H− cyclotrons for radioisotope production

Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland, Amsterdam

951

(1985) 951-956

Section X. Accelerator production of radioisotopes and targets for nuclear research H - CYCLOTRONS FOR RADIOISOTOPE PRODUCTION J.J. ~UK~E~O~ TRIUMF,Vancouwr,B.C., Canada V6T 2.43

Over the past few years H- cyclotrons have been introduced as powerful radioisotope producers. Four of these machines, supplied by The Cyclotron Corporation of Berkeley, California, are now in regular operation in the US, Britain, Germany and Canada. They routinely accelerate protons, variable in energy from 11 to 42 MeV. at beam currents of up to 200 PA after extraction. Negative ion cyclotrons feature simplicity of extraction, ease of varying the energy and potential for more than one extracted beam, Target systems are installed in the external beams, thus e~~~nating much of the usual activation and condonation of the cyclotron. ~erational experience with the presently installed machines is reviewed.

1. Introduction The principle of accelerating and extracting negative hydrogen ions in a cyclotron was demonstrated in 1962 at the University of Colorado 111. Soon it was recognized that this method would have great potential for high efficiency extraction of protons over a wide range of energies [2]. Variable energy H- extraction was subsequently installed at the UCLA [3] and University of Manitoba [4] cyclotrons, and a few years later at the Milan cyclotron [5]. A combination magnet was used to bend the beams of different energies into the beam transport system. In 1975 extraction of two simultaneous beams from the 500 MeV H- cyclotron was demonstrated at TRIUMF ]61. The interest in H- acceleration encouraged the development of stronger II- ion sources [7]. Initially, the more powerful ion sources were too big to be used as internal cyclotron ion sources. In the mid seventies The Cyclotron Corporation of Berkeley developed an internal cyclotron H- source for 300 PA. The stage was set for the construction of powerful, yet compact, Hcyclotrons suitable for commercial radioisotope production.

2. H - cyclotrons in operation Table 1 lists the H- cyclotrons presently in operation and the degree to which their programs are involved with radioisotope production. The older Manitoba and Milan cyclotrons have been producing radioisotopes on a small scale, their low beam current making them uneconomical for commercial production. TRIUMF’s 500 MeV H- cyclotron can produce a 0168-583X/~5/$03.30 (North-Holland

0 Elsevier Science Publishers

Physics Publishing

Division)

B.V.

beam current of 130 PA, because it uses a powerful external H- ion source f7]. In 1979 a proton irradiation facility [8] was installed in the main beam line, just ahead of the “beam dump”. The various targets in the main beam line degrade the energy available for radioisotope production to 450 MeV and scattering reduces the beam current to 80 gA maximum. Twelve radioisotope targets are in the beam whenever the main beam line operates, so one can say that the facility produces radioisotopes 100% of the time. However, because maintenance and the research programme require several shutdown periods a year of up to two months long, the pr~uction of short-lived radioisotopes is not practical. Because the isotopes in this facility are produced by spallation, the chemistry involved to produce sufficiently pure products is difficult and limits the production of certain radiopharmaceuticals. Yet TRIUMF is one of the three producers of lZ7Xe, which is marketed commercially by Atomic Energy of Canada, Ltd. In 1976, before the proton irradiation facility was commissioned, TRIUMF already produced lz31 on a small scale in its 10 pA beam line, distributed free of charge to a number of hospitals in Canada. This project has been discontinued in favour of the construction of a third beam line, which operates at 70-120 MeV. Soon this facility will resume production of tz31 of a better quality. The remaining five cyclotrons in table 1 are all compact cyclotrons (CP-42 and 45) supplied by The Cyclotron Corporation of Berkeley f9]. Their impressive external beam rating of 200 pA, made possible by the powerful internal H- ion source, makes them eminently suitable for commercial radioisotope production and neutron therapy. The machines of M.B. Anderson Hospital in Houston and the Dept. of Radiation Oncology at UCLA are both used primarily for neutron therapy. Since SeptemX. ~DIOISOTOPES~ARGETS

J.J. Burgerjon

952

/ H - cyclotrons for radioisotope production

Table 1 Establishment

Univ. of Manitoba Winnipeg, Canada Univ. of Milan Italy TRIUMF Vancouver, Canada M.B. Anderson Hospital Houston, TX, USA Amersham International Amersham, England TRIUMF Vancouver, Canada Kemforschungszentrum Karlsruhe, Germany UCLA Dept of Oncology Los Angeles. CA, USA

Energy

Maximum beam current

Fraction of time used for radioisotope production

Manufactured by

Radioisotopes produced

lz31, @Kr, ‘lmKr, etc.

25-48

10

c 10%

U. of Man.

18-45

10


U. of Milan

450

100

100%

70 11-42

100

11-42 11-42

BlmG

, 201Tl,

etc., etc.

TRIUMF r parasitic”)

“‘Xe, ‘09Cd. 67Cu “Ge, 83Sr

0%

TCC

isotope production planned

200

100%

TCC

201’Il,67Ga, “‘In

200

100%

TCC

1231, 123~~

11-42

200

12-46

200

100% cyclotron under construction

ber 1983 the MBA machine has been supporting a neutron therapy program at the rate of three days a week with a beam delivery of 0.7 mAh/week. Radioisotope production facilities are still under construction. The UCLA machine is presently being instaIled. It has a slightly higher energy than the others, 45 instead of 42 MeV. The machine installed at Amersham International has been in operation since November 1982 and is used exclusively for commercia1 radioisotope production. Normally the machine is operated 60 h/week with a beam delivery of 6 mAh/week, but during a recent breakdown of the “old” (- 1965) Philips cyclotron it was run for several weeks at 130 h/week and a beam delivery of 14 mA h/week. The beam current varies from 160 to 120 PA. The machine at TRIUMF has been in operation since November 1983. It is used for 90% of the time for commercial radioisotope production’ and 10% for the production of short-lived positron emitters. It regularly operates 100 h per week with an average beam delivery of 10 mAh/week. The beam current varies from 200-150 PA. The commercial radioisotopes are processed and marketed by Atomic Energy of Canada, Ltd. The positron emitters are sent via a fast pneumatic capsule transfer system (“rabbit”) to the positron emission tomograph at the University of B.C. Hospital located 2.5 km from the cyclotron [lo]. The H- cyclotron in Karlsruhe has been in operation since late 1983. It is also used 100% for radioisotope production, but this includes substantial time for the irradiation of machine parts for wear studies for the

201~

1231

1111~

1

67~~

1

TCC

57Co; ‘* F,“SO 1231 8’Rb

TCC

isotope production planned

industry. The latter program was transferred from the “old” (- 1962) AEG cyclotron, which can now be devoted entirely to nuclear physics. The machine is operated 85 h a week with a beam delivery of 1 mAh/week. At present the beam current does not exceed 50 PA. In summary, of the new compact H- cyclotrons only the Amersham and TRIUMF machines are being operated at beam currents well above 100 pA. For the MBA and Karlsruhe machines, running at the higher beam currents has not yet been found necessary. automotive

3. CP-42 operating experience Because of direct experience we will discuss the TRIUMF CP-42 in some detail while referring to the other CP-42’s where there are differences. Table 2 lists the major features of a CP-42 cyclotron and fig. 1 shows its general layout as described in ref. f9]. This general layout shows as many as four fixed energy beams and one variable energy beam that can be extracted from the cyclotron. The actual number of variable and fixed energy beams varies for different machines. Fig. 2 is a layout of the TRIUMF machine and its beam lines. One of the factory acceptance tests for the TRIUMF machine was a 100 h uninterrupted run of 200 PA at full energy on an external target. This test was run successfully in 1981, at 40 MeV. The stability of the beam during that test was excellent. It was also shown, at that time, that two beams of the same energy could

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J.J. Burgerjon / H - cyclotro?s for radioisotope production

Table 2 Major features of CP-42 cyclotron Particle accelerated Beam energy Beam current (external) Energy resolution at max energy External beam emittance Operating frequency (fixed) Accelerating mode Pole diameter Max extraction radius H- ion source Vacuum Pumping speed Method

of extraction

Nominal extraction efficiency Beam exit Control

H11-42 MeV 200 pA 1% (fwbm) 30 mm mr 26.8 MHz fundamental 120 cm 53 cm internal PIG 6 X 10e6 Torr. H, 1.2~10~ l/s, Hz charge exchange

100% multiple computer

both be run simultaneously in a stable mode, while differing in beam current in a ratio of 1 : 100. The site acceptance tests in October 1983 required a 200 PA run of 16 h with 90% availability and 200 /.LA runs of 1 h on each of the 3 external production targets and at 11,20, 30 and 42 MeV on a target in the variable energy beam line. These tests were also passed, but with

26 MeV

r

Fig. 1. (1) Return yoke, (2) dees, (3) hills, (4) inner and (5) outer harmonic coils, (6) variable energy extractor, (7) fixed energy extractors, (8) rf liner, (9) RF tuner, (10) neutral beam baffles, (11) internal beam probe, (12) magnetic channel, (13) diffusion pumps, (14) dee bias feedthrough, (15) combination magnet, (16) variable energy beam probe.

5m

I ’

u

Fig. 2. The 42 MeV radioisotope production facility at TRIUMF. (1) Cyclotron vault. (1A) cyclotron, (1B) 27 MeV solids target station, (1C) 26 MeV gas target, (2) variable energy target cave, (3) service room, (4) active waste storage, (5) active waste holding tank, (6) cooling equipment, (7,lO) personnel change and monitoring areas, (8) power supply room, (9) control room, (11) rabbit tubes to radiochemistry hot cells.

considerably more difficulty because the production targets were smaller and required a better focused beam. Deficiencies were a 10% internal beam loss between 41 and 42 MeV, requiring the 16 h test to be run at 41 MeV, and the external beam loss at 12 MeV exceeding the specified 10%. Also, operator intervention was needed more frequently than specified: 2 per hour instead of 0.25 per hour. However, the results of these difficult acceptance tests must be considered quite impressive. Tightly specified acceptance tests are meant to guarantee a certain operational performance of the cyclotron. However, the performance during the acceptance tests cannot actually be achieved during prolonged operation. This is because the amount of effort and expertise available during acceptance tests is normally not available during routine operation. After nine months of continuous operation at 100 h per week we find that we can normally run at 150 PA, but 200 PA requires some luck and extra care, and cannot be guaranteed. In a negative ion cyclotron it is of great importance to maintain the best vacuum possible to limit internal beam loss due to gas stripping, which activatives the machine. This beam loss is a disadvantage that partially offsets the advantage of a high extraction efficiency and is approximately 15%. The residual gas in the vacuum tank is a result of (4 a 0.1 Torr I/s of hydrogen flow to maintain the ion source arc; (b) a 0.001 Torr l/s of air leakage, to be kept below this figure so as to have a negligible effect on the gas stripping compared to the partial pressure of hydrogen; and (4 water and organic vapours released from the tank wall, dees and other surfaces inside the tank. The X. RADlOlSOTOPES/TARGETS

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J.J. Burgerjon / H - cyc~otmnsfor radioisotope producrion

magnitude of this release is generally unknown. The hydrogen and air gas loads are successfully pumped by the four 30 cm diameter diffusion pumps with a total effective pumping speed of 12300 l/s. The vapour release is meant to be trapped by a fiquid nitrogen trap with an effective pumping speed of I5 700 l/s [9]. However, tests at TRIUMF indicate that running the LN trap cold results only in a drop in vacuum as indicated on an ionization gauge from 5 to 4.7 X 10e6 Torr and a hardly noticeable increase in the beam current of 150 PA. This indicates that the cyclotron tank is quite dry, probably because the tank is always vented with dry nitrogen. Experience with the Amersham cyclotron confirms this observation.

4. Maintenance

The present running mode of the cyclotron consists of four 24 h operating days, followed by one day for cooldown, one day for maintenance, repairs and improvements, and one day to test the cyclotron to make sure everything is in working order for the next operating period. Routine maintenance consists of a change of stripping foils every week and a change of ion source cathodes every three weeks. The anode, with the ion exit slit, can last as long as three months. For these changes the cyclotron has to be opened up, a minor disadvantage, although we can usually run the cyclotron again three hours after pumpdown. To shorten this period the Karlsruhe cyclotron has airlocks so the ion source and extractor mechanisms can be removed without opening the tank. The ion source cathodes normally last the 200 h quoted by the manufacturer, but the stripping foils do not last the 60 h they did during the 1981 factory acceptance tests. This is because the external beam has to be focused better, which means the internal beam also has to be focused to a smaller spot when it passes through the stripper. The foils, made of 25 pm thick pyrolitic graphite, warp and eventually crack after 20-30 h, depending on the beam current. This causes divergence of the beam, resulting in excessive beam loss in the beam line. At present stripping foil-life limits the time we can run the cyclotron without having to open the tank to change foils. The variable energy extractor has a 4-foil carousel and we have just installed a similar carousel on the fixed energy extractor. We are looking for a light material that would last longer in the concentrated beam. However, at this time we know of no better solution to the foil-life problem than to increase the number of foils that can be installed on one extractor.

5. Reliability In new cyclotron installations failure of the electronic support equipment, RF and DC power supplies, etc., is rather frequent initially, until weak parts have been upgraded and a routine replacement schedule has been established by experience, More serious is failure of equipment located in the cyclotron vault and particularly inside the vacuum tank, since its repairs involve personnel exposure to radiation. The most common failure in the TRIUMF CP-42 is caused by the dee insulators. It is common practice for cyclotron designers to avoid dee insulators and support the dees only at the voltage node, at the cost of some mechanical stability. However, the CP-42 dees, which carry an RF voltage of 35 kV to ground, are supported by 12 insulators, at least 8 of which are at the high voltage end. To some extent, the expectation that these insulators would be reliable in a H- cyclotron was justified, because the vacuum has to be better than in a H+ cyclotron, as explained in section 3. During commissioning of the machine the insulator failure rate was indeed very low, in fact much lower than the other CP-42 owners reported. Yet, after nine months of intense operation we have had 13 insulator

d 0

IO

/

r;/

20

30 ED,

i

, 40

I

50

IO9

I

60

MeV

Fig. 3. The yield of various radioisotopes from proton reactions when protons of incident energy EP are stopped in carbon, aluminum, iron and copper. The abscissa scale on the right-hand side is the resulting saturated activity for 100 pA incident beam current.

failures, which is much more than the other CP-42 owners report. However, only the Amersham machine operates at comparable intensity. It used to be run at 170-220 PA by the middle of 1983 and also had a high insulator failure rate. Now it is run at 120-160 PA and at a reduced D-voltage. Amersham staff also inspect all insulators regularly and replace them when they look bad. Hence the Amersham insulator failure rate is now very low.

6. Residual activation The components inside the vacuum chamber as shown in fig. 1 are located in the median plane and consequently exposed to the neutral beam. Fig. 3 shows the yield of various radioisotopes produced as a function of energy in the main elements present in the median plane [ll]. Since high current operation of the machine is at energies below 30 MeV, it is clear that copper and iron (stainless steel) should be avoided and that carbon and aluminum are to be preferred. As generally maintenance periods are preceded by 24 h cooldown periods, fig. 3 also shows that carbon is preferable over aiuminum. Surveys made 24 h after “end of beam” confirm these conclusions. The general field measures - 60 mR/h, but the various copper parts, such as the extractor foil carousels, extractor arm, and the transition piece between dees and dee-stem measure from 3 to 10 R/h on contact. A stainless steel cooling tube that crosses the median plane measures - 1.5 R/h. The stainless steel vacuum chamber is well protected by the neutral beam baffles, which measure - 300 mR/h and the various carbon baffles and collimators are indistinguishable from the general field and measure only a few mR/h after removal. Residual fields in the vault are produced by neutron activation, and after 24 h vary generally from 20 to 50 mR/h and up to several R/h in the vicinity of target stations, even though the targets have, of course, been removed via a remote controlled rabbit system. These fields result in a personnel exposure of approximately 1 R/y/person for a group of 10 persons, involved’with maintenance and repairs to the cyclotron, beam lines and targets.

7. Building layout The TRIUMF 42 MeV facility, shown in fig. 2, consists mainly of a cyclotron vault with two short fixed energy beam lines with energies of 26 and 27 MeV and one target cave for the 11-42 MeV variable energy beam line. The concrete for the walls has a Na content of less than 0.2% to reduce residual activation caused by 24Na. Standard concrete may contain up to 2% of Na.

Fig. 4. (1) Cyclotron vault, (1A) variable energy beam line, (lB-1E) fixed energy beam lines, (2-7) target caves, (8) cooling equipment room, (9) power supply room, (10) control room, (11) change and mo~to~ng room, (12) radi~he~st~ labs.

At the moment the variable energy beam line is shared by a solids target station and a gas target for the production of positron emitters. For the gas target to receive beam the solids target has to be removed from the station. Early in 1985 a switching magnet will be installed, which may serve up to 9 targets. This layout, grown out of existing building constraints, is by no means ideai. The two targets in the cyclotron vault increase the residual activity in the vault and reduce the advantage of the H- cyclotron: that targets need not be placed in the internal beam. The short vault beam lines also lack control over the shape of the beam spot, since there are no focusing or steering elements. The Amersham CP-42 has two 28.5 MeV target stations in the cyclotron vault and is capable of extracting three more beams of 30, 30 and 42 MeV into short vault beam lines. The 11-42 MeV variable energy beam line runs into a small development cave. The Rarlsruhe CP-42 has two variable energy beam lines, both leading into separate target caves, one for 14-30 MeV and the other for 11-42 MeV. There are no plans to locate targets in the cyclotron vault. To fully utilize the multiple beam potential of the H- cyclotron, and keeping residual activity in the cyclotron and its vault to a minimum, the optimum building would have a separate target cave for each beam line. Quadrupoles, steering magnets and switching magnet would be located in the cyclotron vault. One change and monitoring room would provide access to X. ~DIOISO~P~~ARG~

956

J. J. Burgerjon / H - cyclotrons for radioisotope production

target caves, cyclotron vault as well as the radiochemistry labs. The layout for such a building might look like the one in fig. 4.

References

Ill M.E. Rickey and Rodman Smythe, Nucl. Instr. and Meth. 18-19 (1962) 66.

PI A.C. Paul and B.T. Wright, Bull. Am. Phys. Sot. 8 (1963) 8. Conclusions The new generation of powerful H- cyclotrons are potentially highly productive radioisotope machines. The following points would substantially reduce personnel radiation exposure, while further improving productivity. (1) Activation of components can be further reduced by avoiding all copper and stainless steel in the median plane in favour of ahnninum and, where possible, graphite. (2) Future designs should avoid dee insulators until their reliability has been proven. (3) Stripping foil life should be improved by finding a better material, or alternatively, by increasing the number of foils that can be changed without having to enter the cyclotron vault. (4) Targets should not be located in the cyclotron vault but in separate low sodium concrete target caves.

606.

131B.T. Wright, IEEE Trans. NS-13 (1966) 72. 141 J.J. Burgerjon, NucI. Instr. and Meth. 43 (1966) 381. [51 E. Acerbi et al., Proc. 5th Int. Conf. on Cyclotrons, 1969 (Buttetworths, London, 1971) p. 388. 161 J.R. Richardson, Proc. 7th Int. Conf. on Cyclotrons and their Applications (Birkhauser, Basel, 1975) p. 41. [71 K.W. EhIers, Nucl. Instr. and Meth. 32 (1965) 309. PI J.J. Burgerjon et al, Proc. 27th Conf. on Remote Systems Technology (Amer. Nucl. Sot., Illinois, 1980) p. 285. [91 G.O. Hendry et al., Proc. 9th Int. Conf. on Cyclotrons and their Applications (Les Eds. de Physique, Paris, 1982) p. 125. PO1 J.J. Burgerjon et al., Proc. Conf. on Robotics and Remote Handling in the Nuclear Industry (Can. Nucl. Sot., Toronto, 1984) p. 236. 1111 I.M. Thorson, TRIUMF, private communication.