (p,2p)K.P. JACKSON, TRIUMF, Vancouver, B.C., Canada ...................  (n,p)G. JONES, U.B.C., Vancouver, B.C., Canada ................  (p, ir)(p, irx)P. KITCHING, TRIUMF, Vancouver, B.C., Canada .................... (p,2p)J. LISANTTI, TRIUMF, Vancouver, B.C., Canada .................... (p,2p)G.J. LOLOS, U. of Regina, Regina, Sask., Canada ...........  (p, it) (p, irx)E.L. MATHIE, U. of Regina, Regina, Sask., Canada .......... (p, ir)(p, irx)W.J. McDONALD, U. of Alberta, Edmonton, Alta., Canada..(p,2p)(p,n)(p,pn)C.A. MILLER, TRIUMF, Vancouver, B.C., Canada .............  (p,2p)(p,pn)G.A. MILLER, U. of Washington, Seattle, WA, USA .. Theory(n,p)(p, ir)(p, irx)A. MOALEM, Ben-Gurion University, Israel .................  (p,pn)(p, irx)S.I.H. NAQVI, U. of Regina, Regina, Sask., Canada ...............  (p»ff)W.C. OLSEN, U. of Alberta, Edmonton, Alta., Canada ... (p,2p)(p,pn)(p, irx)S. PAGE, U. of Manitoba, Winnipeg, Man., Canada ..... (p,2p)(p,pn)(p, irx)W.D. RAMSAY, TRIUMF, Vancouver, B.C., Canada .............. (p,2p)(p,pn)E.F. REDISH, IUCF, Bloomington, IN, USA ..................... Theory,allP.G. ROOS, U. of Maryland, College Park, MD, USA .........  (p,2p)(p,irx)P. SCHWANDT, Indiana University, Bloomington, IN, USA ..... (p,2p)(p,pn)D.M. SHEPPARD, U. of Alberta, Edmonton, Alta., Canada ..(p,pn)(p,2p)(p,n)W. SHIN, U. of Saskatchewan, Saskatoon, Sask, Canada ............ (p,2p)G. STINSON, U. of Alberta, Edmonton, Alta., Canada........  (P»2p)(p,pn)L.W. SWENSON, Oregon State U., Corvallis, Oregon, USA..(p,py)(p,2p)(p,pn)M. VETTERLI, TRIUMF, Vancouver, B.C., Canada ............... (p»2p)(p,n)P.L. WALDEN, TRIUMF, Vancouver, B.C., Canada ......................  allT.E. WARD, Brookhaven National Lab, Upton, NY, USA .... all except (p,2p)S. YEN, TRIUMF, Vancouver, B.C., Canada ..........................  all: . • *• ' :


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