Neutron detectors, nuclear material detectors

Abstract

To obtain 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 using the TENA method. A neutron source 10 emits primary neutrons N1 toward the sample 100, and a neutron detector 20 detects neutrons (primary neutrons N1 and secondary neutrons N2) emitted from the sample 100. In addition, an analysis unit 30 is used to recognize high-energy neutrons by analyzing the detection results of the neutron detector 20. When the neutron energy is high, 16 O or 12 Inelastic scattering of neutrons by carbon generates particularly high-energy electrons. This neutron detector 40 recognizes high-energy neutrons by utilizing the fact that when particularly high-energy neutrons are incident, the Cherenkov light produced by these high-energy electrons is detected by a photodetector.

Description

[Technical Field]

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

[0002] To prevent the proliferation of nuclear weapons, technologies are needed to detect, for example, concealed nuclear materials (or fissile materials). For this reason, various technologies have been proposed, such as those described in Non-Patent Document 1. Among these, threshold energy neutron analysis (TENA) is effective, as it involves irradiating a sample with highly permeable neutrons and detecting neutrons emitted from the nuclear material within the sample.

[0003] Figure 12 is a simplified diagram showing the measurement configuration in the TENA method. Here, when a neutron (primary neutron N1) emitted from the neutron source 90 is incident on the sample 100 under test, the nuclear material in the sample 100 undergoes a nuclear fission reaction due to the primary neutron N1, thereby generating a new neutron (secondary neutron N2). When the neutron detector 91 detects the neutron emitted from the sample 100, this neutron consists of the primary neutron N1 that has passed through or been scattered by the sample 100, and the secondary neutron N2 generated in the sample 100. If the secondary neutron N2 is detected, it is recognized that the nuclear material that generated this secondary neutron N2 is present in the sample 100.

[0004] In the neutron detector 91, primary neutrons N1 and secondary neutrons N2 are detected without distinction as neutrons; therefore, a configuration that can distinguish between them is required. For this reason, a DD neutron source utilizing the reaction of deuterium atoms is used as the neutron source 90, in which case the energy of primary neutrons N1 is 2.45 MeV (monochromatic). An example of the energy spectrum of secondary neutrons N2 in this case is shown in Figure 13. Here, the low-energy components below 2.45 MeV include primary neutrons N1 (or primary neutrons N1 that have been scattered and slowed down) as well as secondary neutrons N2, whereas neutrons with energies higher than 2.45 MeV are not included in primary neutrons N1. Therefore, if neutrons with energies higher than 2.45 MeV can be significantly recognized, the presence of nuclear material in the sample 100 can be recognized. However, as can be seen from Figure 13, the amount of such high-energy neutrons is less than that of low-energy neutrons, and their integrated amount is 30% of the total. To achieve this, the neutron detector 91 is required to be able to detect neutrons in a specific high-energy range (higher than 2.45 MeV in the example in Figure 13) with high sensitivity.

[0005] In contrast, a neutron detector capable of detecting neutrons is, for example, 3 He detector, 10 B detector, 6 Li detectors are known, and in these, 3 He, 10 B, 6It utilizes the fact that Li has a large neutron reaction cross - section. Also, similar to γ - rays, etc., organic scintillation detectors (plastic scintillation detectors, liquid scintillation detectors) are also used for neutron detection. In this case, a substance that emits fluorescence upon neutron incidence is used. Further, for example, the Tension Metastable Fluid Detector (TMFD) described in Non - Patent Document 2 detects neutrons by utilizing the fact that a fluid with applied tension undergoes cavitation upon neutron incidence. Since the energy of the neutrons to be detected can be set by setting this tension, it is also possible to selectively detect neutrons with energies higher than 2.45 MeV as described above.

Prior Art Documents

Non - Patent Documents

[0006]

Non - Patent Document 1

Non - Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] 3 He, 10 B, 6In a neutron detector using a material with a large neutron reaction cross section such as Li, the large reaction cross section of these materials for thermal neutrons (energy ~0.025 eV) is utilized. Therefore, when detecting high-energy neutrons, it is necessary to slow them down. In this case, as shown in FIG. 13, it has become difficult to selectively detect neutrons with an energy higher than 2.45 MeV. In addition, in organic scintillation detectors such as plastic scintillation detectors and liquid scintillation detectors, high-energy neutrons (energy ~1 MeV or higher) can be mainly detected using elastic scattering reactions. However, due to the low reaction cross section of the elastic scattering reaction, there is a problem that the detection efficiency of such high-energy neutrons is low. Furthermore, similar to the detectors using 3 He, 10 B, 6 Li, etc., it has also become difficult to selectively detect neutrons with an energy higher than 2.45 MeV in these organic scintillation detectors. In TMFD, selective detection of high-energy neutrons can be performed well. However, since the detection efficiency itself is still low, long-time measurement is required for high-precision measurement.

[0008] Therefore, there has been a demand for a neutron detection device that can selectively detect high-energy neutrons with high sensitivity, and a highly sensitive nuclear material detection device using the same.

[0009] The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

Means for Solving the Problems

[0010] In order to solve the above problems, the present invention has the following configuration. The present invention relates to a neutron detection device for detecting 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; and an analysis unit for recognizing high-energy neutrons by recognizing the pulse output corresponding to the Cherenkov light emitted in the detection chamber by electrons having an energy higher than a predetermined electron energy threshold, based on the pulse output obtained when the photodetector detects the Cherenkov light. The electron energy threshold is the energy corresponding to the maximum energy of the electron formed when the neutron undergoes a neutron capture reaction in the detection chamber, and the analysis unit may recognize the neutron having an energy greater than or equal to the threshold energy at which the neutron undergoes an inelastic scattering reaction in the detection chamber by recognizing the pulse output corresponding to the Cherenkov light emitted in the detection chamber by electrons having an energy higher than the electron energy threshold. The aforementioned medium is 16 It may contain O. In the detection tank, the medium is 12 It may be housed in a casing made of a material containing C. The present invention relates to a nuclear material detection device for detecting nuclear material contained in a sample, comprising a neutron source that emits primary neutrons into the sample such that the nuclear material emits secondary neutrons, and a neutron detection device, wherein 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 an energy higher than the electron energy threshold. The neutron source emits primary neutrons in a pulsed manner, repeatedly switching on and off. 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 detector when the primary neutrons are on and the measurement result of the neutron detector when the primary neutrons are off. The medium is water, a DD neutron source is used as the neutron source, and the electron energy threshold may be 1.994 MeV. [Effects of the Invention]

[0011] As the present invention is configured as described above, a neutron detection device capable of detecting high-energy neutrons (neutrons with energies above the threshold energy of inelastic scattering reactions) with high sensitivity and selectivity, and a highly sensitive nuclear material detection device using the same can be obtained. [Brief explanation of the drawing]

[0012] [Figure 1] This diagram shows the configuration of a nuclear material detection device according to an embodiment of the present invention. [Figure 2] This figure shows the configuration of a neutron detector used in a nuclear material detection device according to an embodiment of the present invention. [Figure 3] This shows the neutron energy dependence of the reaction cross-sections for various reactions between neutrons and 16O. [Figure 4] This is the result of calculating the time course of the flux of electrons with energies higher than 0.264 MeV by changing the energy of the incident neutrons. [Figure 5] This shows the neutron energy dependence of the reaction cross-sections for various reactions between neutrons and 12C. [Figure 6] This shows the energy spectrum of electrons generated in the detector chamber by neutron incidence, calculated by varying the neutron energy, while considering inelastic scattering. [Figure 7] This is a histogram of the number of photons of Cherenkov radiation detected from the detector chamber by neutron incidence, considering inelastic scattering, calculated by varying the neutron energy. [Figure 8] This diagram schematically illustrates a method for recognizing high-energy electrons using a threshold value for the pulse output from a photodetector. [Figure 9] This shows the time course of the electron flux with energy higher than 1.994 MeV in the detection chamber when the sample was irradiated with pulsed neutrons (pulse width 50 μs), calculated for each amount of 235 U present in the sample. [Figure 10]Figure 9 shows the results (calibration curve) of calculating the amount of electrons with energies higher than 1.994 MeV generated during neutron irradiation (0-50 μs) over a total of 5 seconds (105 pulses), and the abundance of 235U in the sample. [Figure 11] This figure shows the time course of electron flux at energies higher than 0.264 MeV, as the energy of the incident neutrons is varied, on a shorter timescale than Figure 4. [Figure 12] This diagram shows a simplified configuration of the measurement in the TENA method, which is one 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. [Modes for carrying out the invention]

[0013] The nuclear material detection device 1 according to an embodiment of the present invention detects nuclear material by the TENA method similar to that shown in Figure 12. Figure 1 shows a simplified representation of this configuration. Here, as in Figure 12, a neutron source 10 that emits primary neutrons N1 toward the sample 100 and a neutron detector 20 that detects neutrons (primary neutrons N1 and secondary neutrons N2) emitted from the sample 100 are used. Furthermore, an analysis unit 30, 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, is used. As the neutron source 10, a DD neutron source that can control and emit 2.45 MeV monochromatic neutrons in a pulsed manner is particularly preferred.

[0014] The neutron detector 20 used here is configured to distinguish and detect high-energy neutrons from low-energy neutrons as shown in Figure 13, and the analysis unit 30 detects high-energy neutrons with high sensitivity according to the characteristics of the neutron detector 20. Therefore, the neutron detector 20 and the analysis unit 30 are combined to form the neutron detection device 40. This neutron detection device 40 can also be used independently of the detection of nuclear material.

[0015] The following will first describe the configuration of the neutron detector 20 and the phenomena that occur when neutrons are incident on it. Figure 2 schematically shows the configuration of the neutron detector 20. In this neutron detector 20, the Cherenkov light C emitted in the detection chamber 21 by the incident neutron N is detected. Therefore, the neutron detector 20 comprises a detection chamber 21 into which the neutron to be detected is incident, and a photodetector 22 that detects the Cherenkov light generated by the incident neutrons on the detection chamber 21.

[0016] The detection chamber 21 is constructed by housing a liquid (medium 212) that emits Cherenkov light in response to neutron incidence within a housing 211. As the photodetector 22, a photomultiplier tube or silicon photomultiplier (SiPM) can be used, which detects Cherenkov light, such as visible light or ultraviolet light, with each emission and emits a pulsed output (pulse output). In Figure 2, one photodetector 22 is used for a single detection chamber 21, but in practice, it is preferable to arrange and use multiple photodetectors 22 to detect Cherenkov light with high efficiency. Furthermore, it is preferable to attach a reflective material such as a PTFE sheet to the inner surface of the housing 211 to improve the collection efficiency of Cherenkov light, and it is preferable to connect the light-receiving part of the photodetector 22 to the medium 212 via an optical grease or the like that reduces reflection at the interface with UV-transmitting acrylic or the like, which has a high transmittance of Cherenkov light.

[0017] Here, as is well known, the medium 212 that emits Cherenkov light is one in which the refractive index is greater than 1, but in particular 16 Water containing a large amount of oxygen is particularly preferred. Furthermore, the housing 211 is preferably made of a material with high transmittance to incident neutrons and emitted Cherenkov light (mainly visible and ultraviolet light), but here in particular 12 As a material containing a large amount of carbon, resin materials such as acrylic are particularly preferred.

[0018] There are two main processes by which Cherenkov light is emitted upon the incidence of a neutron. The first is when a neutron is slowed (scattered) by the medium 212 or housing 211, then a gamma ray is emitted by the neutron capture reaction of a hydrogen nucleus, and as a result of Compton scattering of the gamma ray in the medium 212 or housing 211, a high-energy electron is generated, and the velocity of this electron in the medium 212 or housing 211 exceeds the speed of light in the medium 212 or housing 211. The second is when a neutron undergoes an inelastic scattering reaction in the medium 212 or housing 211, generating a gamma ray, and as a result of Compton scattering of the gamma ray in the medium 212 or housing 211, a high-energy electron is generated, and the velocity of this electron in the medium 212 or housing 211 exceeds the speed of light in the medium 212 or housing 211. In either case, the emitted photon is detected by the photodetector 22, and the aforementioned 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 that time. This emission intensity depends on the energy of the high-energy electrons that emitted Cherenkov light.

[0019] Here, if the neutron energy is high ( 16 O or 12 (If the energy is above the reaction threshold for inelastic scattering of neutrons by C) 16 O or 12 Inelastic scattering of neutrons by C generates particularly high-energy electrons. In this neutron detector 40, particularly high-energy ( 16 O or 12 When a neutron (with an energy above the reaction threshold for inelastic scattering of neutrons by C) is incident, Cherenkov light from such high-energy electrons is detected by the photodetector 22. By utilizing the fact that these inelastic scattering reactions are threshold reactions, high-energy neutrons (neutrons with an energy above the reaction threshold energy for inelastic scattering) are selectively recognized. This point will be explained below.

[0020] Here, the Cherenkov radiation in the medium 212 and enclosure 211 is generated by electrons produced by neutrons that possess sufficient energy to produce Cherenkov radiation. When the medium 212 is water, the energy threshold for electrons that produce Cherenkov radiation in this way is 0.264 MeV. When the enclosure 211 is acrylic, the energy threshold for electrons that produce Cherenkov radiation in this way is 0.178 MeV. Therefore, the following explanation will primarily focus on the circumstances of such electron generation.

[0021] First, we will explain the mechanism leading to the generation of Cherenkov light when low-energy neutrons, which do not cause the inelastic scattering described above, are incident on the medium 212 (water) or the enclosure 211 (acrylic). In this case, when the neutron is slowed down in the water or acrylic and captured by a hydrogen nucleus, a capture gamma ray (energy 2.223 MeV) is emitted. The Compton scattering of this gamma ray in the water or acrylic generates an electron with a maximum energy of 1.994 MeV. Since this energy is higher than the aforementioned 0.264 MeV and 0.178 MeV, this electron emits Cherenkov light, which is detected by the photodetector 22. This allows for the detection of the incident neutron.

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

[0023] This inelastic scattering emits gamma rays with higher energy than the aforementioned capture gamma rays, and similarly, the Compton scattering of these gamma rays generates high-energy electrons. The energy of these electrons includes those higher than the 1.994 MeV of the neutron capture reaction, corresponding to the energy of the gamma rays. Therefore, the emission intensity of the Cherenkov light generated by these electrons includes those higher than the emission intensity of Cherenkov light in the case of the neutron capture reaction (without inelastic scattering). Thus, the presence of electrons with energies higher than 1.994 MeV indicates that 16 This means that inelastic scattering of neutrons occurred due to O. The neutron energy threshold that causes this inelastic scattering (6.43 MeV mentioned above) is shown as A in Figure 13, and this corresponds to neutrons with higher energies than the 2.45 MeV that should be detected (secondary neutrons N2).

[0024] Figure 4 shows the time distribution of the flux (electron number density) of electrons with energies higher than 0.264 MeV that can produce Cherenkov light, calculated immediately after neutron incidence, taking into account the characteristics of Figure 3. Here, the neutron incidence time is set to time 0 s, and the neutron energies 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 which becomes the medium 212) is set to 30 cm × 30 cm × 30 cm, and the housing 211 is assumed to be made of 5 mm thick acrylic plate. From these results, it can be confirmed that when the neutron energy is lower than the aforementioned 6.43 MeV, this flux is small in the time immediately after incidence (within approximately 0.5 μs), while when the energy is 6.43 MeV or higher, this flux increases. Such high-energy electrons can be recognized as Cherenkov light with high emission intensity. Furthermore, the generation of electrons resulting from the neutron capture reaction is delayed by the amount of time required for the high-energy neutron to heat up after the neutron incidence, so it is hardly visible immediately after incidence (within approximately 0.5 μs) as shown in Figure 4.

[0025] Figure 5 shows 12It exhibits similar characteristics to those shown in Figure 3 for C (Source: Modified from 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). In this case, the neutron energy threshold is 4.812 MeV. This threshold energy is also higher than 2.45 MeV in Figure 13, therefore, as mentioned above... 16 As with case O 12 High-energy electrons generated by inelastic scattering by 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 Figure 1, the primary neutron N1 component incident on the neutron detector 20 does not undergo the inelastic scattering described above, while the secondary neutron N2 component does. Therefore, if electrons with energies higher than 1.994 MeV are generated by the incidence of neutrons, these neutrons can be estimated to be secondary neutrons N2. In other words, in the configuration shown in Figure 1, if the analysis unit 30 recognizes Cherenkov light from such high-energy electrons, it can be estimated that nuclear material is present in the sample 100.

[0027] Considering the above points, the energy spectrum (relationship between electron energy and flux) of electrons generated by the incidence of neutrons of specific energies was calculated in the detection tank 21 in Figure 2. Here, the internal volume of the housing 211 (volume of water which becomes the medium 212) was set to 30 cm × 30 cm × 30 cm, and the housing 211 was assumed to be made of 5 mm thick acrylic plate. Figure 6 shows the case when monochromatic neutrons with energies of (1) 2.45 MeV, (2) 6.4 MeV, and (3) 6.5 MeV were incident, and (4) 252 This calculation is based on the case where neutrons with an energy spectrum similar to that of fission neutrons emitted from fission material (a continuous distribution) are incident on Cf. (1) 2.45 MeV corresponds to the energy of the primary neutron N1.

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

[0029] Furthermore, 6.4 MeV(2) is the above 12 Although it is greater than the neutron energy threshold for inelastic scattering of C (4.812 MeV), 16 The neutron energy threshold for inelastic scattering of O (6.43 MeV) is lower, and 6.5 MeV (3) is higher than both of these thresholds. For this reason, in Figure 6, the results of (2) and (3) diverge above approximately 4.2 MeV, and this difference is mainly due to 16 This reflects the effect of inelastic scattering by O. In (4), since the neutron energies are continuously distributed within the range that includes these thresholds, a gentle distribution similar to (3) is obtained in the region where the energy is greater than 1.994 MeV.

[0030] Based on 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 electrons generated after neutron incidence. That is, if we set the threshold energy for the electrons to be detected (electron energy threshold) to 1.994 MeV, electrons with higher energies than this can be estimated to have been generated by inelastic scattering of neutrons, i.e., in this case, neutrons with energies of 4.812 MeV (or 6.43 MeV) or higher were incident.

[0031] The calculation above was for the energy spectrum (relationship between flux and energy) of electrons generated in the detection chamber 21 after neutron incidence. However, what is actually observed is not the electrons themselves, but the Cherenkov light generated by these electrons. Therefore, when four 6.35 cm aperture photomultiplier tubes were installed on the top surface of the detection chamber 21 as described above, 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 the photodetector 22. Figure 7 shows the histogram of monochromatic neutrons with energies of (5) 2.45 MeV, (6) 6.0 MeV, (7) 8.0 MeV, and (8) 10 MeV. 5 This is the result of calculating the distribution of the number of photons (emission intensity) detected after the incidence of neutrons. Reflecting the results in Figure 6, a distribution is obtained in which the emission intensity is higher as the neutron energy increases. From this result, for example, if a significant amount of emission is detected on the side with a higher number of photons than approximately 40 photons, it can be recognized that high-energy electrons due to inelastic scattering as described above were present. The presence of such high-energy electrons means that secondary neutrons N2 (fission neutrons) were emitted from sample 100 in Figure 1, that is, nuclear material is present in sample 100.

[0032] Figure 7 corresponds to a histogram of emission intensity obtained by detecting numerous emission events with the photodetector 22 within a certain measurement period. However, the above determination can also be made from the pulse output obtained by detecting a single emission event with the photodetector 22. Figure 8 schematically illustrates this situation. Here, it is assumed that pulse outputs (voltages) P1 and P2 are obtained from the photodetector 22. The number of photons in the aforementioned single emission corresponds to the pulse height or pulse area of ​​these pulse outputs P1 and P2. When making this determination using pulse height, if a threshold PHT for pulse height is set corresponding to the aforementioned threshold for the number of photons (approximately 40), then a pulse output P2 where the pulse height PH2 exceeds PHT (high emission intensity) can be estimated to correspond to the aforementioned high-energy electrons (the presence of nuclear material). The above describes identification using pulse height, but the same identification can be made by setting a threshold using pulse area. Such pulse height or pulse area corresponds to the emission intensity of a single emission (number of photons in a single emission). From the results in Figure 7, this emission intensity corresponds to the energy of the electrons that generated it. In other words, the presence of high-energy neutrons can be detected by the high-intensity light emitted by the high-energy electrons produced by them.

[0033] The number of photons and pulse height of the pulse output mentioned 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, measurements can be performed using standard samples (with and without nuclear material), and the threshold values ​​for determining the emission intensity and pulse height can be set accordingly.

[0034] The above demonstrates that the presence or absence of nuclear material in sample 100 can be determined using the principle described above. Next, we will explain the results of a similar simulation to estimate the amount of nuclear material present in sample 100 (quantitative analysis).

[0035] Figure 9 shows the calculated time course of the flux of high-energy electrons (energy higher than the electron energy threshold of 1.994 MeV in Figure 6) in the detector chamber 21 when sample 100 was irradiated with a 2.45 MeV monochromatic neutron as a primary neutron N1 in pulse form (pulse width 50 μs) as described above. Here, sample 100 is a highly enriched uranium sample with an enrichment degree of 93%, and in sample 100 235 For the cases where the abundance of U is (9) 0g, (10) 50g, (11) 100g, (12) 300g, and (13) 500g, the results were calculated from the on period (0-50μs) to the off period (50-100μs) of the primary neutron N1. From these results, it can be confirmed that high-energy electrons are generated in sync with the on / off state of the primary neutron N1, and that the amount of these electrons generated clearly depends on the abundance of uranium.

[0036] When using pulsed primary neutrons N1 in this way, the number of high-energy electrons with energies higher than 1.994 MeV can be calculated by using, for example, only the number of electrons generated when primary neutron N1 is on, or by using the difference between the number of electrons generated when primary neutron N1 is on and the number of electrons generated when primary neutron N1 is off, thereby obtaining a more accurate number of electrons with reduced background influences. Figure 10 shows the integral value of the number of electrons generated during the period when primary neutron N1 is on (0 to 50 μs) in the results of Figure 9, totaled over 5 s (10 5 The vertical axis represents the cumulative value (of pulses). 235 This is a calibration curve showing the abundance of U on the horizontal axis. The number of primary neutron N1 irradiations is 10 per second. 8The results showed a good proportional relationship between the two, and the simulation demonstrated that the above method can be used not only to determine the presence or absence of nuclear material but also to perform quantitative analysis of nuclear material. In Figure 10, the vertical axis represents the number of (high-energy) electrons produced, but as mentioned above, this can actually be recognized as part of the emission intensity or its distribution as detected by the photodetector 22. Therefore, in practice, a calibration curve can be obtained using, 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 above a certain value during this period, as the vertical axis of Figure 10 instead of the number of electrons produced. In this case, since the detected light intensity changes depending on the measurement conditions, etc., as mentioned above, this calibration curve can be created using, for example, a standard sample.

[0037] In Figure 9, primary neutrons N1 are emitted in a pulsed manner, and the on and off periods of primary neutrons N1 are clearly distinguishable. In this case, for example, the difference between the average number of electrons generated (or integral value) during the on period and the average number of electrons generated (or integral value) during the off period is used as the number of electrons generated for quantification (corresponding to the vertical axis in Figure 10), thereby improving the accuracy of quantitative analysis. Furthermore, if the primary neutrons N1 are repeatedly emitted with a constant pulse width, the statistical error in the measurement results for each period can be reduced by integrating the results of the on and off periods for each period in Figure 9, thereby particularly improving the accuracy of quantification. Furthermore, even when using a neutron source that steadily emits primary neutrons N1, by knowing the number of neutrons produced 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 produced when nuclear material is present, and it also becomes possible to quantify the amount of nuclear material from the increase in the number of neutrons produced. In other words, a neutron source 10 that emits steady-state (constant intensity over time) or pulsed primary neutrons N1 is particularly effective for quantitative analysis.

[0038] Figure 11 shows the result of Figure 4 (time course of electron flux generated by neutrons) magnified on a shorter timescale. As mentioned above, electrons generated here by neutrons with energies of 6.5 MeV or higher are mainly produced by inelastic scattering of neutrons. As shown here, these electrons are mainly produced in a short time, within 10 ns from the incident point (origin). Therefore, when the primary neutron N1 is pulsed as described above, its transition time only needs to be 10 ns or longer, and by using such pulsed primary neutron N1, the above measurement can be performed with high precision.

[0039] On the other hand, in the results shown in Figure 9, 235 When the abundance of U is high, 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. Alternatively, a constant value corresponding to the background component can be set in advance, and quantitative analysis can be performed using the difference between this constant value and the actual value. In this case, the primary neutron N1 may be emitted continuously (or steadily) as described above, rather than in a pulsed manner. In this case, a DD neutron source that continuously (or steadily) emits primary neutron N1 can be used as the neutron source 10. Furthermore, the above uses nuclear material as an example. 235 I mentioned the results for U, 235 Like U, it is a nuclear material or fissile material that undergoes a nuclear fission reaction upon irradiation with neutrons. 239 Pu, 233 U, 241 Am, 237 Np, 243 It is clear that the same determination of presence or absence and estimation of abundance (quantitative analysis) are possible for Am and other substances.

[0040] Furthermore, in the above example, a monochromatic neutron of 2.45 MeV was used as the primary neutron N1. However, as long as the detection results of Cherenkov radiation using neutrons with energies of 4.812 MeV (or 6.43 MeV) or higher can be significantly distinguished from the detection results of Cherenkov radiation using neutrons with lower energies that do not cause inelastic scattering, it is not necessary to use a monochromatic neutron as the primary neutron N1. In other words, as long as similar analytical methods can be applied, neutron sources other than DD neutron sources may be used.

[0041] The above configuration uses a neutron detector that detects Cherenkov light emitted by neutrons, and an analysis unit that detects high-energy neutrons or nuclear material through the above analysis. Here, the configuration of this neutron detector is the same as that known conventionally, and here 16 O or 12 The same applies to the use of carbon (C), and it is clear that this method allows for highly sensitive detection of neutrons compared to the technologies described in Non-Patent Documents 1 and 2.

[0042] Furthermore, in the above configuration, high-energy electrons generated by inelastic scattering are detected, 3 He, 10 B, 6 Unlike neutron detectors that use neutron capture reactions with materials such as lithium, moderators are not required for detecting high-energy neutrons. Therefore, it is easy to miniaturize and lighten the nuclear material detection device, and it can be transported to various locations for measurements.

[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 through the above analysis can be easily realized using a conventional computer. In other words, the nuclear material detection device 1 can be easily obtained.

[0044] Also, 16 O or 12Not limited to the case of C, a threshold exists for neutron energy in inelastic scattering of neutrons. Therefore, by appropriately adding elements (nuclides) that cause such inelastic scattering to the detector tank 21 (medium 212, housing 211), high-energy electrons (high-energy neutrons) can be detected by the same principle. Accordingly, the primary neutron energy (2.45 MeV) and the electron energy threshold are set as appropriate.

[0045] The present invention has been described above based on embodiments. These embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in the combination of these components, and that such modifications also fall within the scope of the present invention. [Explanation of symbols]

[0046] 1. Nuclear material detection device 10, 90 neutron source 20, 91 Neutron detectors 21 Detection tank 22 Photodetector 30 Analysis Department 40 Neutron detection device 100 samples 211 enclosure 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, A detection tank provided with the aforementioned medium, A photodetector for detecting the Cherenkov light emitted from the detection chamber, An analysis unit recognizes high-energy neutrons by recognizing the pulse output obtained when the photodetector detects the Cherenkov light, and the pulse output corresponding to the Cherenkov light emitted in the detection chamber by electrons with energy higher than a predetermined electron energy threshold, A neutron detection device characterized by comprising the following:

2. The electron energy threshold is the energy corresponding to the maximum energy of the electron formed when the neutron undergoes a neutron capture reaction in the detector. The neutron detection apparatus according to claim 1, characterized in that the analysis unit recognizes the pulse output corresponding to the Cherenkov light emitted in the detection chamber by electrons having an energy higher than the electron energy threshold, thereby recognizing neutrons having an energy above the threshold energy at which neutrons undergo an inelastic scattering reaction in the detection chamber.

3. The aforementioned medium is １６ The neutron detection device according to claim 1 or 2, characterized in that it contains O.

4. In the detection tank, the medium is １２ The neutron detection device according to claim 1 or 2, characterized in that it is housed in a casing made of a substance containing C.

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

6. The neutron source emits the primary neutrons in a pulsed manner, which is repeatedly switched on and off. 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 using the difference between the measurement result of the neutron detection device when the primary neutron is on and the measurement result of the neutron detection device when the primary neutron is off.

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

Images