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A cyclotron isotope production facility designed to maximize production and minimize radiation dose
Nuclear Instruments and Methods in Physics Research B79 (1993) 929-932 North-Holland
Beam Interactions with Materials A Atoms
A cyclotron isotope production facility designed to maximize production and minimize radiation dose W.J. Dickie Nordion International Inc., 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2.43
N.R. Stevenson and F.F. Szlavik T~UMF,
4004 Wesbrook ~a~
Vancouver, L?C, Canada V6T 243
Continuing increases in requirements from the nuclear medicine industry for cyclotron isotopes is increasing the demands being put on an aging stock of machines. In addition, with the 1990 recommendations of the ICRP publication in place, strict dose limits will be required and this will have an effect on the way these machines are being operated. Recent advances in cyclotron design combined with lessons learned from two decades of commercial production mean that new facilities can result in a substantial charge on target, low personnel dose, and minimal residual activation. An optimal facility would utilize a well engineered variable energy/high current H- cyclotron design, multiple beam extraction, and individual target caves. Materials would be selected to minimize activation and absorb neutrons. Equipment would be designed to minimize ma~tenance activities performed in high radiation fields.
1. Introduction Demand for cyclotron isotopes has grown significantly since the mid 1980s [l]. The most commonly produced isotopes on commercial compact cyclotrons (CCC) include uflTl, 67Ga, “‘In, ‘=I and 57c0. These cyclotron-made isotopes are largely being produced on an aging stock of H+ machines utilizing internal targetry 111.In the past few years these have been supplemented by a number of a new generation of Hcyclotrons employing external targetry. A significant challenge for the isotope manufacturers will be to continue to maximize productivi~ while meeting the new international limits for radiation exposure to personnel (ICRP publication 60 recommends an average of 20 mSv/yr). Currently these limits, soon to take effect, are being exceeded in many facilities. At TRIUMF the technical staff have extensive experience with the technology involved in the operation of H- cyclotron based systems (likely to be the dominant method for future pr~uction [2]). The TRIUMF accelerator, a 520 MeV isochronous H- cyclotron that runs nominally at 140 PA, has given over 20 years of operating (and upgrading) experience. In addition, 10 years of production has proceeded on the first generation CCC H- machine - the CP42 (42 MeV; 200 PA). The lessons learned have since been incorporated in the design and operation of another cyclotron facility the TR30 (30 MeV, 2 x 200 pA) that has now been in Old-583X/93/$06.~
operation for over two years. The purpose of this paper is to propose an optimum design, given existing technology, for a commercial isotope production facility which is both productive and low dose to personnel. We advocate the position that the objectives of productivity and radiation safety can be complementary and not in conflict.
2. Cyclotron concept The H- cyclotron has a number of advantages over its earlier H+ generation and has therefore been adopted by the major cyclotron manufacturers worldwide. The major advantages are: 1) The ability to more easily extract high currents with variable energy. This allows the elimination of internal targetry which had been the only real practical means of achieving high productivity with the H+ machines. Internal targetry is by far the largest source of activation and ~nta~nation inside the older cyclotrons. Extensive cooldown and maintenance periods are required and the work usually results in high personnel doses. 2) The ability of H- cyclotrons to extract high current multiple beams (and energies) simultaneously. When combined with high power targetry [3] and appropriate shielding this allows cool down for maintenance while production continues on other beamlines.
0 1993 - Elsevier Science Publishers B.V. All rights
reserved
XVIII. ISOTOPE PRODUCTION
W;J. Dickie et al. / Cyclotron isotope production facili@
930 3. Cyclotron design features
Historically, more than half of the CP42 worker’s dose came from tank work [4]. This was due to the nature of the activation in this machine which typically consisted of localized sources from proton induced reactions within a dispersed neutron induced background. In contrast, the TR30 was designed such that the activation of the tank and the vault is an order of magnitude lower. This is achieved in several ways: In an H- cyclotron it is of great im~rtan~ to maintain a high vacuum to limit internal beam loss due to gas stripping which activates the internal components of the machine. On the CP42, which operates at a vacuum of - 3 pTorr, gas stripping losses are about 15%. On the TR30, operating at - 0.3 PTorr this is about 3%. The better vacuum results from the use of an external rather than internal ion source and also high efficiency cryopumps rather than diffusion pumps (thereby avoiding any residual oil fumes in the tank). Another major point in reducing radiation dose lies in the prudent selection of materials. Given that some neutral beam will exist in any design it is best to keep such materials as steel and copper out of the median plane [Z]. Even the choice of steel can be important, e.g., in the TR30 the magnet was constructed of low cobalt-nickel-copper steel. The CP42 tank is made of steel but the TR3O’s is aluminum. A third major point in reducing dose and increasing productivity is reducing repairs and maintenance. The basic design must have this aspect of operation as a goal. For example, consider the CP42 which utilizes Dee insulators that have frequently failed in the harsh environment to which they are subjected. The resulting dose to personnel over the years was a significant portion of the overall exposure (une~ected failures are generally more dose intensive to repair than
i
160
k 1982
1984
/
/
1986
1988 ‘/FAf
1
1390
,.-
/
I
I
1992
1994
Fig. 1. Person-dose history of the CP42 (solid line) and TR30 (dashed line) facilities. Histogram bars are for quarterly periods.
19kl
l&4
!98f, '
19bs YEAR
19bo
IdQ
19b-l
Fig. 2. Charge on target history of CP42 and TR30 facilities.
planned maintenances [5]). The TR30 avoids this problem by removing the insulators outside of the tank. Also, the CP42’s central region requires a lid-up maintenance every six weeks. The TR30 has an external ion source and the only maintenance required is to change the ion source filament and the extraction foils - all of which can be done without opening the tank. One significant improvement in the TR30 system (and since retrofitted to the CP42) is the removal from the active areas of all water and air manifolds so that adjustment, maintenance, and emergency shutoff can be done without entering a radiation area. Simply having these outside of active areas means that previously frequently failing com~nents now rarely do so. Fig. 1 compares the person-dose accrued over the years from maintenance work, major repairs, and upgrading activities to the CP42 cyclotron and target systems with that of the TR30. The histogram shows that on average the dose expenditure for the CP42 is - 35 person-mSv per quarter. Specific isolated columns, noticeable in the last few years, result from major dose intensive activities requiring extensive downtime. For the TR30 the average dose requirement is much lower (- 5 person-mSv per quarter) predominantly resulting from target servicing work on that system. Fig. 2 shows the corresponding charge on target (productivity in mAh) for both the CP42 and TR30 systems. The lessons learned from operating the CP42 and subsequently applied to the TR30 facility result in an overall increase in production which is more than double the CP42’s yet as mentioned above the resulting dose to workers is significantly reduced.
4. Layout of radiation areas The most important design consideration for the cyclotron vault is that it only be used to house the cyclotron itself. In some facilities target stations have
W.J. LX&e et al. / Cyclotronisotopeproductionfacility
;
-c
1 I?
Fig. 3. Isodose curves of the CP42 vault (mSv/h) for the situation where two target stations were placed in the vault adjacent to the cyclotron (in 1987).Axes represent dimensions of vault in meters.
been situated adjacent to cyclotrons or within the same radiation room. This has resulted in increased damage to equipment requiring higher levels of maintenance. The maintenance/repair work itself is also significantly more dose intensive [4]. This is illustrated in figs. 3 and 4 which compare the residual radiation field levels in the CP42 vault for the two cases when a) two target stations were present; and b) after these were removed to external caves. The second issue is the reduction in the amount of “Na created by thermal neutrons via =Na(n, yfz4Na. Low sodium concrete was used in both the CP42 and TR30 (0.2% vs 2% in normal concrete). In addition 0.2% boron was added in the TR30 as a thermal neutron absorber. This has contributed to the reduction of almost an order of magnitude in the residual field of the TR30 vault over the CP42. One improvement that should be contemplated in future designs would be to increase the vault size so as to decrease
6
5
0
3
2
1
6
'0
Fig. 4. Same as for fig. 3 except with target stations removed to external caves (in 1992).
931
the isotropic background dose and generally provide more room to work effectively. The targets should be housed in individual caves due to significant activation being produced in the target stations [4]. The caves should have separate entrances with appropriate neutron beam blockers in beamlines if required such that one could work in one target cave while operating in others. The number of separate caves and stations should be at least twice the number of simultaneous beams normally used. In this way required target maintenance can be carried out with appropriate cooldown and without production pressure. The only equipment that should be within these target caves are the target stations themselves to avoid unnecessa~ radiation damage. In particular, the temptation of some designers to position magnets in the cave or cave walls should be avoided if at all possible. These components should be in the cyclotron vault. Of course, good cyclotron designs which include low emittance (such as the TR30) are required for this to be possible.
5. Target systems Most commercial production of cyclotron isotopes is done using the so-called solid targets which involves electroplating a small amount (l-5 g) of enriched target material on a water cooled target (typically silver or copper). This target is manipulated remotely so that it is placed in the beam for irradiation without resulting in dose to the operator. At TRIUMF the solid targets are moved into the irradiation areas using pneumatic transport systems. Targets are briefly placed within carriers (“rabbits”) for the journey. At each target station, a manipulation system removes the target and places it in a vacuum box connected to the beamline. Cooling water is connected to the target through the manipulator arm which holds the target securely in place during the irradiation process. An O-ring, placed around the body of the target, seals against a flange on the vacuum box. After the the pump-down procedure (typically taking lo-15 min) the main beamline valve is opened to the target station and the i~adiation can proceed. This sequence is reversed to return the irradiated targets to the hot cells for processing. Complete remote operation of the system is achieved with a programmable logic controller. The other type of major target system is the gas target whereby the beam passes through a window into a vessel containing the target gas. As with the solid target system, transfer of the gas is best done in a way which does not involve direct operator handling. On both these target systems there are some dose reduction principles which have been followed. First XVIII. ISOTOPE PRODUCTION
932
W.J. Dickie et al. / Cyclotron isotope production facility
nothing except the minimum is permanently in the target caves. In the case of lz31 production, the ‘%Xe target gas is stored in the hot cell when not in use, rather than in the target caves. Components have been designed in modular form for quick change and repair outside the radiation areas. Consumables or high maintenance items have been removed from the stations (e.g., O-rings originally placed on the manipulator head holding solid targets in the irradiation position while permitting cooling water to circulate have been removed and placed within each target. These target O-rings are now routinely changed in the hot cells before each target is irradiated). The systems are generally designed as simply and robustly as possible to eliminate fine tolerances which cause frequent breakdown. The reality of operations is that maintenance is frequently done by inexperienced personnel in an effort to spread dose so work must not be too complex. As mentioned above often the choice of materials will determine the frequency of failures or dose involved in repairs (e.g., we make sure that we only use radiation-hard wiring and water tubing). Target stations are made as much as possible from aluminum rather than steel. It is often best not to try to shield target stations as there would be more dose involved in removing and reinstalling this shielding than would be saved. Finally, to help reduce any remaining person-dose a simple task robot could be used. At TRIUMF we are testing a remotely controlled manipulator arm mounted on a miniature vehicle [6]. This device, which was originally designed for explosives ordinance disposal, has been modified for use in remote handling and inspection within the radiation rooms. Use of several cameras and accessories enables the detailed examination of highly active components at close range. A
modified manipulator enables the performance of simple tasks (e.g., removal of stuck “hot” targets). The advantages are a reduction in personnel dose and the saving in beam time since a cooldown period would now be often unnecessary.
6. Summary After decades of operating cyclotron based isotope production facilities a new generation of high efficiency H- cyclotrons delivering beam to high power external targetry have emerged. In these types of facilities high productivity without compromising radiation safety can be achieved if sufficient attention is paid to applying the lessons learned from the past.
References [l] W.J. Dickie and B.F. Abeysekera, Proc. 4th Int. Symposium on the Synthesis and Applications of Isotopes and Isotopically Labelled Compounds, Toronto, eds. E. Buncel and G.W. Kabalka (Elsevier, 1992) p. 245. [2] J.J. Burgejon, Nucl. Instr. and Meth. BlO/ll (1985) 951. [3] N.R. Stevenson and W.Z. Gelbart, Proc. 8th Int. Conf. on Cyclotrons and their Applications, Vancouver, 1992, TRIUMF preprint TRI-PP-92-64, eds. M.K. Craddock and G. Dutto (World Scientific) in press. [4] F.F. Szlavik, Health Physics, Proc. Mid-Year Topical Meeting, Reno, Nevada, 1987, eds. W.P. Swanson and D.D. Busick (Natl. Tech. Info Service, U.S. Dept. of Commerce, Springfield, VA). [5] T. Grey-Morgan and R.E. Hubbard, Proc. 8th Int. Conf. on Cyclotrons and their Applications, Vancouver, 1992, TRIUMF preprint TRI-PP-92-63, eds. M.K. Craddock and G. Dutto (World Scientific) in press.. [6] N.R. Stevenson et al., ibid.
Beam Interactions with Materials A Atoms
A cyclotron isotope production facility designed to maximize production and minimize radiation dose W.J. Dickie Nordion International Inc., 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2.43
N.R. Stevenson and F.F. Szlavik T~UMF,
4004 Wesbrook ~a~
Vancouver, L?C, Canada V6T 243
Continuing increases in requirements from the nuclear medicine industry for cyclotron isotopes is increasing the demands being put on an aging stock of machines. In addition, with the 1990 recommendations of the ICRP publication in place, strict dose limits will be required and this will have an effect on the way these machines are being operated. Recent advances in cyclotron design combined with lessons learned from two decades of commercial production mean that new facilities can result in a substantial charge on target, low personnel dose, and minimal residual activation. An optimal facility would utilize a well engineered variable energy/high current H- cyclotron design, multiple beam extraction, and individual target caves. Materials would be selected to minimize activation and absorb neutrons. Equipment would be designed to minimize ma~tenance activities performed in high radiation fields.
1. Introduction Demand for cyclotron isotopes has grown significantly since the mid 1980s [l]. The most commonly produced isotopes on commercial compact cyclotrons (CCC) include uflTl, 67Ga, “‘In, ‘=I and 57c0. These cyclotron-made isotopes are largely being produced on an aging stock of H+ machines utilizing internal targetry 111.In the past few years these have been supplemented by a number of a new generation of Hcyclotrons employing external targetry. A significant challenge for the isotope manufacturers will be to continue to maximize productivi~ while meeting the new international limits for radiation exposure to personnel (ICRP publication 60 recommends an average of 20 mSv/yr). Currently these limits, soon to take effect, are being exceeded in many facilities. At TRIUMF the technical staff have extensive experience with the technology involved in the operation of H- cyclotron based systems (likely to be the dominant method for future pr~uction [2]). The TRIUMF accelerator, a 520 MeV isochronous H- cyclotron that runs nominally at 140 PA, has given over 20 years of operating (and upgrading) experience. In addition, 10 years of production has proceeded on the first generation CCC H- machine - the CP42 (42 MeV; 200 PA). The lessons learned have since been incorporated in the design and operation of another cyclotron facility the TR30 (30 MeV, 2 x 200 pA) that has now been in Old-583X/93/$06.~
operation for over two years. The purpose of this paper is to propose an optimum design, given existing technology, for a commercial isotope production facility which is both productive and low dose to personnel. We advocate the position that the objectives of productivity and radiation safety can be complementary and not in conflict.
2. Cyclotron concept The H- cyclotron has a number of advantages over its earlier H+ generation and has therefore been adopted by the major cyclotron manufacturers worldwide. The major advantages are: 1) The ability to more easily extract high currents with variable energy. This allows the elimination of internal targetry which had been the only real practical means of achieving high productivity with the H+ machines. Internal targetry is by far the largest source of activation and ~nta~nation inside the older cyclotrons. Extensive cooldown and maintenance periods are required and the work usually results in high personnel doses. 2) The ability of H- cyclotrons to extract high current multiple beams (and energies) simultaneously. When combined with high power targetry [3] and appropriate shielding this allows cool down for maintenance while production continues on other beamlines.
0 1993 - Elsevier Science Publishers B.V. All rights
reserved
XVIII. ISOTOPE PRODUCTION
W;J. Dickie et al. / Cyclotron isotope production facili@
930 3. Cyclotron design features
Historically, more than half of the CP42 worker’s dose came from tank work [4]. This was due to the nature of the activation in this machine which typically consisted of localized sources from proton induced reactions within a dispersed neutron induced background. In contrast, the TR30 was designed such that the activation of the tank and the vault is an order of magnitude lower. This is achieved in several ways: In an H- cyclotron it is of great im~rtan~ to maintain a high vacuum to limit internal beam loss due to gas stripping which activates the internal components of the machine. On the CP42, which operates at a vacuum of - 3 pTorr, gas stripping losses are about 15%. On the TR30, operating at - 0.3 PTorr this is about 3%. The better vacuum results from the use of an external rather than internal ion source and also high efficiency cryopumps rather than diffusion pumps (thereby avoiding any residual oil fumes in the tank). Another major point in reducing radiation dose lies in the prudent selection of materials. Given that some neutral beam will exist in any design it is best to keep such materials as steel and copper out of the median plane [Z]. Even the choice of steel can be important, e.g., in the TR30 the magnet was constructed of low cobalt-nickel-copper steel. The CP42 tank is made of steel but the TR3O’s is aluminum. A third major point in reducing dose and increasing productivity is reducing repairs and maintenance. The basic design must have this aspect of operation as a goal. For example, consider the CP42 which utilizes Dee insulators that have frequently failed in the harsh environment to which they are subjected. The resulting dose to personnel over the years was a significant portion of the overall exposure (une~ected failures are generally more dose intensive to repair than
i
160
k 1982
1984
/
/
1986
1988 ‘/FAf
1
1390
,.-
/
I
I
1992
1994
Fig. 1. Person-dose history of the CP42 (solid line) and TR30 (dashed line) facilities. Histogram bars are for quarterly periods.
19kl
l&4
!98f, '
19bs YEAR
19bo
IdQ
19b-l
Fig. 2. Charge on target history of CP42 and TR30 facilities.
planned maintenances [5]). The TR30 avoids this problem by removing the insulators outside of the tank. Also, the CP42’s central region requires a lid-up maintenance every six weeks. The TR30 has an external ion source and the only maintenance required is to change the ion source filament and the extraction foils - all of which can be done without opening the tank. One significant improvement in the TR30 system (and since retrofitted to the CP42) is the removal from the active areas of all water and air manifolds so that adjustment, maintenance, and emergency shutoff can be done without entering a radiation area. Simply having these outside of active areas means that previously frequently failing com~nents now rarely do so. Fig. 1 compares the person-dose accrued over the years from maintenance work, major repairs, and upgrading activities to the CP42 cyclotron and target systems with that of the TR30. The histogram shows that on average the dose expenditure for the CP42 is - 35 person-mSv per quarter. Specific isolated columns, noticeable in the last few years, result from major dose intensive activities requiring extensive downtime. For the TR30 the average dose requirement is much lower (- 5 person-mSv per quarter) predominantly resulting from target servicing work on that system. Fig. 2 shows the corresponding charge on target (productivity in mAh) for both the CP42 and TR30 systems. The lessons learned from operating the CP42 and subsequently applied to the TR30 facility result in an overall increase in production which is more than double the CP42’s yet as mentioned above the resulting dose to workers is significantly reduced.
4. Layout of radiation areas The most important design consideration for the cyclotron vault is that it only be used to house the cyclotron itself. In some facilities target stations have
W.J. LX&e et al. / Cyclotronisotopeproductionfacility
;
-c
1 I?
Fig. 3. Isodose curves of the CP42 vault (mSv/h) for the situation where two target stations were placed in the vault adjacent to the cyclotron (in 1987).Axes represent dimensions of vault in meters.
been situated adjacent to cyclotrons or within the same radiation room. This has resulted in increased damage to equipment requiring higher levels of maintenance. The maintenance/repair work itself is also significantly more dose intensive [4]. This is illustrated in figs. 3 and 4 which compare the residual radiation field levels in the CP42 vault for the two cases when a) two target stations were present; and b) after these were removed to external caves. The second issue is the reduction in the amount of “Na created by thermal neutrons via =Na(n, yfz4Na. Low sodium concrete was used in both the CP42 and TR30 (0.2% vs 2% in normal concrete). In addition 0.2% boron was added in the TR30 as a thermal neutron absorber. This has contributed to the reduction of almost an order of magnitude in the residual field of the TR30 vault over the CP42. One improvement that should be contemplated in future designs would be to increase the vault size so as to decrease
6
5
0
3
2
1
6
'0
Fig. 4. Same as for fig. 3 except with target stations removed to external caves (in 1992).
931
the isotropic background dose and generally provide more room to work effectively. The targets should be housed in individual caves due to significant activation being produced in the target stations [4]. The caves should have separate entrances with appropriate neutron beam blockers in beamlines if required such that one could work in one target cave while operating in others. The number of separate caves and stations should be at least twice the number of simultaneous beams normally used. In this way required target maintenance can be carried out with appropriate cooldown and without production pressure. The only equipment that should be within these target caves are the target stations themselves to avoid unnecessa~ radiation damage. In particular, the temptation of some designers to position magnets in the cave or cave walls should be avoided if at all possible. These components should be in the cyclotron vault. Of course, good cyclotron designs which include low emittance (such as the TR30) are required for this to be possible.
5. Target systems Most commercial production of cyclotron isotopes is done using the so-called solid targets which involves electroplating a small amount (l-5 g) of enriched target material on a water cooled target (typically silver or copper). This target is manipulated remotely so that it is placed in the beam for irradiation without resulting in dose to the operator. At TRIUMF the solid targets are moved into the irradiation areas using pneumatic transport systems. Targets are briefly placed within carriers (“rabbits”) for the journey. At each target station, a manipulation system removes the target and places it in a vacuum box connected to the beamline. Cooling water is connected to the target through the manipulator arm which holds the target securely in place during the irradiation process. An O-ring, placed around the body of the target, seals against a flange on the vacuum box. After the the pump-down procedure (typically taking lo-15 min) the main beamline valve is opened to the target station and the i~adiation can proceed. This sequence is reversed to return the irradiated targets to the hot cells for processing. Complete remote operation of the system is achieved with a programmable logic controller. The other type of major target system is the gas target whereby the beam passes through a window into a vessel containing the target gas. As with the solid target system, transfer of the gas is best done in a way which does not involve direct operator handling. On both these target systems there are some dose reduction principles which have been followed. First XVIII. ISOTOPE PRODUCTION
932
W.J. Dickie et al. / Cyclotron isotope production facility
nothing except the minimum is permanently in the target caves. In the case of lz31 production, the ‘%Xe target gas is stored in the hot cell when not in use, rather than in the target caves. Components have been designed in modular form for quick change and repair outside the radiation areas. Consumables or high maintenance items have been removed from the stations (e.g., O-rings originally placed on the manipulator head holding solid targets in the irradiation position while permitting cooling water to circulate have been removed and placed within each target. These target O-rings are now routinely changed in the hot cells before each target is irradiated). The systems are generally designed as simply and robustly as possible to eliminate fine tolerances which cause frequent breakdown. The reality of operations is that maintenance is frequently done by inexperienced personnel in an effort to spread dose so work must not be too complex. As mentioned above often the choice of materials will determine the frequency of failures or dose involved in repairs (e.g., we make sure that we only use radiation-hard wiring and water tubing). Target stations are made as much as possible from aluminum rather than steel. It is often best not to try to shield target stations as there would be more dose involved in removing and reinstalling this shielding than would be saved. Finally, to help reduce any remaining person-dose a simple task robot could be used. At TRIUMF we are testing a remotely controlled manipulator arm mounted on a miniature vehicle [6]. This device, which was originally designed for explosives ordinance disposal, has been modified for use in remote handling and inspection within the radiation rooms. Use of several cameras and accessories enables the detailed examination of highly active components at close range. A
modified manipulator enables the performance of simple tasks (e.g., removal of stuck “hot” targets). The advantages are a reduction in personnel dose and the saving in beam time since a cooldown period would now be often unnecessary.
6. Summary After decades of operating cyclotron based isotope production facilities a new generation of high efficiency H- cyclotrons delivering beam to high power external targetry have emerged. In these types of facilities high productivity without compromising radiation safety can be achieved if sufficient attention is paid to applying the lessons learned from the past.
References [l] W.J. Dickie and B.F. Abeysekera, Proc. 4th Int. Symposium on the Synthesis and Applications of Isotopes and Isotopically Labelled Compounds, Toronto, eds. E. Buncel and G.W. Kabalka (Elsevier, 1992) p. 245. [2] J.J. Burgejon, Nucl. Instr. and Meth. BlO/ll (1985) 951. [3] N.R. Stevenson and W.Z. Gelbart, Proc. 8th Int. Conf. on Cyclotrons and their Applications, Vancouver, 1992, TRIUMF preprint TRI-PP-92-64, eds. M.K. Craddock and G. Dutto (World Scientific) in press. [4] F.F. Szlavik, Health Physics, Proc. Mid-Year Topical Meeting, Reno, Nevada, 1987, eds. W.P. Swanson and D.D. Busick (Natl. Tech. Info Service, U.S. Dept. of Commerce, Springfield, VA). [5] T. Grey-Morgan and R.E. Hubbard, Proc. 8th Int. Conf. on Cyclotrons and their Applications, Vancouver, 1992, TRIUMF preprint TRI-PP-92-63, eds. M.K. Craddock and G. Dutto (World Scientific) in press.. [6] N.R. Stevenson et al., ibid.