- Email: [email protected]
Alpha-emitting radioisotopes for switchable neutron generators
Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
Alpha-emitting radioisotopes for switchable neutron generators K.L. Hertz*, N.R. Hilton, J.C. Lund, J.M. Van Scyoc Sandia National Laboratories, P.O. Box 969, MS 9402, Livermore, CA 94551-0969, USA
Abstract Traditionally, radioisotopic neutron generators mix an alpha-emitting radioisotope with beryllium. The disadvantage of such an alpha–Be source is that they emit neutrons at a steady rate even when stored. These conventional generators are extremely awkward to use in many applications because of the neutron shielding required to prevent exposure to personnel and sensitive electronics. Recently, at our laboratory and others, the possibility of using switchable radioactive neutron sources has been investigated. These sources rely on a mechanical operation to separate the alphaemitting radioisotope from the Be target, thus allowing the source to be switched on and off. The utility of these new switchable sources is critically dependent on the selection of the alpha-emitting radioisotope. In this paper we discuss issues that determine the desirability of an alpha-emitting source for a switchable neutron generator, and select alpha emitters that are best suited for use in this application. r 2003 Published by Elsevier Science B.V. PACS: 23.60.+e Keywords: Radioisotopes; Neutron source
1. Introduction The work discussed in this paper is an investigation into the alpha-emitting radioisotopes for use in a new type of neutron generator, the switchable radioactive neutron source (SRNS) [1]. Unlike traditional mixed 9Be(a, n) generators, these new devices can be switched on and off. Mechanical action is used to alternatively prevent and allow the alpha particles from the radiation source to strike a beryllium target. In the off-state these devices emit a negligible number of neutrons and *Corresponding author. Tel.: +1-925-294-4535; fax: +1925-294-3231. E-mail address: [email protected] (K.L. Hertz).
other types of radiation, and in the on-state produce a neutron yield that meets the requirements of the application. Bowers et al. [1] investigated the feasibility of a SRNS at Argonne National Laboratory. Their patented design uses a magnet-actuated mechanism to rotate disks of a radioisotope and beryllium in and out of alignment to turn the device on and off. This paper concentrated on the fabrication of a high-activity 238Pu source. They demonstrated that a switchable and portable neutron source is possible. At Sandia National Laboratories, a switchable neutron generator designed to have a neutron yield of 106 neutrons/s is under development for use in the laboratory. In an effort to insure the device will be safe in the off-state,
0168-9002/03/$ - see front matter r 2003 Published by Elsevier Science B.V. doi:10.1016/S0168-9002(03)01016-7
42
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
considerable effort was devoted to finding the most appropriate radioisotope for the alpha source. A general discussion of radioisotopes used for neutron sources has been presented by Knoll [2]. Knoll’s discussion concentrates on spontaneous fission (SF) sources, mixed 9Be(a, n) sources and photoneutron sources. However, for a switchable neutron generator, the requirements of the alpha-emitting source may be quite different than those used in a conventional (continuously on) generator. A switchable neutron generator usually imposes three main requirements on the selection of the alpha-emitting source: the useable lifetime of the device should be several years, the neutron yield must meet the needs of the application, and the device must be safe for personnel to transport and store when in the off-state. All three of these characteristics and, therefore, the success of the device are determined by the radiation emitted by the radioisotope.
process. With this resource and the constraints the device imposes on the source, our search was quickly narrowed down to approximately 20 radioisotopes.
2. Radioisotope search
2.2. Neutron yield
Radioisotopes decay via many different processes including alpha-particle emission, SF, beta decay and electron capture. These processes result in the emission of radiation in the form of heavycharged particles, electrons, gamma rays, X-rays, and neutrons. When determining the proper radioisotope to be used in the SRNS, the emissions from the parent and the entire daughter chain must be considered. The useable lifetime of the device is dependent on the half-life of the source. The neutron yield of the generator is dictated by both the energy and intensity of the alpha particles emitted from the source. The safety of the device is determined by the emission of the collateral radiation, all the radiation that is not an alpha particle. Thus, a detailed look at the emissions from the entire decay chain of alpha-emitting radioisotopes is central to determining the device performance. A complete search of radioisotopes can be a daunting task. However, the online table of radioactive isotopes [3] greatly simplifies this
The expected neutron yield of the generator may be calculated from the energy and intensity of the alpha particles emitted from the parent radioisotope and its daughter chain. Anderson and Hertz [4] derived an empirical relationship (Eq. (1)) between the thick target neutron yield for the 9Be(a, n) reaction, Y =neutrons/106 alphas, and the energy of the incident alpha particle based on measurements from six alphaemitting sources. The high dependence of the neutron yield on the alpha energy is clear. This strong energy dependence is an important consideration in the design of the device. The alpha particles emitted from the source will lose energy as they leave the source and in the air between the source and the beryllium target. Low-energy alphas will lose more energy in transport than high-energy alphas and will more dramatically reduce the neutron yield. This established another constraint on the selected radioisotope: the radioisotope should emit alphas with energies above 4 MeV. This requirement eliminated 145Pm
2.1. Lifetime of the device The optimal half-life of the radioisotope used in this device is a compromise between high neutron yield and the lifetime of the device. A short halflife provides a high emission rate of alphas, increasing the neutron yield. However, over time the neutron yield will deteriorate as the radioisotope decays. The source will then need to be replenished or replaced. For the neutron yield to be stable for several years, the half-life of the radioisotope should be on the order of years. Therefore, the search for an acceptable source was limited to radioisotopes with half-lives between 1 and 1000 yr. There are less than 25 radioisotopes that emit alphas somewhere in their decay chain with half-lives in this range. The radioisotopes discussed in this paper are listed in Table 1.
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
43
Table 1 The radioisotopes with half-lives between 1 and 1000 yr considered in this study. Calculations were performed at a time of 10 yr after the creation of a pure parent source Nuclide
Half-life
Ci/g
Neutrons/ms/ g in on-state
Dose at 1 m unshielded in off-state (rem/g/s)
Dose at 1 m with 2.5 mm Pb shield in off-state (rem/g/s)
145
17.7 yr 74.6 yr 2.898 yr 102 yr 22.3 yr 21.773 yr 5.75 yr 1.9116 yr 68.9 yr 396.1 d 2.858 yr 87.7 yr 14.35 yr 432.2 yr 29.1 yr 18.10 yr 351 yr 13.08 yr 898 yr 2.645 yr
144 32.4 595 14.4 76.4 72.6 273 822 22.4 1403 530 17.1 104 3.44 50.6 81.0 4.18 109.4 1.63 537
2.05 108 9.50 119 28.6 144 959 2980 564 604 1.14 104 661 46.2 2.85 103 9.79 149 205 15.1 266 5.58 163
4.41 105 0 1.21 106 6.21 106 3.61 105 0.0040 0.078 0.14 0.0038 1.00 107 0.0038a 9.73 108 3.90 108 1.56 105 5.80 104 2.68 107 1.97 104 1.65 107 1.33 105 2.18 106
5.35 109 0 7.34 107 1.33 106 8.12 108 0.0014 0.059 0.10 0.0029 1.10 1010 0.0029a 5.52 1010 2.22 109 3.10 109 1.12 104 1.36 108 9.29 105 2.64 1022 1.08 106 1.20 109
Pm Gd 208 Po 209 Po 210 Pb 227 Ac 228 Ra 228 Th 232 U 235 Np 236 Pu 238 Pu 241 Pu 241 Am 243 Cm 244 Cm 249 Cf 250 Cf 251 Cf 252 Cf 148
a 236
Pu decays into
232
U, thus the specific dose rate calculations for
(Ea ¼ 2:24 MeV) and possible sources:
148
Gd (Ea ¼ 3:18 MeV) as
Y ¼ 0:080E 4:05 ;
4:1oEp5:7 MeV
ð1aÞ
Y ¼ 0:80E 2:75 ;
5:7oEp10:0 MeV:
ð1bÞ
A high branching ratio for alpha emission from the parent radioisotope and its daughters is critical to a high neutron yield. 145Pm, 235Np and 241Pu emit alphas less than 0.003% of their decays, therefore neither of these radioisotopes were considered as a possible alpha source. The calculated neutron yields for the radioisotopes are listed in Table 1. The neutron yields do not include neutrons emitted via SF. 2.3. Off-state radiation The radiation emitted by the generator in the off-state must be at or below safe levels for
232
U were used for
SF/ms/g
1.63 103 1.08 103 5.68 1010 5.38 107 7.82 105 2.80 6.53 104 1830 — 4.47 104
236
Pu.
personnel transporting and storing the device. To keep the device small, minimal shielding is desired. High-energy gamma rays and neutrons are of primary concern because they are difficult to shield. The following discussion concentrates on two processes. First, gamma rays are emitted when an excited nucleus transits to a lower-energy state. Most radioisotopes emit gamma rays throughout their decay chains. The energy and intensity of gamma rays from all radioisotopes in the decay chain were converted to dose equivalent rate using the ANS/ANSI 1991 standard conversion [5]. The calculation treats Compton scatter as an absorption, therefore the effect of shielding on dose rate is overestimated. The calculations for unshielded and shielded dose rates from the selected radioisotopes are listed in Table 1. The calculations do not include emissions from SF. Second, both neutrons and gamma rays are emitted during SF. The SF events are listed in the last column in Table 1. Typically, 1–4 prompt
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
44
neutrons/SF are emitted. For 252Cf 8–10 gamma rays/SF were measured. SF cannot be switched off; therefore this yield is undesirable. 244Cm, 250Cf and 252Cf were rejected as possible sources due to their large SF branching ratios. Radioisotopes with small SF branching ratios were considered as possible sources.
3. Radioisotope selection Fig. 1 displays the neutron production rate versus the dose equivalent for candidate radioisotopes. Ideal candidates with high neutron yield and low off-state dose rate would appear in the upper left of the plot. Unfortunately, there are no candidates in this region. Nor would one expect an isotope to exist with a long half-life (>1 yr), that emits very energetic alpha particles (>6 MeV), and whose daughters emit negligible photons on theoretical grounds [6]. However, Fig. 1 does reveal several good candidates.
104
Neutron Production Rate (/µs/g)
Unshielded 2.5 mm Pb shielding
228
Ra
227
103 244
10
Cm*
Pu*
Pu*
228
Po Po
249
4. Conclusions
Th
Cm*
Pb
209
241
U
243
210 208
238
10
236 232
2
1
Ac
Dose rates that are greatly reduced by the shielding indicate that the source emits gamma rays of relatively low energy and intensity. Thus, with minimal shielding the source can be safe in the off-state. A comparison of the unshielded and shielded dose rates exposes several radioisotopes that emit gamma rays with energies and intensities too high to allow for a safe off-state device. The radioisotopes that decay through the 228Th series (228Ra, 232U and 236Pu), 227Ac, and several other sources have dose rates that are not lowered with the 2.5 mm Pb shielding. The shielding is very effective for 210Pb, 238Pu and 241Am. These three radioisotopes are the most promising candidates for an SRNS and deserve further discussion. 210Pb decays into 210Po. 210Po has a half-life of 138 d and emits a relatively highintensity 803-keV gamma ray. This gamma ray is not easily shielded; therefore dose rate calculations should be completed for the specific device design before using this radioisotope. The neutron yield for 238Pu is higher than 241Am. However, the SF intensity produces an off-state neutron yield that is 1% of the on-state yield. A 238Pu source will have a high neutron background. Finally, the gamma rays emitted from 241Am can be easily shielded and the neutron background due to SF is small.
Cf*
Am* 251
Cf
10-16 10-14 10-12 10-10 Dose rate per neutron at 1 meter (rem/neutron) Fig. 1. Neutron production rate versus gamma-ray dose rate for the leading candidate alpha emitters unshielded (open circles) and with 2.5 mm Pb shielding (closed circles). These calculations were performed at a time of 10 yr after the creation of the pure parent source. Radioisotopes with an asterisk decay via SF. Emissions due to SF were not included in these calculations.
A thorough search of alpha-emitting radioisotopes with half-lives between 1 and 1000 yr for use in switchable radioactive neutron sources was performed. If it is considered desirable to minimize the radiological dose of the device in the off-state, and the device must be operable for a long period of time, then our calculations indicate that 241Am (with a small amount of external shielding) is the optimal source. Switchable neutron generators can be made with the available sources. The existence of these sources, combined with the great interest in the development of micromachines (whose dimensions are of the same order as the alpha-particle range) bodes well for the future of switchable neutron sources.
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
References [1] D.L. Bowers, E.A. Rhodes, C.E. Dickerman, J. Radioanal. Nucl. Chem. 233 (1998) 161. [2] G.F. Knoll, in: H.H. Barschall, et al., (Eds.), Neutron Sources For Basic Physics and Applications, Pergamon Press, New York, 1983 (Chapter 2). . [3] S.Y.F. Chu, L.P. Ekstrom, R.B. Firestone, WWW table of radioactive isotopes, database version 1999-
45
02-28 from URL http://nucleardata.nuclear.lu.se/nucleardata/toi/ [4] M.E. Anderson, M.R. Hertz, Nucl. Sci. Eng. 44 (1971) 437. [5] B. Shleien, L.A. Slaback, B.K. Birky, Handbook of Health Physics and Radiological Health, 3rd Edition, Williams & Wilkins, Baltimore, MD, 1998. [6] J.M. Blatt, V.F. Weisskopf, Theoretical Nuclear Physics, Dover Publications, Inc., New York, 1979 (Chapter 1).
Alpha-emitting radioisotopes for switchable neutron generators K.L. Hertz*, N.R. Hilton, J.C. Lund, J.M. Van Scyoc Sandia National Laboratories, P.O. Box 969, MS 9402, Livermore, CA 94551-0969, USA
Abstract Traditionally, radioisotopic neutron generators mix an alpha-emitting radioisotope with beryllium. The disadvantage of such an alpha–Be source is that they emit neutrons at a steady rate even when stored. These conventional generators are extremely awkward to use in many applications because of the neutron shielding required to prevent exposure to personnel and sensitive electronics. Recently, at our laboratory and others, the possibility of using switchable radioactive neutron sources has been investigated. These sources rely on a mechanical operation to separate the alphaemitting radioisotope from the Be target, thus allowing the source to be switched on and off. The utility of these new switchable sources is critically dependent on the selection of the alpha-emitting radioisotope. In this paper we discuss issues that determine the desirability of an alpha-emitting source for a switchable neutron generator, and select alpha emitters that are best suited for use in this application. r 2003 Published by Elsevier Science B.V. PACS: 23.60.+e Keywords: Radioisotopes; Neutron source
1. Introduction The work discussed in this paper is an investigation into the alpha-emitting radioisotopes for use in a new type of neutron generator, the switchable radioactive neutron source (SRNS) [1]. Unlike traditional mixed 9Be(a, n) generators, these new devices can be switched on and off. Mechanical action is used to alternatively prevent and allow the alpha particles from the radiation source to strike a beryllium target. In the off-state these devices emit a negligible number of neutrons and *Corresponding author. Tel.: +1-925-294-4535; fax: +1925-294-3231. E-mail address: [email protected] (K.L. Hertz).
other types of radiation, and in the on-state produce a neutron yield that meets the requirements of the application. Bowers et al. [1] investigated the feasibility of a SRNS at Argonne National Laboratory. Their patented design uses a magnet-actuated mechanism to rotate disks of a radioisotope and beryllium in and out of alignment to turn the device on and off. This paper concentrated on the fabrication of a high-activity 238Pu source. They demonstrated that a switchable and portable neutron source is possible. At Sandia National Laboratories, a switchable neutron generator designed to have a neutron yield of 106 neutrons/s is under development for use in the laboratory. In an effort to insure the device will be safe in the off-state,
0168-9002/03/$ - see front matter r 2003 Published by Elsevier Science B.V. doi:10.1016/S0168-9002(03)01016-7
42
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
considerable effort was devoted to finding the most appropriate radioisotope for the alpha source. A general discussion of radioisotopes used for neutron sources has been presented by Knoll [2]. Knoll’s discussion concentrates on spontaneous fission (SF) sources, mixed 9Be(a, n) sources and photoneutron sources. However, for a switchable neutron generator, the requirements of the alpha-emitting source may be quite different than those used in a conventional (continuously on) generator. A switchable neutron generator usually imposes three main requirements on the selection of the alpha-emitting source: the useable lifetime of the device should be several years, the neutron yield must meet the needs of the application, and the device must be safe for personnel to transport and store when in the off-state. All three of these characteristics and, therefore, the success of the device are determined by the radiation emitted by the radioisotope.
process. With this resource and the constraints the device imposes on the source, our search was quickly narrowed down to approximately 20 radioisotopes.
2. Radioisotope search
2.2. Neutron yield
Radioisotopes decay via many different processes including alpha-particle emission, SF, beta decay and electron capture. These processes result in the emission of radiation in the form of heavycharged particles, electrons, gamma rays, X-rays, and neutrons. When determining the proper radioisotope to be used in the SRNS, the emissions from the parent and the entire daughter chain must be considered. The useable lifetime of the device is dependent on the half-life of the source. The neutron yield of the generator is dictated by both the energy and intensity of the alpha particles emitted from the source. The safety of the device is determined by the emission of the collateral radiation, all the radiation that is not an alpha particle. Thus, a detailed look at the emissions from the entire decay chain of alpha-emitting radioisotopes is central to determining the device performance. A complete search of radioisotopes can be a daunting task. However, the online table of radioactive isotopes [3] greatly simplifies this
The expected neutron yield of the generator may be calculated from the energy and intensity of the alpha particles emitted from the parent radioisotope and its daughter chain. Anderson and Hertz [4] derived an empirical relationship (Eq. (1)) between the thick target neutron yield for the 9Be(a, n) reaction, Y =neutrons/106 alphas, and the energy of the incident alpha particle based on measurements from six alphaemitting sources. The high dependence of the neutron yield on the alpha energy is clear. This strong energy dependence is an important consideration in the design of the device. The alpha particles emitted from the source will lose energy as they leave the source and in the air between the source and the beryllium target. Low-energy alphas will lose more energy in transport than high-energy alphas and will more dramatically reduce the neutron yield. This established another constraint on the selected radioisotope: the radioisotope should emit alphas with energies above 4 MeV. This requirement eliminated 145Pm
2.1. Lifetime of the device The optimal half-life of the radioisotope used in this device is a compromise between high neutron yield and the lifetime of the device. A short halflife provides a high emission rate of alphas, increasing the neutron yield. However, over time the neutron yield will deteriorate as the radioisotope decays. The source will then need to be replenished or replaced. For the neutron yield to be stable for several years, the half-life of the radioisotope should be on the order of years. Therefore, the search for an acceptable source was limited to radioisotopes with half-lives between 1 and 1000 yr. There are less than 25 radioisotopes that emit alphas somewhere in their decay chain with half-lives in this range. The radioisotopes discussed in this paper are listed in Table 1.
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
43
Table 1 The radioisotopes with half-lives between 1 and 1000 yr considered in this study. Calculations were performed at a time of 10 yr after the creation of a pure parent source Nuclide
Half-life
Ci/g
Neutrons/ms/ g in on-state
Dose at 1 m unshielded in off-state (rem/g/s)
Dose at 1 m with 2.5 mm Pb shield in off-state (rem/g/s)
145
17.7 yr 74.6 yr 2.898 yr 102 yr 22.3 yr 21.773 yr 5.75 yr 1.9116 yr 68.9 yr 396.1 d 2.858 yr 87.7 yr 14.35 yr 432.2 yr 29.1 yr 18.10 yr 351 yr 13.08 yr 898 yr 2.645 yr
144 32.4 595 14.4 76.4 72.6 273 822 22.4 1403 530 17.1 104 3.44 50.6 81.0 4.18 109.4 1.63 537
2.05 108 9.50 119 28.6 144 959 2980 564 604 1.14 104 661 46.2 2.85 103 9.79 149 205 15.1 266 5.58 163
4.41 105 0 1.21 106 6.21 106 3.61 105 0.0040 0.078 0.14 0.0038 1.00 107 0.0038a 9.73 108 3.90 108 1.56 105 5.80 104 2.68 107 1.97 104 1.65 107 1.33 105 2.18 106
5.35 109 0 7.34 107 1.33 106 8.12 108 0.0014 0.059 0.10 0.0029 1.10 1010 0.0029a 5.52 1010 2.22 109 3.10 109 1.12 104 1.36 108 9.29 105 2.64 1022 1.08 106 1.20 109
Pm Gd 208 Po 209 Po 210 Pb 227 Ac 228 Ra 228 Th 232 U 235 Np 236 Pu 238 Pu 241 Pu 241 Am 243 Cm 244 Cm 249 Cf 250 Cf 251 Cf 252 Cf 148
a 236
Pu decays into
232
U, thus the specific dose rate calculations for
(Ea ¼ 2:24 MeV) and possible sources:
148
Gd (Ea ¼ 3:18 MeV) as
Y ¼ 0:080E 4:05 ;
4:1oEp5:7 MeV
ð1aÞ
Y ¼ 0:80E 2:75 ;
5:7oEp10:0 MeV:
ð1bÞ
A high branching ratio for alpha emission from the parent radioisotope and its daughters is critical to a high neutron yield. 145Pm, 235Np and 241Pu emit alphas less than 0.003% of their decays, therefore neither of these radioisotopes were considered as a possible alpha source. The calculated neutron yields for the radioisotopes are listed in Table 1. The neutron yields do not include neutrons emitted via SF. 2.3. Off-state radiation The radiation emitted by the generator in the off-state must be at or below safe levels for
232
U were used for
SF/ms/g
1.63 103 1.08 103 5.68 1010 5.38 107 7.82 105 2.80 6.53 104 1830 — 4.47 104
236
Pu.
personnel transporting and storing the device. To keep the device small, minimal shielding is desired. High-energy gamma rays and neutrons are of primary concern because they are difficult to shield. The following discussion concentrates on two processes. First, gamma rays are emitted when an excited nucleus transits to a lower-energy state. Most radioisotopes emit gamma rays throughout their decay chains. The energy and intensity of gamma rays from all radioisotopes in the decay chain were converted to dose equivalent rate using the ANS/ANSI 1991 standard conversion [5]. The calculation treats Compton scatter as an absorption, therefore the effect of shielding on dose rate is overestimated. The calculations for unshielded and shielded dose rates from the selected radioisotopes are listed in Table 1. The calculations do not include emissions from SF. Second, both neutrons and gamma rays are emitted during SF. The SF events are listed in the last column in Table 1. Typically, 1–4 prompt
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
44
neutrons/SF are emitted. For 252Cf 8–10 gamma rays/SF were measured. SF cannot be switched off; therefore this yield is undesirable. 244Cm, 250Cf and 252Cf were rejected as possible sources due to their large SF branching ratios. Radioisotopes with small SF branching ratios were considered as possible sources.
3. Radioisotope selection Fig. 1 displays the neutron production rate versus the dose equivalent for candidate radioisotopes. Ideal candidates with high neutron yield and low off-state dose rate would appear in the upper left of the plot. Unfortunately, there are no candidates in this region. Nor would one expect an isotope to exist with a long half-life (>1 yr), that emits very energetic alpha particles (>6 MeV), and whose daughters emit negligible photons on theoretical grounds [6]. However, Fig. 1 does reveal several good candidates.
104
Neutron Production Rate (/µs/g)
Unshielded 2.5 mm Pb shielding
228
Ra
227
103 244
10
Cm*
Pu*
Pu*
228
Po Po
249
4. Conclusions
Th
Cm*
Pb
209
241
U
243
210 208
238
10
236 232
2
1
Ac
Dose rates that are greatly reduced by the shielding indicate that the source emits gamma rays of relatively low energy and intensity. Thus, with minimal shielding the source can be safe in the off-state. A comparison of the unshielded and shielded dose rates exposes several radioisotopes that emit gamma rays with energies and intensities too high to allow for a safe off-state device. The radioisotopes that decay through the 228Th series (228Ra, 232U and 236Pu), 227Ac, and several other sources have dose rates that are not lowered with the 2.5 mm Pb shielding. The shielding is very effective for 210Pb, 238Pu and 241Am. These three radioisotopes are the most promising candidates for an SRNS and deserve further discussion. 210Pb decays into 210Po. 210Po has a half-life of 138 d and emits a relatively highintensity 803-keV gamma ray. This gamma ray is not easily shielded; therefore dose rate calculations should be completed for the specific device design before using this radioisotope. The neutron yield for 238Pu is higher than 241Am. However, the SF intensity produces an off-state neutron yield that is 1% of the on-state yield. A 238Pu source will have a high neutron background. Finally, the gamma rays emitted from 241Am can be easily shielded and the neutron background due to SF is small.
Cf*
Am* 251
Cf
10-16 10-14 10-12 10-10 Dose rate per neutron at 1 meter (rem/neutron) Fig. 1. Neutron production rate versus gamma-ray dose rate for the leading candidate alpha emitters unshielded (open circles) and with 2.5 mm Pb shielding (closed circles). These calculations were performed at a time of 10 yr after the creation of the pure parent source. Radioisotopes with an asterisk decay via SF. Emissions due to SF were not included in these calculations.
A thorough search of alpha-emitting radioisotopes with half-lives between 1 and 1000 yr for use in switchable radioactive neutron sources was performed. If it is considered desirable to minimize the radiological dose of the device in the off-state, and the device must be operable for a long period of time, then our calculations indicate that 241Am (with a small amount of external shielding) is the optimal source. Switchable neutron generators can be made with the available sources. The existence of these sources, combined with the great interest in the development of micromachines (whose dimensions are of the same order as the alpha-particle range) bodes well for the future of switchable neutron sources.
K.L. Hertz et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 41–45
References [1] D.L. Bowers, E.A. Rhodes, C.E. Dickerman, J. Radioanal. Nucl. Chem. 233 (1998) 161. [2] G.F. Knoll, in: H.H. Barschall, et al., (Eds.), Neutron Sources For Basic Physics and Applications, Pergamon Press, New York, 1983 (Chapter 2). . [3] S.Y.F. Chu, L.P. Ekstrom, R.B. Firestone, WWW table of radioactive isotopes, database version 1999-
45
02-28 from URL http://nucleardata.nuclear.lu.se/nucleardata/toi/ [4] M.E. Anderson, M.R. Hertz, Nucl. Sci. Eng. 44 (1971) 437. [5] B. Shleien, L.A. Slaback, B.K. Birky, Handbook of Health Physics and Radiological Health, 3rd Edition, Williams & Wilkins, Baltimore, MD, 1998. [6] J.M. Blatt, V.F. Weisskopf, Theoretical Nuclear Physics, Dover Publications, Inc., New York, 1979 (Chapter 1).