- Email: [email protected]
Neutron threshold activation detectors (TAD) for the detection of fissions
Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron threshold activation detectors (TAD) for the detection of fissions Tsahi Gozani n, John Stevenson, Michael J. King Rapiscan Laboratories, Inc., 520 Almanor Ave., Sunnyvale, CA 94085, USA
a r t i c l e i n f o
a b s t r a c t
Available online 15 January 2011
Prompt fission neutrons are one of the strongest signatures of the fission process. Depending on the fission inducing radiation, their average number ranges from 2.5 to 4 neutrons per fission. They are more energetic and abundant, by about 2 orders of magnitude, than the delayed neutrons (E3 vs. E0.01) that are commonly used as indicators for the presence of fissionable materials. The detection of fission prompt neutrons, however, has to be done in the presence of extremely intense probing radiation that stimulated them. During irradiation, the fission stimulation radiation, X-rays or neutrons, overwhelms the neutron detectors and temporarily incapacitate them. Consequently, by the time the detectors recover from the source radiation, fission prompt neutrons are no longer emitted. In order to measure the prompt fission signatures under these circumstances, special measures are usually taken with the detectors such as heavy shielding with collimation, use of inefficient geometries, high pulse height bias and gamma-neutron separation via pulse-shape discrimination with an appropriate organic scintillator. These attempts to shield the detector from the flash of radiation result in a major loss of sensitivity. It can lead to a complete inability to detect the fission prompt neutrons. In order to overcome the blinding induced background from the source radiation, the detection of prompt fission neutrons needs to occur long after the fission event and after the detector has fully recovered from the source overload. A new approach to achieve this is to detect the delayed activation induced by the fission neutrons. The approach demonstrates a good sensitivity in adverse overload situations (gamma and neutron ‘‘flash’’) where fission prompt neutrons could normally not be detected. The new approach achieves the required temporal separation between the detection of prompt neutrons and the detector overload by the neutron activation of the detector material. The technique, called Threshold Activation Detection (TAD), is to utilize appropriate substances that can be selectively activated by the fission neutrons and not by the source radiation and then measure the radioactively decaying activation products (typically beta and gamma rays) well after the source pulse. The activation material should possess certain properties: a suitable half-life of the order of seconds; an energy threshold below which the numerous source neutrons will not activate it (e.g., 3 MeV); easily detectable activation products (typically 41 MeV beta and gamma rays) and have a usable crosssection for the selected reaction. Ideally the substance would be a part of the scintillator. There are several good material candidates for the TAD, including fluorine, which is a major constituent of available scintillators such as BaF2, CaF2 and hydrogen free liquid fluorocarbon. Thus the fluorine activation products, in particular the beta particles, can be measured with a very high efficiency in the detector. The principles, applications and experimental results obtained with the fluorine based TAD are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Fast neutron detection Fission prompt neutrons Nuclear material detection Delayed fission gamma rays Neutron scintillation detectors Photofission
1. Introduction The detection of fissile materials (e.g., 235U and 239Pu) that may be smuggled in various conveyances is a major priority for national and international security. Great efforts are being made
n
Corresponding author. Tel.: +1 408 961 9711; fax: + 1 408 727 8748. E-mail address: [email protected] (T. Gozani).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.029
to develop reliable means to detect these materials. Paramount among them is active interrogation of large cargo containers using fast neutrons and/or high energy X-rays. These particles stimulate fissions in all fissionable materials, including the Special Nuclear Materials (SNM). The fission process is rich in detectable signatures [1]. The primary signatures are the prompt neutrons and gamma rays and delayed neutrons and gamma rays (see Table 1). The secondary ones are the high multiplicities, i.e. the number of particles
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
335
Intrinsic prompt fission neutron detection efficiency of an ideal plastic detector
Table 1 Accessible fission signatures.
1 Signature
Isotope 235
Prompt neutrons Prompt gamma rays Delayed neutrons Delayed gamma rays
U
239
Pu
238
U
np from (n,f) En r2.5 MeV or (g,f) Eg o9 MeV
2.8
3.2
2.9
gp nd from (n,f) nd from (g,f)
6.7 0.015 0.01
6.7 0.0061 0.004
7.2 0.044 0.028
gd (total) gd w/Eg Z 3 MeV
6.7 0.127
6.7 0.065
7.2 0.11
Fraction of neutron deteccted
Signature name
2MeV 0.1
3MeV
4MeV
0.01
0.001
emitted simultaneously in the fission process. The other secondary feature is the decay time of the delayed radiations. The use of the high prompt neutron and gamma rays multiplicities requires very high detection efficiencies [2] that are very difficult to achieve when scanning large objects such as trucks or marine containers. The delayed radiations consisting of delayed gamma rays and the smaller delayed neutrons are more commonly used as indicators of fissions. The prompt signatures, neutrons and gamma rays, are strong, but they are created when the much more numerous stimulating radiations (X-rays and neutrons) are present. The timing of their emission makes their detection quite difficult. In fact the detection of the prompt gamma rays is currently not feasible. This paper discusses current and a new method of detecting fission prompt neutrons.
0
50
100
150
200
250
Discrimination level (arbitrary units) Fig. 1. Intrinsic detection efficiency of prompt fission neutrons in an ideal plastic scintillator.
efficiency of a thick detector is reduced to about 4%, although it would exceed 50% if zero discrimination bias were allowed. The need to apply PSD to reduce the gamma ray contamination of the neutron spectrum limits also the size of the usable detector to 10–15 cm diameter and thickness because a larger detector leads to lower PSD effectiveness. Reducing the gamma background often requires also heavy shielding, further reducing the neutron detection sensitivity. These measures can result in a major loss of sensitivity and even the complete inability to detect the fission prompt neutrons when the gamma background is high.
2. Current methods of detecting fission prompt neutrons Current methods of detecting prompt fission neutrons can be divided into categories:
Direct detection in a fast neutron detector during the fission event, and/or
Detection within few hundred microseconds after the fission event in a neutron moderated detector. These methods are discussed below. 2.1. Direct prompt neutron detection via proton recoil liquid scintillators The most common and relatively efficient detectors of fast neutrons are hydrogenous plastic or liquid scintillators [3]. The detection process is based on proton recoil as a result of the neutron elastic collision with hydrogen. Certain liquid scintillators such as the NE213 type (San Gobain BC-519 and Eljen EJ325) offer the possibility of distinguishing between neutrons and gamma rays through pulse-shape discrimination (PSD). This is essential since in most cases the fission neutrons are accompanied by many more photons, either from the probing source, as is the case in X-ray photofission, or the inelastic and capture gamma rays from the numerous source neutrons, as is the case, for example, of 2.5 MeV (d,D) neutrons. An ideal plastic-based proton recoil scintillator’s detection relative efficiency is illustrated in Fig. 1 as a function of the discrimination bias for fission neutrons. Discriminating against source neutrons of 2.8 MeV, which is the maximum neutron energy generated at 01 from the D(d,n) reaction, or from the D(g,n) reaction that uses 8 MeV X-rays, results in a severe reduction in the detection efficiencies of the higher energy fission neutrons. In the case of the neutron fission spectrum, the intrinsic
2.2. Differential Die Away Analysis [4] A special case where the prompt fission neutrons can be measured with a very high efficiency is when the Differential Die Away Analysis (DDAA) method is applicable. Here the fissions are induced by thermalized source neutrons in the SNM. The thermalization process is fast, within a fraction to a few microseconds, depending on the size and density of the hydrogenous constituents in the cargo and the design of the source assembly. The resulting thermal neutrons, however, can efficiently stimulate fissions, only in SNM if present, for hundreds of microseconds, as they are slowly absorbed by the cargo material and leak out. The high energy fission neutrons produced by the thermal neutrons can be detected in high efficiency cadmium-covered detectors containing 3He proportional counters or other alternative 1/v detectors based on 10B or 6Li [3]. The only competing background would be the low cosmic ray induced neutron background. 2.3. Threshold Activation Detectors (TAD) for fission neutrons An alternative method of neutron detection is highly desirable when either the use of proton recoil becomes impractical, e.g., because of the inability to sufficiently suppress the background neutrons and gamma rays and/or X-rays present at the same time, or the DDAA is infeasible, e.g., when the thermal neutron flux is too low to induce enough fissions. The alternative technique should be able to detect the prompt neutrons while not being blinded by the source radiation. An efficient way to detect prompt fission neutrons is through fast neutron activation of an appropriate material that is insensitive to source neutrons because of the threshold nature of its cross-section, which should have an energy above that of the
336
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
Table 2 Candidates for Threshold Activation Detectors. Nuclide
Reaction
Threshold (MeV)
6-Li 19-F
6
Li(n,p)6He F(n,p)19O 19 F(n,a)16N
3.18 4.25 1.61
23-Na
23
19
Na(n,p)23Ne Na(n,a)20F
23
Decay products Beta (MeV)
Intensity (%)
0.807 26.9 7.1
3.5 1.4, 2.1 4.3, 10.4
3.75 4.04
37.2 11.1 0.02 59.1 1.07 37
24-Mg 25-Mg 26-Mg
24
Mg(n,p)23Na Mg(n,p)25Na 26 Mg(n,p)26Na 26 Mg(n,a)23Ne
4.93 3.18 8.90 5.62
28-Si 34-S 37-Cl
34
4.00 4.73 4.19 1.6
25
Si(n,p)28Al S(n,p)34P 37 Cl(n,p)37S 37 Cl(n,a)34P 34
Half-life (s)
134 12 303 12.4
Gamma (MeV)
Intensity (%)
100 54.4, 45.4 67, 28
1.36 6.1
50.4 67
4.38, 3.95 5.39
67, 32 100
0.44, 1.64 1.63
33, 1 100
5.99 2.9, 3.8 7.5 3.9, 4.4
0.05 27, 63 88 32, 67
0.47 0.98, 1.6 1.8 0.44
100 15, 9.5 99 33
2.86 3.2, 5.37 1.76, 4.86 3.2, 5.4
100 15, 85 94, 5.6 15, 85
1.78 2.13, 4.1 3.1, 4.0 2.13, 4.1
100 15, 0.18 94, 0.03 15, 0.2
source neutrons. We call the detectors based on this principle ‘‘Threshold Activation Detectors’’ or TAD. A summary of the TAD desirable attributes is given below:
TAD material activating it.
TADs are most efficient when the activation material is itself a scintillator.
Activation products, electrons from beta decay, are detected with close to 100% efficiency.
The radioactivity of interest has short decay times in the range
of tenths to tens of seconds. Prompt fission signatures can be detected after the events that created them without a concern for the enormous overload generated by the primary radiation such as the gamma and/or neutron ‘‘flash’’ associated with the X-ray or neutron sources. The detector is completely insensitive to neutrons with energies below its threshold.
There are several potential candidates for TAD materials (see Table 2). The threshold nature of the cross-sections of some of these candidates is shown in Fig. 2. The most practical and the first selected TAD is based on fluorine. The 19F(n,a) and the lower cross-section 19F(n,p) reactions are responsible for the detection of the prompt neutrons above 3 MeV. Neutron activation of Teflon and measurement of the resulting 6.1 MeV gamma rays with a Ge detector was proposed in 1978 as a method to measure high energy neutrons [5]. However this approach is extremely inefficient and unsuitable for the detection of SNM in large conveyance. The main advantage of the fluorine based TAD is the fact that several scintillators such as BaF2, CaF2, CeF2 and non-hydrogenous fluorocarbon (FC) liquid scintillators (e.g. Eljen EJ-313) contain large amounts of fluorine. This affords the detection of the two beta particles resulting from the fluorine activation with close to 100% efficiency. We have selected the FC liquid scintillator because it contains 58% fluorine by weight, has the highest density and the highest effective atomic number among liquid scintillators, its cost is moderate especially for large detectors, the decay time is very fast 2 ns and it has low neutron induced background activation compared to inorganic detectors. In addition the FC scintillator can also double as a very efficient fission delayed gamma ray detector. Finally, the scintillators can be made in any desirable shape and volume to maximize the overall system detection efficiency.
Cross Secction (barns)
Fast neutrons, above an energy threshold, interact within a
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05 1
2
3
4
5
6
7
8
9
10
Energy (MeV) Fig. 2. Fast neutron cross-sections for threshold activation reactions in some TAD materials.
The main nuclear properties of the fluorine are summarized below:
Primary productive reaction: J 1n + 19F-16N+ 4He
Threshold is 1.6 MeV, but effectively is at 3 MeV for fission neutrons J 16N has a 7.1 s half-life and beta decays to 16O J The decay releases 10.4 MeV energy: 28% as 10.4 MeV b (no g) and 66% as 4.3 MeV b with 6.1 Mev g (in coincidence). Secondary productive reaction: J 1n + 19F-19O+ 1p J Threshold is about 5 MeV for fission neutrons J 19O decays to 19F with T1/2 ¼26.9 s and releases 4.622 MeV beta particles and a 1.36 MeV (50%) and 0.197 (96%) gamma. Detection efficiency of b particles in FC approaches 100% J
The b-ray spectrum induced by fission neutrons from 252Cf source is shown in Fig. 3. It was obtained by repeating 10 s exposures of the FC detector to a 252Cf source followed by a 10 s measurement while the source is away. The two broad peaks in Fig. 3 represent the superimposition of the two b energies of 4.3 and 10.4 MeV. The former contains a contribution from the broad Compton scattering spectrum of the 6.1 MeV gamma rays and 4.84 MeV beta particles from the 19F(n,p)19O reaction.
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
100
Photofission signatures measured between linac pulses with FC and PS scintillators 1000
γ signal (E>2MeV/3MeV) ≈9.4%/2.8% of total signal γ signal (E>5.5MeV)≈0.4% of
1
total γ signal Prompt neutron (n,α) signal (E>5.5MeV)≈15% of the total (n,α)
0.1 Fluorine b activation spectrum from fission prompt neutrons
0.01
0.001 0
1
2
3
4
5
6
7
8
9
10
11
Photon energy (MeV) Fig. 3. Fission prompt neutrons and delayed gamma rays as detected in FC detector between short pulses of 9 MV Bremsstrahlung X-rays.
Counts per second/unit energy
Intensity per unit energy
Delayed γ from photofission
10
337
100 10
FC
1 PS 0.1 0.01 0
5
10
15
Pulse hieght distribution (MeV) Fig. 5. Spectra collected between 9 MeV end point X-ray pulses stimulating photofission in a 2 kg depleted uranium and measured with plastic and fluorocarbon scintillators of equal size.
spectrum is again illustrated in Fig. 5. A plastic scintillator (EJ-204) with identical size as that of the FC scintillator (EJ-313) was placed at equidistance from a sample of depleted uranium that was exposed to a beam of X-rays from 9 MV linac. The spectra that were collected between the linac pulses are shown in Fig. 5. The plastic scintillator shows a pure delayed gamma ray spectrum, while the FC detector shows the superposition of delayed gamma ray spectrum and the (n.a) activation spectrum.
Prompt fission neutrons, with energy above 3MeV, induce F(n,α) activation with 7.1s half-life. Delayed gamma rays ,with half-lives between a fraction of a second and thousand seconds are also created in the fission process. Both radiations are measured between the linac pulses Fig. 4. Sequence of linac pulses indicating the time interval where the prompt neutron activation and the delayed gamma rays are measured.
Fig. 3 also shows the exponentially declining energy spectrum of the delayed fission gamma rays. It is measured along with the 19 F(n,a) activation between the 9 MV linac pulses, as indicated in Fig. 4. In this mode of operation the linac is pulsed 20 to 100 pulses per second; each pulse is typically 2–4 ms wide. The fission delayed gamma rays and the fluorine activation of the FC itself are collected between the pulses. The combined spectrum is shown in Fig. 3, as an exponentially declining spectrum with a broad peak commencing at about 5.5 MeV and ending at about 10.4 MeV. When the ‘‘double-peak’’ 252Cf spectrum is normalized to the former spectrum above 7 MeV and subtracted from the latter, the difference is the exponentially declining spectrum representing typical fission delayed gamma ray spectrum. The fraction of the delayed gamma ray spectrum above 5.5 MeV is very small ( E0.4%), whereas that of prompt neutron fluorine activation is quite substantial (15%). The unique ability to detect the high energy neutrons that coincidentally are the most penetrating components of the fission
3. Conclusions A novel method for detecting high energy prompt fission neutrons is proposed and demonstrated. The means of detection is not hampered by the very strong and blinding radiation associated with the creation of the fission signature, be it X-ray and/or neutrons with energies below 3 MeV. The method is relatively easy to apply and serves equally well for the measurement of the fission delayed gamma rays.
Acknowledgements This work was supported by the U.S. Department of Homeland Security, Transformational and Applied Research Directorate (TARD) Domestic Nuclear Detection Office (DNDO).
References [1] T. Gozani, IEEE Trans. Nucl. Sci. NS-56 (3) (2009) 726. [2] T. Gozani, Active Nondestructive Assay of Nuclear Materials—Principles and Applications, US Nuclear Regulatory Commission, NUREG/CR-0602, January 1981, p. 261. [3] G. Knoll, Radiat. Detect. Meas., third ed., John Wiley & Sons Inc, 2000, p. 558. [4] K.J. Jordan, T. Gozani, Nucl. Instr. and Meth. B 261 (2007) 265. [5] A. Wolf, R. Moreh, Nucl. Instr. and Meth. 148 (1978) 195.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron threshold activation detectors (TAD) for the detection of fissions Tsahi Gozani n, John Stevenson, Michael J. King Rapiscan Laboratories, Inc., 520 Almanor Ave., Sunnyvale, CA 94085, USA
a r t i c l e i n f o
a b s t r a c t
Available online 15 January 2011
Prompt fission neutrons are one of the strongest signatures of the fission process. Depending on the fission inducing radiation, their average number ranges from 2.5 to 4 neutrons per fission. They are more energetic and abundant, by about 2 orders of magnitude, than the delayed neutrons (E3 vs. E0.01) that are commonly used as indicators for the presence of fissionable materials. The detection of fission prompt neutrons, however, has to be done in the presence of extremely intense probing radiation that stimulated them. During irradiation, the fission stimulation radiation, X-rays or neutrons, overwhelms the neutron detectors and temporarily incapacitate them. Consequently, by the time the detectors recover from the source radiation, fission prompt neutrons are no longer emitted. In order to measure the prompt fission signatures under these circumstances, special measures are usually taken with the detectors such as heavy shielding with collimation, use of inefficient geometries, high pulse height bias and gamma-neutron separation via pulse-shape discrimination with an appropriate organic scintillator. These attempts to shield the detector from the flash of radiation result in a major loss of sensitivity. It can lead to a complete inability to detect the fission prompt neutrons. In order to overcome the blinding induced background from the source radiation, the detection of prompt fission neutrons needs to occur long after the fission event and after the detector has fully recovered from the source overload. A new approach to achieve this is to detect the delayed activation induced by the fission neutrons. The approach demonstrates a good sensitivity in adverse overload situations (gamma and neutron ‘‘flash’’) where fission prompt neutrons could normally not be detected. The new approach achieves the required temporal separation between the detection of prompt neutrons and the detector overload by the neutron activation of the detector material. The technique, called Threshold Activation Detection (TAD), is to utilize appropriate substances that can be selectively activated by the fission neutrons and not by the source radiation and then measure the radioactively decaying activation products (typically beta and gamma rays) well after the source pulse. The activation material should possess certain properties: a suitable half-life of the order of seconds; an energy threshold below which the numerous source neutrons will not activate it (e.g., 3 MeV); easily detectable activation products (typically 41 MeV beta and gamma rays) and have a usable crosssection for the selected reaction. Ideally the substance would be a part of the scintillator. There are several good material candidates for the TAD, including fluorine, which is a major constituent of available scintillators such as BaF2, CaF2 and hydrogen free liquid fluorocarbon. Thus the fluorine activation products, in particular the beta particles, can be measured with a very high efficiency in the detector. The principles, applications and experimental results obtained with the fluorine based TAD are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Fast neutron detection Fission prompt neutrons Nuclear material detection Delayed fission gamma rays Neutron scintillation detectors Photofission
1. Introduction The detection of fissile materials (e.g., 235U and 239Pu) that may be smuggled in various conveyances is a major priority for national and international security. Great efforts are being made
n
Corresponding author. Tel.: +1 408 961 9711; fax: + 1 408 727 8748. E-mail address: [email protected] (T. Gozani).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.029
to develop reliable means to detect these materials. Paramount among them is active interrogation of large cargo containers using fast neutrons and/or high energy X-rays. These particles stimulate fissions in all fissionable materials, including the Special Nuclear Materials (SNM). The fission process is rich in detectable signatures [1]. The primary signatures are the prompt neutrons and gamma rays and delayed neutrons and gamma rays (see Table 1). The secondary ones are the high multiplicities, i.e. the number of particles
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
335
Intrinsic prompt fission neutron detection efficiency of an ideal plastic detector
Table 1 Accessible fission signatures.
1 Signature
Isotope 235
Prompt neutrons Prompt gamma rays Delayed neutrons Delayed gamma rays
U
239
Pu
238
U
np from (n,f) En r2.5 MeV or (g,f) Eg o9 MeV
2.8
3.2
2.9
gp nd from (n,f) nd from (g,f)
6.7 0.015 0.01
6.7 0.0061 0.004
7.2 0.044 0.028
gd (total) gd w/Eg Z 3 MeV
6.7 0.127
6.7 0.065
7.2 0.11
Fraction of neutron deteccted
Signature name
2MeV 0.1
3MeV
4MeV
0.01
0.001
emitted simultaneously in the fission process. The other secondary feature is the decay time of the delayed radiations. The use of the high prompt neutron and gamma rays multiplicities requires very high detection efficiencies [2] that are very difficult to achieve when scanning large objects such as trucks or marine containers. The delayed radiations consisting of delayed gamma rays and the smaller delayed neutrons are more commonly used as indicators of fissions. The prompt signatures, neutrons and gamma rays, are strong, but they are created when the much more numerous stimulating radiations (X-rays and neutrons) are present. The timing of their emission makes their detection quite difficult. In fact the detection of the prompt gamma rays is currently not feasible. This paper discusses current and a new method of detecting fission prompt neutrons.
0
50
100
150
200
250
Discrimination level (arbitrary units) Fig. 1. Intrinsic detection efficiency of prompt fission neutrons in an ideal plastic scintillator.
efficiency of a thick detector is reduced to about 4%, although it would exceed 50% if zero discrimination bias were allowed. The need to apply PSD to reduce the gamma ray contamination of the neutron spectrum limits also the size of the usable detector to 10–15 cm diameter and thickness because a larger detector leads to lower PSD effectiveness. Reducing the gamma background often requires also heavy shielding, further reducing the neutron detection sensitivity. These measures can result in a major loss of sensitivity and even the complete inability to detect the fission prompt neutrons when the gamma background is high.
2. Current methods of detecting fission prompt neutrons Current methods of detecting prompt fission neutrons can be divided into categories:
Direct detection in a fast neutron detector during the fission event, and/or
Detection within few hundred microseconds after the fission event in a neutron moderated detector. These methods are discussed below. 2.1. Direct prompt neutron detection via proton recoil liquid scintillators The most common and relatively efficient detectors of fast neutrons are hydrogenous plastic or liquid scintillators [3]. The detection process is based on proton recoil as a result of the neutron elastic collision with hydrogen. Certain liquid scintillators such as the NE213 type (San Gobain BC-519 and Eljen EJ325) offer the possibility of distinguishing between neutrons and gamma rays through pulse-shape discrimination (PSD). This is essential since in most cases the fission neutrons are accompanied by many more photons, either from the probing source, as is the case in X-ray photofission, or the inelastic and capture gamma rays from the numerous source neutrons, as is the case, for example, of 2.5 MeV (d,D) neutrons. An ideal plastic-based proton recoil scintillator’s detection relative efficiency is illustrated in Fig. 1 as a function of the discrimination bias for fission neutrons. Discriminating against source neutrons of 2.8 MeV, which is the maximum neutron energy generated at 01 from the D(d,n) reaction, or from the D(g,n) reaction that uses 8 MeV X-rays, results in a severe reduction in the detection efficiencies of the higher energy fission neutrons. In the case of the neutron fission spectrum, the intrinsic
2.2. Differential Die Away Analysis [4] A special case where the prompt fission neutrons can be measured with a very high efficiency is when the Differential Die Away Analysis (DDAA) method is applicable. Here the fissions are induced by thermalized source neutrons in the SNM. The thermalization process is fast, within a fraction to a few microseconds, depending on the size and density of the hydrogenous constituents in the cargo and the design of the source assembly. The resulting thermal neutrons, however, can efficiently stimulate fissions, only in SNM if present, for hundreds of microseconds, as they are slowly absorbed by the cargo material and leak out. The high energy fission neutrons produced by the thermal neutrons can be detected in high efficiency cadmium-covered detectors containing 3He proportional counters or other alternative 1/v detectors based on 10B or 6Li [3]. The only competing background would be the low cosmic ray induced neutron background. 2.3. Threshold Activation Detectors (TAD) for fission neutrons An alternative method of neutron detection is highly desirable when either the use of proton recoil becomes impractical, e.g., because of the inability to sufficiently suppress the background neutrons and gamma rays and/or X-rays present at the same time, or the DDAA is infeasible, e.g., when the thermal neutron flux is too low to induce enough fissions. The alternative technique should be able to detect the prompt neutrons while not being blinded by the source radiation. An efficient way to detect prompt fission neutrons is through fast neutron activation of an appropriate material that is insensitive to source neutrons because of the threshold nature of its cross-section, which should have an energy above that of the
336
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
Table 2 Candidates for Threshold Activation Detectors. Nuclide
Reaction
Threshold (MeV)
6-Li 19-F
6
Li(n,p)6He F(n,p)19O 19 F(n,a)16N
3.18 4.25 1.61
23-Na
23
19
Na(n,p)23Ne Na(n,a)20F
23
Decay products Beta (MeV)
Intensity (%)
0.807 26.9 7.1
3.5 1.4, 2.1 4.3, 10.4
3.75 4.04
37.2 11.1 0.02 59.1 1.07 37
24-Mg 25-Mg 26-Mg
24
Mg(n,p)23Na Mg(n,p)25Na 26 Mg(n,p)26Na 26 Mg(n,a)23Ne
4.93 3.18 8.90 5.62
28-Si 34-S 37-Cl
34
4.00 4.73 4.19 1.6
25
Si(n,p)28Al S(n,p)34P 37 Cl(n,p)37S 37 Cl(n,a)34P 34
Half-life (s)
134 12 303 12.4
Gamma (MeV)
Intensity (%)
100 54.4, 45.4 67, 28
1.36 6.1
50.4 67
4.38, 3.95 5.39
67, 32 100
0.44, 1.64 1.63
33, 1 100
5.99 2.9, 3.8 7.5 3.9, 4.4
0.05 27, 63 88 32, 67
0.47 0.98, 1.6 1.8 0.44
100 15, 9.5 99 33
2.86 3.2, 5.37 1.76, 4.86 3.2, 5.4
100 15, 85 94, 5.6 15, 85
1.78 2.13, 4.1 3.1, 4.0 2.13, 4.1
100 15, 0.18 94, 0.03 15, 0.2
source neutrons. We call the detectors based on this principle ‘‘Threshold Activation Detectors’’ or TAD. A summary of the TAD desirable attributes is given below:
TAD material activating it.
TADs are most efficient when the activation material is itself a scintillator.
Activation products, electrons from beta decay, are detected with close to 100% efficiency.
The radioactivity of interest has short decay times in the range
of tenths to tens of seconds. Prompt fission signatures can be detected after the events that created them without a concern for the enormous overload generated by the primary radiation such as the gamma and/or neutron ‘‘flash’’ associated with the X-ray or neutron sources. The detector is completely insensitive to neutrons with energies below its threshold.
There are several potential candidates for TAD materials (see Table 2). The threshold nature of the cross-sections of some of these candidates is shown in Fig. 2. The most practical and the first selected TAD is based on fluorine. The 19F(n,a) and the lower cross-section 19F(n,p) reactions are responsible for the detection of the prompt neutrons above 3 MeV. Neutron activation of Teflon and measurement of the resulting 6.1 MeV gamma rays with a Ge detector was proposed in 1978 as a method to measure high energy neutrons [5]. However this approach is extremely inefficient and unsuitable for the detection of SNM in large conveyance. The main advantage of the fluorine based TAD is the fact that several scintillators such as BaF2, CaF2, CeF2 and non-hydrogenous fluorocarbon (FC) liquid scintillators (e.g. Eljen EJ-313) contain large amounts of fluorine. This affords the detection of the two beta particles resulting from the fluorine activation with close to 100% efficiency. We have selected the FC liquid scintillator because it contains 58% fluorine by weight, has the highest density and the highest effective atomic number among liquid scintillators, its cost is moderate especially for large detectors, the decay time is very fast 2 ns and it has low neutron induced background activation compared to inorganic detectors. In addition the FC scintillator can also double as a very efficient fission delayed gamma ray detector. Finally, the scintillators can be made in any desirable shape and volume to maximize the overall system detection efficiency.
Cross Secction (barns)
Fast neutrons, above an energy threshold, interact within a
1.E+00
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Energy (MeV) Fig. 2. Fast neutron cross-sections for threshold activation reactions in some TAD materials.
The main nuclear properties of the fluorine are summarized below:
Primary productive reaction: J 1n + 19F-16N+ 4He
Threshold is 1.6 MeV, but effectively is at 3 MeV for fission neutrons J 16N has a 7.1 s half-life and beta decays to 16O J The decay releases 10.4 MeV energy: 28% as 10.4 MeV b (no g) and 66% as 4.3 MeV b with 6.1 Mev g (in coincidence). Secondary productive reaction: J 1n + 19F-19O+ 1p J Threshold is about 5 MeV for fission neutrons J 19O decays to 19F with T1/2 ¼26.9 s and releases 4.622 MeV beta particles and a 1.36 MeV (50%) and 0.197 (96%) gamma. Detection efficiency of b particles in FC approaches 100% J
The b-ray spectrum induced by fission neutrons from 252Cf source is shown in Fig. 3. It was obtained by repeating 10 s exposures of the FC detector to a 252Cf source followed by a 10 s measurement while the source is away. The two broad peaks in Fig. 3 represent the superimposition of the two b energies of 4.3 and 10.4 MeV. The former contains a contribution from the broad Compton scattering spectrum of the 6.1 MeV gamma rays and 4.84 MeV beta particles from the 19F(n,p)19O reaction.
T. Gozani et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 334–337
100
Photofission signatures measured between linac pulses with FC and PS scintillators 1000
γ signal (E>2MeV/3MeV) ≈9.4%/2.8% of total signal γ signal (E>5.5MeV)≈0.4% of
1
total γ signal Prompt neutron (n,α) signal (E>5.5MeV)≈15% of the total (n,α)
0.1 Fluorine b activation spectrum from fission prompt neutrons
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Photon energy (MeV) Fig. 3. Fission prompt neutrons and delayed gamma rays as detected in FC detector between short pulses of 9 MV Bremsstrahlung X-rays.
Counts per second/unit energy
Intensity per unit energy
Delayed γ from photofission
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FC
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Pulse hieght distribution (MeV) Fig. 5. Spectra collected between 9 MeV end point X-ray pulses stimulating photofission in a 2 kg depleted uranium and measured with plastic and fluorocarbon scintillators of equal size.
spectrum is again illustrated in Fig. 5. A plastic scintillator (EJ-204) with identical size as that of the FC scintillator (EJ-313) was placed at equidistance from a sample of depleted uranium that was exposed to a beam of X-rays from 9 MV linac. The spectra that were collected between the linac pulses are shown in Fig. 5. The plastic scintillator shows a pure delayed gamma ray spectrum, while the FC detector shows the superposition of delayed gamma ray spectrum and the (n.a) activation spectrum.
Prompt fission neutrons, with energy above 3MeV, induce F(n,α) activation with 7.1s half-life. Delayed gamma rays ,with half-lives between a fraction of a second and thousand seconds are also created in the fission process. Both radiations are measured between the linac pulses Fig. 4. Sequence of linac pulses indicating the time interval where the prompt neutron activation and the delayed gamma rays are measured.
Fig. 3 also shows the exponentially declining energy spectrum of the delayed fission gamma rays. It is measured along with the 19 F(n,a) activation between the 9 MV linac pulses, as indicated in Fig. 4. In this mode of operation the linac is pulsed 20 to 100 pulses per second; each pulse is typically 2–4 ms wide. The fission delayed gamma rays and the fluorine activation of the FC itself are collected between the pulses. The combined spectrum is shown in Fig. 3, as an exponentially declining spectrum with a broad peak commencing at about 5.5 MeV and ending at about 10.4 MeV. When the ‘‘double-peak’’ 252Cf spectrum is normalized to the former spectrum above 7 MeV and subtracted from the latter, the difference is the exponentially declining spectrum representing typical fission delayed gamma ray spectrum. The fraction of the delayed gamma ray spectrum above 5.5 MeV is very small ( E0.4%), whereas that of prompt neutron fluorine activation is quite substantial (15%). The unique ability to detect the high energy neutrons that coincidentally are the most penetrating components of the fission
3. Conclusions A novel method for detecting high energy prompt fission neutrons is proposed and demonstrated. The means of detection is not hampered by the very strong and blinding radiation associated with the creation of the fission signature, be it X-ray and/or neutrons with energies below 3 MeV. The method is relatively easy to apply and serves equally well for the measurement of the fission delayed gamma rays.
Acknowledgements This work was supported by the U.S. Department of Homeland Security, Transformational and Applied Research Directorate (TARD) Domestic Nuclear Detection Office (DNDO).
References [1] T. Gozani, IEEE Trans. Nucl. Sci. NS-56 (3) (2009) 726. [2] T. Gozani, Active Nondestructive Assay of Nuclear Materials—Principles and Applications, US Nuclear Regulatory Commission, NUREG/CR-0602, January 1981, p. 261. [3] G. Knoll, Radiat. Detect. Meas., third ed., John Wiley & Sons Inc, 2000, p. 558. [4] K.J. Jordan, T. Gozani, Nucl. Instr. and Meth. B 261 (2007) 265. [5] A. Wolf, R. Moreh, Nucl. Instr. and Meth. 148 (1978) 195.