Neutron detection devices, nuclear material detection devices

Abstract

To provide a neutron detection device capable of detecting high-energy neutrons with high sensitivity, and a highly sensitive nuclear material detection device using the same. [Solution] This nuclear material detection device 1 detects nuclear material by the TENA method. It uses a neutron source 10 that emits primary neutrons N1 toward a sample 100, and a neutron detector 20 that detects neutrons (primary neutrons N1 and secondary neutrons N2) emitted from the sample 100. It also uses an analysis unit 30 that recognizes high-energy neutrons by analyzing the detection results of the neutron detector 20. When the energy of the neutrons is high, 16 O and 12 Electrons with particularly high energy are generated by inelastic scattering of neutrons by C. In the case where a neutron with particularly high energy is incident, the neutron detection device 40 recognizes the high-energy neutron by utilizing the fact that Cherenkov light caused by such high-energy electrons is detected by a photodetector.

Description

[Technical field]

[0001] The present invention relates to a neutron detection device that detects neutrons, and a nuclear material detection device using the same. [Background technology]

[0002] In order to prevent the proliferation of nuclear weapons, etc., there is a demand for technology to detect, for example, concealed nuclear material (or fissile material). For this purpose, various technologies have been proposed, such as those described in Non-Patent Document 1. Among these, the Threshold Energy Neutron Analysis (TENA) method is effective in that it irradiates a test sample with neutrons that have high material penetration and detects neutrons emitted from the nuclear material in the test sample.

[0003] 12 is a simplified diagram showing the measurement configuration in the TENA method. Here, when neutrons (primary neutrons N1) emitted from a neutron source 90 enter a measured sample 100, the nuclear material in the measured sample 100 undergoes a nuclear fission reaction due to the primary neutrons N1, which generates new neutrons (secondary neutrons N2). When a neutron detector 91 detects neutrons emitted from the measured sample 100, the neutrons include the primary neutrons N1 that have passed through or been scattered by the measured sample 100, and the secondary neutrons N2 that have been generated in the measured sample 100. When the secondary neutrons N2 are recognized, it is recognized that the nuclear material that generated the secondary neutrons N2 is present in the measured sample 100.

[0004] In the neutron detector 91, the primary neutrons N1 and the secondary neutrons N2 are detected as neutrons without distinction, and therefore a configuration capable of distinguishing between them is required. For this reason, a DD neutron source utilizing the reaction between deuterium is used as the neutron source 90, and in this case, the energy of the primary neutrons N1 is 2.45 MeV (monochromatic). An example of the energy spectrum of the secondary neutrons N2 in this case is shown in FIG. 13. Here, the low energy components below 2.45 MeV include the secondary neutrons N2 in addition to the primary neutrons N1 (or the primary neutrons N1 that have been scattered and slowed down), whereas neutrons with energies higher than 2.45 MeV are not included in the primary neutrons N1. Therefore, if neutrons with energies higher than 2.45 MeV can be significantly recognized, the presence of nuclear material in the measured sample 100 can be recognized. However, as shown in FIG. 13, the amount of neutrons on such a high energy side is less than that of neutrons on the low energy side, and the integral amount is 30% of the total. For this purpose, the neutron detector 91 is required to be capable of detecting neutrons in a specific high energy region (energy higher than 2.45 MeV in the example of FIG. 13) with high sensitivity.

[0005] In contrast, neutron detectors that can detect neutrons include, for example, 3 He detector, 10 B detector, 6 Li detectors are known, in which 3 He, 10 B. 6This makes use of the large reaction cross section of Li neutrons. As with gamma rays, organic scintillation detectors (plastic scintillation detectors, liquid scintillation detectors) are also used to detect neutrons. In this case, a material that emits fluorescence when neutrons are incident is used. For example, the Tension Metastable Fluid Detector (TMFD) described in Non-Patent Document 2 detects neutrons by utilizing the fact that a fluid to which tension is applied is bumped by the incidence of neutrons. The energy of the neutrons to be detected can be set by setting this tension, so that neutrons with higher energy than 2.45 MeV as mentioned above can be selectively detected. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] "Introduction to Nuclear Security, Vol. 3: Nuclear Material Detection Technology for Nuclear Security", Yoshiyuki Takahashi, Mitsuo Koizumi, Journal of the Atomic Energy Society of Japan, Vol. 62, No. 8 (2020) [Non-Patent Document 2] “Nondestructive and active interrogation system for special nuclear material: proof of principle and initial results”, Mahmoud Bakr, Kai Masuda, Yoshiyuki Takahashi, Tsuyoshi Misawa, Norio Yamakawa and Tomas Scott, Nuclear Science Technology, volume 35, article number 87, (2024) Summary of the Invention [Problem to be solved by the invention]

[0007] 3 He, 10 B. 6Neutron detectors using materials with a large neutron reaction cross section, such as Li, take advantage of the large reaction cross section of these materials with thermal neutrons (energy up to about 0.025 eV). For this reason, when detecting high-energy neutrons, they need to be slowed down, but in this case, it becomes difficult to selectively detect neutrons with energies higher than 2.45 MeV, as shown in Figure 13. In addition, organic scintillation detectors such as plastic scintillation detectors and liquid scintillation detectors are mainly capable of detecting high-energy neutrons (energy up to 1 MeV or more) using elastic scattering reactions, but there is a problem in that the detection efficiency of such high-energy neutrons is low due to the low reaction cross section of the elastic scattering reaction. Furthermore, the above 3 He, 10 B. 6 As with detectors using Li etc., it became difficult for these organic scintillation detectors to selectively detect neutrons with energies higher than 2.45 MeV. Although the TMFD was able to selectively detect high-energy neutrons well, the detection efficiency itself was still low, so long periods of time were required to perform highly accurate measurements.

[0008] For this reason, there was a demand for a neutron detection device capable of selectively detecting high-energy neutrons with high sensitivity, and a highly sensitive nuclear material detection device using this device.

[0009] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an invention that solves the above problems. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention has the following configuration. The present invention is a neutron detection device that detects neutrons by detecting Cherenkov light emitted by neutrons in a medium, and comprises a detection cell in which the medium is provided, a photodetector that detects the Cherenkov light emitted from the detection cell, and an analysis unit that recognizes the high-energy neutrons by recognizing the pulse output obtained when the photodetector detects the Cherenkov light, which corresponds to the Cherenkov light emitted in the detection cell by electrons having energy higher than a predetermined electron energy threshold. The electron energy threshold is an energy corresponding to the maximum energy of electrons formed when the neutrons undergo a neutron capture reaction in the detection cell, and the analysis unit may recognize neutrons having energy equal to or greater than the threshold energy at which the neutrons undergo an inelastic scattering reaction in the detection cell by recognizing the pulse output corresponding to the Cherenkov light emitted in the detection cell by electrons having energy higher than the electron energy threshold. The medium is 16 It may contain O. In the detection chamber, the medium is 12 It may be housed in a housing made of a material containing C. The present invention is a nuclear material detection device for detecting nuclear material contained in a sample, comprising a neutron source for emitting primary neutrons to the sample such that the nuclear material emits secondary neutrons, and the neutron detection device, and the analysis unit detects the presence of the nuclear material in the sample by recognizing the pulse output corresponding to the Cherenkov light emitted in the medium by electrons having energy higher than the electron energy threshold. The neutron source may emit the primary neutrons in a pulsed manner that is repeatedly turned on and off, and the analysis unit may detect the presence of the nuclear material or calculate the amount of the nuclear material using the difference between the measurement result of the neutron detection device when the primary neutrons are on and the measurement result of the neutron detection device when the primary neutrons are off. The medium may be water, a DD neutron source may be used as the neutron source, and the electron energy threshold may be 1.994 MeV. Effect of the Invention

[0011] Since the present invention is configured as described above, it is possible to obtain a neutron detection device that can selectively detect high-energy neutrons (neutrons with energy equal to or higher than the threshold energy of the inelastic scattering reaction) with high sensitivity, and a highly sensitive nuclear material detection device using the same. [Brief description of the drawings]

[0012] [Figure 1] 1 is a diagram showing a configuration of a nuclear material detection device according to an embodiment of the present invention. [Diagram 2] 1 is a diagram showing a configuration of a neutron detector used in a nuclear material detection device according to an embodiment of the present invention. [Diagram 3] The neutron energy dependence of the cross sections for various reactions between neutrons and 16O is shown. [Figure 4] The energy of the incident neutrons was changed, and the flux of electrons with energies higher than 0.264 MeV was calculated over time. [Diagram 5] The neutron energy dependence of the cross sections for various reactions between neutrons and 12C is shown. [Figure 6] This is the result of calculating the energy spectrum of electrons generated in the detection chamber by incident neutrons, taking into account inelastic scattering, by changing the neutron energy. [Figure 7] This shows a histogram of the number of Cherenkov photons detected in the detection cell in response to incident neutrons, taking inelastic scattering into account, calculated by varying the neutron energy. [Figure 8] FIG. 13 is a diagram showing a schematic diagram of a method for recognizing high-energy electrons using a threshold value for a pulse output from a photodetector. [Figure 9] This is the result of calculating the time course of the flux of electrons with energies higher than 1.994 MeV in the detection cell for each amount of 235U present in the sample when the sample is irradiated with pulsed neutrons (pulse width 50 μs). [Figure 10]This is the result (calibration curve) of calculating the amount of electrons with energies higher than 1.994 MeV generated during neutron irradiation (0 to 50 μs) in Figure 9 over a total of 5 s (105 pulses) and the amount of 235U present in the sample. [Figure 11] This figure shows the time course of the electron flux with energies higher than 0.264 MeV, with the energy of the incident neutrons changed, on a time scale shorter than that of Figure 4. [Figure 12] FIG. 1 is a simplified diagram showing a measurement configuration in the TENA method, which is an example of a nuclear material detection method. [Figure 13] This is an example of the energy spectrum of secondary neutrons incident on a neutron detector in the TENA method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The nuclear material detection device 1 according to the embodiment of the present invention detects nuclear material by the TENA method similar to that shown in FIG. 12. FIG. 1 shows a simplified configuration. Here, as in FIG. 12, a neutron source 10 that emits primary neutrons N1 toward a sample 100 and a neutron detector 20 that detects neutrons (primary neutrons N1 and secondary neutrons N2) emitted from the sample 100 are used. In addition, an analysis unit 30 is used, which is a computer that recognizes high-energy neutrons by analyzing the detection results of the neutron detector 20, and thereby recognizes the presence or absence of nuclear material in the sample 100. As the neutron source 10, a DD neutron source that can emit monochromatic neutrons of 2.45 MeV in a controlled pulsed manner is particularly preferably used.

[0014] The neutron detector 20 used here is configured to be able to distinguish and detect neutrons on the high energy side from those on the low energy side in Fig. 13, and the analysis section 30 detects neutrons on the high energy side with high sensitivity according to the characteristics of the neutron detector 20. For this reason, the neutron detector 20 and the analysis section 30 are combined to form a neutron detection device 40. This neutron detection device 40 can also be used independently of the detection of nuclear material.

[0015] In the following, we will first explain the configuration of this neutron detector 20 and the phenomenon that occurs when neutrons are incident on it. Fig. 2 shows a schematic configuration of this neutron detector 20. In this neutron detector 20, Cherenkov light C emitted in a detection cell 21 by incident neutrons N is detected. For this reason, this neutron detector 20 is equipped with a detection cell 21 into which neutrons to be detected are incident, and a photodetector 22 that detects Cherenkov light generated by the incidence of neutrons on this detection cell 21.

[0016] The detection tank 21 is configured by housing 211 and containing a liquid (medium 212) that emits Cherenkov light when neutrons are incident thereon. As the photodetector 22, for example, a photomultiplier tube or a silicon photomultiplier element (SiPM) that detects Cherenkov light, such as visible light or ultraviolet light, for each emission and emits a pulsed output (pulse output) can be used. In FIG. 2, one photodetector 22 is used for a single detection tank 21, but in order to detect Cherenkov light with high efficiency, it is preferable to actually use a plurality of photodetectors 22. In addition, it is preferable to attach a reflective material such as a PTFE sheet to the inner surface of the housing 211 in order to increase the light collection efficiency of Cherenkov light, and it is preferable that the light receiving portion of the photodetector 22 is connected to the medium 212 via a UV-transmitting acrylic or the like that has a high transmittance of Cherenkov light and optical grease or the like that reduces reflection at the boundary surface.

[0017] As is well known, the medium 212 that emits Cherenkov light has a refractive index greater than 1. 16 It is particularly preferable to use water containing a large amount of O. Furthermore, it is preferable that the housing 211 is made of a material having a high transmittance for incident neutrons and emitted Cherenkov light (mainly light in the visible and ultraviolet regions). 12 As a material containing a large amount of C, a resin material such as acrylic is particularly preferably used.

[0018] Here, there are mainly two processes in which Cherenkov light is emitted by the incidence of neutrons. The first is a case in which neutrons are decelerated (scattered) in the medium 212 or the housing 211, and then gamma rays are emitted by a neutron capture reaction of hydrogen atomic nuclei, and as a result of Compton scattering of the gamma rays in the medium 212 or the housing 211, high-energy electrons are generated, and the speed of these electrons in the medium 212 or the housing 211 exceeds the speed of light in the medium 212 or the housing 211. The second is a case in which neutrons cause an inelastic scattering reaction in the medium 212 or the housing 211 to generate gamma rays, and as a result of Compton scattering of the gamma rays in the medium 212 or the housing 211, high-energy electrons are generated, and the speed of these electrons in the medium 212 or the housing 211 exceeds the speed of light in the medium 212 or the housing 211. In either case, the emitted photons are detected by the photodetector 22, and the above-mentioned pulse output is obtained. In this case, the pulse height and pulse area of ​​the pulse output obtained by the photodetector 22 correspond to the number of photons or the emission intensity at this time. This emission intensity depends on the energy of the high-energy electrons that emitted the Cherenkov light.

[0019] Here, when the neutron energy is high ( 16 O and 12 When the energy is above the reaction threshold for inelastic scattering of neutrons by C, 16 O and 12 Inelastic scattering of neutrons by C generates electrons with particularly high energy. 16 O and 12 When neutrons (with an energy equal to or greater than the reaction threshold for inelastic scattering of neutrons by C) are incident, Cherenkov light due to such high-energy electrons is detected by the photodetector 22, and by utilizing the fact that these inelastic scattering reactions are threshold reactions, high-energy neutrons (neutrons with an energy equal to or greater than the reaction threshold for inelastic scattering) are selectively recognized. This point will be explained below.

[0020] Here, what generates Cherenkov light in medium 212 and housing 211 are electrons that are generated by neutrons and have enough energy to generate Cherenkov light. When medium 212 is water, the energy threshold of electrons that generate Cherenkov light in this way is 0.264 MeV. When housing 211 is acrylic, the energy threshold of electrons that generate Cherenkov light in this way is 0.178 MeV. For this reason, the following mainly describes the situation in which such electrons are generated.

[0021] First, a mechanism for generating Cherenkov light when low-energy neutrons that do not cause the above-mentioned inelastic scattering are incident on the medium 212 (water) or the housing 211 (acrylic) will be described. In this case, when the neutrons are decelerated in the water or acrylic and captured by hydrogen nuclei, capture gamma rays (energy 2.223 MeV) are emitted. Compton scattering of these gamma rays in the water or acrylic generates electrons with a maximum energy of 1.994 MeV. Since this energy is higher than the above-mentioned 0.264 MeV or 0.178 MeV, these electrons emit Cherenkov light, which is detected by the photodetector 22. This makes it possible to detect the incidence of neutrons.

[0022] Figure 3 shows the neutron (n) and 16 The figure shows the neutron energy dependence of the reaction cross section (σ) of various reactions of O (denoted as (n, α), etc.) (Source: Nuclear Data Research Group (Japan Atomic Energy Agency) website (JENDL-5): https: / / wwwndc.jaea.go.jp / jendl / j5 / fig3 / n_008-O-016_f3.jpg modified). As shown in the figure, the reaction cross section σ of inelastic scattering has a threshold of 6.43 MeV for neutron energy, and is zero at energies lower than this and becomes larger at higher energies.

[0023] This inelastic scattering causes the emission of gamma rays with higher energy than the aforementioned captured gamma rays, and similarly to the above, high-energy electrons are generated by Compton scattering of these gamma rays. The energies of these electrons include those higher than 1.994 MeV in the aforementioned neutron capture reaction, corresponding to the energies of the gamma rays. Therefore, the emission intensity of Cherenkov light generated by these electrons includes those higher than the emission intensity of Cherenkov light in the aforementioned neutron capture reaction (when there is no inelastic scattering). For this reason, the existence of such electrons with energies higher than 1.994 MeV means that 16 This means that there was inelastic scattering of neutrons by O. The neutron energy threshold (6.43 MeV mentioned above) that causes this inelastic scattering is indicated by A in Fig. 13, which corresponds to the neutrons (secondary neutrons N2) with energies higher than 2.45 MeV that should be detected.

[0024] FIG. 4 shows the result of calculation of the time distribution of the flux (electron number density) of electrons having energy higher than 0.264 MeV that can generate Cherenkov light, which are generated immediately after the neutron incidence, taking into consideration the characteristics of FIG. 3. Here, the time of incidence of the neutron is set to time 0 s, and the energies of the neutrons are set to 2.45 MeV, 6.5 MeV, 8.5 MeV, and 14.1 MeV. The internal volume of the housing 211 (volume of water that becomes the medium 212) is set to 30 cm×30 cm×30 cm, and the housing 211 is made of an acrylic plate with a thickness of 5 mm. From this result, it can be confirmed that when the neutron energy is lower than the above-mentioned 6.43 MeV, the flux is small immediately after the incidence (within about 0.5 μs), whereas when the energy becomes 6.43 MeV or more, the flux increases. Such high-energy electrons can be recognized as Cherenkov light with high emission intensity. In addition, the generation of electrons due to the neutron capture reaction is delayed from the incidence of the neutrons by the time it takes for the high-energy neutrons to decelerate due to thermalization, so it hardly appears immediately after the incidence (within about 0.5 μs) as shown in Figure 4.

[0025] Figure 5 shows 12The characteristics are similar to those shown in Figure 3 for C (Source: Nuclear Data Research Group (Japan Atomic Energy Agency) HP (JENDL-5): https: / / wwwndc.jaea.go.jp / jendl / j5 / fig3 / n_006-C-012_f3.jpg modified). In this case, the neutron energy threshold is 4.812 MeV. This threshold energy is also higher than the 2.45 MeV shown in Figure 13, so 16 As in the case of O, 12 High-energy electrons generated by inelastic scattering from C can be used to identify neutrons with energies higher than 2.45 MeV in Figure 13. This threshold energy of 4.812 MeV is shown as B in Figure 13.

[0026] In Fig. 1, the primary neutron N1 component incident on the neutron detector 20 does not undergo the above-mentioned inelastic scattering, while the secondary neutron N2 component does undergo the above-mentioned inelastic scattering. Therefore, when an electron with an energy higher than 1.994 MeV is generated by the incidence of a neutron, it can be assumed that this neutron is a secondary neutron N2. That is, in the configuration of Fig. 1, when the analysis unit 30 recognizes Cherenkov light due to such high-energy electrons, it can be assumed that nuclear material is present in the sample 100.

[0027] Taking the above into consideration, the energy spectrum (relationship between electron energy and flux) of electrons generated by the incidence of neutrons of specific energies in the detection chamber 21 of Fig. 2 was calculated. Here, the internal volume of the housing 211 (volume of water that becomes the medium 212) was 30 cm x 30 cm x 30 cm, and the housing 211 was made of an acrylic plate with a thickness of 5 mm. Fig. 6 shows the results when monochromatic neutrons of energies (1) 2.45 MeV, (2) 6.4 MeV, and (3) 6.5 MeV were incident, and when 252 The calculation results are as follows, assuming that neutrons with the energy spectrum of neutrons emitted by spontaneous fission of Cf (a continuous distribution similar to that of fission neutrons emitted from fissionable materials) are incident on the device: (1) 2.45 MeV corresponds to the energy of the primary neutron N1.

[0028] From these results, the distributions below 1.994 MeV in the results of (1) to (4) almost overlap. This value of 1.994 MeV corresponds to the maximum energy of electrons produced by Compton scattering of capture gamma rays that occur when a neutron is captured by a hydrogen nucleus. Furthermore, in the region of energies greater than 1.994 MeV, the presence of electrons is significantly observed in (2) to (4), whereas it is not observed in (1). Therefore, the generation of electrons with energies greater than 1.994 MeV is due to the inelastic scattering of the above-mentioned neutrons, and can be distinguished from electrons generated by the primary neutron N1(1), which does not undergo inelastic scattering.

[0029] In addition, 6.4 MeV(2) is the above 12 Although it is larger than the neutron energy threshold for inelastic scattering of C (4.812 MeV), 16 This is smaller than the neutron energy threshold for inelastic scattering of O (6.43 MeV), and 6.5 MeV (3) is larger than both of these thresholds. For this reason, in Figure 6, the results of (2) and (3) diverge at approximately 4.2 MeV or higher, but this difference is mainly due to 16 This reflects the effect of inelastic scattering by O. In (4), the neutron energies are distributed continuously in the range including these thresholds, so a gradual distribution similar to (3) is obtained in the region of energies greater than 1.994 MeV.

[0030] From the above results, in the region where the electron energy is greater than 1.994 MeV, 12 Inelastic scattering by C 16 Both effects of inelastic scattering by O are reflected in the energy distribution of the electrons produced after the neutron is incident. In other words, the threshold energy of the electrons to be detected (electron energy threshold) is set to 1.994 MeV, and electrons with higher energies than this are generated by inelastic scattering of neutrons, i.e., in this case, it can be assumed that neutrons with energies of 4.812 MeV (or 6.43 MeV) or more were incident.

[0031] What was calculated above was the energy spectrum (relationship between flux and energy) of the electrons generated in detection cell 21 after neutron incidence, but what is actually observed is not the electrons themselves, but the Cherenkov light generated by these electrons. For this reason, four photomultiplier tubes with an aperture of 6.35 cm were installed on the top surface of detection cell 21 as described above as photodetectors 22, and the number of photons detected for each neutron incidence (emission intensity) was calculated. This result corresponds to a histogram of the emission intensity of the light detected by photodetector 22. Figure 7 shows the histogram of the emission intensity of the light detected by photodetector 22 for monochromatic neutrons (10 5 6, the distribution of the number of photons (emission intensity) detected after the incidence of neutrons (number of photons) is calculated. Reflecting the results of FIG. 6, the distribution is expanded toward the side of higher emission intensity as the neutron energy increases. From these results, for example, a threshold of about 40 photons is set, and if significant emission is detected on the side with more photons than this, it can be recognized that high-energy electrons caused by inelastic scattering as described above are present. The presence of such high-energy electrons means that secondary neutrons N2 (fission neutrons) have been emitted from sample 100 in FIG. 1, i.e., nuclear material is present in sample 100.

[0032] FIG. 7 corresponds to a histogram of the emission intensity obtained by detecting a large number of emission by the photodetector 22 within a certain measurement period, but the above judgment can also be made from the pulse output obtained by detecting one emission by the photodetector 22. FIG. 8 is a diagram showing this situation in a schematic manner. Here, it is assumed that pulse outputs (voltages) P1 and P2 are obtained from the photodetector 22. The number of photons in one emission corresponds to the pulse height or pulse area of ​​the pulse outputs P1 and P2. Here, when making this judgment using the pulse height, if a threshold value PHT for the pulse height is set corresponding to the threshold value of the number of photons (about 40), it can be estimated that the pulse output P2 whose pulse height PH2 exceeds PHT (high emission intensity) corresponds to the high energy electrons (nuclear material exists). Although the above description has been made regarding the discrimination using the pulse height, the discrimination can also be made by setting a threshold value using the pulse area in a similar manner. Such a pulse height or pulse area corresponds to the emission intensity of one emission (the number of photons in one emission). From the results of FIG. 7, this emission intensity corresponds to the energy of the electron that generated it. That is, the presence of high-energy neutrons can be detected by the intense light emitted by the high-energy electrons they produce.

[0033] The number of photons and the pulse height of the pulse output described above depend on the measurement conditions and the output circuit of the photodetector 22, and can be set appropriately according to the conditions. In this case, for example, a standard sample (with or without nuclear material) is used for measurement, and the judgment threshold for the emission intensity and pulse height described above can be set appropriately.

[0034] It has been shown above that the presence or absence of nuclear material in sample 100 can be determined based on the above principle. Next, the results of a similar simulation conducted to estimate the amount of nuclear material present in sample 100 (quantitative analysis) will be described.

[0035] 9 shows the results of calculating the flux of high-energy electrons (energy higher than the electron energy threshold of 1.994 MeV in FIG. 6) over time in the detection chamber 21 when the sample 100 is irradiated with pulsed (pulse width 50 μs) monoenergetic neutrons of 2.45 MeV as described above as primary neutrons N1. Here, the sample 100 is a highly enriched uranium sample with an enrichment level of 93%, and 235 The results were calculated for the periods when the primary neutron N1 was on (0-50 μs) and off (50-100 μs) for the U abundances of (9) 0 g, (10) 50 g, (11) 100 g, (12) 300 g, and (13) 500 g. From these results, it can be confirmed that high-energy electrons are generated in synchronization with the on / off of the primary neutron N1, and that the amount of generated electrons clearly depends on the amount of uranium present.

[0036] When a pulsed primary neutron N1 is used in this way, for example, only the number generated when the primary neutron N1 is on can be used as the number of high-energy electrons generated that have energies higher than 1.994 MeV, or by using the difference between the number generated when the primary neutron N1 is on and the number generated when the primary neutron N1 is off, a more accurate number of electrons can be obtained with reduced influence of background, etc. Figure 10 shows the integral value of the number of electrons generated during the period when the primary neutron N1 is on (0 to 50 μs) in the results of Figure 9 for a total of 5 s (10 5 The vertical axis shows the cumulative total of the pulses. 235 The horizontal axis shows the amount of U present (calibration curve). The number of primary neutrons N1 irradiated was 10 per second. 8The results show that the two are in a good proportional relationship, and the above-mentioned method can be used to not only determine the presence or absence of nuclear material, but also to quantitatively analyze the nuclear material. In FIG. 10, the vertical axis represents the number of (high-energy) electrons produced, but as described above, this can actually be recognized as the emission intensity recognized by the photodetector 22 or as part of its distribution. For this reason, in practice, a calibration curve can be obtained in which, for example, the integral value of the light intensity during the period when the primary neutron N1 is on, or the number of events with an emission intensity equal to or greater than a certain value during this period, is used as the vertical axis of FIG. 10 instead of the number of electrons produced. In this case, since the detected light intensity varies depending on the measurement conditions, etc., as described above, this calibration curve can be created using, for example, a standard sample.

[0037] In Fig. 9, the primary neutrons N1 are emitted in a pulsed manner, and the on-period and off-period of the primary neutrons N1 can be clearly distinguished. In this case, for example, as described above, the difference between the average number of electrons generated (or the integral value) during the pulse on-period and the average number of electrons generated (or the integral value) during the pulse off-period can be used as the number of electrons generated for quantification (corresponding to the vertical axis in Fig. 10), thereby improving the accuracy of quantitative analysis. Also, if the primary neutrons N1 are repeatedly emitted with a constant pulse width, the statistical error of the measurement results in each period can be reduced by integrating the results of the pulse on-period and the pulse off-period for each cycle in Fig. 9, thereby improving the accuracy of quantification in particular. Furthermore, even when using a neutron source that steadily emits primary neutrons N1, by knowing in advance the number of neutrons generated when no nuclear material is present (i.e. when only 2.45 MeV neutrons are incident on the neutron detector), it becomes possible to detect nuclear material by recognizing a significant increase in the number of neutrons generated when nuclear material is present, and the amount of nuclear material can be quantified from the increase in the number of neutrons generated. In other words, a neutron source 10 that steadily emits (with constant intensity over time) or pulsed primary neutrons N1 is particularly effective for quantitative analysis.

[0038] Figure 11 shows the results of Figure 4 (time course of electron flux generated by neutrons) enlarged on a shorter time scale. As mentioned above, the electrons generated by neutrons with energies of 6.5 MeV or more are mainly generated by inelastic scattering of neutrons. As shown here, these electrons are mainly generated within a short time of 10 ns from the point of incidence (origin). For this reason, when the primary neutrons N1 are pulsed as described above, the transition time only needs to be 10 ns or more, and by using such pulsed primary neutrons N1, the above measurements can be performed with high precision.

[0039] On the other hand, in the results of Figure 9, 235 When the amount of U present is large, the number of electrons generated during the off period is negligible compared to the number of electrons generated during the on period, so similar results can be obtained without using the measurement results from the off period. Also, a constant value corresponding to the background component can be preset, and quantitative analysis can be performed using the difference between this constant value. In this case, the primary neutrons N1 may be emitted continuously (or steadily) as described above, rather than in a pulsed manner. In this case, a DD neutron source that emits primary neutrons N1 continuously (or steadily) can also be used as the neutron source 10. Also, in the above, as an example of nuclear material, 235 We mentioned the results of U. 235 Like U, it is a nuclear material or fissile material that undergoes a nuclear fission reaction when irradiated with neutrons. 239 Pu, 233 U, 241 Am, 237 Np, 243 It is clear that it is similarly possible to determine the presence or absence and estimate the amount of Am, etc. (quantitative analysis).

[0040] Furthermore, in the above example, monoenergetic neutrons of 2.45 MeV are used as the primary neutrons N1, but as long as the detection results of Cherenkov light from neutrons with energies of 4.812 MeV (or 6.43 MeV) or more as described above can be significantly distinguished from the detection results of Cherenkov light from neutrons with lower energies that do not cause inelastic scattering, it is not necessary to use monoenergetic neutrons as the primary neutrons N1. In other words, as long as the same analysis method is applicable, a neutron source other than a DD neutron source may be used.

[0041] The above-mentioned configuration uses a neutron detector that detects Cherenkov light emitted by neutrons, and an analysis unit that detects high-energy neutrons or nuclear material by the above-mentioned analysis. Here, the configuration of this neutron detector is the same as that of a conventionally known device, and 16 O and 12 The same is true for the use of C, and it is clear that this allows neutrons to be detected with higher sensitivity than the techniques described in Non-Patent Documents 1 and 2.

[0042] In addition, the above configuration detects high-energy electrons generated by inelastic scattering, which is different from the conventional 3 He, 10 B. 6 Unlike neutron detectors that use neutron capture reactions with Li etc., no moderator is required to detect high-energy neutrons. This makes it easy to make the nuclear material detection device smaller and lighter, and it can be transported to perform measurements at various locations.

[0043] On the other hand, the processing time required for the above analysis is short, and the analysis unit that detects high-energy neutrons and nuclear material by the above analysis can be easily realized using a normal computer. In other words, the nuclear material detection device 1 can be easily obtained.

[0044] Also, 16 O and 12Since there is a threshold value for the energy of neutrons in the inelastic scattering of neutrons, not only for C, it is possible to detect high-energy electrons (high-energy neutrons) by appropriately adding an element (nuclide) that causes such inelastic scattering to the detection tank 21 (medium 212, housing 211) in accordance with the same principle. Accordingly, the energy of the primary neutrons (2.45 MeV) and the electron energy threshold value are also appropriately set.

[0045] The present invention has been described above based on an embodiment. This embodiment is merely an example, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components, and that such modifications are also within the scope of the present invention. [Explanation of symbols]

[0046] 1. Nuclear material detection device 10, 90 neutron source 20, 91 Neutron detector 21 Detection tank 22 Photodetector 30 Analysis Department 40 Neutron detection equipment 100 samples 211 Cabinet 212 Medium C Cherenkov light N Neutron N1 primary neutron N2 secondary neutron

Claims

1. A neutron detection device that detects neutrons by detecting Cherenkov light emitted by neutrons in a medium, comprising: a detection chamber provided with the medium; a photodetector for detecting the Cherenkov light emitted from the detection chamber; an analysis unit that recognizes the high-energy neutrons by recognizing a pulse output corresponding to the Cherenkov light emitted in the detection chamber by electrons having energy higher than a predetermined electron energy threshold from a pulse output obtained by the photodetector detecting the Cherenkov light; A neutron detection device comprising:

2. the electron energy threshold is an energy corresponding to a maximum energy of electrons formed when the neutrons cause a neutron capture reaction in the detection chamber; The neutron detection device according to claim 1, characterized in that the analysis unit recognizes the neutrons having energy equal to or greater than a threshold energy at which the neutrons cause an inelastic scattering reaction in the detection cell by recognizing the pulse output corresponding to the Cherenkov light emitted in the detection cell by electrons having energy higher than the electron energy threshold.

3. The medium is １６ 3. The neutron detection device according to claim 1, further comprising O.

4. In the detection chamber, the medium is １２ 3. The neutron detection device according to claim 1, wherein the neutron detection device is accommodated in a housing made of a C-containing material.

5. A nuclear material detection device for detecting nuclear material contained in a sample, comprising: a neutron source for emitting primary neutrons to the sample such that the nuclear material emits secondary neutrons; A neutron detection device according to claim 1 or 2; Equipped with The analysis unit detects the presence of the nuclear material in the sample by recognizing the pulse output corresponding to the Cherenkov light emitted in the medium by electrons having energy higher than the electron energy threshold.

6. The neutron source emits the primary neutrons in a pulsed on-off manner, The nuclear material detection device according to claim 5, characterized in that the analysis unit detects the presence of the nuclear material or calculates the amount of the nuclear material by using a difference between the measurement result of the neutron detection device when the primary neutrons are on and the measurement result of the neutron detection device when the primary neutrons are off.

7. 7. The nuclear material detection device according to claim 6, wherein the medium is water, a DD neutron source is used as the neutron source, and the electron energy threshold is set to 1.994 MeV.

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