JP2013130418A - Nuclear material detection device and nuclear material detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a nuclear material in a non-contact and non-destructive manner with high accuracy.SOLUTION: A witness foil 20 is irradiated with transmitted γ rays 12 transmitted through a specimen 100. The witness foil 20 is a membrane containing a large quantity of nuclear species (e.g., Pu-239) subjected to detection. According to a nuclear material detection method, neutron rays 13 emitted by the witness foil 20 are detected by a neutron detector 30. The neutron rays 13 are emitted when photofission occurs in atomic nuclei of the nuclear species contained in the witness foil 20 and subjected to detection. The incident γ rays 11 are set such that photofission occurs in the atomic nuclei of the nuclear species subjected to detection.

Description

  The present invention relates to a nuclear material detection apparatus and a nuclear material detection method for detecting nuclear fuel material (nuclear material) with high accuracy, non-contact and non-destructive.

  Nuclear fuel materials (nuclear materials) used in nuclear power can also be used in weapons, and there is concern that such weapons will be used in terrorist activities. For this reason, for example, technology for detecting such substances at airports is extremely effective for preventing terrorist activities. Particularly in such a case, it is necessary to determine whether or not the nuclear material is contained in the article hidden in the container in a non-contact and non-destructive manner.

  Further, not only in such a case, a similar technique is required for analysis of molten nuclear fuel generated by, for example, a nuclear accident. For example, it is virtually impossible for humans to approach molten nuclear fuel or to take out molten nuclear fuel for analysis due to the presence of harmful radiation. Even in such a case, for example, if the molten nuclear fuel in the inside can be analyzed in a non-contact and non-destructive manner from the outside of the shield, it is extremely effective in the processing.

  The nuclide to be analyzed here is Pu-239 and U-235 which are fission materials. For example, Patent Document 1 describes a technique for detecting these nuclides in a non-destructive and non-contact manner by irradiating a sample with X-rays or the like and obtaining a transmission image thereof. However, although this technique is effective for element identification, since the X-ray transmission characteristics do not depend on the difference in nuclides (isotopes), it is impossible to identify isotopes in practice.

  On the other hand, these nuclides can be detected by detecting γ-rays and neutrons spontaneously emitted by these nuclides. For example, Patent Document 2 describes a technique for detecting γ rays, and Patent Document 3 describes a technique for detecting neutrons. In these cases, by examining the spectrum (energy) of γ-rays and neutrons, the nuclide that emitted them can be identified. However, the energy of γ rays and neutron rays spontaneously emitted by these nuclides is often as low as 1 MeV or less. For this reason, for example, when the sample is in the shield, the γ rays and neutron rays cannot pass through the shield, and it is difficult to detect them with sufficient intensity. In addition, unlike γ-rays with specific energy, neutron energy generally has a continuous spread, so it is difficult to measure the neutron energy and identify the nuclide that emitted the neutron. For this reason, it is difficult to obtain high detection sensitivity.

A Witness foil method described in Non-Patent Document 1 is known as a method capable of performing analysis of a sample thus shielded with high accuracy. FIG. 5 is a diagram showing the principle of the witness foil method. In the witness foil method, the sample 100 is irradiated with incident γ rays 91. As the incident γ-ray 91, laser Compton γ-ray is used. For example, as described in Non-Patent Document 2, laser Compton γ-rays cause energy (energy particles) to collide with electrons (high energy particles) that have been increased in energy by an accelerator and laser light (visible light). Is a high-energy γ-ray. This γ-ray (energy E _γ ) has a quasi-monochromatic spectrum, and its peak energy can be appropriately set by adjusting the energy of electrons. Here, as shown in FIG. 5, the peak energy is E _γ0 and the peak intensity is a. The peak energy E _γ0 is matched with the resonance energy E _{NRF at} which the nucleus of the nuclide to be detected causes nuclear resonance fluorescence scattering. In nuclear resonance fluorescence scattering, an atomic nucleus absorbs a photon (γ-ray) having a certain resonance energy determined by a nuclide to be in an excited state, and emits γ-ray to be de-excited. For example, in the case of Pu-239, the resonance energy E _NRF can be 2.143 MeV. Since this energy γ-ray has a high transmission power, it can be transmitted even if there is a shield.

Since the photon that causes nuclear resonance fluorescence scattering is absorbed in the incident γ-ray 91, the intensity of E _γ0 is c (= a) in the spectrum of the transmitted γ-ray 92 transmitted through the sample 100 as shown in FIG. Absorption lines are formed to reduce to -b). b increases according to the abundance in the sample 100 of the nucleus of the nuclide to be detected, and c decreases conversely. That is, only photons having energy in a narrow range near E _γ0 (= E _NRF ) in the spectrum of the incident γ-ray 91 contribute to this nuclear resonance fluorescence scattering and are absorbed.

Here, the transmission γ-ray 92 is configured to enter the witness foil 110. The witness foil 110 is a thin film containing a large amount of a nuclide to be detected (for example, Pu-239 in the above case). Therefore, the transmitted γ-ray 92 causes nuclear resonance fluorescence scattering in the witness foil 110 as in the sample 100. For this reason, in the spectrum of γ rays (witness foil transmission γ rays 93) after passing through the witness foil 110, the absorption at E _γ0 (= E _NRF ) is further increased. However, in this case, if the abundance of the nuclide to be detected in the sample 100 is large, c will be small, and the absolute amount of photons at E _γ0 will be small, so that the occurrence of nuclear resonance fluorescence scattering will be small.

Therefore, if the fluorescent γ-ray 94 (peak energy E _γ1 , peak intensity d) generated from the witness foil 110 by nuclear resonance fluorescence scattering is detected by the γ-ray detector 120 placed close to the witness foil 110, the peak is obtained. The intensity d depends on c (= a−b). Here, the γ-ray detector 120 detects from the optical axis of the incident γ-ray 91 so that the γ-ray detector 120 selectively detects only the fluorescent γ-ray 94 without detecting the witness foil transmission γ-ray 93. Removed and placed. If the detection target nuclide does not exist in the sample 100, d increases. If the detection target nuclide exists in a large amount, d decreases. For this reason, it is possible to calculate b from the output of the γ-ray detector 120 (the number of counts by the fluorescent γ-ray 94), whereby the abundance of the nuclide to be detected can be obtained. Note that the γ-ray detector 120 detects not only photons having peak energy in the spectrum of the fluorescent γ-ray 94 in FIG. 5, but all photons included in the hatched region.

  At this time, the fluorescent gamma ray 94 is emitted from the sample 100 as well as from the witness foil 110. For example, when an article containing a nuclide to be detected is shielded in the sample 100, the fluorescent gamma ray It is difficult to perform detection with high efficiency. On the other hand, the relationship between the witness foil 100 and the γ-ray detector 120 can be set so that the fluorescent γ-ray 94 can be detected with high efficiency.

  If the spectrum of the transmitted γ-ray 92 including the absorption line (dip) shown in FIG. 5 can be measured accurately, b and c in FIG. 5 can be obtained from this, and in this case There is no need to use the witness foil 110 or the like. However, since the width of this absorption line is very narrow, there is no actual γ-ray detector with high energy resolution to the extent that this spectrum can be accurately measured. In the witness foil method, b and c can be indirectly measured by detecting the fluorescent γ rays 94 emitted from the witness foil 110 instead of measuring the spectrum precisely. Thereby, it is possible to detect the nuclide to be detected included in the sample 100 or to calculate the content thereof.

In the above example, the peak energy E _γ0 of the incident γ-ray 91 is assumed to be equal to the resonance energy E _NRF of nuclear resonance fluorescence scattering, but it is not necessary to make these exactly equal. However, it is clear that the detection accuracy is improved by matching them. Further, in FIG. 5, the absorption line by nuclear resonance fluorescence scattering is simplified to be one, but actually, there are a plurality of absorption lines (energy E _NRF1 , E _NRF2 ,...) _Having close energy. . Even in such a case, since the above relationship is established in each absorption line, the correlation between the intensity of the fluorescent γ-ray 94 and the content of the nuclide to be detected is the same. In this case, for example, E _γ0 may be adapted to the energy of the absorption line where the absorption is _maximized .

  As described above, in particular, according to the witness foil method, the nuclear material can be detected in a non-contact and non-destructive manner, and the nuclear material in the shielded article can be detected with high accuracy.

J. et al. Pruet, D.C. P. McNabb, C.I. A. Hagmann, F.M. V. Hartmann, and C.H. P. J. et al. Barty, "Detecting Clandestine Material with Nuclear Resonance Fluorescence", journal of Applied Physics, vol. 99, p123102, 2006 R. Hajima, T .; Hayagawa, N .; Kikuzawa, and E.K. Minehara, “Proposal of nondestructive radical assembly using a high-flux gamma-ray source and nuclear resonance fluence, JournalSensorship. 45, no. 5, p441, 2008

JP 2010-203863 A JP-A-2005-180936 JP 2009-198469 A

  However, incident γ rays react with nuclei and atoms other than the nuclide to be detected by a mechanism other than nuclear resonance fluorescence scattering. At this time, the reaction cross section of nuclear resonance fluorescence scattering used in the witness foil method is significantly larger than the reaction cross section when various nuclei, atoms and γ-rays react by other mechanisms. It is not. For this reason, for example, when this detection is performed through a shield, the transmitted γ-ray is not only affected by the nuclear resonance fluorescence scattering of the nucleus of the target nuclide as shown in FIG. 5, but is a detection target. It is also greatly affected by atoms other than those with nuclides. For this reason, it has been difficult to detect nuclear materials with high accuracy.

  That is, it has been difficult to detect nuclear material with high accuracy in a non-contact and non-destructive manner.

  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.

In order to solve the above problems, the present invention has the following configurations.
The nuclear material detection device of the present invention is a nuclear material detection device that detects nuclear material contained in a sample by irradiating the sample with γ rays, and the nuclear material detection device irradiates the nucleus of the nuclear material to be detected. Γ-ray generating means for generating an incident γ-ray that generates a photo-fission reaction in the nucleus and irradiating the sample; and a transmitted γ-ray after the incident γ-ray passes through the sample is irradiated; A witness foil that emits a neutron beam when irradiated, and a neutron detection means that detects the neutron beam emitted from the witness foil in a direction away from the optical axis of the incident γ-ray. Features.
In the nuclear material detection device of the present invention, the γ-ray generation means is a laser Compton γ-ray source that generates the incident γ-rays by a Compton effect when high energy particles collide with laser light. To do.
In the nuclear material detection device of the present invention, the witness foil includes a nuclear material to be detected.
In the nuclear material detection device of the present invention, the witness foil contains deuterium.
In the nuclear material detection device according to the present invention, the peak energy of the incident γ-ray is in the range of 3 MeV to 5 MeV.
The nuclear material detection method of the present invention is a nuclear material detection method for detecting nuclear material contained in a sample by irradiating the sample with γ-rays, and the nuclear material is detected by irradiating the nucleus of the nuclear material to be detected. A material that irradiates the sample with incident γ-rays that generate a photo-fission reaction in the nucleus, and emits neutrons by irradiating the transmitted γ-rays after the incident γ-rays have passed through the sample. The nuclear material contained in the sample is detected from the intensity of the neutron beam emitted from the witness foil in a direction deviating from the optical axis of the incident γ-ray. Features.
In the nuclear material detection method of the present invention, the witness foil includes a nuclear material to be detected.
In the nuclear material detection method of the present invention, the witness foil contains deuterium.
The nuclear material detection method of the present invention is characterized in that the peak energy of the incident γ-ray is in the range of 3 MeV to 5 MeV.

  Since the present invention is configured as described above, the nuclear material can be detected with high accuracy in a non-contact and non-destructive manner.

It is a figure which shows the principle of the nuclear material detection method used as embodiment of this invention. It is a figure which shows typically the gamma ray energy dependence of the reaction cross section of a photo-fission and nuclear resonance fluorescence scattering. It is a figure which shows the structure of the nuclear material detection apparatus used as embodiment of this invention. It is a figure which shows the gamma ray energy dependence of the reaction cross section of the photo-fission in Pu-239. It is a figure which shows the principle of a witness foil method.

  Hereinafter, a configuration of a nuclear material detection device and a nuclear material detection method according to an embodiment of the present invention will be described. The nuclear material detected here is a fission material such as Pu-239 or U-235. This nuclear matter detection method is the same as the witness foil method described above in that the sample is irradiated with laser Compton γ rays and a witness foil is used. However, the reaction used for detection of the nuclear material is different, and the target to be detected by the detector is also different according to this.

  FIG. 1 is a diagram illustrating the principle of the nuclear material detection method according to the first embodiment. Also in this nuclear matter detection method, laser Compton γ rays are particularly preferably used as the incident γ rays 11 used as probes. The transmitted γ-rays 12 that have passed through the sample 100 are irradiated to the witness foil 20. The witness foil 20 is a thin film containing a large amount of nuclides (for example, Pu-239) to be detected.

  In the conventional witness foil method shown in FIG. 5, the fluorescent γ-ray 94 emitted from the witness foil 110 is detected, whereas in this nuclear material detection method, the neutron beam emitted from the witness foil 20 is detected. 13 is detected by a neutron detector (neutron detection means) 30. This neutron beam 13 is emitted when the nucleus of the nuclide to be detected included in the witness foil 20 undergoes photo-fission. For this reason, the incident γ-ray 11 is set so that the nucleus of the nuclide to be detected undergoes photo-fission. This setting is easy for laser Compton γ rays, as in the case of the witness foil method.

  Below, the effect | action in this nuclear material detection method is demonstrated. First, in the witness foil method described in Non-Patent Document 1, the nuclear resonance fluorescence scattering of γ rays by the nuclide to be detected is used, whereas in this nuclear matter detection method, γ rays are used. Utilize photo-fission. Photo-fission is a phenomenon in which atomic nuclei split by absorbing γ-rays, a phenomenon peculiar to actinoid elements. When this photofission occurs, several prompt neutrons and delayed neutrons are emitted, so this neutron is detected here.

FIG. 2 is a result schematically showing the reaction cross section of nuclear resonance fluorescence scattering and photofission in the same nuclide with the horizontal axis as the energy E _γ of incident γ rays. The characteristic of FIG. 2 is that, first, the reaction cross section of nuclear resonance fluorescence scattering is a delta function, whereas the reaction cross section of photo-fission continuously increases above a certain threshold. Is a point. In addition, the energy region where photo-fission occurs is higher than the energy region where nuclear resonance fluorescence scattering occurs. Nuclear resonance fluorescence scattering occurs in many nuclides, whereas photo-fission is a phenomenon unique to actinoids containing nuclear materials.

In FIG. 1, as in FIG. 5, the spectra of incident γ rays 11, transmitted γ rays 12, neutron rays 13, and witness foil transmitted γ rays 14 are shown. The shape of the incident γ-ray 11 (peak energy E _γ0 , peak intensity a) is the same as in FIG. 5, but from FIG. 2, E _γ0 does not need to match a specific energy and the reaction cross-section of photofission is large. It can be set in a range, for example, 3 to 5 MeV. However, this energy is higher than the energy of nuclear resonance fluorescence scattering. That is, E _γ0 is increased as compared with the conventional witness foil method.

  Since the reaction cross section of the photofission continuously shows a large value, unlike the case of FIG. 5 using nuclear resonance fluorescence scattering, photons in a wide energy range in the spectrum of the incident γ-ray 11 are involved in the photofission reaction. Lost to contribute. Therefore, as shown in FIG. 1, the spectrum of the transmitted γ-ray 12 has a shape that is significantly different from the spectrum of the transmitted γ-ray 92 in FIG. The transmitted γ-ray 12 is not in a form in which the intensity is reduced only in a narrow energy range (absorption line) like the transmitted γ-ray 92 in FIG. 5, but in a shape that decreases over the entire energy range (peak intensity y: y = a -X).

  When this transmitted γ-ray 12 is incident on the witness foil 20, as in the sample 100, the nucleus of the nuclide to be detected causes photo-fission. Again, photons with a wide energy range in the spectrum of the transmitted γ-ray 12 contribute to this photofission and are lost. Therefore, the spectrum of the witness foil transmission γ-ray 14 has a shape in which the transmission γ-ray 12 is further uniformly reduced over the entire energy range, as shown in FIG.

A photo-fission reaction occurs by the amount of the lost photons, and neutrons are emitted in proportion to the amount of generation. This neutron is detected as a neutron beam 13 by a neutron detector (neutron detection means) 30. Here neutrons to be detected are all neutrons spectrum neutron 13 in FIG. 1 (peak at neutron energy E _n is E _n1) in the hatching areas of.

  As the neutron detector 30, for example, a proportional counter or a combination of a scintillator and a photomultiplier can be used. In this case, not only neutrons but also γ rays and the like are detected, so that the witness foil transmission γ rays 14 after the transmission γ rays 12 further pass through the witness foil 20 are not detected as in the case of FIG. Similarly, it is preferable that the neutron detector 30 be arranged off the optical axis of the incident γ-ray 11.

In general, when the detection efficiency of the γ-ray detector 120 used in FIG. 5 is compared with the neutron detector 30 used in FIG. 1, the detection efficiency of the γ-ray detector 120 is remarkably low. This is because, in a Ge semiconductor detector used as a detector of a γ-ray detector, the detection efficiency depends on the size of the Ge semiconductor crystal used, and currently reaches the upper limit of this size. It is. For this reason, the detection efficiency of the γ-ray detector is typically about 0.5%. On the other hand, the neutron detector 30, ³ the He proportional counter and BF ₃ proportional counter is used, the detection efficiency in this case is obtained as high as 50% values.

In addition, the incident γ-ray 11 is pulsed, and neutrons generated during a short period of time when the γ-ray 11 is irradiated are decelerated by a modulator, and a neutron is detected in a ³ He proportional counter with very low sensitivity to γ-rays. It can be measured using as a device 30. In this case, since the neutron is decelerated, the neutron is delayed more than the γ-ray, so that the neutron can be detected at a time when the γ-ray pulse does not come. For this reason, the neutron beam 13 can be detected in a low background state where the background of γ rays is low. From these points, the neutron beam 13 can be detected with high efficiency.

Furthermore, in fission, delayed neutrons may be emitted with a delay from the occurrence of fission. Also in this case, the incident γ-rays are pulsed and the delayed neutrons are detected at the timing when the γ-rays do not come, so that the influence of γ-rays can be removed and the measurement sensitivity of neutron detection can be further increased.

  Thus, in said structure, the detection efficiency by the neutron detector 30 of the neutron beam 13 can be made very high. On the other hand, in the configuration of FIG. 5, it is impossible to detect the fluorescent γ-ray 94 with high sensitivity and low background by the same method.

  According to this configuration, the intensity of the transmitted γ-ray 12 (corresponding to y) can be calculated from the output of the neutron detector 30 (the number of counts by the neutron beam 13). Thereby, it is possible to calculate the intensity of γ rays lost in the sample 100 (corresponding to x), and from this, it is possible to calculate the abundance of the detection target nuclide in the sample 100. That is, the nuclear material in the sample 100 can be detected without contact. At this time, as shown in FIG. 2, the number of photons contributing to the photo-fission reaction is larger than that in the case of using nuclear resonance fluorescence scattering, and the detection efficiency thereof is higher. Can be detected.

  At the time of this detection, the nucleus of the nuclide to be detected in the sample 100 undergoes a photo-fission reaction and is destroyed. However, since other nuclei that do not cause photo-fission do not decay, the sample 100 is not substantially destroyed in this nuclear material detection method. That is, it is possible to detect nuclear material nondestructively. For example, in a nuclear reactor, neutrons generated by fission induce fission of another nuclear nucleus and continuously generate fission. However, in order to do so, it is necessary to reduce the neutron energy with a moderator such as water and to maintain conditions that allow fission to continue. In this detection apparatus, unless such a moderator is present in the sample 100 under specific conditions, the fission reaction does not occur continuously. Therefore, even if the photofission reaction occurs in the sample 100 or the witness foil 20, there is no danger. If laser Compton γ rays are used as the incident γ rays 11, the directivity of the laser Compton γ rays is extremely high, so that it is easy to irradiate only a specific region in the sample 100, and the photofission reaction is performed only in a narrow region. Can be generated.

Here, as described above, only the actinide that causes the photo-fission reaction can be detected by this nuclear material detection method. Among them, for example, in order to selectively detect Pu-239, for example, the reaction cross-section of photo-fission is within an energy range in which the reaction cross-section of Pu-239 is larger than that of other nuclides that cause a photo-fission reaction. The incident γ ray 11 may be set. However, for example, when it is desired to detect both Pu-239 and U-235, which are representative examples of nuclear materials, the incident γ-ray 11 is set to an energy range in which the reaction cross sections of photo-fission in these nuclides are both increased. That's fine. In any case, the presence of nuclei other than actinoids does not affect the generation of the neutron beam 13 in the witness foil 20, so that the nuclei have higher sensitivity than the conventional witness foil method using nuclear resonance fluorescence scattering. Substances can be detected. Thus, the energy E _γ0 of the incident γ-ray 11 or its half-value width can be appropriately set according to the object to be detected and the purpose. In the case of using laser Compton γ rays, this setting can be easily performed by adjusting the energy of high energy electrons to be used.

  In addition, as described above, the energy region of the incident γ-ray 11 is set so that the nucleus of the nuclide to be detected causes photo-fission (the reaction cross-section of photo-fission increases). From FIG. 2, this energy region is on the higher energy side than the energy region where nuclear resonance fluorescence scattering occurs. For this reason, the energy range of the incident γ-ray 11 is higher than in the case of the conventional witness foil method, and the transmission capability with respect to the sample 100 is higher. That is, the detection capability of the nuclear material contained in the shielded article becomes higher.

  Next, the configuration of a nuclear material detection apparatus that realizes the above-described nuclear material detection method will be described. FIG. 3 is a cross-sectional view along the optical axis of the incident γ-ray 11 of the configuration of the nuclear material detection device 10. Here, the incident γ-ray 11 emitted from the γ-ray source (γ-ray generating means) 40 is irradiated onto the sample 100 through an opening in the radiation shield 51 made of lead or the like. The radiation shield 51 is used to shield a part of the incident γ rays 11 and γ rays emitted from the sample 100. As the γ-ray source 40, a laser Compton γ-ray source is particularly preferably used as in the conventional witness foil method.

  As described above, the incident γ rays 11 pass through the sample 100 and become transmission γ rays 12. At this time, scattered γ rays and neutron rays are emitted in a direction away from the optical axis from the sample 100. Since these become noise when detecting the neutron beam 13 emitted from the witness foil 20, they are blocked by the radiation shield 52 and the neutron shield 53.

  Thereafter, the transmitted γ-ray 12 passes through the monitor γ-ray detector 60 and its intensity is detected. As the monitor γ-ray detector 60, a gas counter, a detector using a thin plastic scintillator, or the like can be used so that attenuation of the transmitted γ-ray 12 due to this can be ignored. However, even if the attenuation of the transmitted γ-ray 12 is not negligible, data processing can be performed in consideration of the attenuation state as long as the attenuation state can be predicted. Further, the detection timing of the neutron detector 30 can be set by the output of the monitor γ-ray detector 60. Since γ-rays and neutrons are also emitted from the monitoring γ-ray detector 60, a radiation shield 54 is provided around the radiation shield 55 and a radiation shield 55 and a neutron shield 56 are provided behind the radiation shield 54 to shield them.

  As described above, the transmitted γ rays 12 are applied to the witness foil 20. As described above, the neutron detector 30 is provided around the witness foil 20. The neutron detector 30 surrounds the witness foil 20 and is provided around the optical axis in order to detect neutrons in a direction deviating from the optical axis of the incident γ-ray 11 (transmission γ-ray 12). Further, by providing a monitoring γ-ray detector 61 adjacent to the neutron detector 30, if scattered γ-rays and fluorescent γ-rays emitted from the witness foil 20 are detected, information on these γ-rays is also analyzed. It can also be used.

  As long as the incident γ-ray 11, the sample 100, the transmitted γ-ray 12, the witness foil 20, and the neutron detector 30 are provided in the same relationship as in FIG. 1, the presence or absence of other components and their arrangement are arbitrary. is there. For example, the monitoring γ-ray detectors 60 and 61 do not need to be provided unless the information on the γ-rays detected by these is used in the operation and analysis of the neutron detector 30. Alternatively, the monitor γ-ray detector 61 can be provided behind the witness foil 20. In addition, the radiation shield and the neutron shield are appropriately arranged so that the witness foil 20 is not irradiated with radiation (γ rays, neutron rays) other than the transmitted γ rays 12 or γ rays and neutrons are not leaked to the outside. be able to.

  Further, as described above, the witness foil 20 preferably contains a large amount of nuclides to be detected. This is the same as the conventional witness foil method. However, unlike the conventional witness foil method, which was intended to measure the intensity of transmitted γ-rays in a narrow absorption line, this nuclear material detection method uses integrated intensity over a wide energy range of transmitted γ-rays. Can be measured. For this reason, if the witness foil 20 contains a nuclide that emits the neutron beam 13 by being irradiated with the transmitted γ-rays 12, it can be used in the same manner. In this case, materials other than the nuclear material to be detected can be used. An example of such a material is a material containing deuterium (heavy water or the like). In this case, handling of the witness foil 20 is safe and easy, and can be made inexpensive. When the witness foil 20 is made of such a material, for example, a gas or liquid containing such a material can be sealed to form a thin plate.

  As described above, as the γ-ray source 40, a laser Compton γ-ray source with variable peak energy and half-value width is particularly preferably used. However, even with other γ-ray sources, a desired energy range and intensity are used. It is clear that any γ-ray source that can emit γ-rays and can irradiate the sample locally can be used in the same manner.

  As described above, according to the nuclear material detection method and the nuclear material detection device, the nuclear material in the shielded article can be detected with high accuracy in a non-destructive and non-contact manner. This can be used to detect hidden nuclear weapons. In addition, for example, the molten fuel of a nuclear reactor generated by an accident exists in various structures in order to protect the high radiation generated from the molten fuel, so that it is extremely effective in the analysis of the molten fuel.

(Example)
A comparison was made by calculation between the case of using the conventional nuclear fluorescence resonance scattering method and the measurement accuracy of the nuclear material detection method of the present invention using photo-fission. Here, the sample and the measurement conditions that can be unified were the same. As a sample, γ rays were irradiated from a 1 cm × 1 cm surface of a rectangular parallelepiped (1 cm × 1 cm × 4 cm, weight 64 g). The main component of the sample is uranium oxide, and the detection target is plutonium oxide (Pu-239) with a content of 1%. The energy width of the incident laser Compton γ-ray was dE / E = 3%, and the entire incident γ-ray had a flux of 10 ⁸ photons / s. This flux is 7 × 10 ⁴ photons / s / eV in luminance. The measurement time was 300 seconds. For the nuclear resonance fluorescence scattering method, the center energy of incident γ rays was 2.2 MeV, and in the nuclear material detection method of the present invention, the energy was 5.4 MeV, which is higher than this. The detection efficiency of the detector for measuring scattered γ rays from the witness foil is assumed to be 0.5%, and as described above, the detection efficiency of the neutron detector is set to 50%, which is higher than this. In addition, FIG. 4 shows the γ-ray energy dependence (corresponding to FIG. 2) of the photo-fission reaction cross section of Pu-239 used here.

  Under the above assumption, according to the conventional photonuclear resonance scattering method, 1% Pu-239 is obtained with an error of about 25% (statistical error in counting of the γ-ray detector) in a measurement time of 300 seconds. It can be measured. On the other hand, according to the nuclear material detection method of the present invention, measurement can be performed with an error of 12% (statistical error of neutron detector count), and the accuracy is improved more than twice. That is, it is possible to detect Pu-239 with high accuracy.

DESCRIPTION OF SYMBOLS 10 Nuclear material detection apparatus 11, 91 Incident gamma ray 12, 92 Transmission gamma ray 13 Neutron ray 14, 93 Witness foil Transmission gamma ray 20, 110 Witness foil 30 Neutron detector (neutron detection means)
40 gamma ray source (gamma ray generation means)
51, 52, 54, 55 Radiation shield 53, 56 Neutron shield 60, 61 Monitor γ-ray detector 94 Fluorescent γ-ray 120 γ-ray detector

Claims (9)

A nuclear material detection device that detects nuclear material contained in a sample by irradiating the sample with gamma rays,
Γ-ray generating means for generating incident γ-rays that irradiate the nucleus of the nuclear material to be detected to generate a photofission reaction in the nucleus and irradiating the sample;
A witness foil that emits a neutron beam when irradiated with the transmitted γ-ray after the incident γ-ray passes through the sample, and
Neutron detection means for detecting the neutron beam emitted from the witness foil in a direction away from the optical axis of the incident γ-ray;
A nuclear material detection apparatus comprising:   2. The nuclear material according to claim 1, wherein the γ-ray generation means is a laser Compton γ-ray source that generates the incident γ-rays by a Compton effect when high energy particles collide with laser light. Detection device.   The nuclear material detection apparatus according to claim 1, wherein the witness foil includes a nuclear material to be detected.   The nuclear material detection apparatus according to claim 1, wherein the witness foil contains deuterium.   5. The nuclear material detection device according to claim 1, wherein a peak energy of the incident γ-ray is in a range of 3 MeV to 5 MeV. A nuclear material detection method for detecting nuclear material contained in a sample by irradiating the sample with gamma rays,
The sample is irradiated with incident γ-rays that generate a photofission reaction in the nucleus by irradiating the nucleus of the nuclear material to be detected,
Irradiating a witness foil made of a material that emits a neutron beam by irradiating the transmitted γ-ray after the incident γ-ray has passed through the sample,
A nuclear material detection method, comprising: detecting nuclear material contained in the sample from the intensity of the neutron beam emitted from the witness foil in a direction off the optical axis of the incident γ-ray.   The nuclear material detection method according to claim 6, wherein the witness foil includes a nuclear material to be detected.   The nuclear material detection method according to claim 6, wherein the witness foil contains deuterium.   The nuclear material detection method according to any one of claims 6 to 8, wherein a peak energy of the incident γ-ray is set in a range of 3 MeV to 5 MeV.

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