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A convenient target for the preparation of bromine-77 for TDPAC studies Hutter, Jeffrey Lee 1990

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A CONVENIENT TARGET FOR THE PREPARATION OF BROMINE-77 FOR TDPAC STUDIES B y J E F F R E Y L E E H U T T E R B . Sc. (Physics) The University of Br i t i sh Columbia, 1987 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF M A S T E R O F S C I E N C E i n THE FACULTY OF GRADUATE STUDIES PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA M a y 1990 © J E F F R E Y L E E H U T T E R , 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library, shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of p h y s i c s  The University of British Columbia Vancouver, Canada Date August 10, 1990 DE-6 (2/88) Abstract This paper describes the design and construction of a convenient and inexpensive appara-tus for the production of the radioisotope bromine-77. The applications of this isotope in various fields of research are discussed and its suitability for use in the field of perturbed angular correlations is demonstrated. ii Table of Contents Abstract ii List of Tables v List of Figures vi Acknowledgement vii 1 Introduction 1 2 Applications of the Radioisotope Bromine-77 2 3 Production Methods for Bromine-77 6 4 A Simple Target for Bromine-77 Production 10 4.1 Radiobromine Production at T R I U M F 10 4.2 A New Radiobromine Target 11 4.3 Target Construction 13 4.4 Injection/Extraction System 22 4.5 Target Operation 25 4.6 Safety 26 5 Bromine-77 Production 33 6 The TDPAC Technique 39 i i i 7 Preparation of Silver Bromide Samples 44 8 Application of the TDPAC Technique to AgBr 46 9 Results 55 10 Conclusions 60 Bibliography 61 iv List of Tables I Some Properties of Isotopes of Bromine 4 II Product ion Methods for Bromine-77 7 III Some Properties of Selenium and Compounds 31 I V Reactions Occurring i n the Target 34 V Y i e l d Calculations 35 V I Yields 36 v List of Figures 1 Target Schematic 12 2 Plate A 14 3 Plate B 15 4 Beam-line Coupling 16 5 Pressure Vessel 17 6 Target Chamber 18 7 Beam Stop 19 8 Water Cooling Plate 20 9 Exploded View 23 10 Target Solution Injection/Extraction System 24 11 Operation of the Bromine-77 Production Target 27 12 Par t ia l Level Structure of Selenium-77 47 13 Energy Spectrum of Bromine-77 Measured by a N a l ( T l ) Detector . . . . 49 14 Energy Spectrum of Bromine-77 Measured by a Ge(Li) Detector 50 15 Delayed Coincidence Energy Spectrum of Bromine-77 Gated by the Gamma 1 Transition Measured by a N a l ( T l ) Detector 51 16 Delayed Coincidence Energy Spectrum of Bromine-77 Gated by the Gamma 2 Transition Measured by a N a l ( T l ) Detector 53 17 Exponential Decay of the Isomeric Level . 56 18 Perturbation Factor for Bromine-77 Decay i n A g B r 57 v i Acknowledgement I would like to thank Dr. Peter W. Martin for his support and encouragement, without which this work would not have been completed. Dr. Tom Ruth of TRIUMF also deserves credit for his help with the fine details of target design and isotope production. I am also indebted to the crew at the CP42 cyclotron for their role in the process of installing an operational target. Finally, I would like to thank NSERC for their financial support of this work. vii Chapter 1 Introduction The technique of perturbed angular correlation spectroscopy has been used since 1951 as a probe of the nuclear environment. Although less well known than several more recent nuclear spectroscopic methods, many interesting experiments using this method have yet to be done. The radioisotope bromine-77 is a relatively recent nucleus to angular correlation studies, although it has been used in several medical applications. Due to a combination of a fairly short half-life and a low branching ratio for the de-sired nuclear de-excitation, this isotope is not the most convenient for correlation studies. There are, however, a number of interesting systems in which bromine is involved which lend themselves to a perturbed angular correlation (PAC) study. This, in addition to the established medical uses of the various radioisotopes of bromine, provides sufficient motivation for the design and building of a convenient target for the production of radio-bromines. The presence of the CP42 production cyclotron, located on the TRIUMF site at the University of British Columbia provides an excellent opportunity for this research. This thesis discusses the production and uses of bromine isotopes, details a convenient method for the production of small quantities of bromine-77, briefly describes the field of time-differential perturbed angular correlation studies, and demonstrates the suitability of the bromine-77 probe to these studies. 1 Chapter 2 Applications of the Radioisotope Bromine-77 Several radioisotopes have been employed i n the field of nuclear medicine. Ideally, one would like to use isotopes of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sul-fur, as these atoms make up the vast majority of al l organic chemicals. This would make i t possible to label any organic compound which can be synthesized, without altering its chemical properties. Unfortunately, al l of the radioisotopes of these ba-sic building blocks have inconveniently short half-lives. Al though some nuclei, such as carbon-11 (Ti/2 = 20 min) , nitrogen-13 (10 min), oxygen-15 (2.0 min), and phosphorus-30 (2.5 min) [1], have been used, the short half-lives require the compounds to be labelled and used quickly, making it difficult to achieve a high specific activity and l imi t ing the applications to short-term studies. For this reason, the analogue approach to nuclear medicine is usually employed. In this approach, a convenient radionuclide is substituted for a stable nucleus at a convenient location i n an organic molecule. For example fluorine-18 (which has a half-life of 110 min) has been used i n the molecule 2-fluoro-2-deoxyglucose (which is chemically similar to 2-deoxyglucose) for in vivo studies of glucose uti l ization [2]. Al though care must be taken that such substitutions do not change the biological behaviour of the compound, this approach has proven useful and greatly increases the variety of labelled compounds available to nuclear medicine. One of the most useful classes of atoms to be used i n this approach has been the radiohalogens. The halogens are useful chemically because they bond covalently wi th 2 Chapter 2. AppRcations of the Radioisotope Bromine-77 3 carbon atoms and thus can be considered to be organic atoms. There are a wide vari-ety of radioactive isotopes of the halogens and several of them have convenient nuclear properties (such as convenient half-lives and decay energies). Of the radiohalogens, the isotopes of iodine, particularly iodine-123 [3], have received the most attention. Al though these isotopes have been useful, part of the reason for their success has been historical; the radioiodines were among the first radiohalogens to be applied to nuclear medicine and production systems have long been i n place. It is only recently that the radioisotopes of bromine have received attention. Al though there are problems associated wi th the use of radiobromines, i n particu-lar the typically high gamma energies which make sharp imaging difficult wi th common scintillation cameras, these isotopes have their own particular advantages. The ma-jor chemical advantage is that the C - B r bond has a binding energy which is typically 40-60 k J / m o l stronger than that of corresponding C - I bonds [3]. This greater stablil-i ty of bromine compounds makes labelling more convenient. Another advantage is that bromine which is released by the labelled compounds is not accumulated i n the thyroid, as iodine is, so no one organ receives a greater total dose. There are a number of bromine isotopes available, wi th a range of properties summarized i n table I [1]. Most of these have potential applications. The short l ived isotopes bromine-74, 75, and 76 have potential as positron emitters for positron emission tomography ( P E T ) studies. For other studies, though, the positron emission is a disadvantage as i t increases the total radiation dose. The short half-lives cause practical difficulties as well . Bromine-77 has long been recognized as being the radiobromine wi th the most poten-t ia l i n nuclear medicine. Among its useful properties are a half-life long enough that i t can be used for long term studies such as clot localization and that i t can be transported to regions far removed from the cyclotron. Chapter 2. Apphcations of the Radioisotope Bromine-77 Table I: Some Properties of Isotopes of Bromine Isotope Natural Abundance Half-life Decay Mode Conversion Electrons* 7 2 Br 1.31 min 8+ (100) n.a. 7 3 Br 3.4 min 8+/EC (95/5) n.a. 74m B r 28.0 min 8+/EC (85/15) < 1 7 4 Br 41.5 min 8+/EC (91/9) < 1 7 5 Br 101 min 8+1'EC (76/24) < 1 7 6 Br 15.9 hr P+/EC (57/43) < 1 7 7 Br 56 hr 8+/EC (0.7/99.3) < 1.2 7 8 Br 6.46 min 8+/EC (92.4/7.6) n.a. 7 9 B r 50.69% 8 0 Br 17.4 min /3-/J3+/EC (91.6/5.8/2.6) n.a. 8 1 Br 49.31% 8 2 Br 35.3hr 8- (100) n.a. *Per 100 decays Properties of natural bromine: Atomic Mass: 79.909 Density: 3.12 g/mL Melting Point: -7.2 °C Boiling Point: 58 °C Chapter 2. Applications of the Radioisotope Bromine-77 5 Bromine-77 has already been used as an organic label i n a variety of medical ap-plications including studies of the brain, l iver, and gall bladder, as well as for steroid metabolism studies [4,5]. In addition, i t has been used i n the form of an inorganic anion for the estimation of extra-cellular fluid volumes [4]. Studies of soil adsorption and plant uptake of bromine from rainwater have also been performed uti l izing this isotope [5]. Certainly, this isotope wi l l be applied to other problems i n the future as routine production is now beginning at a few facilities. More recently, bomine-77 has attracted attention i n the field of perturbed angular correlation ( P A C ) studies [6]. The decay scheme of this isotope contains the 250 keV isomeric level of selenium-77, which has a convenient half-life of about 9.6 ns. This is long enough that the time differential method ( T D P A C ) can be applied, but not so long as to make data acquisition times unreasonably long. Only about 3% of the decay events of this nucleus proceed by the appropriate gamma cascades and the energies involved are rather close to other major lines i n the gamma spectrum, making this radioisotope more difficult to use than the traditional isotopes of T D P A C studies. These disadvantages can, however, be overcome by increased run times and are outweighed by the large number of unstudied systems made available by this newer probe. For example, an interesting system being considered for future study i n this laboratory is the graphite-bromine intercalation system. It is expected that interesting behaviour including a phase transition would be observed. These oportunities for T D P A C studies provided the motivation to design and build a target system for radiobromine production at T R I U M F . Chapter 3 Production Methods for Bromine-77 Due to the recent interest in radioisotopes of bromine, a number of production methods have been developed. Targets consisting of arsenic, selenium, bromine, and molybdenum, all of varying degrees of enrichment, have been irradiated by proton, deuteron, alpha particle, and helium-3 beams [7]. A summary of the reactions and methods used to produce bromine-77 is included in table II. As can be seen, there are essentially four classes of reactions. The first listed involves alpha particles incident on a target containing arsenic. This particular reaction has been used for some time and has recently been used in a high current target for large scale production with a batch yield of about 50 mCi [8]. Radiobromides have also been pro-duced by both proton and helium-3 beams using both natural and enriched selenium targets [9]. Another method that has been employed involves proton, deuteron, or al-pha particle beams on bromine targets [10,11]. The bromine-77 is then formed via a krypton-77 precursor. In order to keep the bromine-77 separate from the target bromine, the krypton-77 must be removed from the target before it decays. As this isotope has a short half-life of 1.2 hours, this requirement limits the batch yield to a few millicuries. The final method involves a high energy (800 MeV), high current (500 (J,A) proton beam incident on a molybdenum sheet in a parasitic position [12]. The irradiation is continued for several days and the bromine-77 is formed by a spallation process. After the target is allowed to cool, the radiobromine is separated chemically. This method has been used for production with batch yields of about 300 mCi. This is an attractive means of production 6 Chapter 3. Production Methods for Bromine-77 7 Table II: Production Methods for Bromine-77 Nuclear Reaction Energy Range (MeV) Th ick Target Y i e l d ( m C i / / i A - h ) Impurities at E O B (%) Comments 7 5 A s ( a , 2 n ) 7 7 B r 30-14 0.64 7 6 Br(5 .5) Estimated from yield of an A S 2 O 3 target 7 7 S e ( p , n ) 7 7 B r 12-9 0.43 7 6 Br(7 .7 ) , 8 2 Br(1 .4 ) 92.4% enriched 7 7 S e metal target 7 8 Se(p ,2n ) 7 7 Br 25-20 4.3 7 6 Br(0 .5 ) , 8 2 Br(0 .01) 97.9% enriched 7 8 S e " a f Se(p,a ;n) 7 7 Br 30-15 3.5 7 6 Br(148) , 8 2 B r ( 1 6 ) Estimated from yield of a target consisting of various selenium oxides " a ' S e ( 3 H e , p x n ) 7 7 B r 38-0 0.01 7 6 Br(41) Estimated from yield of a Na2Se target 7 6 S e ( 3 H e , p n ) 7 7 B r 22-15 0.2 7 6 B r ( 5 ) Estimated from yield of a 96.9% enriched 7 6 S e target 7 7 S e ( 3 H e , p 2 n ) 7 7 B r 30-15 0.45 7 6 B r ( l . l ) Est imated from yield of a 94.4% enriched 7 7 S e target 7 9 - 8 1 B r ( p , x n ) 7 7 K r 7 7 K r ^ 7 7 B r 65-25 1.6 Low current K B r target; batch removal of radiokrypton 7 9 - 8 1 B r ( d , x n ) 7 7 K r 7 7 K r i 4 * 7 7 B r 50-25 0.64 7 6 Br(4 .0) Estimated from yields of a medium current N a B r target; on-line removal of radiokrypton 7 9 - 8 1 B r ( a , x n ) 7 7 R b 7 7 R b ^ » 7 7 K r l 4 ' 7 7 B r 102-70 0.04 Mo(p , spa l l ) 7 7 Br 800 0.012 7 6 B r ( < 2), 8 2 Br(0 .4 ) Large-scale production using a high-current, thick target Chapter 3. Production Methods for Bromine-77 8 i f suitable facilities are available because the half-life of bromine-77 is long relative to the other spallation products, so the long production process results i n a low level of impurities. As can be seen from the table, most of the possible methods give a yield of the order of one millicurie per micro-amp hour. It is also evident that, i n addition to the bromine-77, impurities consisting of other radiobromines are produced. The amount of other isotopes formed can be controlled by appropriate selection of the energy range and by using targets wi th a high enrichment of the desired target nucleus. The use of enriched selenium targets, however, greatly increases the expense and adds to the complexity of the extraction process as the enriched selenium must be recovered for re-use. The choice of a production method is dictated mostly by the facilities available, the yield required, and the puri ty level desired. In the case of bromine-77 production for this laboratory, the availability of the C P 4 2 production cyclotron on the T R I U M F site at the University of Br i t i sh Columbia indicated the use of a proton beam reaction. As this isotope was not being produced by any group in the area at the time, i t was necessary to devise a method for its production. However, as target design was not the author's specialty, this apparatus would ideally be simple to bui ld and operate. It was decided that the reaction 7 8 Se(p ,2n ) 7 7 Br held the most promise for a convenient production system. A s T D P A C studies only record gamma de-excitations which fall within two selected energy ranges, the presence of other isotopes of bromine or small amounts of other elements are not a major concern. In addition, T D P A C methods can only use a few tens of microcuries of activity at a time, so large yields were not required. Thus it was possible to use natural selenium, which contains 23.5% selenium-78, i n the target compound. A s a result, several different reactions of the type (p,xn) on several different isotopes of selenium wi l l occur i n the target. These reactions wi l l result i n the production of various isotopes of bromine including bromine-77 by processes such as 7 7 S e ( p , n ) 7 7 B r . If this is Chapter 3. Production Methods for Bromine-77 9 undesirable for a particular application, selenium enriched in selenium-78 could be used. If radiobromines other than bromine-77 are desired, one could use a different enrichment and select different beam energies. With these criteria, a target system for the small scale production of bromine-77 was designed. Chapter 4 A Simple Target for Bromine-77 Production 4.1 Radiobromine Production at TRIUMF The ini t ia l attempt to produce bromine-77 was made using a target which had been designed for the production of radiobromines at the C P 4 2 facility, but had not been used. This target allowed the irradiation of a thick sample of solid selenium by the proton beam. The concept for this target was to use a heater i n the target to melt the selenium during the irradiation, allowing any bromine produced to diffuse to the surface. Tubing connected to the target was used to blow helium across the surface of the molten metal i n order to flush out the bromine. The helium was then bubbled through a basic solution to trap the product. Other tubing was used to inject solution into the vessel and to recover it after a run. The active solution would then contain carrier free bromine as bromide ion. The advantages of this design were that the target would not require frequent reload-ing and that the product could be recovered without access to the target area. The latter point is an important consideration i n the CP42 facility, where access to the target area is sometimes not available for periods of up to two months. Al though direct analysis of the irradiated selenium showed that bromine-77 was being produced i n the expected amounts, none of the recovered solutions contained any usable amount of radiobromides. After several runs i n which the temperature of the selenium 10 Chapter 4. A Simple Target for Bromine-77 Production 11 was held at 400 °C (well above its melting point) for periods of several hours, it be-came apparent that the bromine dissolved in the molten selenium could not be recovered without removal of the target. Thus, it was necessary to design a new target. A simple option would have been to prepare targets of selenium foil which could be inserted into the beam-line remotely. The foil could then be recovered and the selenium extracted chemically. There are difficulties associated with this method, however, because the nonmetallic behaviour of selenium makes it difficult to electroplate and this together with its low melting point would likely cause the target to deteriorate when irradiated. For this reason, it was decided to retain the original concept of the recovery of the product in a solution which could be extracted through tubing from outside of the target area. 4.2 A New Radiobromine Target This led to the design of the target illustrated schematically in figure 1. It consists of three parts; a beam-line coupling, a pressurized vessel, and a target section. The target components are separated by 0.0254 mm (0.001 inch) Havar foil windows which are sealed by two O-rings each. The design and operation of the target are discussed extensively below. To summarize, the major attractions of the target are its ease of construction and operation. Selenium is loaded into the target in the form of a solution such as sodium selenite. Although the density of selenium in this solution is only about 0.14 g/mL, which is quite small compared to the density of solid selenium (4.79 g/cm 3), the entire solution and hence the entire radioisotope yield is recovered in carrier-free form after the irradiation. As the range of protons in water is not much greater than their range in selenium, this low density of selenium nuclei will result in a yield which is much lower than that for a F I G U R E 1 T A R G E T S C H E M A T I C 3 O 3 •a I I N . I ? H S3 8 COOLING CHAMBER TARGET CHAMBER BEAM LINE COUPLING PRESSURE VESSEL TARGET SECTION Chapter 4. A Simple Target for Bromine-77 Production 13 thick solid selenium target. This is not a problem for TDPAC studies where only small activities are required. This target makes a quick and convenient system for small-scale radiobromine production. The first target section attaches to the end of the beam-line by means of a 227 viton O-ring and a dependex ring clamp flange. This connection is, of course dependent on the facility. Modifications for use at a different cyclotron would be simple to make. During operation, the second section is pressurized to between one and two atmo-spheres with helium to reduce the pressure differential across the window which isolates the target. If the target is run at increased beam currents, this section can be pressur-ized by flowing helium which is directed by a metal tube onto the beam-line window to provide additional cooling. Also, in the event of the target window being punctured by the beam, this section of the target will prevent contamination of the beam-nne. The target section consists of two chambers. The first is the target chamber which contains the solution to be irradiated. Connections for a thermocouple and the tubing used to transport the solution are threaded through the target wall into this section. Behind this section is a cooling chamber which is connected to water lines. Separating these two chambers is a 13 mm thick aluminum plate which is quite sufficient to stop 30 MeV protons (which have a 4.3 mm range in aluminum) [13]. 4.3 Target Construction Detailed drawings of the target components are shown in figures 2 through 8. Although all of the information required by a machinist to build the target is included in these drawings, a brief description of the various pieces is still helpful. Figures 2 and 3 detail two items labelled "plate A" and "plate B". These plates are used to bolt successive sections of the target together. Adjacent faces contain one O-ring DEPTH 0.065 R1.200 R0.965 ALL TOLERANCES + / - 0.005" UNLESS OTHERWISE STATED 231 O-RING GROOVE DEPTH 0.113 +0, -0.005 I THREADED M8 i I 0.750 | I T 2.609 2.992 —| |— 0.200 F I G U R E 2 P L A T E A SCALE = 1:1 DEPTH 0.065 4.000 J L • - 0 0 . J I i n i l R1.200 R0.965 ALL TOLERANCES + / - 0.005" UNLESS OTHERWISE STATED 218 O-RING GROOVE DEPTH 0.113 +0, -0.005 8mm DIAMETER i 0.750 I i 1 T T 1.234 1.610 J i *• i T r F I G U R E 3 P L A T E B —-| 1—0.200 S C A L E = 1 : 1 Chapter 4. A Simple Target for Bromine-77 Production 16 F I G U R E 4 B E A M - L I N E C O U P L I N G K 1 .535 -J 2 . 3 7 5 -ri S C A L E = 1:1 ALL TOLERANCES + / - 0.005" UNLESS OTHERWISE STATED END DETAIL X3 0 . 2 0 0 0 . 1 0 0 O-RING FACE 5 ° FROM VERTICAL Chapter 4. A Simple Target for Bromine-77 Production 18 > ^3 CD 3] m O ro m 33 o 5 in > Chapter 4. A Simple Target for Bromine-77 Production 19 Chapter 4. A Simple Target for Bromine-77 Production 21 groove each to isolate the target interior from the atmosphere and to seal the foil windows which allow the passage of the proton beam while separating the target components. A s i t is essential that O-ring grooves have a uniform depth and smooth surfaces, it is important to face the plates and machine these grooves after the plates have been welded to the appropriate target sections. This is because the process of welding generally causes some warping, especially for a difficult material such as aluminum. Similarly, it is recommended that the threading of the bolt holes be performed after the welding. The grooves on the sides opposite the O-rings are less cri t ical as their purpose is to join the plates to the target components. The fourth figure shows the target component used to connect the target to the beam line. As was discussed, the details wi l l vary from site to site. This design involves the use of a ring flange held to the target by a snap ring inserted i n the groove shown. A vi ton 227 O-ring is placed at the end of the section and held i n place by an insulating spacer. The inside of this section has a step machined at the appropriate depth so that the insulating ring allows the O-ring to be compressed by the appropriate amount when the flange is bolted to the matching insulated flange at the end of the beam line. Again , the final stages of machining should be completed after this section is welded to plate A . This system of joining two pipe sections is known as a "dependex ring clamp". Figure 5 shows the pressure vessel body, which is simply a short section wi th two pipe threads for the connection of tubing. The main target body is illustrated i n figure 6. It is welded to one of the "plate B " components and has steps machined inside for the positioning of two smaller plates described below. Three holes threaded for a 1 / 8 - N P T fitting (for 1/8 inch tubing) pass through the wall for access to the target chamber by the target solution. Figure 7 details the cooling chamber isolation plate. This piece is welded inside of the target section to separate the two chambers. The plate is designed to be thick enough to Chapter 4. A Simple Target for Bromine-77 Production 22 stop the proton beam. The reason for its shape is to minimize the volume of the target chamber while leaving room for the fittings for the target solution tubing. The final detailed drawing shows the plate welded to the back of the target section. This plate closes the water cooling chamber while the threaded holes allow the connection of fittings to attach to the coolant flow lines. Figure 9 is an exploded view of the completed target showing the order of assembly. The parts of each section weld together while the individual sections are bolted to each other by 8 m m bolts. Two each of foil windows, 231 O-rings, and 218 O-rings are required for the final assembly. 4.4 Injection/Extraction System Once assembled and installed on the beam-line, the target is connected to the injec-tion/extraction system. This assembly of valves and tubing is diagrammed i n figure 10. Four lines lead from the laboratory outside of the cyclotron cave to the target. Two of them make up the helium flow lines to the pressure vessel as discussed above. The remaining two allow for injection and extraction of target chamber solutions. The sample line passes through a solenoid valve and ends i n a stainless steel tube leading to the bot tom of the target chamber. The remaining line, also controlled by a solenoid valve, is used to pressurize the target chamber i n order to recover the solution. A third valve provides a vent for the target chamber allowing i t to be filled through the sample line. W i r i n g to monitor the target temperature and to control the solenoid valves also leads out of the cyclotron cave. F I G U R E 9 E X P L O D E D V I E W o Chapter 4. A Simple Target {or Bromine-77 Production 24 FIGURE 1 0 TARGET SOLUTION INJECTION/EXTRACTION SYSTEM WATER LINE BEAM COLLIMATOR V2 COOLING WATER OUTLET 5 V3 V1 VENT COOLING WATER INPUT He FLOW FROM He BOTTLE He FLOW FOR SAMPLE RECOVERY SAMPLE LINE THERMOCOUPLE PROBE He RETURN LINE SEALED FOR PRESSURIZATION CYCLOTRON CAVE WALL Chapter 4. A Simple Target for Bromine-77 Production 25 4.5 Target Operation The irradiation schedule planned for this target was for half hour runs at a current of one microampere of 29 M e V protons. This current is small enough to prevent damage to the th in foil windows while providing enough beam to produce sufficient yields for T D P A C studies. If higher yields are required or less beam time is available, the current can be increased considerably as the windows used are capable of withstanding currents of up to perhaps eight microamperes. Whi le any soluble selenium compound could be used as a target material, relatively few are suitable because most are only slightly soluble. The solutions used consisted of approximately 2.2 M sodium selenite (Na2Se0s). These were prepared by dissolving about 12 g of selenium dioxide (which has a solubility l imit of 38.4g per lOOcc of water) i n 50 m L of water and neutralizing the resulting acidic solution wi th sodium hydroxide to form the sodium selenite solution. A n excess of 0.1 m o l / L of sodium hydroxide was added to make the solution slightly basic i n order to ensure that any bromine produced remained i n solution as bromide or bromate ion. A total of 25 m L of solution is required to fill the target chamber. Operation of the target consists of injecting the selenium solution into the target chamber through the teflon sample line while the vent valve allows the helium inside the target chamber to be displaced. This was easily done using a plastic syringe, even though over ten meters of 2 m m (inside diameter) tubing connected the injection point and the target. Once the solution had been forced into the tubing, the syringe was used to pump air through the system i n order to force the solution into the target. The advantage of this crude technique is that a sharp change i n the force required to compress the syringe signals the arrival of a l l of the solution at the target chamber. After the irradiation, during which al l valves are closed, the target solution is recovered by pressurizing the Chapter 4. A Simple Target for Bromine-77 Production 26 target with helium and driving the irradiated solution out through the sample line. A summary of the steps involved in the safe operation of this target is shown in figure 11. 4.6 Safety Although the method described above is both simple and convenient, it does involve the handling of a solution which is both toxic and radioactive. It is, therefore, important to consider the safety features designed into the system, as well as to discuss the necessary precautions to be observed in its operation. Because of their simplicity, the target and related apparatus contain only a few easily identifiable safety hazards. Any problems that do arise are therefore easily rectified. Below is a discussion of safety concerns that were considered in the target design, as well as related observations that were made during the actual operation of the target. One concern is the possible contamination of the beam-line. Obviously, a target must be fully leak-tested to ensure that beam-line vacuum can be maintained. Of equal concern is that the foil windows also be leak-tight to prevent beam-line contamination by the target contents. These windows can be ruptured by applying too great a current, so this had to be considered in the design. If the first window, separating the beam-line from the helium vessel, were to rupture, the beam-line vacuum would fail, but no damage would occur and repairs could be easily made. Were the target foil to be similarly punctured, the active target solution would drain into the helium vessel. This would be a more serious problem because care would have to be taken in the removal and decontamination of the target, but most of the solution could be recovered through the helium flow lines. Finally, in the unlikely event Chapter 4. A Simple Target for Bromine-77 Production 27 Figure 11: Operation of the Bromine-77 Production Target • Loading of Target 1. Connect valve power supply 2. Open valves V I , V 2 , and V 3 3. F i l l lOcc syringe wi th Na2Se03 target solution 4. Force solution into sample line 5. Seal sample line 6. Repeat steps 3-5 unt i l ~28 m L of solution has been injected 7. Pump air into line wi th syringe unt i l a sharp change i n restriction is noted 8. Close al l valves 9. Disconnect power supply • Target Irradiation 1. A standard run irradiates the target wi th 1 pA of 29 M e V protons for 0.5 h 2. Spread beam over as wide an area as possible (ie. 1 pA on target and 1 pA on collimator) 3. Moni tor temperature • Sample Recovery 1. Connect valve power supply 2. Open valves V I and V 2 (not V3) 3. Place sample une i n sample bottle wi th a small volume of water 4. Pressurize target wi th ~ 2 atmospheres of helium flow through helium tube 5. Collect irradiated solution i n sample bottle 6. Flush target twice wi th water (see "Loading of Target" above) 7. Collect rinse water wi th sample 8. Disconnect helium supply 9. Close all valves 10. Disconnect power supply Chapter 4. A Simple Target for Bromine-77 Production 28 of a simultaneous window failure, some beam-line contamination could occur and require a small amount of cleaning. For small beam currents, i t is quite unlikely for the window to fail by direct action of the beam as the energy deposited by the beam i n the window is quite small. The energy lost by the protons as they travel through the window wi l l be of the order of 0.1 M e V (using an energy loss of 38.9 M e V / c m for 30 M e V protons i n aluminum [13] and a thickness of 0.00254 cm). Thus, the total energy deposited i n the window wi l l only be about 0.1 W for a current of one micro-amp. Obviously, this can easily be carried away by the window seal and target solution. As the danger of a window being destroyed by the beam increases wi th the concentration of the beam as well as wi th its total current, the target was operated wi th the proton beam spread across as much of the window as possible. It is conceivable that the window could fail i f its temperature became great enough due to heating of the target itself by the beam. However, the temperature is never l ikely to become large enough to harm a window as the same design of window has been used i n other targets, such as the original molten selenium target, at temperatures i n excess of 3 0 0 ° C . A s discussed below, temperatures this large wi l l not be attained i n the target. A calculation of the actual temperature expected to be reached by the target is quite difficult to perform. It is intuit ively obvious that this temperature wi l l not be very high because the target is quite large, water cooled, and subject to an input power of only 30 W (deposited by 1 pA of 30 M e V protons). Using the (unreasonable) assumption of uniform heat input one can use the Stefan-Boltzmann law to calculate the equilibrium temperature. From the formula P = eaT4A where P is the radiated power Chapter 4. A Simple Target for Bromine-77 Production 29 e = 0.3 is the emissivity of aluminum c = 5.67032 x 10" 8 W / m 2 K 4 is the Stefan-Boltzmann constant T is the absolute temperature and A = 542 cm is the surface area of the target one can calculate an equilibrium temperature of around 150°C . In fact, the average temperature could not even reach this level as the total energy deposited i n an ordinary run (30MeV x Ipk x 0.5h = 54kJ) is only sufficient to raise the temperature of the 580g aluminum target wi th 25 m L of water by 86 °C. This is found by applying the formula C = AE/AT and using heat capacities of 0.9 J / g . K for aluminum and 4.18 J / g . K for water. Thus, even wi th no additional cooling, the temperature of the target can not reach dangerous levels. Of course, the target is water cooled so its temperature wi l l never reach the values calculated above, even though they were simple estimates made using unreasonable as-sumptions. The highest density of energy deposition possible for this target would occur if al l of the beam hit the back plate of the target chamber rather than being attenuated by the solution. If this were to happen the energy would be deposited i n a area on the plate of the order of one square centimeter and within a depth of 4.3 m m (which is the range of 30 M e V protons i n aluminum [13]). The maximum temperature differential between the two chambers of the target section is easily calculated to be of the order of 20 °C. This is found from the formula P = k(Ti — T2)A/d) for heat conduction wi th k = 2.1 W / c m . K for aluminum and the over-estimate of d = 13 m m (ie. assuming that al l of the energy is deposited at the surface of the aluminum plate). A total of eight runs have been performed wi th the target to date and the temperature Chapter 4. A Simple Target for Bromine-77 Production 30 of the target chamber was never observed to increase above 25 °C. Another means by which the target window could fail is by pressure build-up i n the target chamber due to localized heating of the solution by the beam. It is very difficult to estimate i n advance the pressure which wi l l result during irradiation. As a precaution, the target chamber was never completely filled and a total length of approximately one meter of tubing was left between the chamber and the valves. This gives any vapor formed a volume i n which to expand. In practice, the pressure appeared to be too low to do any damage. It was noticed that pressure build-up i n the target chamber forced the target solution to rise through the tubing to the closed solenoid valves. As the solution is ionic, this provided a path from the target to ground making i t impossible for the beam current to be accurately monitored by the control room. Thus, i t was necessary to insulate these valves from the beam-line structure and to disconnect the valves when they were not i n use. The only failure of the target system which could occur outside of the target itself would be a leak of the selenium solution. The amount of solution is small and none of the radioactive components are produced i n unmanageable quantities, so such a spill would not be difficult to deal wi th . To guard against such an eventuality, al l fittings and tubes were examined and a continuous length of pre-tested tubing was used as the sample line. A s operation of this target involves the use of selenium compounds, i t is necessary to discuss the chemical hazards involved. A l l selenium compounds are considered to be toxic wi th a maximum acceptable level i n air of 0.2 m g / c m . A s sodium selenite is non-volatile, there is no danger of inhalation in the event of a spill even i f it were to occur i n an area not immediately accessible. Selenium compounds irritate the skin and respiratory system so i t is necessary to prepare the solutions i n a fume hood and to use gloves when they are being handled. Table III lists some of the properties of selenium compounds [14]. Chapter 4. A Simple Target for Bromine-77 Production Table III: Some Properties of Selenium and Compounds Selenium: Elemental selenium has several allotropes and can exist i n both metal and nonmetallic forms. A l l selenium compounds are considered to be toxic wi th a maximum allowable level i n air of 0.2 m g / m 3 . Physical properties: natural selenium Se02 Atomic Mass: 78.96 110.96 Density: 4.79 g / m L 3.95 g / m L Mel t ing Point: 2 1 7 ° C 3 1 5 ° C subl. Boi l ing Point : 685 °C Isotopic composition (%) of natural selenium: 7 4 S e 0.87 7 6 S e 9.02 7 7 S e 7.58 7 8 S e 23.52 8 0 S e 49.82 8 2 S e 9.19 Chapter 4. A Simple Target for Bromine-77 Production 32 In view of this chemical hazard, as much care must be taken i n the injection of the target solution into the target as i n the extraction of the irradiated solution. For the short, low-yield runs that were performed, i t was sufficient to perform these processes on a lab bench wi th a tray to catch possible spills and a stop-cock to seal the sample line. For this to be done safely, it must be remembered that the sample line itself becomes slightly contaminated wi th radioactivity over several runs. It was observed that during the irradiation, certain chemical changes occurred i n the sample solution. The extracted irradiated solution generally contained a suspension of selenium, giving it a red-brown color. This was not completely unexpected as the selenite ion can be reduced by the aluminum of the target chamber. This is not of great concern i n itself as the amount of selenium formed (and hence of a luminum oxidized) was observed to be quite small over the short period of time that the solution remained i n the target. Also, as selenium takes several hours to settle i n water, the elemental selenium wi l l remain i n the path of the beam and the yield wi l l not be affected. However, this suspended selenium can make extraction of the solution difficult i f i t is not thoroughly flushed out of the system after each irradiation. Occasional cleaning of the target is therefore recommended. Chapter 5 Bromine-77 Production Although the target was designed for the production of the isotope bromine-77, this is not the only radioisotope produced. A variety of reactions, including those of the type (p,xn), (p,pn), (p,p2n), and (p,a) can occur, resulting in radioisotopes of bromine, selenium, and arsenic (from reactions on the target selenium) as well as isotopes resulting from reactions on other materials, particularly water. A list of the reactions expected is included as table IV. The production of any particular isotope can be enhanced by proper selection of the beam energy, as well as by using targets enriched in a specific isotope. The production of the isotopes showed a small amount of variation from run to run, which is to be expected because of small changes in the experimental conditions between runs. The precise yield will obviously depend on the concentration of the target solution. Other important factors include the volume of solution injected as well as the beam current applied. This is not surprising considering that both of these quantities affect the amount of target solution which will vaporize during the irradiation, and hence the density of the material being hit by the beam. Nevertheless, the actual yields were consistent to within about ten percent. The results of the analysis of a typical run are shown in tables V and VI. These results were obtained by identifying several the principal gamma lines measured by a Ge(Li) detector. The quantity of each isotope produced during the irradiation was determined from the measured intensity of each line together with its branching ratio and the energy 33 Chapter 5. Bromine-77 Production 34 Table IV: Reactions Occurring in the Target Reaction Q-value Reaction Q-value (MeV) (MeV) 82Se(p,n)82Br -0.8 76Se(p,n)76Br -5.4 82Se(p,2n)81Br -8.5 76Se(p,2n)75Br -14.7 82Se(p,3n)80Br -18.6 76Se(p,3n)74Br -26.8 82Se(p,a)79As -1.0 76Se(p,pn)75Br -11.2 80Se(p,n)80Br -2.7 74Se(p,n)74Br -7.6 80Se(p,2n)79Br -10.5 74Se(p,2n)73Br -17.6 80Se(p,3n)78Br -21.2 80Se(p,a)77As -1.0 2 7Al(p,n)2 7Si +5.6 78Se(p,n)78Br -4.4 27Al(p,2n)26Si +18.9 78Se(p,2n)77Br -12.6 2 7Al(p,a) 2 4Mg -1.6 78Se(p,3n)76Br -23.3 1 60(p,n) 1 6F +16.4 78Se(p,pn)77Br -10.5 1 60(p,pn) 1 50 +4.1 78Se(p,p2n)76Br -17.9 1 60(p,a) 1 3N +5.2 78Se(p,a)75As -0.9 1 70(p,n) 1 7F +3.5 77Se(p,n)77Br -3.1 1 70(p,a) 1 4N -1.2 77Se(p,2n)76Br -12.8 1 80(p,n) 1 8F +2.4 77Se(p,3n)75Br -23.3 1 80(p,a) 1 5N -1.2 77Se(p,pn)76Br -7.4 23Na(p,n)23Mg +4.8 77Se(p,p2n)75Br -18.6 23Na(p,pn)22Na +12.4 77Se(p,a)74As -1.1 Note that the given energies are Q-values in the center of mass frame. The values in the laboratory frame are greater by a factor of (Mtarget + rnproton)/Mtarget- To calculate the threshold energies, the Coulomb barrier must be added. For the reaction 78Se(p,2n)77Br, the proton must overcome a Coulomb barrier of approximately 7.6 MeV. Chapter 5. Bromine-77 Production Table V : Y i e l d Calculations Isotope Major Branching Measured Ac t iv i t y 7-lines ratio counts* (keV) (%) (cps) (pd) 8 2 B r 554.3 70.6 1.499 x 10 4 0.5738 619.1 43.1 8.821 x 10 3 0.5531 698.3 27.9 6.078 x 10 3 0.5888 776.5 83.4 1.739 x 10 4 0.5635 827.8 24.2 4.697 x 10 3 0.5246 1044 27.4 6.062 x 10 3 0.5979 7 7 B r 239.0 23.9 4.047 x 10 4 4.5765 249.8 3.08 5.386 x 10 3 4.7262 297.2 4.30 7.674 x 10 3 4.8234 520.7 23.2 3.997 x 10 4 4.6563 578.9 3.06 4.558 x 10 3 4.0258 7 6 B r 559.1 72.3 4.857 x 10 4 1.8156 657.0 15.5 9.882 x 10 3 1.7231 1129.8 4.34 2.702 x 10 3 1.6827 1216.1 8.68 5.278 x 10 3 1.6434 1853.7 14.0 7.764 x 10 3 1.4988 7 5 S e 136.0 59.0 1.137 x 10 3 0.0521 264.6 59.1 1.521 x 10 3 0.0696 7 2 S e 834.0 91.3 2.189 x 10 3 0.0648 2 2 N a 1274.5 99.9 2.037 x 10 3 0.0551 "including corrections for detector efficiency and geometry The data for the decay energies was obtained from the Nuclear Data Sheets [1]. Chapter 5. Bromine-77 Production 36 Table V I : Yields Isotope Y i e l d ( / iC i / / iA-h ) ( M B q / ^ A . h ) ( /^Ci.cm 2 / / /A.h.g) 8 2 B r 2 2 ± 1 0.83±0.04 189±9 7 7 B r 127±6 4 .7±0 .2 1070±50 7 6 B r 230±10 8 .3±0.4 1900±90 7 5 S e 0 .9±0 .1 0.034±0.004 7 .8±0.9 7 2 S e 1.2±0.2 0.043±0.006 1 0 ± 1 2 2 N a 0 .8±0 .1 0.031±0.004 Chapter 5. Bromine-77 Production 37 dependent efficiency of the detector. The activity at end of beam was then obtained for each nuclear species as an average over the value obtained from each line considered. The uncertainties were calculated using the uncertainties i n the gamma spectrum (given by the program used to operate the detector system) as well as the uncertainty i n the integrated beam time. They do not reflect the additional variation observed for different irradiated samples. Table V summarizes the measured results for each of the gamma lines considered and includes the branching ratio for each line [1], while table V I lists the final measured results. In table V I , the yield for each radioisotope per unit beam is listed i n the normal fashion as well as i n the unusual units of / /Ci .cm 2 / juA.h .g . The reason for this column is to allow an easy comparison wi th the yields of other targets indicated i n table II. In the past, targets which use selenium as the target nucleus have irradiated selenium metal or compounds in solid form. For this target, however, a much lower density of selenium nuclei is presented to the beam. Thus, although this target can be considered to be a thick target i n that the beam is degraded to zero energy by the target material, most of the energy is lost by passage of the beam through water wi th relatively l i t t le loss through collisions wi th selenium nuclei. It is therefore useful to consider the rate of isotope production per cross-sectional density of selenium present. This was approximated by assuming the range of the beam i n solution to be equal to its range i n water. From the results listed i n table V I , we see that while the absolute yield of bromine-77 is down by a factor of ten from typical yields of more usual thick selenium targets, the yield per cross-sectional density of target nuclei compares favorably wi th past results. It is seen that the major impurities are bromine-76 and bromine-82. In addition, a small amount of radioactive selenium is present along wi th sodium-22 from the N a O H used to make up the solution. The bromine impurities are a usual result for the proton beam reaction on a selenium target and could be drastically decreased by the use of Chapter 5. Bromine-77 Production 38 selenium enriched i n selenium-78 (see table II). Al though the amount of radioactive sele-n ium impurities produced is small, they are relatively long l ived. However, the chemical preparation of samples can eliminate vir tual ly al l of their activity from the final product. Likewise, activity due to the long-lived sodium-22 is not a major concern. In addition, a quick assay of one batch of active target solution was performed within a few hours of irradiation to judge the yield of short-lived isotopes. A large amount of 511 k e V radiation due to positron annihilation was observed, i n agreement wi th the expected production of positron emitters by the action of the proton beam on water. It is expected that oxygen-15, nitrogen-13, and fluorine-18 would be produced, although a gamma spectrum is unable to determine the relative amounts. The results of this test also revealed the production of bromine-75 wi th a yield approximately seven times that of the desired bromine-77. Again , this could be reduced by the use of an enriched target, but i n practice this was not necessary due to its relatively short 1.6 hour half-life. In fact, bromine-77 has the longest half-life of al l of the radiobromides produced, causing the relative amount of impuri ty to decrease wi th time. Thus, i t is seen that the target system discussed above was successful i n producing small amounts of bromine-77 of sufficient activity and puri ty for T D P A C studies. Chapter 6 The T D P A C Technique A s has been stated, the motivation for the production of bromine-77 in this laboratory was for its use in perturbed angular correlation studies. Al though only preliminary work has been performed towards this goal, a brief discussion of the technique is i n order. A full discussion of the theory of this method is beyond the scope of this work, but the review article by Rinneberg [15] serves as a good introduction to the field while articles by Steffen and Frauenfelder [16,17] are usually quoted i n discussions of the general theory. Essentially, P A C experiments are a probe for the nuclear environment. As is well known, there is, i n general, a directional correlation between successive gamma quanta emitted during the de-excitation of an excited nuclear state. In P A C studies, radioactive nuclei which decay to nuclei having a metastable state i n their energy level scheme are incorporated i n the sample of interest. After the decay of their parent, some of the daughter nuclei de-excite via this isomeric level. However, i f the nuclei are not free, but rather are subject to some chemical environment, the angular correlation wi l l be perturbed by the fields present at the nucleus. Classically, this may be thought of as a precession of the nuclear magnetic (or nuclear quadrupole) moment while the nucleus is trapped i n its intermediate state. A measurement of the actual angular correlation as a function of time provides useful information about the local environment of the probe nucleus. The usual experimental apparatus involves the fast-slow coincidence technique. Each gamma detector used for the measurement is set up to produce a fast signal used for 39 Chapter 6. The TDPAC Technique 40 t iming and a slow signal used to measure the energy of the detected radiation to ensure that the measured event is part of the appropriate delayed decay. The fast signals enter a time-to- amplitude converter ( T A C ) to measure the time spent i n the isomeric state. The T A C is set to wait for a stop signal for five to ten lifetimes of the intermediate level after receiving a start signal from a gamma quanta wi th an energy corresponding to the first de-excitation of the cascade being exploited. If no stop pulse is detected i n this time it is most l ikely that the second quanta has been emitted i n a direction away from the stop detectors and the T A C is reset. When a stop pulse does arrive, the resulting signal is sent to a multichannel analyzer which is gated by the slow side electronics. The signals are recorded only i f exactly two pulses are detected during a preset time period and i f the two energies are within energy windows corresponding to decay via the isomeric level. The result is a measurement of the number of decays involving the isomeric level which emit gamma quanta i n the direction of the two detectors as a function of the time spent i n the intermediate state (ie. a delayed coincidence spectrum). As should be obvious, the dominant feature is an exponential decay of detected events wi th time owing to the finite lifetime of the isomeric level. In fact, for a fixed angle between the detectors, this is al l that would be seen i f the angular correlation were constant. However, the effect of the electro-magnetic fields present at the nuclear site is to provide a perturbation to the correlation causing it to be a function of the time delay between the two gamma events. Removing the exponential decay allows this fluctuation i n the correlation function to be observed. In addition to the requirement of an isomeric level, there are certain restrictions on the nuclei which may be successfully applied i f accurate information about the time dependence of the perturbation is to be measured. Experiments i n which this time dependence is measured are known as time-differential perturbed angular correlation ( T D P A C ) studies. The lifetime of the isomeric level must be long enough that it is much Chapter 6. The TDPAC Technique 41 larger than the time resolution of the detectors and associated electronics. On the other hand, if the half-life is too long, the source strength must be kept low to avoid accidental coincidences, resulting in long data collection times. Practically, these considerations demand isomeric half-lives of between two nanoseconds and one microsecond. In addition to these limitations, it is important that the interaction of the nucleus with the fields present be large enough that the periods of the components of the periodic perturbation factor be no greater than the lifetime of the state so that the frequencies can be accurately measured. On the other hand, this precession frequency must be low enough that it is not lost owing to the finite time resolution of the detector. There are at least two dozen gamma-gamma cascades suitable for the TDPAC method. Of these, bromine-77 is a relatively recent nucleus to be applied. This makes it a probe of current interest despite the fact that only 3% of its decay events proceed through one of the required de-excitations. The important theoretical results are included in the articles previously mentioned. It can be shown that the unperturbed correlation is given in general by W{9) = —[1+ £ AkkPk(cos0)} 4 7 F fc=2,4,... where the functions Pk(x) axe the Legendre polynomials and the coefficients Akk are parameters which depend on the nuclear spins of the levels involved in the de-excitation. The value of kmax, which determines the number of terms in the sum, also depends on the nuclear spins of the three nuclear levels involved in the cascade. If the multipole characters of the two transitions are L\ and L2, the value of kmax is given by Kax = Min(2I,2Li,2L 2) where J is the angular momentum of the isomeric level. The possible values of L\ and Z/2 can be determined from the angular momentum eigenvalues of the initial and final Chapter 6. The TDPAC Technique 42 levels involved i n the de-excitation process and correspond to the angular momentum carried away by the radiation field. When this correlation is perturbed by interactions of the nucleus wi th the fields near i t (such as by the interaction of the nuclear magnetic dipole moment wi th an applied magnetic field or by the interaction of the nuclear electric quadrupole moment wi th the electric field gradient), an appropriate correlation factor must be included. Including a factor for the exponential decay of the isomeric level gives us the expression W(0,t) = - [1+ AkkGkk(t)Pk(cos6)] 4 7 r T * fc=2,4,... where TN is the lifetime of the intermediate state and Gkk(t) is the periodic, time-dependent perturbation factor. Of course, an expression for the number of counts actually observed must also include the efficiency of the detectors as well as an additive constant to represent the contribution of random coincidences. One may then write N(9,t) = NQestartestopW(6,t) + C It can be shown that for a powder sample wi th randomly oriented static internal magnetic fields and electric field gradients, under certain assumptions, the perturbation factor may be calculated from the expression A&ectiveG22(t) = ±(JlW)-l) 3 where R(t) is the ratio = [iV(180V)i3 - C13l[iV(180V)24 - C24] = ^(180V) 1 3^(180V) 2 4  U [iV(90°,0i4-C 1 4][iV(90 o,i)23-C 2 3] W(90°,t)14W{90°,t)23 Here, the subscripts identify the detectors used to measure each of the orthogonal cor-relations. Note that this ratio results i n the cancellation of the detector efficiencies and the exponential decay of the isomeric level, so these do not need to be considered i n Chapter 6. The TDPAC Technique 43 the experimental analysis. In addition, for accurate results it is necessary to include a correction for the limited time resolution of the experimental apparatus. Once the perturbation factor has been obtained experimentally it can be fitted to theoretical expressions to determine which models for the system and interactions are the most reasonable. Information about the system is found by the interpretation of the expression to which the data is fitted and of the fitting parameters. A discussion of the interpretation of several typical cases is given in the review articles previously discussed. Chapter 7 Preparation of Silver Bromide Samples Once the bromine-77 has been obtained in the form of bromide ion i n solution, i t is simply a matter of chemistry to incorporate it into samples of interest. It was decided to study silver bromide as an in i t ia l application of the target product. As this type of experiment has not been previously performed wi th silver bromide, this study holds some interest on its own. In addition, beginning wi th a simple system makes i t easier to set up the angular correlation spectrometer for the bromine-77 probe. The first step is the preparation of silver bromide. The solution extracted from the bromine production target consists mostly of the original sodium selenite solution wi th a 0.1 M concentration of sodium hydroxide. It was also observed that some solid selenium was suspended i n the solution. The total volume of solution used was approximately 50 m L which includes 20 m L of water used to flush the target after the irradiated solution was removed. Whi le the preparation of silver bromide from such a solution is quite a simple chemical synthesis, there are a few problems which must be addressed. One is that both the selenite and hydroxide ion form an insoluble compound wi th silver, so these ions must be removed from the solution. For this to be useful, the chemical process of removing these ions obviously must not remove the radiobromide or leave behind other ions that wi l l precipitate wi th silver. In order to physically handle the prepared sample, i t is necessary to add nonactive bromine to the solution to increase the size of the sample. It is best for T D P A C studies to keep the samples as small as possible, though. In order to keep 44 Chapter 7. Preparation of Silver Bromide Samples 45 the volumes of the solutions involved i n the synthesis small, it is recommended that any reagents used have a high concentration. The first step was to filter and wash the nonactive suspended selenium. To the so-lut ion was added concentrated copper nitrate to precipitate the selenite and hydroxide as insoluble copper compounds. The result was a large quantity of powder wi th two dis-tinct shades of blue-green corresponding to the two compounds formed. Enough copper nitrate was added to give the solution a blue colour, indicating that no more copper was being precipitated. The solution was again filtered and the filtrate washed thoroughly. Next, bromide was added i n the form of potassium bromide so that a visible quantity of silver bromide could be produced. A n excess of silver nitrate was used to precipitate the desired silver bromide from the solution. After filtering and washing, the precipitate was allowed to dry, after which i t was collected i n a sample tube. A t each step in this process, some radioactive bromine is lost to the waste material. A rough measurement of the amount of act ivi ty i n the waste product made wi th a radiation monitor led to an estimate that 80% of the radiobromide was included i n the final product. Better yields could probably be attained by additional washing of filtrates, but this would require more elaborate apparatus able to handle larger volumes of solution. Chapter 8 Application of the TDPAC Technique to AgBr In order to tune an angular correlation spectrometer for a specific probe nucleus, certain information about the nucleus and its level structure is required. Below is a summary of the data needed for the use of bromine-77 in TDPAC studies. The main information needed for the experimental apparatus is the energy level scheme for the daughter nucleus of the probe nucleus. A partial level diagram for selenium-77 is shown in figure 12 [18]. As can be seen, the decay of bromine-77 into the selenium daughter is rather complicated with several transitions involved. Not shown in this chart are de-excitations by conversion electrons, which ocurr in less than 1.2% of the decays [3]. As shown by this diagram, the 250 keV isomeric level is populated mainly by three transitions with energies of 568 keV (with a branching ratio of 0.89), 575 keV (1.23), and 755 keV (1.73). Unfortunately, the desired 250 keV transition (gamma 2) is quite close to the main line in the spectrum (239 keV with a branching ratio of 24.2). Worse, several of the transitions which feed this interfering level have energies close to those which feed the isomeric level. This makes it difficult to set energy windows to select only the desired cascade. In TDPAC experiments, it is the time dependence of the perturbations of the angular correlations that is being measured, so good time resolution is required. However the en-ergy resolution is also very important because it must be determined whether the detected radiation is part of a decay through the isomeric level. Without this energy selection, the correlation spectra will be dominated by a prompt peak from the de-excitations which 46 Chapter 8. Application of the TDPAC Technique to AgBr FIGURE 12 PARTIAL LEVEL STRUCTURE OF S E L E N I U M - 7 7 SSRJS o o — o pass 3 / 2 -(3/2- ,5/2-)_ 1/2- — 3 / 2 -5 / 2 -5 / 2 -3 / 2 -7/2+ 1/2-$!nio5?5!/>i8?3 tNO O CN«-<N cor*. 5 i 8 e keV %EC -1005 4.64 824 4.74 818 10.4 521 19.4 439 1.5 N w 250 0.54 v 239 15.7 8 162 0.12 X Z 0 41.8 Chapter 8. Apphcation of the TDPAC Technique to AgBr 48 involve no isomeric level. The detectors used i n this experiment were N a l ( T l ) detectors, which represent an acceptable compromise between good time resolution and good energy resolution. It is not possible to separate completely the gamma lines involved i n the ap-propriate cascade from the interfering prompt cascades wi th these detectors. A n energy spectrum of the radiobromides i n the A g B r sample is shown i n figure 13. This gamma spectrum is i n sharp contrast wi th the one shown is figure 14. This latter spectrum was measured wi th a Ge(Li) detector and shows excellent energy resolution. However, the selection of the desired gamma cascade is much more complete than might be expected. This is because a pulse is only sent to the multichannel analyzer if two gammas are detected and both fall within the energy windows. Obviously, this drastically decreases the amount of 239 keV gammas which gate the M C A because a gate pulse wi l l only be sent i f the 239 keV gamma was preceded by a gamma wi th an energy within the gamma 1 window. A s the source strength is kept low enough i n T D P A C studies that the amount of random coincidences is small, only cascades where both gamma energies are close to the desired ones wi l l interfere. As can be seen from the level diagrams, i f the energy windows for gamma 1 are set to include the 568 keV and 575 k e V transitions, the interference from the 579-239 k e V and 585-239 keV cascades wi l l be of the same order as the contribution of the desired cascades. This is certainly significant, but wi l l not make it impossible to obtain usable data. If a smaller prompt peak is required, the 755-250 k e V cascade, which is only interfered wi th by the weak 766-239 k e V cascade (note the 0.04 branching ratio for the 766 keV transition), can be used. The 271-250 keV cascade also involves the isomeric level but is too weak compared to the interfering 282-239 k e V de-excitation to be of much use. To illustrate these points, gated energy spectra measured by the N a l ( T l ) detectors are included. Figure 15 shows a measured energy spectrum gated by a single channel analyzer FIGURE 13 Energy Spectrum of Bromine-77 Measured by a Nal(Tl) Detector 7000 -i Gamma Energy (keV) Chapter 8. Application of the TDPAC Technique to AgBr 50 Counts o o o ro o o o co o o o 03 3 3 03 m 3 °' CD o < co o g 8 -J o _ o o FIGURE 15 Delayed Coincidence Energy Spectrum of Bromine-77 Gated by the Gamma 1 Transition Measured by a Nal(Tl) Detector 2000 T G a m m a Energy (keV) Chapter 8. Application of tie TDPAC Technique to AgBr 52 wi th an energy window corresponding to the first gamma event of the cascade. A second S C A was used to select only those decays which were detected more than 5 ns after the gamma 1 transition. As can be seen, the relative contribution of the 250 keV peak to the spectrum is greatly enhanced. Similarly, figure 16 shows a delayed coincidence energy spectrum gated by an energy window set around the 250 keV transition. It is observed that the relative contribution of the 755 keV transition is considerably greater than that i n the original ungated spectrum. Note that i n both cases, the selection is not complete. For these spectra most of the interference is due to false coincidences involving the major peaks of the de-excitation spectrum rather than to prompt cascades involving energies close to the selected energies as the latter were filtered out by the time-delay window. In actual T D P A C studies, the prompt cascades are more of a concern as they make the analysis of the perturbation factor for short times more difficult. For the analysis of the correlation spectra, it is useful to know the half-life of the isomeric level. Measurements of the anisotropy, A22, are needed i f the nuclear quadrupole moment ( N Q M ) interaction wi th the electric field gradient ( E F G ) is being studied. For experiments involving the interaction of the magnetic dipole moment wi th magnetic fields, i t is necessary to know the value of the g factor. Measurements of a l l of these quantities for selenium-77 are discussed i n a paper by Zamboni and Saxena [19]. F rom their data, they determined that A22 = -0 .30 ± 0.02 g = 0.447 ± 0.010 and T = 9.56 ± 0.10 ns The large value of the anisotropy makes bromine-77 an attractive probe for T D P A C studies. However, there is some disagreement as to the correct value of A22- Other recent values are A22 = —0.354 ± 0.021, found by Braga and Sarantites (1974) [20], and FIGURE 16 Delayed Coincidence Energy Spectrum of Bromine-77 Gated by the Gamma 2 Transition Measured by a Nal(Tl) Detector 500 T G a m m a Energy (keV) Chapter 8. Application of the TDPAC Technique to AgBr 54 A22 = -0 .454 ± 0.009, measured by Mohsen and Pleiter (1987) [21]. Further research to fix an accepted value would therefore be useful. W i t h the energy levels set and the parameters required for data analysis known, experiments using bromine-77 as a probe nucleus may be performed. Chapter 9 Results After preparing the spectrometer system for bromine-77, correlation spectra of the silver bromide sample were measured. These preliminary studies show the suitability of the bromine-77 probe to TDPAC studies. Figure 17 shows a typical plot of the number of measured delayed coincidences versus time for a pair of detectors. The prompt peak from cascades not involving the isomeric level is clearly seen and is followed by an exponential curve with a decay constant equal to the lifetime of the isomeric level. Analysis of the perturbation factor relies on the small deviations from a purely exponential decay. In an ideal situation, one would expect there to be no perturbation to the angular cor-relation for a crystal with cubic symmetry as the electric field gradient would be expected to be zero at any lattice site. However, there can be slight variations in the crystalline fields owing to the presence of impurities as well as lattice vibrations. Crystalline AgBr has a crystal structure similar to that of NaCl (two interpenetrating face-centered cubic lattices). Thus, we expect to see no perturbation to the correlation function. In figure 18, a plot of the experimental A ^ G ^ W v a i u e is shown. This was calculated in the normal fashion by assuming that the values of Akk with A; > 2 are small relative to A22- Although the uncertainty in this data could be decreased by more data runs, it appears to be low enough that the form of any perturbation factor could be seen. In order to obtain this plot, it was useful.to subtract the prompt peaks in the delayed coincidence spectra. This was done by fitting the sum of an exponential decay and a normal curve (centered at the time zero channel) to the experimental data for each detector pair and 55 FIGURE 17 Exponential Decay of the Isomeric Level 10000q 1000: A typical delayed coincidence spectrum for a pair of detectors is shown along with the fitted curves for the prompt peak and exponential decay. 100: 10: The starting point of the time axis is arbitrary. ~T~ 5 10 45 —I 50 25 Time (ns) Chapter 9. Results Perturbation p p o o p o oo b> A Ka o Ks Chapter 9. Results 58 subtracting the normalized Gaussian curve thus determined from the experimental data. The parameters for the curve fitted to the prompt peak yield an estimate of the time resolution of the apparatus. For this experiment, the standard deviation of the Gaussian curve was determined to be 0.829 ns. A more usual measure of the time resolution is the full width at half-maximum ( F W H M ) of the prompt peaks. This is found to be ires = 2 \ / 2 C T ln2 = 1.95 ns for this experiment. A s can be seen from the scatter plot, any perturbation to the angular correlation which does exist must be quite small. Al though the poor statistics of the data is clearly seen from the scatter of the points, one can put a l imit of roughly 0.2 on the maximum deviation of G22 from unity. This indicates that, as expected, the angular correlation for successive gamma quanta emitted by bromine-77 in A g B r is unperturbed (aside from the possibility of a small contribution from nuclei which happen to be near impur i ty sites). The obvious physical interpretation is that the electric field gradient at the bromine lattice sites i n A g B r is zero to the l imit of precision allowed by the experimental data. If one makes the assumption that there is no perturbation, so Gii(t) = G22 = 1, an experimental value of the anisotropy A22 can be calculated as the average of the A22G22 curve. For the given data, a value of A22 = —0.334 ± 0.006 was found. The uncertainty is an estimate based on the distribution of the data points about the mean value. In addition, this value wi l l have a slight systematic error as no correction for the finite solid angles of the detectors was made. A more accurate calculation was not performed because the intent of this preliminary study was not to obtain a new value for the anisotropy, so the detector distances were adjusted to maintain an optimal count-rate over the data acquisition time. This experimental value lies midway between the values quoted by Zamboni and Saxena [19] and by Braga and Sarantites [20], and is within the error bounds given i n the latter work. Al though this study is not a particularly important example of T D P A C studies, i t Chapter 9. Results 59 does demonstrate the general principles involved. Relatively few angular correlation studies using bromine-77 have yet been performed, but this will likely change as more production systems become available. A recent example of work involving this isotope is given in the paper by Mohsen and Pleiter [21] where the hyperfine field of selenium in nickel was investigated. Future work in this laboratory may involve the investigation of graphite intercalation compounds containing bromine. Chapter 10 Conclusions The radioisotope bromine-77 has received recent attention as a suitable probe for T D P A C studies. Very few studies have yet been made using this nucleus, leaving an abundance of interesting experiments to be performed. The use of bromine-77 has also been increasing i n the field of radiopharmaceuticals, adding further interest to the production of this isotope. Al though routine production is now being performed at a few facilities, i t is st i l l not a readily available isotope. This work has described a simple, inexpensive target which can be quickly installed i n a proton cyclotron for the production of small amounts of bromine-77. Prel iminary studies show that although long data aquisitions are necessary, this isotope is well suited to perturbed angular correlation studies. 60 Bibliography [1] Reus, U., and Westmeier, W . , Atomic Data and Nuclear Data Tables, 29 Sept. 1983. [2] Phelps, M . E . , Huang, S. C , Hoffman, E . J . , Selin, C , Sokoloff, L . , K u h l , D . E . , Ann. Neurol, 6, 371-388 (1979). [3] Stocklin, G . , Int. J. appl. Radiat. Isotopes, 28, 131-148 (1977). [4] Bosch, R . L . P . van den, Production of1231, 77Br and 87 Y with the Eindhoven A. V.F. Cyclotron Eindhoven University of Technology, Eindhoven, The Netherlands (1979). [5] Nozaki , T . , Iwamoto, M . , and Itoh, Y . , Int. J. appl. Radiat. Isotopes, 30, 79-83 (1979). [6] Vianden, R. , Hyperfine Interactions, 4, 956 (1978). [7] Quaim, S. M . , and Stocklin, G . , Radiochimica Acta, 34, 25-40 (1983). [8] Blessing, G . , Weinreich, R. , Quaim, S. M . , Stocklin, G . , Int. J. appl. Radiat. Isotopes, 33, 333-339 (1982). [9] Janssen, A . G . M . , Bosch, R . L . P. van den, Goeij, J . J . M . , and Theelen, H . M . J . , Int. J. appl Radiat. Isotopes, 31, 405-409 (1980). [10] Jong, D . de, Kooiman, H . , Lindner, L . , Kaspersen, F . M . , Br inkman, G . A . , J. lab. Comp. Radiopharmac, 16, 223 (1979). [11] Quaim, S. M . , Stocklin, G . , and Weinreich, R. , em Int. J . appl. Radiat . Isotopes, 28, 947-953 (1977). [12] Grant , P. M . , Whipple , R . E . , Barnes, J . W . , Bentley, G . E . , Wanek, P. M . , O 'Br ien , Jr., H . A . , J. Inorg. Nucl. Chem., 43, 2217-2222 (1981). [13] Janni, J . F . , Atomic Data and Nuclear Data Tables 27 ( M a r c h / M a y / J u l y / S e p t . 1982). [14] Bretherick, L . , ed. Hazards in the Chemical Laboratory, 3rd ed. The Royal Society of Chemistry, London (1981). [15] Rinneberg, H . H . , Atomic Energy Review, 17, 477-595 (1979). 61 Bibliography 62 [16] Steffen, R . M . and Frauenfelder, H . , Alpha-, Beta and Gamma-Ray Spectroscopy Ed. K . Siegbahn, Amsterdam: North-Holland (1964). [17] Steffen, R . M . and Alder , K . . The Electromagnetic Interaction in Nuclear Spec-troscopy E d . W . D . Hamil ton, C h . 12-13, Amsterdam: Nor th Hol land (1975). [18] Singh, B . and Viggars, D . A . , Atomic Data and Nuclear Data Sheets, 29, 75-168 (1980). [19] Zamboni, C . B . and Saxena, R . N . , J. Phys. G: Nucl. Phys., 10, 1571-1577 (1984). [20] Braga, R . A . and Sarantites, D . G . , Phys. Rev. C, 9, 1493-1505 (1974). [21] Mohsen, M . and Pleiter, F . , Hyperfine Interactions, 39, 123-128 (1988). 

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