neutron detector

Abstract

To provide a neutron detector which can highly efficiently measure a neutron of high dose with the high n / γ selectivity.SOLUTION: A scintillator 10 includes a lamination structure in which a fluorescent layer 11 and a light transmission layer 12 are alternately laminated in a z direction. The fluorescent layer 11 is constituted of a phosphor which emits fluorescent light by absorbing a neutron, and a material thereof is, for example, a conventionally known material of a scintillator for neutron detection. The light transmission layer 12 is constituted of a material which has the high transmittance of the fluorescent light emitted from the phosphor and the small absorption of the neutron. In the scintillator 10, when the neutron and the gamma-ray photon are incident thereon, the light emission intensity (pulse height) is largely different between the neutron and the gamma-ray photon. With this, discrimination of output of the neutron and the gamma-ray photon can be easily performed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a neutron detector that detects neutrons by detecting scintillation light emitted by the neutrons. [Background technology]

[0002] When measuring neutron doses using a neutron detector under high neutron doses, a detector with fast response and high detection efficiency is advantageous in order to reduce neutron count losses. However, in such environments, not only neutron but also gamma ray doses are generally high. For example, when a moderator containing a large amount of hydrogen is used in an environment with a high dose of fast neutrons, the moderated neutrons capture and react with hydrogen to generate 2.2 MeV gamma rays, which inevitably increases the gamma ray dose. For this reason, a fast-response, highly efficient neutron detector is desired that can distinguish (detect) neutrons from gamma rays even when the doses of both gamma rays and neutrons are high.

[0003] Neutrons have no electric charge and therefore have a much higher penetrating power than charged particles. Therefore, in order to detect slow neutrons, a detector that uses an isotope (neutron absorbing isotope) with a large cross section for the neutron absorption reaction, which accompanies the emission of high-energy secondary charged particles, is required to increase the neutron absorption probability. Even when using such a neutron absorbing isotope, in order to detect neutrons with sufficient sensitivity, especially those with energies equal to or greater than epithermal neutrons, the density length (g / cm), which is the product of density and thickness, is required. 2 ) conversion, several to several tens of g / cm 2 In some cases, a detector of the order of magnitude is required.

[0004] Common neutron detectors include proportional counters that detect the ionization of gases due to interaction with radiation by amplifying the charge in proportion to the amount of ionization, and scintillation detectors that use a photodetector to detect light emission (fluorescence) in a scintillator that corresponds to the amount of energy imparted by the radiation. Because these detectors also have detection sensitivity to gamma rays, increasing the sensitivity to neutrons also increases the sensitivity to the above-mentioned gamma rays that become background events. For this reason, a neutron detector that can detect neutrons separately from gamma rays is desired.

[0005] In a proportional counter, as described in Patent Document 1, for example, the neutron absorption cross section is large and the neutron has a low atomic number, so it is difficult for the neutron to interact with gamma rays. 3 When He gas is used, the selectivity between neutrons and gamma rays (hereinafter referred to as n / γ selectivity) can be improved. However, in this case, the peak value of the output pulse becomes lower until the proportional counter is charged, and the time it takes for it to become difficult to detect (resolved time) becomes longer. If this resolved time is long, it becomes difficult to recognize individual output pulses that are consecutive in time, especially under high doses, making it difficult to count properly. Furthermore, in order to measure epithermal neutrons and fast neutrons with high efficiency, it is necessary to make the layer of this gas sufficiently thick (to have a large density length), which results in an increase in the size of the device, or 3 The problem is that the cost of helium makes the equipment expensive.

[0006] Scintillation detectors use phosphors that emit fluorescence by absorbing the energy of charged particles, and by selecting the phosphor, the decomposition time can be shortened, making them suitable for measurements under high doses. 6 Neutron-absorbing isotopes such as Li were added. 6 Li-glass:Ce 3+Phosphors of this type are widely used for neutron detection due to their relatively fast decay time (up to 60 nanoseconds) and moderate thermal neutron emission intensity (up to 1.6 MeVee) in units of electron equivalent energy (MeVee). The reason why the emission intensity is stated in units of electron equivalent energy (MeVee) above is that even if the light intensity itself is large, if the light intensity of gamma rays (electron beams) is similarly large, the gamma ray signal and the neutron signal will end up being mixed together, and electron equivalent energy is an important parameter when evaluating n / γ selectivity. By using a phosphor that emits a very large amount of light in units of electron equivalent energy (MeVee) when a neutron is absorbed, n / γ selectivity can be improved. Examples of such phosphors include 6 Neutron-absorbing isotopes such as Li were added. 6 LiF / ZnS:Ag (thermal neutron emission ~10 MeVee) is widely known. However, this phosphor has a relatively long decay time (~200 ns) and also contains a very long decay time constant component, which makes it difficult to achieve a fast response.

[0007] On the other hand, Patent Document 2 describes a scintillator with improved n / γ selectivity through structural innovation. This scintillator has a configuration in which inorganic phosphor particles made of the above-mentioned phosphor are dispersed in a resin material. In this case, neutrons can be detected by the fluorescence emitted by the phosphor when neutrons are absorbed, just as in the above case. However, compared to conventional scintillators made uniformly of phosphor, the intensity of the fluorescence emitted when energy from gamma rays is absorbed can be reduced. This makes it easier to distinguish between neutrons and gamma rays. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-14947 [Patent Document 2] International Publication No. 2015 / 064588 Summary of the Invention [Problem to be solved by the invention]

[0009] As a phosphor 6 When LiF / ZnS:Ag was used, the decay time constant of the fluorescence was long, making it difficult to measure under high doses. In addition, the transmittance of the fluorescence in this material was not high, so using large crystals also resulted in a problem of low fluorescence extraction efficiency. 6 Li-glass:Ce 3+ Although this system is capable of measuring at high doses as mentioned above, the amount of light emitted in MeVee units is insufficient, and the n / γ selectivity is low. For this reason, it has been difficult to realize a neutron detector that can detect neutrons with high n / γ selectivity at high doses by simply selecting the phosphor material.

[0010] For this reason, in addition to selecting the phosphor material, it is effective to devise a scintillator structure as described in Patent Document 2. However, because the specific gravities of inorganic phosphor particles and resin materials are different, it is not easy to uniformly disperse inorganic phosphor particles in the resin material, and manufacturing such a scintillator has been difficult. Furthermore, because the resin material contains a large amount of hydrogen, which has a mass similar to that of a neutron, neutrons are elastically scattered and thermalized, increasing the probability that the neutrons will be absorbed outside the inorganic phosphor particles, thereby reducing detection efficiency. Furthermore, such thermalization (scattering) increases the transit time of neutrons within the scintillator, degrading the time resolution of detection when measuring the timing of neutron detection. This makes it difficult to use, for example, in neutron TOF measurements, which require high time resolution.

[0011] For this reason, a neutron detector that can measure high doses of neutrons with high n / γ selectivity and high efficiency is desired.

[0012] The present invention has been made in view 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]

[0013] In order to solve the above problems, the present invention has the following configurations. The neutron detector of the present invention detects neutrons by fluorescence emitted when a phosphor containing a neutron-absorbing isotope that absorbs neutrons and emits secondary charged particles absorbs the neutrons, and comprises: a scintillator having a laminated structure in which a thin-film phosphor layer made of the phosphor and a thin-film light-transmitting layer adjacent to the phosphor layer in the film thickness direction and made of a light-transmitting material that transmits the fluorescence are provided in multiple sets along the incident direction of the neutrons; and a photodetector that emits an output pulse as an output that detects the fluorescence. The density length, which is the product of the density and thickness of the phosphor layer, is 0.0625 g / cm 2 ~0.5g / cm 2 and the density of the light transmitting layer is 0.2 g / cm 2 ~1.3g / cm 2 The range was It is characterized by: In the neutron detector of the present invention, the photodetector detects the fluorescence emitted from the scintillator along the incident direction. In the neutron detector of the present invention, the photodetector detects the fluorescence emitted from the scintillator along an in-plane direction of the phosphor layer and the light transmitting layer. The neutron detector of the present invention is characterized in that the photodetectors include a first photodetector and a second photodetector that face each other along the in-plane direction with the scintillator sandwiched between them, and further includes a coincidence counting unit that, when synchronism is recognized between a first output pulse that is the output pulse from the first photodetector and a second output pulse that is the output pulse from the second photodetector, newly outputs an output based on the first output pulse and the second output pulse as the output pulse. The neutron detector of the present invention is characterized in that in the scintillator, the phosphor layer and the light-transmitting layer adjacent to the phosphor layer are combined to form a plurality of segments along the incident direction that propagate the fluorescence emitted by the phosphor layer, and in that the stacked structure, a light-shielding layer that blocks the fluorescence is provided between the segments that are adjacent in the incident direction so that the fluorescence within each segment propagates in the in-plane direction within the segment and does not propagate between adjacent segments, and the photodetector is provided for each segment. The neutron detector of the present invention is characterized by comprising a non-coincidence counting unit that outputs an output pulse for each segment that has not been found to be simultaneous with other output pulses. The neutron detector of the present invention is characterized in that the ratio of the refractive index of the light transmitting layer with respect to the fluorescence to the refractive index of the phosphor layer with respect to the fluorescence is in the range of 0.90 to 1.10. In the neutron detector of the present invention, the phosphor layer and the light-transmitting layer are mainly composed of silicon oxide (SiO2), and the phosphor layer is doped with the neutron-absorbing isotope. In the neutron detector of the present invention, the neutron absorbing isotope is 6 Li or 10 B 。 [Effects of the Invention]

[0014] Since the present invention is configured as described above, it is possible to obtain a neutron detector that has high n / γ selectivity and can measure high dose neutrons with high efficiency. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view showing the structure of a scintillator used in a neutron detector according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a diagram schematically showing pulse height distributions when both neutrons and gamma rays are detected using a conventional scintillator and a scintillator according to the present invention. [Figure 3] FIG. 1 shows the state of light emission due to interactions of gamma-ray photons (A) and neutrons (B) and (C) in a phosphor. [Figure 4] This shows the relationship between the film thickness and the absorption rate of the energy of charged particles emitted by neutron absorption in the phosphor layer. [Figure 5] This shows the relationship between the sensitivity of the phosphor layer to 2.2 MeV gamma rays and the film thickness. [Figure 6] 1A to 1C are diagrams showing two types of configurations of a neutron detector according to a first embodiment. [Figure 7] This shows the results of measuring the pulse height distribution according to the number of light-transmitting layers through which the fluorescence passes. [Figure 8] 1 shows the results of comparing the relationship between the sensitivity to 2.2 MeV gamma rays and the film thickness of the light transmitting layer for each film thickness of the phosphor layer in the scintillator used in the neutron detector according to the first embodiment. [Figure 9] 1 shows the results of comparing the relationship between the sensitivity to 5.0 MeV gamma rays and the film thickness of the light transmitting layer for each film thickness of the phosphor layer in the scintillator used in the neutron detector according to the first embodiment. [Figure 10] FIG. 4 is a cross-sectional view showing the structure of a scintillator used in a neutron detector according to a second embodiment of the present invention. [Figure 11] FIG. 10 is a diagram showing the configuration of a neutron detector according to a second embodiment. [Figure 12] 10 shows the results of comparing the relationship between the sensitivity to 2.2 MeV gamma rays and the thickness of the phosphor layer with and without segmentation in the scintillator used in the neutron detector according to the second embodiment. [Figure 13] FIG. 10 is a diagram showing the configuration of a modified example of the neutron detector according to the second embodiment. [Figure 14] 10 shows the results of comparing the relationship between the sensitivity to 5.0 MeV gamma rays and the thickness of the phosphor layer with and without segmentation in the scintillator used in the neutron detector according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] The neutron detector according to the embodiment of the present invention is a scintillation detector that detects neutrons. For this reason, similar to the neutron detector described in Patent Document 2, for example, a scintillator that absorbs neutrons and emits fluorescence is used in combination with a photodetector that detects this fluorescence. Here, the neutron detector is characterized by the structure of the scintillator or the form of combination with the photodetector. Below, two embodiments classified based on the basic structure of the scintillator will be described.

[0017] (First embodiment) FIG. 1 is a plan view (a) and a cross-sectional view (b) showing the structure of a scintillator 10 used in a neutron detector according to a first embodiment. Neutrons to be detected enter the scintillator 10 from the negative side in the z direction in the figure. The scintillator 10 has a layered structure in which phosphor layers 11 and light-transmitting layers 12 are alternately stacked in the z direction. Both the phosphor layers 11 and the light-transmitting layers 12 have a thin-film configuration that extends in the x and y directions in the figure. Although FIG. 1 shows five phosphor layers 11 and four light-transmitting layers 12, these numbers can be set appropriately.

[0018] The phosphor layer 11 is made of a phosphor that emits fluorescence by absorbing the energy of charged particles, and the charged particles can be detected by detecting this fluorescence. In this case, a phosphor to which, for example, a neutron-absorbing isotope is added is used in order to provide sensitivity to neutrons, which have no charge. Such phosphors can be used, for example, scintillation glass for detecting neutrons, which is conventionally known. 6 Li-glass:Ce 3+Specifically, GS20, KG2 (manufactured by Scintacor), etc. are used. In order to obtain sufficient n / γ selectivity, it is preferable that the amount of light emitted upon neutron absorption is 1.5 MeVee or more in electron equivalent energy (MeVee) units. The light-transmitting layer 12 is made of a material that has high transmittance for the fluorescence emitted by this phosphor and low neutron absorption. As will be described later, it is preferable that the refractive index for this fluorescence is close to that of the phosphor layer 11 and that the material is transparent to the fluorescence. As a material for the light-transmitting layer 12, synthetic quartz, lead glass containing a small amount of lead oxide, etc. are preferably used.

[0019] With this configuration, the scintillator 10 can detect neutrons with high efficiency while improving n / γ selectivity when detecting neutrons. This point will be explained below. Generally, the phosphor that makes up the scintillator absorbs the energy of charged particles and emits fluorescence. When this fluorescence is detected with a photodetector (photomultiplier tube) or the like with high time resolution, photoelectrons are generated by the fluorescence at the photocathode, and these are amplified to obtain a pulsed electrical output corresponding to the emission time distribution. The number of photoelectrons corresponds to the emission intensity, which in turn corresponds to the output pulse, the so-called pulse height or charge integral value.

[0020] Although there are slight differences in the output pulse waveforms due to the difference in the form of energy transfer between neutrons and gamma rays to the phosphor, it is generally difficult to distinguish between the output pulses due to fluorescence when neutrons are absorbed and the output pulses due to fluorescence when energy is transferred by gamma rays, provided that the light quantities (total number of photons) are similar. For example, even if the energy released when a neutron is absorbed by a phosphor is 4.78 MeV, as described below, the electron equivalent energy for this light quantity is approximately 1.6 MeVee in the GS20 described above. On the other hand, for example, gamma rays of 2.2 MeV primarily undergo Compton scattering in the phosphor, transferring continuous energy of approximately 2.0 MeV or less to the electrons in the phosphor. Therefore, in principle, it is impossible to distinguish between these pulses based on their pulse heights where they overlap. Even under these circumstances, it is necessary to detect neutrons and gamma rays separately.

[0021] In contrast, this scintillator 10 is designed so that when neutrons and gamma-ray photons are incident, the emission intensities (pulse heights) of neutrons and gamma-ray photons differ greatly, making it easy to distinguish between them based on the pulse heights.

[0022] First, we will explain the output (distribution of pulse height values ​​of output pulses) when neutrons and gamma rays are mixed when a typical scintillator is used. Figure 2 is a diagram that schematically explains this situation. Here, the horizontal axis represents the pulse height of the output pulse, which corresponds to the energy absorbed by the phosphor by a single neutron or gamma ray photon. The vertical axis represents the frequency of detection of multiple neutrons or gamma ray photons.

[0023] In Figure 2, D1 is the pulse height distribution of the neutron output pulse when a conventional scintillator made entirely of a thick phosphor is used. Here, the peak energy of this distribution is 6This corresponds to a certain amount of energy released by the nuclear reaction of Li with neutrons (the electron equivalent energy of the light emission is 1.6 MeVee for GS20, as mentioned above). On the other hand, D2 is the distribution of pulse heights due to gamma rays (2.2 MeV) when using a similar scintillator. Here, D1 due to neutrons has a single peak as mentioned above, whereas gamma rays have a continuous spectrum from near the maximum energy of gamma rays to the lower energy side due to Compton scattering, etc., resulting in a broader distribution. As shown here, in the overlapping range of D1 and D2, it is difficult to distinguish between neutrons and gamma rays from the pulse height value alone.

[0024] Generally, the phosphors that make up a scintillator emit light (fluorescence) by absorbing the energy of charged particles with energy sufficiently higher than that required to raise the electrons in the phosphor from the ground state to an excited state. The emission wavelength corresponds to the difference in energy between the excited state and the ground state, and photons with this emission wavelength are generated in a number that depends on the energy absorbed by the phosphor and the amount of energy absorbed per unit length, etc.

[0025] Here, gamma-ray photons transfer their energy to the electrons in the phosphor through electromagnetic interaction, so the charged particles become electrons, and these electrons transfer their kinetic energy to the phosphor, causing it to emit fluorescence. At this time, the electrons ejected by the gamma rays tend to travel forward due to the law of conservation of momentum, and the more energy an electron receives, approaching the peak value of a neutron, the closer its travel direction will be to the direction of the original gamma ray incident.

[0026] In contrast, neutrons hardly interact with each other electromagnetically, and the probability of neutron absorption by general materials is low. However, if a neutron-absorbing isotope that emits high-energy secondary charged particles when it absorbs neutrons is contained in a fluorescent material, the high-energy secondary charged particles will be generated when this isotope absorbs neutrons. For example, 6 When Li is used, the reaction is as shown in equation (1).

[0027]

number

[0028] In other words, the secondary charged particle in this case is an α particle ( 4 He nuclei) and tritium ( 3 H nuclei), and unlike the electrons in the case of gamma rays, their emission distribution does not depend on direction, and according to the law of conservation of momentum, they are emitted in opposite directions with kinetic energies of 2.05 MeV and 2.73 MeV, totaling about 4.78 MeV. These secondary charged particles then excite the electrons in the material, causing fluorescence in the same way. Although the type and energy of the secondary particles are different, other neutron-absorbing isotopes (for example 10 In B), energy is also imparted by secondary particles.

[0029] Here, since the mass of the secondary charged particle (atomic nucleus) is greater than the mass of the electron (for example, electron mass: alpha particle mass = 1:7300), if they have the same energy, the speed of the secondary charged particle is less than that of the electron. For this reason, there is a difference between the situation in which electrons generated by gamma ray photons cause light emission in a phosphor and the situation in which secondary charged particles generated by neutrons cause light emission in a phosphor. Figure 3 is a diagram that shows these situations. Here, gamma ray photons and neutrons are assumed to be incident from the left side of the diagram.

[0030] In Figure 3, (A) in the upper part of the figure shows a schematic diagram of high-energy electrons generated by gamma-ray photons transferring their energy to produce light in a scintillator 100 composed of a thick phosphor. Meanwhile, (B) in the middle part and (C) in the lower part show a similar situation for secondary charged particles generated by neutrons. Note that this figure illustrates the light-emitting region and does not depict the total amount of light emitted. As mentioned above, electrons have a higher speed than secondary charged particles, and their charge is, for example, half that of alpha particles (secondary charged particles). The energy transferred per unit travel distance (dE / dx) of a charged particle (electron, secondary charged particle) is proportional to the product of the transit time (inversely proportional to the speed) and the square of the charge, so dE / dx due to electrons << dE / dx due to secondary charged particles. For this reason, in the scintillator 100, the range of electrons >> the range of secondary charged particles; for example, the maximum range of 1.5 MeV electrons in the GS20 phosphor is approximately 2.5 mm. However, because the electrons are light (or have the same mass as the electrons in the phosphor), they are easily scattered, and as shown in Figure 2(A), their paths become complex, resulting in a shorter actual range. If the scintillator 100 is thin compared to the range of the electrons, there is a high probability that the electrons will not be able to impart all of their energy to the scintillator 100, and therefore the pulse height will be reduced compared to when the scintillator 100 is sufficiently thick.

[0031] On the other hand, the energy transferred per unit flight distance of the high-energy secondary charged particles generated by neutron absorption is large, so the distance required to transfer the total energy is short. For example, in the case of α particles, it is only a few μm within the GS20 phosphor. 3 In the case of H, it is several tens of μm.

[0032] Therefore, when a neutron-absorbing isotope is encapsulated in a phosphor, its thickness can be set to a level that allows almost all of the kinetic energy of the secondary charged particles to be transferred during neutron absorption, and that allows the high-energy electrons generated during interaction with gamma-ray photons to quickly escape from the phosphor. In this case, as shown in Figure 2, ideally, the pulse-height distribution D1 due to neutrons remains unchanged, while the pulse-height distribution due to gamma rays changes from D2 to the lower energy D3. In this case, if a threshold T is set between distributions D1 and D3, it can be determined that neutrons have been detected if the detected pulse-height value is equal to or greater than T, and that gamma rays have been detected if it is less than T.

[0033] Furthermore, as mentioned above, since the traveling direction of high-energy electrons is almost the same as the incident (traveling) direction of gamma rays, in order to maximize this effect, when the phosphor is in the form of a thin film, it is preferable that the incident direction of gamma rays is the same as the film thickness direction of the phosphor. In many cases where neutrons and gamma rays coexist, the neutron source and the gamma ray source overlap, so this condition is met.

[0034] Furthermore, because the probability of neutron absorption is not high, neutrons may be absorbed near the surface and generate secondary charged particles, as shown in (B) in Figure 3, or they may be absorbed deep within the phosphor and generate secondary charged particles, as shown in (C). When only a single thin layer of phosphor is provided, neutrons can be detected in case (B). However, in case (C), where neutrons pass through the phosphor without any interaction, neutrons cannot be detected, which may result in a decrease in neutron detection efficiency. In contrast, by using a multilayer structure of phosphor layer 11 and light-transmitting layer 12 as shown in Figure 1(b), ensuring the total thickness of phosphor layer 11 allows neutron detection even in case (C), thereby maintaining high neutron detection efficiency.

[0035] The following explains the results of a specific study to clarify the above points. Figure 4 shows the secondary charged particles (α particles, 3This is the result of calculating the probability that the energy of the H nucleus is absorbed in the fluorescent material. 6 Li, i.e., the secondary charged particles are the α particles mentioned above, 3 The absorptance is 0.98 when the thickness of the phosphor is 1.0 mm, and even at a thickness of 0.25 mm, the absorptance is about 0.94, but it drops sharply when the thickness is thinner than this.

[0036] On the other hand, as mentioned above, when GS20 is used, the electron-equivalent energy equivalent to the light emission of neutrons (the aforementioned secondary charged particles) is 1.6 MeVee. In other words, the pulse heights in the output of electrons of this energy and secondary charged particles are approximately the same, so as shown by D1 and D2 in Figure 2, it is not possible to distinguish between gamma rays and neutrons (or high-energy electrons and secondary charged particles) based on the pulse height.

[0037] To distinguish between neutrons and gamma-ray photons, which generate electrons of similar energy, using the pulse peak value of the output pulse, the energy imparted to the phosphor by gamma rays (electrons generated by the gamma rays) can be set sufficiently smaller than 1.6 MeV to achieve the distribution D3 in Figure 2. For example, consider a typical detector consisting of the GS20 phosphor (scintillator) and a photomultiplier tube, whose energy resolution (the value obtained by dividing the peak value of the pulse-height distribution by the full width at half maximum of the pulse-height distribution) is approximately 16% for the secondary charged particles and approximately 24% for 1.2 MeV electrons. In this case, if T is set so that 99% of the neutron distribution D1 is above T in the pulse-height distribution of Figure 2, and the energy imparted to the phosphor layer by gamma rays is set to 1.2 MeV or less, the gamma-ray pulse peak value will hardly exceed the threshold T, thereby improving n / γ selectivity at the pulse peak value. Therefore, the phosphor layer 11 should be thinned so that the energy imparted to the phosphor by electrons is 1.2 MeV or less.

[0038] As mentioned above, the probability of absorption of neutrons with energies equal to or greater than epithermal neutrons is not high, so in order to detect neutrons efficiently, a certain total thickness of the phosphor layers 11 is required, and for this reason, the multilayer structure shown in Figure 1(b) is effective. However, by using a multilayer structure, the probability of interaction with gamma rays also increases, and even if the pulse-height distribution shifts to the lower energy side as mentioned above, the frequency itself increases, which may result in an increase in gamma-ray sensitivity. For this reason, we investigated the gamma-ray sensitivity (the probability that energy exceeding 1.2 MeV is imparted to the phosphor) when the number of layers (and the thickness of each layer) of the phosphor layers 11 was changed while keeping the total thickness of the phosphor layers 11 constant. Here, the total thickness of the phosphor layer 11 is fixed at 5 mm, and it is dispersed into a single layer (5 mm thick), three layers (1.67 mm thick phosphor layer 11), five layers (1 mm thick phosphor layer), ten layers (0.5 mm thick phosphor layer), and twenty layers (0.25 mm thick phosphor layer). When the phosphor layer 11 is multilayered, a 2 mm thick light-transmitting layer 12 made of synthetic quartz is sandwiched between the phosphor layers 11 to form the multilayer structure shown in Figure 1. Figure 5 shows the relative gamma-ray sensitivity calculation results. The gamma-ray energy used here is 2.2 MeV, which is emitted by the reaction between moderated neutrons and hydrogen in the moderator, as described above. This 2.2 MeV gamma ray is one of the gamma rays that requires the most attention in practical applications because it can be a major background phenomenon under various measurement conditions.

[0039] In Figure 5, the case of a thickness of 5 mm (single layer) corresponds to the conventional case where the entire scintillator is made uniformly of phosphor and does not have a light-transmitting layer 12. Therefore, it is clear from Figure 5 that by making it multilayered with the light-transmitting layer 12 sandwiched in between, the luminescence intensity due to gamma-ray photons can be significantly reduced compared to conventional cases.

[0040] The amount of electron energy absorption is largely dependent on the product (density length) of the density of the phosphor and the thickness (depth) through which it passes. Therefore, more generally, the density of GS20 (2.5 g / cm3) is plotted on the horizontal axis of Figure 5. 3) is used as an index. Even when using a phosphor other than GS20, the same results as GS20 can be obtained by using a phosphor with an equivalent density length. For example, if the density length is 1.25 g / cm 2 Compared with the case where one phosphor layer 11 (5 mm thick in GS20 equivalent) is used, 2 By forming the five phosphor layers 11 into a laminated structure as described above, the energy absorption is reduced, and the rate at which energy of 1.2 MeV or more is absorbed is reduced to about 1 / 10. When such density length is used as an index, the preferable range of density length of the phosphor layer 11 to obtain the above-mentioned effect is 0.0625 g / cm. 2 ~0.5g / cm 2 This becomes:

[0041] Therefore, even if the total thickness of the phosphor 11 is the same, if the individual phosphor layers 11 are made thinner and the total number of layers is increased, the difference between the pulse height of the output pulse due to neutrons and the pulse height of the output pulse due to gamma-ray photons will become larger.

[0042] However, if a neutron is absorbed by the light transmitting layer 12, the energy from this reaction does not contribute to light emission, and therefore the neutron is not detected. Therefore, in the scintillator 10 having the structure shown in Figure 1, the neutron detection efficiency decreases by the amount of neutrons absorbed by the light transmitting layer 12. Therefore, it is desirable to use a material with a low neutron absorption probability for the light transmitting layer 12.

[0043] For this reason, the light-transmitting layer 12 is preferably made of a material that does not contain the neutron-absorbing isotope and is transparent to fluorescence. However, in the neutron detector described below, the fluorescence extracted from the scintillator 10 is detected externally. In this case, reflection at the interface between the phosphor layer 11 and the light-transmitting layer 12 hinders the fluorescence from being extracted to the outside of the scintillator 10. To suppress such interface reflection, it is preferable that the refractive indexes of the phosphor layer 11 and the light-transmitting layer 12 are similar to each other with respect to fluorescence. Specifically, the refractive index of the light-transmitting layer 12 at the wavelength of the fluorescence relative to the phosphor layer 11 is preferably in the range of 0.90 to 1.10. For example, when the phosphor layer 11 is made of GS20, synthetic quartz can be used as a material that satisfies the above requirements. That is, by using a material primarily composed of silicon oxide (SiO2) as the material for the phosphor layer 11 and the light-transmitting layer 12, reflection at the interface can be suppressed.

[0044] As described above, when the scintillator 10 shown in Fig. 1 is used, it is possible to significantly reduce the pulse height of the gamma-ray photon pulse output without reducing the neutron detection efficiency or the pulse height of the neutron output pulse. This makes it possible to distinguish between neutrons and gamma-ray photons based solely on the pulse height. The decay time constant of the output pulse is determined by the material that constitutes the phosphor layer 11, and at least the above-described configuration does not lengthen this decay time constant. Therefore, if a material with a short decay time constant is used for the phosphor layer 11, measurements under high doses will be possible, just like conventional scintillators.

[0045] Next, an embodiment in which the above-mentioned scintillator 10 is actually used in a detector will be described. Figures 6(a) and 6(b) schematically show two types of neutron detectors 1 and 2 that use the above-mentioned scintillator 10. In the figures, arrow A indicates the incident direction of neutrons to be detected or gamma rays that interfere with the detection of these neutrons, and arrows B, C, and D indicate the traveling directions of fluorescence emitted by neutrons or gamma rays in this scintillator 10, which are the targets of detection by the photodetector.

[0046] The photodetector used here has high time resolution and is capable of receiving the fluorescence emitted by the phosphor layer 11 as described above and emitting an output pulse, and specifically, a photomultiplier tube or the like is used.

[0047] In the neutron detector 1 in Fig. 6(a), the incident direction of neutrons (arrow A) and the incident direction of light to be detected by the photodetector 21 (arrow B) are both aligned with the stacking direction (z-axis direction) in Fig. 1. For this reason, this configuration can be particularly easily realized when, for example, the scintillator 10 has a planar shape extending in the xy plane in Fig. 1.

[0048] 6(a), a single photodetector 21 is used, but in reality, a multi-terminal photodetector or the like may be used in the xy plane in FIG. 1. In this case, the neutron absorption position (light emission position) in the xy plane of the scintillator 10 can be recognized with a resolution approximately equal to the terminal spacing of the photodetector. In this case, not only the photodetector but also the scintillator 10 may be divided and arranged in a similar manner.

[0049] Here, as shown in Figures 3(B) and (C), the depth of the neutron absorption varies due to the low probability of neutron absorption, so the phosphor layer 11 and the light-transmitting layer 12 are formed into a multilayer structure as described above. In Figure 6(a), the fluorescence emitted from the phosphor layer 11 on the surface side (the leftmost side in Figure 6(a)) reaches the photodetector 21 after passing through all of the layers (four light-transmitting layers 12 and four phosphor layers 11) closer to the photodetector 21, as shown by path R1. In contrast, the fluorescence emitted from the phosphor layer 11 closest to the photodetector 21 (the rightmost side) reaches the photodetector 21 directly in a short distance, as shown by path R2. Therefore, when absorption and attenuation of fluorescence in the phosphor layer 11 and the light-transmitting layer 12, reflection between layers, etc. cannot be ignored, the fluorescence from the former will be detected by the photodetector 21 as having a lower intensity (lower pulse height) than the fluorescence from the latter. For this reason, even when the same energy is applied, the detected emission intensity (pulse height) may differ depending on which phosphor layer 11 in Fig. 1 the energy is applied to. This increases the horizontal spread of D1 in Fig. 2.

[0050] Figure 7 shows the results of actual measurements of the influence of the light-transmitting layer 12 on the detection of such fluorescence. To investigate this influence, only the leftmost phosphor layer 11 in Figure 1(b) was provided as the phosphor layer 11, with no other phosphor layers 11 provided, and the number of light-transmitting layers 12 to the right of that was varied, using the configuration shown in Figure 6(a). The results show the pulse-height distribution corresponding to D1 in Figure 2 in the output of the photodetector 21. The difference in the number of light-transmitting layers 12 here results in differences in the total film thickness of the light-transmitting layers through which the fluorescence passes, the influence of reflection between layers, and other factors. The light-transmitting layer 12 is made of synthetic quartz, and its individual thickness is 2.5 mm. In Figure 7, "0 Glass Layers" corresponds to the case where the fluorescence emitted from the phosphor layer 11 reaches the photodetector 21 without passing through any light-transmitting layers 12. "4 Glass Layers" corresponds to the case where four light-transmitting layers 12 (total thickness: 10 mm) are provided, and "12 Glass Layers" corresponds to the case where 12 light-transmitting layers 12 (total thickness: 30 mm) are provided. This result shows that even when a material with sufficiently high light transmittance, such as synthetic quartz, is used, its presence can significantly reduce the detected pulse height value. When the scintillator 10 with the structure shown in Figure 1(b) is used in Figure 6(a), the fluorescence detected by the photodetector 21 varies depending on which phosphor layer 11 it originates from. Therefore, the number of layers (total thickness) of the light-transmitting layers 12 through which the fluorescence passes varies accordingly. Therefore, the pulse height distribution obtained by the photodetector 21 is actually a superposition of the pulse height distributions shown in Figure 7. In this case, even if the spread (full width at half maximum) of the pulse-height distribution for each phosphor layer in Fig. 7 is small, the spread of the superimposed pulse-height distribution becomes large, which is not preferable from the viewpoint of separating distribution D1 and distribution D3 in Fig. 2.

[0051] 6(a), it is particularly preferable that the light transmittance of the phosphor layer 11 and the light-transmitting layer 12 is high and the reflectance at the interface between these layers is low (the difference in refractive index between these layers is small).It is also preferable not to increase the total number of layers too much.

[0052] 7 shows the change in pulse height distribution during neutron detection depending on the number of light transmitting layers 12 through which the fluorescence passes, as an effect of the light transmitting layers 12 on the detection of fluorescence by the photodetector 21. Next, the sensitivity to gamma rays when the thickness of the light transmitting layers 12 is changed will be described.

[0053] As in Figure 5, Figures 8 and 9 show the results of calculating the gamma-ray sensitivity when the thickness of the light-transmitting layer 12 is changed for the following cases: a single layer (5 mm thick), three layers (single layer thickness 1.67 mm), five layers (single layer thickness 1 mm), ten layers (single layer thickness 0.5 mm), and twenty layers (single layer thickness 0.25 mm), with the total thickness of the phosphor layer 11 fixed at 5 mm. Figure 8 shows the case where the gamma-ray energy is 2.2 MeV, and Figure 9 shows the case where the gamma-ray energy is 5 MeV. The light-transmitting layer 12 is made of synthetic quartz. Unlike Figure 5, the vertical axis is a logarithmic scale.

[0054] 8 and 9, in the case of multi-layered structures, the sensitivity (emission intensity) to gamma rays decreases as the total thickness of the light-transmitting layers 12 that do not contribute to light emission increases. From this perspective, when neutron absorption by the light-transmitting layer 12 can be ignored, it is preferable that the light-transmitting layer 12 is thick in order to reduce the emission intensity due to gamma-ray photons. However, although the absorption of neutrons by the light-transmitting layer 12 is slight, the neutron detection efficiency decreases little by little as the thickness increases. Furthermore, particularly in the embodiment of FIG. 6(b) described later, when the light-transmitting layer 12 is thick, the photosensitive area of ​​the photodetector must be increased, which also creates a disadvantage in terms of cost. For this reason, it is not preferable to make the light-transmitting layer 12 unnecessarily thick. Specifically, the thickness of the light-transmitting layer 12 should be set to 6 mm (density length equivalent: 1.3 g / cm). 2 8, it is preferable that the phosphor layer 11 has a thickness of about 0.25 mm (density equivalent length: 0.0625 g / cm 2), even if the light transmission layer 12 is 1 mm thick (0.2 g / cm in density length equivalent), it is expected that the effect of suppressing 2.2 MeV gamma ray sensitivity will be sufficient. However, if the phosphor layer 11 is 0.25 mm thick, the total number of layers must be increased to ensure neutron detection efficiency, and as mentioned above, the pulse height distribution will become broader in the configuration of FIG. 6(a), which is not preferable. Even when the configuration of FIG. 6(b) is used, if the light transmission layer 12 is too thin, the light propagation efficiency in the in-plane direction will decrease, and the pulse height distribution will also become broader, which is not preferable from the perspective of separating the distributions D1 and D3 in FIG. 2. Therefore, as with the phosphor layer 11, the preferable range for the light transmission layer 12, expressed in terms of density length, is 0.2 g / cm 2 ~1.3g / cm 2 This becomes:

[0055] On the other hand, in the neutron detector 2 of Fig. 6(b), the incident direction of the fluorescence to be detected by the photodetectors (arrows C and D) is different by 90° from that in Fig. 6(a), and two photodetectors (first photodetector) 31A and (second photodetector) 31B are used that face each other across the scintillator 10 in the y direction. When neutrons are absorbed in the phosphor layer 11, the light emitted has no particular directionality and is emitted in all directions, so this light emission can also be detected by the photodetectors 31A and 31B.

[0056] In this case, the light emitted by the leftmost phosphor layer 11 and the light emitted by the rightmost phosphor layer 11 in Fig. 6(b) as described above both pass through the phosphor layer 11 that emitted this light and the adjacent light-transmitting layer 12 along the y direction, and there is no difference in the path length from when this light is generated until it reaches the photodetectors 31A and 31B. Therefore, in this case, the pulse height does not differ depending on which of the five phosphor layers 11 emitted light (which absorbed the neutron), as in the case of Fig. 6(a), and the broadening of the pulse height distribution due to the multilayer structure does not occur.

[0057] On the other hand, even in this case, if the above-mentioned light absorption in the phosphor layer 11 and the light-transmitting layer 12 cannot be ignored, the pulse height of the output pulse obtained by the photodetectors 31A and 31B will be affected. In FIG. 6(b), if a neutron is absorbed in the phosphor layer 11 on the photodetector 31A side (upper side in the figure), the light reaches the photodetector 31A via a short path R3 and reaches the photodetector 31B via a long path R4. On the other hand, if a neutron is absorbed in the phosphor layer 11 on the photodetector 31B side (lower side in the figure), the light reaches the photodetector 31A via a long path R5 and reaches the photodetector 31B via a short path R6. Therefore, if the light absorption in the phosphor layer 11 cannot be ignored, if a neutron is absorbed on the photodetector 31A side, the pulse height at the photodetector 31A will be high and the pulse height at the photodetector 31B will be low. If a neutron is absorbed on the photodetector 31B side, the opposite will occur. That is, a distribution occurs in the pulse heights of the output pulses of the photodetectors 31A and 31B depending on the incident position of the neutron in the y direction.

[0058] On the other hand, in the neutron detector 2, the output pulse (first output pulse) P of the photodetector 31A A and the output pulse (second output pulse) P of the photodetector 31B. B is input to the coincidence circuit (coincidence unit) 32. The coincidence circuit 32 outputs the output pulse P A and output pulse P B If all of these are recognized at the same time, the sum P A +P B Output P A +P BThe pulse height of P is almost independent of the incident position of the neutron in the y direction, and corresponds to the energy absorbed by the phosphor layer 11 due to neutron absorption. In other words, by using such a coincidence circuit 32, even if light absorption occurs in the phosphor layer 11, it is possible to obtain an output pulse that is almost independent of the incident position of the neutron in the y direction, thereby suppressing the spread of the pulse height distribution during neutron detection. This makes it easy to distinguish between neutrons and gamma-ray photons. However, since attenuation due to light absorption, for example, is nonlinear with the distance from the incident position to the photodetector, P A and P B In a strict sense, the simple sum of P A and P B It is more preferable to use a method for calculating and using an appropriate pulse height that is independent of the incident position. The coincidence circuit (coincidence unit) 32 may be configured as an electric circuit, or may be configured with a computer or the like that processes the digitized output pulses. In particular, when a computer is used, this processing does not need to be performed in real time during detection. For example, data on output pulses within a certain period of time may be stored, and then such processing may be performed collectively on this series of data. In this case, the coincidence circuit 32 may be separated from the photodetector or the like, and this processing may be performed offline, away from the measurement environment.

[0059] In addition, even in the configuration of Figure 6(b), by arranging photodetectors 31A and 31B and corresponding coincidence circuits 32 in the x direction, it is possible to recognize the neutron absorption position (light emission position) in the x direction.

[0060] 6(b), the fluorescence generated by neutron absorption is, in principle, always detected simultaneously by the photodetectors 31A and 31B, and therefore noise components unrelated to the detection of radiation such as neutrons in the outputs of the photodetectors 31A and 31B are removed from the output of the coincidence circuit 32 unless they are accidentally output simultaneously. Note that the coincidence of the outputs of the photodetectors 31A and 31B in the coincidence circuit 32 is recognized as simultaneous if they occur within an appropriately set short time interval.

[0061] 6(b), if light absorption in the phosphor layer 11 can be ignored, it is possible to use only one of the detectors 31A and 31B without using the simultaneous detection circuit 32 in FIG. 6(b). Also, as described above, the phosphor layer 11 is formed thin in the z direction, and by providing a thicker light-transmitting layer 12 adjacent to it, the structure of FIG. 1 is particularly effective, in that it can receive not only the phosphor layer 11 but also light propagating through the light-transmitting layer 12 in the y direction.

[0062] (Second embodiment) Next, a second embodiment will be described, in which a scintillator having a structure different from that of scintillator 10 shown in Fig. 1 is used. Fig. 10 is a cross-sectional view corresponding to Fig. 1(b), showing the structure of this scintillator 50. This scintillator 50 also has five phosphor layers 11 and five light-transmitting layers 12, and is similar in that phosphor layers 11 interact with neutrons (or gamma-ray photons) to emit fluorescence.

[0063] However, in this figure, a thin light-shielding layer 13 that reflects but does not transmit fluorescence is formed on the right side of each light-transmitting layer 12. The light-shielding layer 13 is preferably made of a material that does not transmit fluorescence but has negligible neutron absorption (e.g., aluminum, etc.). While it is generally difficult for a thin metal to absorb neutrons, it is easy to block visible light and ultraviolet light, making it easy to form such a light-shielding layer 13. The energy of gamma rays or the high-energy electrons generated by them may also be absorbed by the light-shielding layer 13. Meanwhile, as will be described later, light detection is performed for each segment in this scintillator 50, and the light-shielding layer 13 serves as a boundary between the segments. In this case, the thickness of the light-shielding layer 13 may be set thick to facilitate light detection for each segment. For example, such a setting is easy because aluminum, etc., has low neutron absorption.

[0064] In Fig. 10, light emitted from one phosphor layer 11 is incident on the adjacent light-transmitting layer 12, but is prevented from entering the adjacent phosphor layer 11 by the light-shielding layer 13. For this reason, the scintillator 50 is divided into five segments S1 to S5 in the z direction with respect to fluorescence, with the light-shielding layer 13 as boundaries. In Fig. 10, one phosphor layer 11 and one light-transmitting layer 12 are provided in each segment, and light emitted from one phosphor layer 11 travels only within the segment to which it belongs, particularly along the y direction. Furthermore, due to the presence of the light-shielding layer 13, light emitted from the phosphor layer 11 is not emitted in the direction of arrow B in Fig. 6(a), but is emitted only in the directions of arrows C and D in Fig. 6(b).

[0065] 1, the number of stacked phosphor layers 11, etc. is actually set appropriately. In the example of Fig. 10, one segment is composed of one phosphor layer 11 and one light-transmitting layer 12, but one segment (area partitioned by light-shielding layer 13) may have multiple phosphor layers 11 and light-transmitting layers 12.

[0066] As described above, light emission due to the absorption of one neutron occurs only in a single phosphor layer 11, whereas light emission due to one gamma-ray photon may occur across multiple phosphor layers 11. In this case, if segments S1 to S5 are provided as in the structure of Fig. 10 and light emission is detected in segment units, the influence of light emission in other segments on light emission due to gamma-ray photons can be eliminated, thereby further reducing the intensity of light emission due to gamma-ray photons.

[0067] In this case, since it is not possible to extract light in the direction of arrow B in Fig. 6(a), this scintillator 50 cannot be used in place of the scintillator 10 in Fig. 6(a), but it can be used in place of the scintillator 10 in Fig. 6(b). Furthermore, this scintillator 50 can be used in a form different from that shown in Fig. 6(b). Fig. 11 shows the configuration of this neutron detector 3 in correspondence with Fig. 6(b). Here, the scintillator 50 is depicted in a simplified manner, with only the segments S1 to S5 described above being depicted.

[0068] 6(b), photodetectors 61A and 61B are provided in segment S1, photodetectors 62A and 62B are provided in segment S2, photodetectors 63A and 63B are provided in segment S3, photodetectors 64A and 64B are provided in segment S4, and photodetectors 65A and 65B are provided in segment S5. Also, corresponding to the coincidence counting circuit 32 in FIG. 6(b), the outputs P 1A , P 1B The coincidence circuit (coincidence unit) 71 receives the output P of the photodetectors 62A and 62B. 2A , P 2B The coincidence circuit (coincidence unit) 72 receives the output P of the photodetectors 63A and 63B. 3A , P 3B The coincidence circuit (coincidence unit) 73 receives the output P of the photodetectors 64A and 64B. 4A , P 4B The coincidence circuit (coincidence unit) 74 receives the output P of the photodetectors 65A and 65B. 5A , P 5Bare input to the coincidence circuit (coincidence unit) 75, respectively.

[0069] Therefore, the coincidence circuit 71 outputs an output pulse P 1A , P 1B The coincidence circuits 72 to 75 similarly output P2 to P5, which are the sums of the output pulses of the two connected photodetectors. That is, in this neutron detector 3, the configuration of the neutron detector 2 in FIG. 6(b) is realized for each segment, and the outputs P1 to P5 are obtained from different channels. As mentioned above, P1 (etc.) is converted into P 1A , P 1B It is not necessary to make it the sum (or the sum of) P 1A , P 1B It is also possible to appropriately calculate and use a value P1 (or the like) obtained by correcting the nonlinear component that does not depend on the incident position.

[0070] For this reason, when the scintillator 50 of FIG. 10 is used, output pulses P1 to P5 corresponding to the segments S1 to S5 are extracted individually through different channels CH1 to CH5, and the P extracted as an output in FIG. 6(b) A +P B correspond to P1 to P5 in this case, respectively.

[0071] Figure 12 shows the results of calculations for the point at which the light emission intensity due to gamma-ray photons can be particularly reduced in the configuration of Figure 11, corresponding to Figure 5. Here, the sensitivity to gamma rays is shown when the total film thickness of the phosphor layers 11 is kept constant and the number of layers (or the thickness of each layer) of the phosphor layers 11 is changed, depending on whether or not the phosphor layers are segmented.

[0072] FIG. 12 shows the results of a similar calculation for a scintillator 50 (with segmentation) obtained by segmenting the scintillator 10 using a light-shielding layer 13, along with the results of FIG. 5 (without segmentation). Here, a thickness of 5 mm corresponds to the case where only a single phosphor layer 11 is used, so the results for the scintillator with and without segmentation are essentially the same. In the case of a multilayer structure, the calculation results are shown for the first phosphor layer 11 (segment S1), which has the highest luminescence intensity (absorbed energy) due to gamma-ray photons. This result shows that segmentation can significantly reduce the luminescence intensity, particularly due to gamma-ray photons, compared to a case without segmentation, when the phosphor layer 11 is thin and the number of layers is large. On the other hand, when the absorption of neutron energy by the light-shielding layer 13 is negligible, the luminescence intensity due to neutron absorption is the same as that of the scintillator 10. Therefore, the configuration of FIG. 11 can particularly improve n / γ selectivity.

[0073] In the configuration of Figure 11, since the output is extracted from five channels (CH1 to CH5), it is sufficient to discriminate and detect neutrons for each channel, and in this case, discrimination is particularly easy. Also, if the output is divided into five channels, for example, the counting rate for each segment also decreases, making it effective even under high doses. Furthermore, when it is desired to measure neutron velocity with high precision, such as in TOF measurement, it is possible to know with which phosphor layer (segment) the neutron reacted, which reduces the uncertainty of the distance, making it effective.

[0074] Fig. 13 shows the configuration of a neutron detector 4, which is a modified example of the neutron detector 3 in Fig. 11. In the neutron detector 3 in Fig. 11, outputs are extracted for each of five channels, but in this neutron detector 4, a non-coincidence counting circuit (non-coincidence counting unit) 81 is used to produce a single output.

[0075] The non-coincidence counting circuit 81 receives CH1 (P1) through CH5 (P5) as inputs and, unlike the coincidence counting circuit 32, outputs only those CH1 (P1) through CH5 (P5) for which no coincidence was detected. Therefore, the non-coincidence counting circuit 81 outputs one of P1 through P5. The lack of coincidence in this output means that no other segments were emitted simultaneously except for the segment corresponding to the target output pulse (P1 through P5). As mentioned above, neutrons and gamma-ray photons are distinguished using pulse height. However, since gamma-ray photons may simultaneously emit multiple segments, the use of the non-coincidence counting circuit 81 also suppresses gamma-ray detection and further enhances n / γ selectivity. Like the coincidence counting circuit 32, the non-coincidence counting circuit 81 can also be configured using a computer. In this case, the processing does not need to be performed in real time but can also be performed offline. Particularly under high radiation doses, the probability of accidentally counting multiple signals simultaneously in each segment increases, so care must be taken when using the non-coincidence counting circuit 81.

[0076] The effect of such segmentation differs depending on the energy of the gamma rays. Figure 14 shows the results of a calculation for 5.0 MeV gamma rays, similar to the results of Figure 12 for 2.2 MeV gamma rays. When the incident gamma rays have high energy, there is a high probability that high-energy electrons ejected from the phosphor layer 11 will pass through this phosphor layer 11, then also pass through the light-transmitting layer 12, and will again enter the subsequent phosphor layer 11 and contribute to light emission. There is also a very high probability that high-energy electrons ejected from the light-transmitting layer 12 will not be completely absorbed by the light-transmitting layer 12, and will also be absorbed in the subsequent phosphor layer 11. Therefore, in the case of no segmentation in Figure 14, when the thickness of the phosphor layer 11 is 1 mm (density length equivalent: 0.25 g / cm 2 ) and 1.67 mm (density length equivalent: 0.42 g / cm 2 ) the sensitivity of gamma rays is 5 mm (density length equivalent 1.25 g / cm 2) (conventional example). However, segmentation causes the light emitted across multiple phosphor layers 11 to be distributed among the outputs of each segment, lowering the output of a single segment and significantly reducing sensitivity. For this reason, the effect of segmentation becomes greater as the energy of gamma rays increases. Furthermore, the non-coincidence counting circuit 81 described above is particularly effective against such high-energy gamma rays.

[0077] In the second embodiment, the preferred range of the thickness (density length) of the phosphor layer 11 is the same as in the first embodiment. On the other hand, in the second embodiment, the fluorescence does not propagate between segments, so the restriction on the thickness (density length) of the light-transmitting layer 12 is loose. However, in the second embodiment, if each segment is thick, the photosensitive area of ​​the photodetector must be increased, which also causes a disadvantage in terms of cost. For this reason, it is not preferred to make the light-transmitting layer 12 unnecessarily thick, and the thickness of the light-transmitting layer 12 is set to 6 mm (density length equivalent: 1.3 g / cm 2 ) or less.

[0078] In the second embodiment, the light-transmitting layer 12 functions to transmit fluorescence to the photodetector, but the phosphor layer 11 can also guide fluorescence to the photodetector. Therefore, particularly when segmentation is performed, the light-transmitting layer 12 does not need to be provided within the segment. However, because the phosphor layer 11 is set to be thin as described above, the efficiency of light (fluorescence) propagation to the photodetector in the in-plane direction is not high. From this perspective, it is also preferable to provide the light-transmitting layer 12 within the segment.

[0079] In the technology described in Patent Document 2, inorganic phosphor particles and a resin material are used, with the inorganic phosphor particles corresponding to the phosphor layer 11 and a layer made of the resin material corresponding to the light-transmitting layer 12. However, unlike the light-transmitting layer 12, there is a high probability that neutrons will be scattered, thermalized, or absorbed in a resin material containing hydrogen. Therefore, the neutron detection efficiency in this case is lower than that of the present invention. Furthermore, although the inorganic phosphor particles and the resin material are generally made of completely different materials, their densities must be similar in order to uniformly mix them. Under these conditions, further approximating their refractive indices and suppressing fluorescence reflection at their interface, as between the phosphor layer 11 and the light-transmitting layer 12 in the present invention, imposes strict restrictions on the resin material or inorganic phosphor particles, making it difficult to select and use such materials in practice. In contrast, in the present invention, there are no such restrictions on the densities of the phosphor layer 11 and the light-transmitting layer 12, thereby increasing the freedom of material selection.

[0080] Furthermore, in the case of the present invention, the phosphor layer 11 is made thin in the direction of incidence of gamma rays (neutrons) and has a laminated structure, which makes it possible to significantly reduce the sensitivity to gamma rays without reducing the sensitivity to neutrons as described above, whereas in the technology described in Patent Document 2, the size of the inorganic phosphor particles is isotropic and unrelated to the direction of incidence, so the effect of reducing the sensitivity to gamma rays without reducing the sensitivity to neutrons is small.

[0081] The specific configuration of the neutron detector is arbitrary as long as it can perform the same operation as described above. For example, any combination of phosphor layers and light-transmitting layers may be used, and the main components of these layers may be different, as long as they can perform the same function as described above. The configuration of the photodetector can also be set appropriately. [Explanation of symbols]

[0082] 1~4 Neutron detector 10, 50, 100 Scintillators 11 Phosphor layer 12 Light transmission layer 13 Light blocking layer 21 Photodetector 31A, 61A, 62A, 63A, 64A, 65A Photodetector (first photodetector) 31B, 61B, 62B, 63B, 64B, 65B Photodetector (second photodetector) 32, 71~75 Coincidence circuit (coincidence unit) 81 Non-coincidence counting circuit (non-coincidence counting section) S1~S5 segments

Claims

1. A neutron detector that detects neutrons by fluorescence emitted when a phosphor containing a neutron-absorbing isotope that absorbs neutrons and emits secondary charged particles absorbs the neutrons, a scintillator having a laminated structure in which a thin-film phosphor layer made of the phosphor and a thin-film light-transmitting layer adjacent to the phosphor layer in a film thickness direction and made of a light-transmitting material that transmits the fluorescence are provided in multiple sets along the incident direction of the neutrons; a photodetector that detects the fluorescence and emits an output pulse as an output; Equipped with A neutron detector characterized in that the density length, which is the product of the density and thickness of the phosphor layer, is in the range of 0.0625 g / cm 2 to 0.5 g / cm 2 , and the density length of the light transmitting layer is in the range of 0.2 g / cm 2 to 1.3 g / cm 2 .

2. 2. The neutron detector according to claim 1, wherein the photodetector detects the fluorescence emitted from the scintillator along the incident direction.

3. 2. The neutron detector according to claim 1, wherein the photodetector detects the fluorescence emitted from the scintillator along an in-plane direction of the phosphor layer and the light-transmitting layer.

4. The photodetectors include a first photodetector and a second photodetector that face each other along the in-plane direction with the scintillator interposed therebetween, 4. The neutron detector according to claim 3, further comprising a coincidence counting unit that, when synchronism is recognized between a first output pulse that is the output pulse from the first photodetector and a second output pulse that is the output pulse from the second photodetector, newly outputs an output based on the first output pulse and the second output pulse as the output pulse.

5. In the scintillator, a plurality of segments are formed along the incident direction by combining the phosphor layer with the light-transmitting layer adjacent to the phosphor layer, through which the fluorescence emitted from the phosphor layer propagates; and a light-shielding layer that blocks the fluorescence is provided between the segments adjacent to each other in the incident direction in the laminated structure so that the fluorescence propagates in the in-plane direction within each segment and does not propagate between adjacent segments; 5. The neutron detector according to claim 3, wherein the photodetector is provided for each of the segments.

6. 6. The neutron detector according to claim 5, further comprising a non-coincidence counting unit that outputs an output pulse for each segment that has not been found to be simultaneous with other output pulses.

7. 7. The neutron detector according to claim 1, wherein the ratio of the refractive index of the light transmitting layer to the refractive index of the fluorescent material layer to the fluorescent material is in the range of 0.90 to 1.

10.

8. The main components of the phosphor layer and the light-transmitting layer are silicon oxide (SiO ２ 8. The neutron detector according to claim 1, wherein the phosphor layer is doped with the neutron absorbing isotope.

9. The neutron absorbing isotope ６ Li or １０ 9. The neutron detector according to claim 8, wherein the neutron detector is B.

Images