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Development of a sealed-accelerator-tube neutron generator
Applied Radiation and Isotopes 53 (2000) 801±809
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Development of a sealed-accelerator-tube neutron generator J.M. Verbeke a,b,*, K.N. Leung b, J. Vujic a a
Nuclear Engineering Department, University of California, Berkeley, CA 94720, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
b
Abstract Sealed-accelerator-tube neutron generators are being developed in Lawrence Berkeley National Laboratory (LBNL) for applications ranging from neutron radiography to boron neutron capture therapy and neutron activation analysis. The new generation of high-output neutron generators is based on the D±T fusion reaction, producing 14.1-MeV neutrons. The main components of the neutron tube Ð the ion source, the accelerator and the target Ð are all housed in a sealed metal container without external pumping. Thick-target neutron yield computations are performed in this paper to estimate the neutron yield of titanium and scandium targets. With an average deuteron beam current of 1 A and an energy of 120 keV, a time-averaged neutron production of 01014 n/s can be estimated for a tritiated target, for both pulsed and cw operations. In mixed deuteron/triton beam operation, a beam current of 2 A at 150 keV is required for the same neutron output. Recent experimental results on ion sources and accelerator columns are presented and discussed. Published by Elsevier Science Ltd. Keywords: Neutron generator; D±T neutron source; Solid target neutron yield
1. Introduction The RF-driven multicusp ion source developed at Lawrence Berkeley National Laboratory (LBNL) has found numerous applications ranging from neutral beam injection systems for fusion reactors to particle accelerators, proton therapy machines and ion implantation systems (Leung, 1996). Such sources are simple to operate, have long lifetimes, high gas eciencies and provide high-density plasmas with high monatomic species yields. These characteristics make the RF-driven ion source a viable candidate for the next generation of compact, high-output, sealed-tube neu-
* Corresponding author. Tel.: +1-510-495-2792; fax: +1510-486-5105. E-mail address: [email protected] (J.M. Verbeke). 0969-8043/00/$ - see front matter Published by Elsevier Science Ltd. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 2 6 2 - 1
tron generators, utilizing the fusion of deuterium and tritium, or deuterium and deuterium. Recently, LBNL has developed a compact, sealedaccelerator-tube neutron generator capable of producing 109±1010 D±T neutrons per second (Perkins et al., 1996a, 1996b). The ion source, a miniaturized variation of earlier RF-driven multicusp ion sources, is designed to ®t within a 5-cm-diameter borehole. Typical operating parameters include repetition rates up to 100 pps, with pulse widths between 10 and 80 ms (limited only by the available RF power supply) and source pressure as low as 5 mTorr. There are several new developments which enable the neutron tube to provide a higher neutron yield: (a) H+ yields over 95% have recently been achieved using a 5-cm-diameter RF-driven multicusp source. High monatomic yields are essential for high neutron outputs; (b) The source could be operated at low gas
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pressures (1 to 2 mTorr). Low gas pressure operation is necessary to reduce both charge exchange processes and high-voltage breakdown in the accelerator column. These experimental ®ndings will enable one to develop a new generation of compact, high-output, sealed-tube 14-MeV neutron generators based on the D±T fusion reaction. This article is divided into four parts, the ®rst one gives a general description of the neutron generator, the others describe the target, the ion source and the accelerator column. 2. Neutron tube design In order to achieve a time-averaged neutron yield of 1014 n/s over long periods of time, a large multicusp source together with a multi-aperture extraction system to produce an average ion beam current of 2 A, accelerated to 150 kV, and impinging on a well-cooled target is required. Both pulsed and cw operations are possible for the neutron tube. The main components of the sealed D±T neutron tube are the ion source, the 150-kV accelerator column, the water-cooled target and the vacuum system. Fig. 1 shows a schematic diagram of the sealed D±T neutron generator. It is a scale-up version of the compact neutron tube that was developed at LBNL. In this new system, the ion source, the accelerator and the target are all housed in a sealed metal container without external pumping. During operation, deuterium and tritium will be released from the reservoir element and the target. After operation, both gases will return to the reservoir element and target
due to their lower temperatures. The absence of gaseous tritium in the neutron tube between operations is a safety feature in case of mechanical failure during transportation. Time-averaged neutron yields of 1014 n/s can be achieved by bombarding a tritiated target with an average 1-A deuteron beam at 120 keV. However, this neutron output deteriorates as deuterium coming from the beam slowly replaces the tritium occluded in the target. In order to maintain a constant neutron output, a beam-loading target will be used and the multicusp source will be operated with a 50%±50% mixture of deuterium and tritium. The concentrations of deuterium and tritium in the target decrease with temperature. A large-area target will be used in order to reduce the power density deposited by the impinging ions and to lower the target temperature. The lower temperature will result in a higher neutron output per unit ion beam current. With a 150-keV and 2-A average mixed beam current hitting a well-cooled target, a neutron yield of 1014 n/s can be achieved over long periods of time. 3. Target design The target is a copper substrate coated with a thin ®lm of either scandium or titanium. Film thicknesses can range from 10 to 50 mm. The neutron production eciency of materials depends mainly on their capacity to retain deuterium and tritium and on their stopping power. The more deuterium and tritium they can retain, the more nuclear fusion reactions can occur between incoming ions and occluded gas. The lower the stopping power of the material, the less energy the ions will lose by interacting with it. Scandium and titanium form metal hydrides and can thus be used to produce neutrons from the D±T reactions. Due to their low atomic number, their stopping power is relatively low compared to other higher-Z metals. Moreover, the ratio of hydrogen atoms to metal atoms Ð refered to as atomic ratio (A.R.) Ð for these metal hydrides can be as high as 2.0. These two properties make them the most ecient metal hydrides for neutron generation. Other metals form hydrides of atomic ratios up to 3.0, and even 3.75 for thorium, but their higher Z and thus higher stopping power result in an overall lower neutron production eciency. 3.1. Thick-target neutron yield computations
Fig. 1. Schematic diagram of the sealed D±T neutron generator.
Even though titanium targets have been used sively for neutron generation, the outcome thorough literature search for thick titanium neutron yields resulted in only two papers: one by Shope (Shope, 1966) and a paper by Kim
extenof a target report (Kim,
J.M. Verbeke et al. / Applied Radiation and Isotopes 53 (2000) 801±809
1977). Due to the lack of published experimental data for the stopping power of titanium in 1966, Shope estimated it by using two sets of data for elements that bracket titanium in atomic weight, namely, carbon and copper, and assuming that the atomic stopping power is proportional to the square root of the atomic weight. Using these estimated stopping powers, he computed neutron yields for deuteron and triton beams driven into titanium targets. Kim studied thicktarget neutron yields for mixed beams driven into titanium targets. No information was found in the literature concerning neutron yields of scandium targets. In order (a) to improve and complete Shope's calculations for the neutron yields of deuteron and triton beams on titanium targets using experimental data sets for the stopping power and (b) to compare the eciency of titanium and scandium as solid targets for neutron production, we performed calculations of neutron yields for deuteron beams, triton beams and mixed beams driven into these two metals using the stopping powers based on experimental data sets published by Andersen (Andersen et al., 1968). Calculations of neutron yields for a metal target onto which deuterium and tritium particles are bombarded, are involved because of the complexity of the concentration buildup inside the target. In steady state, the deuterium and tritium concentrations are believed to reach certain saturation values, which depend on the target material and the target temperature. In the calculations which follow, it is assumed that the saturation concentrations are constant over the particle range, and that the target thickness is larger than the particle range. For a deuteron beam of current I and energy E composed of monatomic and molecular species impinging on a target loaded with tritium, the total number of neutrons produced per second can be computed using the integral form of the thick-target yield equation: Y
E=k 3 A:R: I X sDÿT E k fk dE dE e 0 dx E k1
1
where A.R. is the atomic ratio of the tritium in the target, and e is the electronic charge. The integrals represent the contributions of each ion species. They are weighted by their fraction fk and their number of nuclei k per ion. sDÿT E is the neutron production cross-section of the fusion reaction D±T and is taken from Shope (Shope, 1966). dE=dx E is the molecular stopping power of the target material loaded with tritium. Similar equations can be written for deuteron and triton beams bombarding targets loaded with deuterium. The stopping power of the target material is assumed to follow Bragg's law of additivity, namely, the molecular stopping power is the sum of the atomic
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stopping powers of the constituents: dE=dx MHA:R: dE=dx M A:R: dE=dx H
2
where M indicates the metal occluder and H the hydrogen isotope. The atomic stopping powers of deuterons and tritons in deuterium or tritium are taken directly from Shope (Shope, 1966). They were computed from the well-established stopping power of protons in hydrogen (Phillips, 1953; Reynolds et al., 1953) and the experimentally established and accepted facts that (1) isotopes have the same atomic stopping power for the same incident particles and (2) isotopes of same velocity lose energy at the same rate in the same material. The stopping powers of deuterons in titanium and scandium were taken from Andersen (Andersen et al., 1968). Fact (2) was used to compute the stopping power of tritons in both metal occluders. 3.2. Thick-target neutron yields of monoisotopic ion beams The integral in Eq. (1) is evaluated numerically for atomic ratios ranging from 0.2 to 2.0, and the results are plotted in Figs. 2 and 3 for the D±T and the T±D nuclear reactions, respectively. Neutron yields are traditionally computed per unit impinging ion beam current. In this study, we are interested in maximizing the neutron yield per unit ion beam power, because this last quantity determines the power required by the high-voltage power supplies, the heat load on the target and, thus, the heat exchanger requirements to cool down the target. Hence, the ®gures show the neutron yields per unit ion beam power versus ion beam energy. For the D±T nuclear reaction, the highest neutron yields per unit ion beam power are obtained for a deuteron beam energy of 175 keV for the titanium target and 180 keV for the scandium target. These optimal energies are the same for all atomic ratios. The neutron yields per unit beam power decrease slowly around the optimal energies. Half of the neutron yields per unit beam power are still obtained at 80 keV for both targets and for all atomic ratios. For the T±D reaction, the optimal triton beam energy is 265 keV for titanium targets and 270 keV for scandium targets. Since scandium has a higher stopping power than titanium, scandium targets have lower neutron yields than titanium targets. As shown in the ®gures, the neutron yields increase with the atomic ratio. The energy scales in the ®gures are in keV per number of nuclei k per ion. The neutron yields per unit beam power of diatomic ion beams D 2 and T 2 or tri atomic ion beams D3 and T 3 can be computed from these ®gures by doubling and tripling, respectively, the energy scales. Thus, the optimal energy for a diatomic
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Fig. 2. Neutron yield per unit beam power versus beam energy for deuteron beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
deuteron beam on a titanium target would be 350 keV and its neutron yield per unit power would be identical to the neutron yield of the optimal 175 keV monatomic deuteron beam. This higher energy is much more dicult to obtain with compact accelerators. Therefore, it is advantageous to produce mostly monatomic deuteron ions in the ion source. If the ion source provides mostly diatomic deuteron ions, the neutron output per unit power at 175 keV will be less than half the neutron output for monatomic deuteron ions at the same energy. Similar conclusions can easily be drawn for diatomic triton beams, as well as triatomic deuteron and triton beams.
3.3. Thick-target neutron yields of mixed ion beams With titanium and scandium targets loaded with tritium to an atomic ratio of 2.0, neutron yields of 01014 n/s/A can be obtained with 120-keV deuteron beams. However, this neutron yield deteriorates with time due to dilution of tritium in the target by deuterons from the ion beam. Indeed, a deuteron beam bombarding a tritiated target will gradually lead to mixedgas target operation and a decrease in neutron output with time. In order to maximize the lifetime of the target and maintain a constant neutron output, a beamloading target will be used and the multicusp ion
Fig. 3. Neutron yield per unit beam power versus beam energy for triton beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
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source and accelerator column will be operated with a 50%±50% mixture of deuterium and tritium. This mode of operation solves the dilution problem. However, the neutron yield is lower. For mixed beams composed of deuterons and tritons, the neutron yield at the target is the sum of several terms of the same form as Eq. (1) " E=k 3 3 X I X A:R:T sD±T E Y k fDk k dE dE e k1 0 dx E k1 E=k
fTk
0
fDT
3E=5 0
A:R:D sTÿD E dE dE dx E
2E=5 0
A:R:D sTÿD E dE dE dx E
f D 2 T 2
3E=7 0
fDT2 2
2E=7 0
E=4 0
!
A:R:T sD±T E dE dE dx E
A:R:D sTÿD E dE dE dx E
3E=8 0
A:R:T sD±T E dE dE dx E
!
A:R:T sD±T E dE dE dx E
A:R:D sTÿD E dE dE dx E
!# 3
where the f's denote the fractions of each species in
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the ion beam, A:R:D and A:R:T refer to the ratios of occluded deuterium and tritium atoms, respectively, to metal atoms in the target. The ®rst term is the neutron yield produced by high-energy deuterons from the beam interacting with tritium in the target through D±T reactions, the second term is related to T±D reactions. The other terms correspond to the interactions of DT , D2 T and DT 2 ions also present in the beam with deuterium and tritium in the target. The contributions of the D±D and T±T fusion reactions to the total neutron yield are neglected in this computation because the neutron production cross-sections of the D±D and T±T reactions are much smaller than the ones of the D±T and T±D reactions. In the case of mixed ion beams (see Fig. 4a and b), the atomic ratio A.R. in Eq. (2) is the ratio of the sum of deuterium and tritium occluded atoms in the target to metal occluder atoms. The neutron yields per unit power for mixed beams are about twice lower than the ones for monoisotopic ion beams. The ion beam energy for which the neutron yield per unit power is the highest is 0230 keV for titanium and 0240 keV for scandium. This energy is the same for all atomic ratios. The eect of the monatomic fraction in the ion beam hitting the target is analyzed. For the sake of simplicity, the triatomic ion species are neglected in this analysis. For the monatomic ion species, we assume fD fT : There are three dierent diatomic ion species, D fT2 2 , T2 and DT : We assume fD 2 0:5fDT : Fig. 5a and b show plots of the neutron yields per unit power for mixed beams of dierent compo-
Fig. 4. Neutron yield per unit beam power versus beam energy for monatomic, mixed 50% deuteron/50% triton beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
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sitions hitting titanium and scandium targets loaded with 50% deuterium/50% tritium to an atomic ratio of 2.0. The neutron yield per unit power is independent of the beam composition at 0330 keV. The neutron yield per unit power is higher for monatomic ion beams than for diatomic ones below 330 keV and lower for them above 330 keV. Therefore, depending on the ions species produced by the ion source, accelerator columns must operate at very dierent energies in order to maximize the neutron output. The neutron output per unit power is maximized at 230/240 keV for monatomic ions, at 460/480 keV for diatomic ions, and at 690/720 keV for triatomic ions bombarding titanium/scandium targets. The neutron output per unit beam power is lower at other energies, regardless of the ion beam composition. Therefore, if for instance monatomic ion species are produced by the ion source, it is optimal to operate at 230/240 keV and there is no gain to operate at higher energies. One might consider operating at lower energies because of the diculty to hold high voltages in compact accelerators. The neutron output per unit unit beam power decreases slowly from 230/240 keV down to 150 keV, and more rapidly down to lower energies. Considering the size of the proposed compact neutron generator and the diculties related to high voltage holding, the accelerator column will have to operate at energies lower than 200 keV. Whether the beam should be accelerated to 150 or 200 keV is arguable. In order to maximize the neutron output at this energy, the ion source will have to produce a highly monatomic ion beam. This is the subject of the next section. A few words should be said about the maximum atomic ratios achievable for both scandium and tita-
nium. As metal hydrides heat up, they release the deuterium and tritium occluded gases. If the energy deposited by the ion beam is not removed, the temperature of the target will rise, the atomic ratio and consequently the neutron yield will decrease. To reduce the peak temperature to which the ion beam drives the target, the target will be cooled by circulating water. The atomic ratio of a metal in equilibrium with hydrogen gas can be determined by the gas pressure and the system temperature from PTC (pressure±temperature± composition) curves, which show the hydrogen gas pressure in equilibrium with metals as a function of the atomic ratio for dierent temperatures. For titanium and scandium at a given temperature, the hydrogen pressure increases rapidly with the atomic ratio in the two ranges A.R. < 0.6 and A.R. > 1.3 where there are only single metal phases, and is constant between these two values where two metal phases are present. The two limits depend on the metal and the temperature. The temperature corresponding to this plateau decreases with decreasing hydrogen pressure and can be calculated using the enthalpy and entropy of formation of the metal hydride. Scandium hydride is more stable than titanium hydride. For titanium in equilibrium with hydrogen at 1 mTorr, the equilibrium temperature of the plateau is 2448C, while it is 5058C for scandium (Beavis, 1969). The atomic ratio will be higher than 01.3 for lower temperatures, and lower than 00.6 for higher temperatures. Very little information is available for PTC curves at lower temperatures, but from these ®rst considerations, we conclude that higher atomic ratios can be obtained with scandium hydride than with titanium hydride, especially for target temperatures of a few hundred degrees. If
Fig. 5. Neutron yield per unit beam power versus beam energy for multispecies, mixed 50% deuteron/50% triton beams impinging on (a) titanium and (b) scandium targets. The atomic ratio is set to 2.0.
J.M. Verbeke et al. / Applied Radiation and Isotopes 53 (2000) 801±809
807
Fig. 6. (a) Hydrogen ion species distribution versus cw RF power for gas pressure of 4 mTorr, with and without magnetic ®lter. (b) Minimum cw RF power to sustain plasma for dierent gas pressures.
the atomic ratio is 1.9 for scandium and 1.6 for titanium, higher neutron yields will be obtained with scandium than with titanium. The neutron yields will be measured experimentally for both target materials.
4. Ion source The multicusp ion source will be a 30-cm-diameter cylindrical stainless-steel chamber surrounded with columns of samarium-cobalt magnets. The plasma is produced by RF induction discharge. In order to deliver RF power to the plasma, a coupler in the form of a multi-turn induction coil is used. The RF power supply is a broad band power ampli®er driven at 13.56 MHz by a signal generator. To maximize the neutron output at the target, it is necessary to produce high D+ and T+ fractions in the extracted beam. Experiments have been carried out with a 5-cm-diameter ion source to determine the distribution of ion species. In order to avoid safety hazards related to radioactivity and neutron production, hydrogen was used instead of deuterium and tritium for most experiments. For a hydrogen pressure of 4 mTorr, the results are plotted in Fig. 6a. The monatomic ion species H+ can be enhanced by installing a permanent magnetic ®lter in the source chamber (Perkins, 1996b). Fig. 6a shows that the ®lterequipped source produces a higher H+ fraction than the source without magnetic ®lter. In the best case, 97% of H+ were obtained, to compare with 79% without magnetic ®lter. In order (a) to avoid voltage breakdown in the accelerator column, (b) to decrease the ion beam losses
by electron capture reaction with the neutral gas in the accelerator column, the gas pressure cannot exceed a few mTorr. The use of a magnetic ®lter decreases the minimum gas pressure required to sustain the plasma at a given cw RF power (see Fig. 6b).The minimum gas pressure goes down from 4 mTorr for the source without ®lter to 2 mTorr for the ®lter-equipped source. Also, lower RF power is needed to sustain the plasma with the magnetic ®lter: 660 W instead of 1800 W at 4 mTorr. Fig. 7 shows the current density increasing linearly with the RF power. The presence of the magnetic ®lter decreases the current density at all RF powers and all gas pressures.
Fig. 7. Current density versus cw RF power for ion source with and without magnetic ®lter at a gas pressure of 6 mTorr.
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5. Accelerator column The beam current is scheduled to increase in four steps from 1 mA to 15 mA, 150 mA and eventually 1.5 A. The ®rst step has already been completed. A single hole accelerator column was designed, built and operated last year. It was designed to extract a proton beam of 01 mA and to accelerate it to an energy in the range 150±200 keV. The 5-cm-diameter ion source operated at a pressure of 6 mTorr and a cw RF power of 1900 W without magnetic ®lter. These operating conditions result in a current density of 27 mA/cm2 (see Fig. 7). With the 2-mm-diameter extraction hole, the extracted current can be calculated and is equal to 0.85 mA. The accelerator column could be operated up to 165 kV but diculties due to voltage breakdown made operation at higher voltages very dicult. The current was measured in a Faraday cup with magnetic suppression and was equal to 0.8 mA, very close to the expected current. For the 15 mA, 150 mA and 1.5 A beams, the beam extraction system which closes o the other end of the ion source chamber will consist of a multi-aperture extraction system as represented in Fig. 1. The ion beam current is proportional to the number of apertures in the plasma and extraction electrodes. The shape, separation and the voltage distribution on the electrodes were determined by the IGUN computation code (Becker, 1998). Source and electrode damages caused by high-energy backstreaming electrons can be avoided by using a suppressor electrode. The beamlets cross over and expand at the exit of the suppressor electrode. With the optics shown, the current density on the target is 0350 W/cm2. Ion beam losses are of concern in the neutron tube because the accelerator column must operate at the same pressure as the ion source in a sealed system. For the accelerator column shown in Fig. 1, the beam
Fig. 8. Proton beam losses as a function of distance from the plasma electrode and neutral gas pressure.
losses due to electron capture reactions H H2 4 H H 2 have been computed and are shown in Fig. 8. Since the electron capture reactions do not scatter signi®cantly the incoming H+ ions, the neutral atoms H formed continue in the same direction as the H+ ions. Therefore, the H atoms produced by electron capture reactions occuring at energies greater than 150 keV past the suppressor electrode will still reach the target with full energy, and not contribute to the beam losses. The total beam loss is thus the dierence between the beam current at the plasma electrode and the remaining beam current at 6 cm. It is equal to 14, 26 and 54% for 1, 2 and 5 mTorr, respectively. One has shown by IGUN simulations that the same accelerator column could be used for deuteron and mixed deuteron/triton beams just by varying the extraction electrode potential by a few kilovolts. The beam losses for these beams were comparable to the ones for proton beams. 6. Conclusion A sealed-accelerator-tube neutron generator based on the D±T fusion reaction is being developed at Lawrence Berkeley National Laboratory. In this neutron generator, the ion source, the accelerator column and the target will all be housed in a sealed metal container without external pumping to avoid realease of radioactive tritium to the environment. Time-averaged neutron yields of 01014 n/s can be obtained with an average 1-A, 120-keV deuteron beam bombarding a tritiated target. However, this neutron output deteriorates over time due to the replacement of tritium occluded in the target by deuterium coming from the beam. In order to maintain a constant neutron output, mixed beams composed of 50% deuterons/50% tritons will be used for neutron production. From thick-target neutron yield computations, 230 keV mixed beams lead to the highest neutron yields per unit beam power. High monatomic fractions are required to maximize the neutron output at this energy. Monatomic fractions of 97% have recently been obtained with a 5-cm-diameter ion source equipped with a magnetic ®lter. With this monatomic fraction, one can estimate a neutron production of 01014 n/s for a 150-keV, 2-A mixed beam bombarding titanium and scandium targets. Cooling of the target is essential for the neutron yield. Due to the greater thermal stability of scandium hydride, scandium targets will lead to greater neutron yields than titanium targets. Accelerator columns are being designed to expand the ion beams onto large-area targets. The beam losses are of concern because the accelerator column is operating at the same gas pressure as the ion source. Using
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a 8-cm-long accelerator column, beam losses of 14 and 26% were computed for gas pressures of 1 and 2 mTorr, respectively.
Acknowledgements This work is supported by Sandia National Laboratory and the US Department of Energy under contract number DE-AC03-76SF00098.
References Andersen, H.H., Hanke, C.C., Simonsen, H., Sorensen, H., Vajda, P., 1968. Stopping power of the elements Z 20 through Z 30 for 5±12 MeV protons and deuterons. Phys. Rev. 175, 389.
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Beavis, L.C., 1969. Characteristics of some binary transition metal hydrides. J. of Less-Common Metals 19, 315±328. Becker, R., 1998. New features in the simulation of ion extraction with Igun. In: Proc. EPAC 98, Stockholm, Sweden. Kim, J., 1977. Neutron sources using D±T mixed beams driven into solid target. Nucl. Instr. and Meth. 145, 9±17. Leung, K.N., 1996. Rev. Sci. Instrum. 67, 1302. Perkins, L.T., Herz, P.R., Leung, K.N., Pickard, D., 1996a. Rev. Sci. Instrum. 65, 1186. Perkins, L.T., et al., 1996b. Rev. Sci. Instrum. 67, 1057. Phillips, J.A., 1953. The energy loss of low energy protons in some gases. Phys. Rev. 90, 532. Reynolds, H.K., Dunbar, D.N.F., Wenzel, W.A., Whaling, W., 1953. The stopping cross section of gases for protons, 30±600 keV. Phys. Rev. 92, 742. Shope, L.A., 1966. Theoretical thick target yields for the D± D, D±T, and T±D Nuclear Reactions Using the Metal Occluders Ti and Er and Energies up to 300 keV, SC-TM66-247. Sandia Laboratory, Albuquerque, New Mexico.
www.elsevier.com/locate/apradiso
Development of a sealed-accelerator-tube neutron generator J.M. Verbeke a,b,*, K.N. Leung b, J. Vujic a a
Nuclear Engineering Department, University of California, Berkeley, CA 94720, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
b
Abstract Sealed-accelerator-tube neutron generators are being developed in Lawrence Berkeley National Laboratory (LBNL) for applications ranging from neutron radiography to boron neutron capture therapy and neutron activation analysis. The new generation of high-output neutron generators is based on the D±T fusion reaction, producing 14.1-MeV neutrons. The main components of the neutron tube Ð the ion source, the accelerator and the target Ð are all housed in a sealed metal container without external pumping. Thick-target neutron yield computations are performed in this paper to estimate the neutron yield of titanium and scandium targets. With an average deuteron beam current of 1 A and an energy of 120 keV, a time-averaged neutron production of 01014 n/s can be estimated for a tritiated target, for both pulsed and cw operations. In mixed deuteron/triton beam operation, a beam current of 2 A at 150 keV is required for the same neutron output. Recent experimental results on ion sources and accelerator columns are presented and discussed. Published by Elsevier Science Ltd. Keywords: Neutron generator; D±T neutron source; Solid target neutron yield
1. Introduction The RF-driven multicusp ion source developed at Lawrence Berkeley National Laboratory (LBNL) has found numerous applications ranging from neutral beam injection systems for fusion reactors to particle accelerators, proton therapy machines and ion implantation systems (Leung, 1996). Such sources are simple to operate, have long lifetimes, high gas eciencies and provide high-density plasmas with high monatomic species yields. These characteristics make the RF-driven ion source a viable candidate for the next generation of compact, high-output, sealed-tube neu-
* Corresponding author. Tel.: +1-510-495-2792; fax: +1510-486-5105. E-mail address: [email protected] (J.M. Verbeke). 0969-8043/00/$ - see front matter Published by Elsevier Science Ltd. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 2 6 2 - 1
tron generators, utilizing the fusion of deuterium and tritium, or deuterium and deuterium. Recently, LBNL has developed a compact, sealedaccelerator-tube neutron generator capable of producing 109±1010 D±T neutrons per second (Perkins et al., 1996a, 1996b). The ion source, a miniaturized variation of earlier RF-driven multicusp ion sources, is designed to ®t within a 5-cm-diameter borehole. Typical operating parameters include repetition rates up to 100 pps, with pulse widths between 10 and 80 ms (limited only by the available RF power supply) and source pressure as low as 5 mTorr. There are several new developments which enable the neutron tube to provide a higher neutron yield: (a) H+ yields over 95% have recently been achieved using a 5-cm-diameter RF-driven multicusp source. High monatomic yields are essential for high neutron outputs; (b) The source could be operated at low gas
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pressures (1 to 2 mTorr). Low gas pressure operation is necessary to reduce both charge exchange processes and high-voltage breakdown in the accelerator column. These experimental ®ndings will enable one to develop a new generation of compact, high-output, sealed-tube 14-MeV neutron generators based on the D±T fusion reaction. This article is divided into four parts, the ®rst one gives a general description of the neutron generator, the others describe the target, the ion source and the accelerator column. 2. Neutron tube design In order to achieve a time-averaged neutron yield of 1014 n/s over long periods of time, a large multicusp source together with a multi-aperture extraction system to produce an average ion beam current of 2 A, accelerated to 150 kV, and impinging on a well-cooled target is required. Both pulsed and cw operations are possible for the neutron tube. The main components of the sealed D±T neutron tube are the ion source, the 150-kV accelerator column, the water-cooled target and the vacuum system. Fig. 1 shows a schematic diagram of the sealed D±T neutron generator. It is a scale-up version of the compact neutron tube that was developed at LBNL. In this new system, the ion source, the accelerator and the target are all housed in a sealed metal container without external pumping. During operation, deuterium and tritium will be released from the reservoir element and the target. After operation, both gases will return to the reservoir element and target
due to their lower temperatures. The absence of gaseous tritium in the neutron tube between operations is a safety feature in case of mechanical failure during transportation. Time-averaged neutron yields of 1014 n/s can be achieved by bombarding a tritiated target with an average 1-A deuteron beam at 120 keV. However, this neutron output deteriorates as deuterium coming from the beam slowly replaces the tritium occluded in the target. In order to maintain a constant neutron output, a beam-loading target will be used and the multicusp source will be operated with a 50%±50% mixture of deuterium and tritium. The concentrations of deuterium and tritium in the target decrease with temperature. A large-area target will be used in order to reduce the power density deposited by the impinging ions and to lower the target temperature. The lower temperature will result in a higher neutron output per unit ion beam current. With a 150-keV and 2-A average mixed beam current hitting a well-cooled target, a neutron yield of 1014 n/s can be achieved over long periods of time. 3. Target design The target is a copper substrate coated with a thin ®lm of either scandium or titanium. Film thicknesses can range from 10 to 50 mm. The neutron production eciency of materials depends mainly on their capacity to retain deuterium and tritium and on their stopping power. The more deuterium and tritium they can retain, the more nuclear fusion reactions can occur between incoming ions and occluded gas. The lower the stopping power of the material, the less energy the ions will lose by interacting with it. Scandium and titanium form metal hydrides and can thus be used to produce neutrons from the D±T reactions. Due to their low atomic number, their stopping power is relatively low compared to other higher-Z metals. Moreover, the ratio of hydrogen atoms to metal atoms Ð refered to as atomic ratio (A.R.) Ð for these metal hydrides can be as high as 2.0. These two properties make them the most ecient metal hydrides for neutron generation. Other metals form hydrides of atomic ratios up to 3.0, and even 3.75 for thorium, but their higher Z and thus higher stopping power result in an overall lower neutron production eciency. 3.1. Thick-target neutron yield computations
Fig. 1. Schematic diagram of the sealed D±T neutron generator.
Even though titanium targets have been used sively for neutron generation, the outcome thorough literature search for thick titanium neutron yields resulted in only two papers: one by Shope (Shope, 1966) and a paper by Kim
extenof a target report (Kim,
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1977). Due to the lack of published experimental data for the stopping power of titanium in 1966, Shope estimated it by using two sets of data for elements that bracket titanium in atomic weight, namely, carbon and copper, and assuming that the atomic stopping power is proportional to the square root of the atomic weight. Using these estimated stopping powers, he computed neutron yields for deuteron and triton beams driven into titanium targets. Kim studied thicktarget neutron yields for mixed beams driven into titanium targets. No information was found in the literature concerning neutron yields of scandium targets. In order (a) to improve and complete Shope's calculations for the neutron yields of deuteron and triton beams on titanium targets using experimental data sets for the stopping power and (b) to compare the eciency of titanium and scandium as solid targets for neutron production, we performed calculations of neutron yields for deuteron beams, triton beams and mixed beams driven into these two metals using the stopping powers based on experimental data sets published by Andersen (Andersen et al., 1968). Calculations of neutron yields for a metal target onto which deuterium and tritium particles are bombarded, are involved because of the complexity of the concentration buildup inside the target. In steady state, the deuterium and tritium concentrations are believed to reach certain saturation values, which depend on the target material and the target temperature. In the calculations which follow, it is assumed that the saturation concentrations are constant over the particle range, and that the target thickness is larger than the particle range. For a deuteron beam of current I and energy E composed of monatomic and molecular species impinging on a target loaded with tritium, the total number of neutrons produced per second can be computed using the integral form of the thick-target yield equation: Y
E=k 3 A:R: I X sDÿT E k fk dE dE e 0 dx E k1
1
where A.R. is the atomic ratio of the tritium in the target, and e is the electronic charge. The integrals represent the contributions of each ion species. They are weighted by their fraction fk and their number of nuclei k per ion. sDÿT E is the neutron production cross-section of the fusion reaction D±T and is taken from Shope (Shope, 1966). dE=dx E is the molecular stopping power of the target material loaded with tritium. Similar equations can be written for deuteron and triton beams bombarding targets loaded with deuterium. The stopping power of the target material is assumed to follow Bragg's law of additivity, namely, the molecular stopping power is the sum of the atomic
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stopping powers of the constituents: dE=dx MHA:R: dE=dx M A:R: dE=dx H
2
where M indicates the metal occluder and H the hydrogen isotope. The atomic stopping powers of deuterons and tritons in deuterium or tritium are taken directly from Shope (Shope, 1966). They were computed from the well-established stopping power of protons in hydrogen (Phillips, 1953; Reynolds et al., 1953) and the experimentally established and accepted facts that (1) isotopes have the same atomic stopping power for the same incident particles and (2) isotopes of same velocity lose energy at the same rate in the same material. The stopping powers of deuterons in titanium and scandium were taken from Andersen (Andersen et al., 1968). Fact (2) was used to compute the stopping power of tritons in both metal occluders. 3.2. Thick-target neutron yields of monoisotopic ion beams The integral in Eq. (1) is evaluated numerically for atomic ratios ranging from 0.2 to 2.0, and the results are plotted in Figs. 2 and 3 for the D±T and the T±D nuclear reactions, respectively. Neutron yields are traditionally computed per unit impinging ion beam current. In this study, we are interested in maximizing the neutron yield per unit ion beam power, because this last quantity determines the power required by the high-voltage power supplies, the heat load on the target and, thus, the heat exchanger requirements to cool down the target. Hence, the ®gures show the neutron yields per unit ion beam power versus ion beam energy. For the D±T nuclear reaction, the highest neutron yields per unit ion beam power are obtained for a deuteron beam energy of 175 keV for the titanium target and 180 keV for the scandium target. These optimal energies are the same for all atomic ratios. The neutron yields per unit beam power decrease slowly around the optimal energies. Half of the neutron yields per unit beam power are still obtained at 80 keV for both targets and for all atomic ratios. For the T±D reaction, the optimal triton beam energy is 265 keV for titanium targets and 270 keV for scandium targets. Since scandium has a higher stopping power than titanium, scandium targets have lower neutron yields than titanium targets. As shown in the ®gures, the neutron yields increase with the atomic ratio. The energy scales in the ®gures are in keV per number of nuclei k per ion. The neutron yields per unit beam power of diatomic ion beams D 2 and T 2 or tri atomic ion beams D3 and T 3 can be computed from these ®gures by doubling and tripling, respectively, the energy scales. Thus, the optimal energy for a diatomic
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Fig. 2. Neutron yield per unit beam power versus beam energy for deuteron beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
deuteron beam on a titanium target would be 350 keV and its neutron yield per unit power would be identical to the neutron yield of the optimal 175 keV monatomic deuteron beam. This higher energy is much more dicult to obtain with compact accelerators. Therefore, it is advantageous to produce mostly monatomic deuteron ions in the ion source. If the ion source provides mostly diatomic deuteron ions, the neutron output per unit power at 175 keV will be less than half the neutron output for monatomic deuteron ions at the same energy. Similar conclusions can easily be drawn for diatomic triton beams, as well as triatomic deuteron and triton beams.
3.3. Thick-target neutron yields of mixed ion beams With titanium and scandium targets loaded with tritium to an atomic ratio of 2.0, neutron yields of 01014 n/s/A can be obtained with 120-keV deuteron beams. However, this neutron yield deteriorates with time due to dilution of tritium in the target by deuterons from the ion beam. Indeed, a deuteron beam bombarding a tritiated target will gradually lead to mixedgas target operation and a decrease in neutron output with time. In order to maximize the lifetime of the target and maintain a constant neutron output, a beamloading target will be used and the multicusp ion
Fig. 3. Neutron yield per unit beam power versus beam energy for triton beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
J.M. Verbeke et al. / Applied Radiation and Isotopes 53 (2000) 801±809
source and accelerator column will be operated with a 50%±50% mixture of deuterium and tritium. This mode of operation solves the dilution problem. However, the neutron yield is lower. For mixed beams composed of deuterons and tritons, the neutron yield at the target is the sum of several terms of the same form as Eq. (1) " E=k 3 3 X I X A:R:T sD±T E Y k fDk k dE dE e k1 0 dx E k1 E=k
fTk
0
fDT
3E=5 0
A:R:D sTÿD E dE dE dx E
2E=5 0
A:R:D sTÿD E dE dE dx E
f D 2 T 2
3E=7 0
fDT2 2
2E=7 0
E=4 0
!
A:R:T sD±T E dE dE dx E
A:R:D sTÿD E dE dE dx E
3E=8 0
A:R:T sD±T E dE dE dx E
!
A:R:T sD±T E dE dE dx E
A:R:D sTÿD E dE dE dx E
!# 3
where the f's denote the fractions of each species in
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the ion beam, A:R:D and A:R:T refer to the ratios of occluded deuterium and tritium atoms, respectively, to metal atoms in the target. The ®rst term is the neutron yield produced by high-energy deuterons from the beam interacting with tritium in the target through D±T reactions, the second term is related to T±D reactions. The other terms correspond to the interactions of DT , D2 T and DT 2 ions also present in the beam with deuterium and tritium in the target. The contributions of the D±D and T±T fusion reactions to the total neutron yield are neglected in this computation because the neutron production cross-sections of the D±D and T±T reactions are much smaller than the ones of the D±T and T±D reactions. In the case of mixed ion beams (see Fig. 4a and b), the atomic ratio A.R. in Eq. (2) is the ratio of the sum of deuterium and tritium occluded atoms in the target to metal occluder atoms. The neutron yields per unit power for mixed beams are about twice lower than the ones for monoisotopic ion beams. The ion beam energy for which the neutron yield per unit power is the highest is 0230 keV for titanium and 0240 keV for scandium. This energy is the same for all atomic ratios. The eect of the monatomic fraction in the ion beam hitting the target is analyzed. For the sake of simplicity, the triatomic ion species are neglected in this analysis. For the monatomic ion species, we assume fD fT : There are three dierent diatomic ion species, D fT2 2 , T2 and DT : We assume fD 2 0:5fDT : Fig. 5a and b show plots of the neutron yields per unit power for mixed beams of dierent compo-
Fig. 4. Neutron yield per unit beam power versus beam energy for monatomic, mixed 50% deuteron/50% triton beams impinging on (a) titanium and (b) scandium targets. k in the units of the energy scale refers to the number of nuclei per ion.
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sitions hitting titanium and scandium targets loaded with 50% deuterium/50% tritium to an atomic ratio of 2.0. The neutron yield per unit power is independent of the beam composition at 0330 keV. The neutron yield per unit power is higher for monatomic ion beams than for diatomic ones below 330 keV and lower for them above 330 keV. Therefore, depending on the ions species produced by the ion source, accelerator columns must operate at very dierent energies in order to maximize the neutron output. The neutron output per unit power is maximized at 230/240 keV for monatomic ions, at 460/480 keV for diatomic ions, and at 690/720 keV for triatomic ions bombarding titanium/scandium targets. The neutron output per unit beam power is lower at other energies, regardless of the ion beam composition. Therefore, if for instance monatomic ion species are produced by the ion source, it is optimal to operate at 230/240 keV and there is no gain to operate at higher energies. One might consider operating at lower energies because of the diculty to hold high voltages in compact accelerators. The neutron output per unit unit beam power decreases slowly from 230/240 keV down to 150 keV, and more rapidly down to lower energies. Considering the size of the proposed compact neutron generator and the diculties related to high voltage holding, the accelerator column will have to operate at energies lower than 200 keV. Whether the beam should be accelerated to 150 or 200 keV is arguable. In order to maximize the neutron output at this energy, the ion source will have to produce a highly monatomic ion beam. This is the subject of the next section. A few words should be said about the maximum atomic ratios achievable for both scandium and tita-
nium. As metal hydrides heat up, they release the deuterium and tritium occluded gases. If the energy deposited by the ion beam is not removed, the temperature of the target will rise, the atomic ratio and consequently the neutron yield will decrease. To reduce the peak temperature to which the ion beam drives the target, the target will be cooled by circulating water. The atomic ratio of a metal in equilibrium with hydrogen gas can be determined by the gas pressure and the system temperature from PTC (pressure±temperature± composition) curves, which show the hydrogen gas pressure in equilibrium with metals as a function of the atomic ratio for dierent temperatures. For titanium and scandium at a given temperature, the hydrogen pressure increases rapidly with the atomic ratio in the two ranges A.R. < 0.6 and A.R. > 1.3 where there are only single metal phases, and is constant between these two values where two metal phases are present. The two limits depend on the metal and the temperature. The temperature corresponding to this plateau decreases with decreasing hydrogen pressure and can be calculated using the enthalpy and entropy of formation of the metal hydride. Scandium hydride is more stable than titanium hydride. For titanium in equilibrium with hydrogen at 1 mTorr, the equilibrium temperature of the plateau is 2448C, while it is 5058C for scandium (Beavis, 1969). The atomic ratio will be higher than 01.3 for lower temperatures, and lower than 00.6 for higher temperatures. Very little information is available for PTC curves at lower temperatures, but from these ®rst considerations, we conclude that higher atomic ratios can be obtained with scandium hydride than with titanium hydride, especially for target temperatures of a few hundred degrees. If
Fig. 5. Neutron yield per unit beam power versus beam energy for multispecies, mixed 50% deuteron/50% triton beams impinging on (a) titanium and (b) scandium targets. The atomic ratio is set to 2.0.
J.M. Verbeke et al. / Applied Radiation and Isotopes 53 (2000) 801±809
807
Fig. 6. (a) Hydrogen ion species distribution versus cw RF power for gas pressure of 4 mTorr, with and without magnetic ®lter. (b) Minimum cw RF power to sustain plasma for dierent gas pressures.
the atomic ratio is 1.9 for scandium and 1.6 for titanium, higher neutron yields will be obtained with scandium than with titanium. The neutron yields will be measured experimentally for both target materials.
4. Ion source The multicusp ion source will be a 30-cm-diameter cylindrical stainless-steel chamber surrounded with columns of samarium-cobalt magnets. The plasma is produced by RF induction discharge. In order to deliver RF power to the plasma, a coupler in the form of a multi-turn induction coil is used. The RF power supply is a broad band power ampli®er driven at 13.56 MHz by a signal generator. To maximize the neutron output at the target, it is necessary to produce high D+ and T+ fractions in the extracted beam. Experiments have been carried out with a 5-cm-diameter ion source to determine the distribution of ion species. In order to avoid safety hazards related to radioactivity and neutron production, hydrogen was used instead of deuterium and tritium for most experiments. For a hydrogen pressure of 4 mTorr, the results are plotted in Fig. 6a. The monatomic ion species H+ can be enhanced by installing a permanent magnetic ®lter in the source chamber (Perkins, 1996b). Fig. 6a shows that the ®lterequipped source produces a higher H+ fraction than the source without magnetic ®lter. In the best case, 97% of H+ were obtained, to compare with 79% without magnetic ®lter. In order (a) to avoid voltage breakdown in the accelerator column, (b) to decrease the ion beam losses
by electron capture reaction with the neutral gas in the accelerator column, the gas pressure cannot exceed a few mTorr. The use of a magnetic ®lter decreases the minimum gas pressure required to sustain the plasma at a given cw RF power (see Fig. 6b).The minimum gas pressure goes down from 4 mTorr for the source without ®lter to 2 mTorr for the ®lter-equipped source. Also, lower RF power is needed to sustain the plasma with the magnetic ®lter: 660 W instead of 1800 W at 4 mTorr. Fig. 7 shows the current density increasing linearly with the RF power. The presence of the magnetic ®lter decreases the current density at all RF powers and all gas pressures.
Fig. 7. Current density versus cw RF power for ion source with and without magnetic ®lter at a gas pressure of 6 mTorr.
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5. Accelerator column The beam current is scheduled to increase in four steps from 1 mA to 15 mA, 150 mA and eventually 1.5 A. The ®rst step has already been completed. A single hole accelerator column was designed, built and operated last year. It was designed to extract a proton beam of 01 mA and to accelerate it to an energy in the range 150±200 keV. The 5-cm-diameter ion source operated at a pressure of 6 mTorr and a cw RF power of 1900 W without magnetic ®lter. These operating conditions result in a current density of 27 mA/cm2 (see Fig. 7). With the 2-mm-diameter extraction hole, the extracted current can be calculated and is equal to 0.85 mA. The accelerator column could be operated up to 165 kV but diculties due to voltage breakdown made operation at higher voltages very dicult. The current was measured in a Faraday cup with magnetic suppression and was equal to 0.8 mA, very close to the expected current. For the 15 mA, 150 mA and 1.5 A beams, the beam extraction system which closes o the other end of the ion source chamber will consist of a multi-aperture extraction system as represented in Fig. 1. The ion beam current is proportional to the number of apertures in the plasma and extraction electrodes. The shape, separation and the voltage distribution on the electrodes were determined by the IGUN computation code (Becker, 1998). Source and electrode damages caused by high-energy backstreaming electrons can be avoided by using a suppressor electrode. The beamlets cross over and expand at the exit of the suppressor electrode. With the optics shown, the current density on the target is 0350 W/cm2. Ion beam losses are of concern in the neutron tube because the accelerator column must operate at the same pressure as the ion source in a sealed system. For the accelerator column shown in Fig. 1, the beam
Fig. 8. Proton beam losses as a function of distance from the plasma electrode and neutral gas pressure.
losses due to electron capture reactions H H2 4 H H 2 have been computed and are shown in Fig. 8. Since the electron capture reactions do not scatter signi®cantly the incoming H+ ions, the neutral atoms H formed continue in the same direction as the H+ ions. Therefore, the H atoms produced by electron capture reactions occuring at energies greater than 150 keV past the suppressor electrode will still reach the target with full energy, and not contribute to the beam losses. The total beam loss is thus the dierence between the beam current at the plasma electrode and the remaining beam current at 6 cm. It is equal to 14, 26 and 54% for 1, 2 and 5 mTorr, respectively. One has shown by IGUN simulations that the same accelerator column could be used for deuteron and mixed deuteron/triton beams just by varying the extraction electrode potential by a few kilovolts. The beam losses for these beams were comparable to the ones for proton beams. 6. Conclusion A sealed-accelerator-tube neutron generator based on the D±T fusion reaction is being developed at Lawrence Berkeley National Laboratory. In this neutron generator, the ion source, the accelerator column and the target will all be housed in a sealed metal container without external pumping to avoid realease of radioactive tritium to the environment. Time-averaged neutron yields of 01014 n/s can be obtained with an average 1-A, 120-keV deuteron beam bombarding a tritiated target. However, this neutron output deteriorates over time due to the replacement of tritium occluded in the target by deuterium coming from the beam. In order to maintain a constant neutron output, mixed beams composed of 50% deuterons/50% tritons will be used for neutron production. From thick-target neutron yield computations, 230 keV mixed beams lead to the highest neutron yields per unit beam power. High monatomic fractions are required to maximize the neutron output at this energy. Monatomic fractions of 97% have recently been obtained with a 5-cm-diameter ion source equipped with a magnetic ®lter. With this monatomic fraction, one can estimate a neutron production of 01014 n/s for a 150-keV, 2-A mixed beam bombarding titanium and scandium targets. Cooling of the target is essential for the neutron yield. Due to the greater thermal stability of scandium hydride, scandium targets will lead to greater neutron yields than titanium targets. Accelerator columns are being designed to expand the ion beams onto large-area targets. The beam losses are of concern because the accelerator column is operating at the same gas pressure as the ion source. Using
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a 8-cm-long accelerator column, beam losses of 14 and 26% were computed for gas pressures of 1 and 2 mTorr, respectively.
Acknowledgements This work is supported by Sandia National Laboratory and the US Department of Energy under contract number DE-AC03-76SF00098.
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Beavis, L.C., 1969. Characteristics of some binary transition metal hydrides. J. of Less-Common Metals 19, 315±328. Becker, R., 1998. New features in the simulation of ion extraction with Igun. In: Proc. EPAC 98, Stockholm, Sweden. Kim, J., 1977. Neutron sources using D±T mixed beams driven into solid target. Nucl. Instr. and Meth. 145, 9±17. Leung, K.N., 1996. Rev. Sci. Instrum. 67, 1302. Perkins, L.T., Herz, P.R., Leung, K.N., Pickard, D., 1996a. Rev. Sci. Instrum. 65, 1186. Perkins, L.T., et al., 1996b. Rev. Sci. Instrum. 67, 1057. Phillips, J.A., 1953. The energy loss of low energy protons in some gases. Phys. Rev. 90, 532. Reynolds, H.K., Dunbar, D.N.F., Wenzel, W.A., Whaling, W., 1953. The stopping cross section of gases for protons, 30±600 keV. Phys. Rev. 92, 742. Shope, L.A., 1966. Theoretical thick target yields for the D± D, D±T, and T±D Nuclear Reactions Using the Metal Occluders Ti and Er and Energies up to 300 keV, SC-TM66-247. Sandia Laboratory, Albuquerque, New Mexico.