Neutron detector

Abstract

The present invention relates to a neutron detector capable of measuring a high dose of neutrons with high efficiency and n / γ selection capability. The scintillator (10) has a laminated structure in which a phosphor layer (11) and a light-transmitting layer (12) are alternately laminated in the z direction. The phosphor layer (11) is made of a phosphorous material emitting fluorescent light by absorbing neutrons, the material being, for example, a scintillator material used in previously known neutron detectors. The light-transmitting layer (12) is made of a material having a high transmittance for fluorescent light emitted by the phosphor and only weakly absorbing neutrons.In the scintillator (10), when neutrons and gamma-ray photons enter it, the intensity of the luminescence (pulse height) due to the neutrons is significantly different from that due to the gamma-ray photons. It is thus easy to distinguish the outputs of the two types of radiation. Figure for abstract: Fig. 1B.

Description

Title of the invention: Neutron detector

[0001] The present invention relates to a neutron detector which detects neutrons by detecting scintillation light emitted by the neutrons. Technological background

[0002] When measuring the neutron dose using a neutron detector under a high neutron dose condition, it is advantageous to have a fast-response detector and a high detection efficiency, in order to reduce neutron counting losses. However, in such an environment, in general, it often happens that not only the neutron dose, but also the gamma ray dose are high. For example, when a neutron moderating material containing a large amount of hydrogen is used in an environment where the fast neutron dose is high, gamma rays of 2.2 MeV (Megaelectron-Volt) are generated by the capture reaction of the moderated neutrons with hydrogen, causing the gamma ray dose to necessarily become high.Therefore, there is a demand for a neutron detector with fast response and high detection efficiency that can detect neutrons by distinguishing them from gamma rays (neutron-gamma ray discrimination) even when both the gamma ray and neutron doses are high.

[0003] Since neutrons have no charge and therefore have a very high penetrating power compared to charged particles, it is generally necessary, for the detection of slow neutrons, to use a detector employing an isotope with a large neutron absorption reaction cross-section accompanied by the emission of high-energy secondary charged particles or the like (neutron-absorbing isotope) and whose probability of neutron absorption is thus increased. Even in this case of using a neutron-absorbing isotope, in order to detect in particular neutrons of an energy equal to or greater than that of epithermal neutrons with sufficient sensitivity, it may be necessary to use a detector having a density length of a value of the order of several to several tens of g / cm2 corresponding to the product of the density and the thickness.

[0004] Commonly used neutron detectors include a proportional counter detecting the ionization of the gas caused by its interaction with radiation, by amplifying the charge proportionally to the amount of ionization, and a scintillation detector detecting luminescence (fluorescence) based on the amount of energy transferred by the radiation in the scintillator using a photodetector. These detectors also having a detection sensitivity for gamma rays, when the sensitivity to neutrons is increased, the sensitivity to the gamma rays described above, corresponding to a background event, also increases. Therefore, a neutron detector capable of detecting neutrons by distinguishing them from gamma rays is required.

[0005] In a proportional counter, as described in patent literature 1, for example, the ability to distinguish between neutrons and gamma rays (hereinafter referred to as n / y distinguishing ability) can be increased when using 3 He gas, which has a large neutron absorption reaction cross-section and hardly interacts with gamma rays due to its low atomic number. However, in this case, the height of the pulses corresponding to the output is small until charges are loaded into the proportional counter, and the time, during which detection is difficult to achieve (resolution time), becomes long. When the resolution time is long, it becomes difficult, especially under a high dose condition, to identify each individual pulse of the temporally continuous output pulses, and correct counting becomes difficult.Furthermore, in order to measure epithermal neutrons and fast neutrons with high efficiency, it becomes necessary to make the gas layer sufficiently thick (large density length), causing the problem of a device becoming large, or a device becoming expensive due to the high price of 3He.

[0006] The scintillation detector is suitable for measurement under high dose conditions, as it uses a phosphor emitting fluorescent light by absorbing the energy of charged particles, where the resolution time described above can be reduced by the selection of the phosphor. For example, phosphors of the glass family 6Li:Ce 3+ to which a neutron-absorbing isotope such as 6Li is added, are widely used for neutron detection, thanks to their relatively fast decay time (about 60 nanoseconds) and the moderate amount of luminescence due to thermal neutrons in terms of equivalent electron energy (MeVee for "Megaelectron-Volt electron equivalent") (about 1.6 MeVee).The reason why the amount of luminescence was mentioned in terms of electron equivalent energy (MeVee) in the above description is that even when the amount of luminescence itself is large, if the amount of luminescence due to gamma rays is also large, it results in a mixing of the signals for gamma rays and those for neutrons, therefore, the electron equivalent energy is an important parameter for judging the n / y discrimination ability. The n / y discrimination ability can be increased by using a phosphor whose amount of luminescence in terms of electron equivalent energy (MeVee) upon absorption of neutrons is very large. Such a phosphor, for example, . 6LiF / ZnS:Ag (equivalent energy of an electron of a thermal neutron of about 10 MeVee) to which a neutron-absorbing isotope such as 6Li is added is widely known. However, this phosphor has a relatively long decay time (about 200 nanoseconds) and, in addition, a very long decay time constant component, which poses a problem of fast response.

[0007] On the other hand, patent literature 2 discloses a scintillator whose n / y discrimination ability is increased by an improvement in the structure. The scintillator has a configuration in which inorganic phosphor particles made of such a phosphor as described above are dispersed in a resin material. In this case, neutrons can be detected by the fluorescent light emitted by the phosphor upon absorption of neutrons as in the case described above, but it is possible to reduce the intensity of the fluorescent light emitted upon absorption of gamma ray energy, compared to previous cases where the scintillator is uniformly made of a phosphor. Therefore, the distinction between neutrons and gamma rays becomes easy. [Prior art document] [Patent Literature] [Patent Literature 1] JP 2008-14947 A [Patent Literature 2] WO 2015 / 064588 [Technical problem]

[0008] When 6LiF / ZnS:Ag is used as a phosphor, measurement under high-dose conditions is difficult because the decay time constant of fluorescent light is long. In addition, the extraction efficiency of fluorescent light is reduced when a large crystal is used because the transmittance of fluorescent light in this material is low. On the other hand, although the +Li-glass:Ce3+ family allows measurement under a high-dose condition as described above, its luminescence amount in terms of MeVee is not sufficient, and its n / y discrimination ability is poor. For this reason, it has been difficult to realize a neutron detector capable of detecting neutrons with high n / y discrimination ability under a high-dose condition only by adjusting the phosphor material.

[0009] Therefore, in addition to this adjustment of the phosphor material, it is effective to design a scintillator structure as described in patent literature 2. However, due to the difference in specific gravity between the inorganic phosphor particles and the resin material, it is difficult to disperse uniformly disperse the inorganic phosphorus particles in the resin material, and therefore the scintillator is not easy to manufacture. In addition, since the resin material contains a large amount of hydrogen whose mass is approximately equal to that of neutrons, the neutrons are elastically scattered and thermalized, which increases the probability of neutron absorption by elements other than the inorganic phosphorus particles, thereby reducing the detection efficiency. In addition, this thermalization (scattering) causes an increase in the neutron transit time in the scintillator, resulting in a deterioration in the detection time resolution for neutron detection time measurement, and therefore it is difficult to use the scintillator in, for example, a neutron time-of-flight (TOF) measurement requiring high time resolution.

[0010] For the reasons described above, a neutron detector capable of measuring a high dose of neutrons with high efficiency and n / y selection capability is desired.

[0011] The present invention has been made in view of the problems described above, and aims to provide an invention which solves these problems. Objectives of the invention

[0012] The present invention is configured as follows, in order to solve the problems.

[0013] The neutron detector of the present invention is a neutron detector configured to detect neutrons by means of fluorescent light emitted when the neutrons are absorbed by a phosphor, the phosphor containing a neutron-absorbing isotope which emits secondary charged particles upon absorbing the neutrons, the neutron detector comprising a scintillator configured to have a layered structure comprising, along a neutron incidence direction, a plurality of combinations of a thin-film phosphor layer made of the phosphor and a thin-film light-transmitting layer made of a light-transmitting material transmitting fluorescent light and being adjacent to the phosphor layer in the thickness direction, and a photodetector configured to emit an output pulse as an output generated upon detection of the fluorescent light.

[0014] The neutron detector of the present invention can be characterized in that the photodetector detects the fluorescent light emitted by the scintillator in the direction of incidence.

[0015] The neutron detector of the present invention can be characterized in that the photodetector detects fluorescent light emitted by the scintillator along a direction in the plane of the phosphor layers and the light transmission layers.

[0016] The neutron detector of the present invention can be characterized by comprising, as a photodetector, a first photodetector and a second photodetector opposite to each other in the plane direction, the scintillator being inserted therebetween, and a coincidence counting unit configured to output, as a new output pulse, an output based on a first output pulse corresponding to the output pulse of the first photodetector and a second output pulse corresponding to the output pulse of the second photodetector when synchronization is recognized between the first and second output pulses.

[0017] This neutron detector of the present invention can be characterized in that in the scintillator, a plurality of segments each composed of a combination of the phosphor layer and the light-transmitting layer neighboring the phosphor layer and transmitting the fluorescent light emitted from the phosphor layer are formed along the incident direction, and, in the layered structure, a light-shielding layer for shielding the fluorescent light is provided between the segments neighboring each other in the incident direction so that the fluorescent light in one segment propagates in the plane direction, but does not propagate between the neighboring segments, and the photodetectors are provided according to each of the segments.

[0018] The neutron detector of the present invention can be characterized in that it comprises an anti-coincidence counting unit configured to output, among the output pulses of the respective segments, the output pulse whose synchronism with other output pulses has not been recognized.

[0019] The neutron detector of the present invention can be characterized in that a ratio of the light transmission layer to the phosphor layer in refractive index for fluorescent light is in a range of 0.90 to 1.10.

[0020] The neutron detector of the present invention can be characterized in that a main component of the phosphor layer and that of the light transmission layer are silicon dioxide (SiO2), and the neutron absorbing isotope has been added into the phosphor layer.

[0021] The neutron detector of the present invention can be characterized in that the neutron-absorbing isotope is 6Li or 10B.

[0022] The neutron detector of the present invention can be characterized in that a density length, corresponding to the product of the density and the thickness, is set in a range of 0.0625 to 0.5 g / cm2 and in a range of 0.2 to 1.3 g / cm2, respectively, for the phosphor layer and the light transmission layer. Advantageous effects of the invention

[0023] Configured as described above, the present invention provides a neutron detector capable of measuring a high dose of neutrons with high efficiency and n / y selection capability. Brief description of the figures

[0024] [Fig.lA] is a plan view showing a structure of a scintillator used in a neutron detector according to a first embodiment of the present invention.

[0025] [Fig.lB] 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.

[0026] [Fig.2] is a diagram schematically showing the pulse height distributions where neutrons and gamma rays are detected, in the case of using a prior technology scintillator or that according to the present invention.

[0027] [Fig.3] is a diagram showing the luminescence states by interaction of gamma ray photons (A) and neutrons (B) (C) in a phosphor.

[0028] [Fig.4] shows the relationship between the rate of absorption of the energy of charged particles emitted by absorption of neutrons in a layer of phosphorus and the thickness of this layer.

[0029] [Fig.5] shows the relationship between the sensitivity to 2.2 MeV gamma rays in a phosphorus layer and the thickness of this layer.

[0030] [Fig.6A] is a diagram showing a first type of configuration of a neutron detector (first example) according to the first embodiment.

[0031] [Fig.6B] is a diagram showing a second type of configuration of a neutron detector (second example) according to the first embodiment.

[0032] [Fig.7] shows the result of an actual measurement of the pulse height distributions as a function of the number of light transmission layers through which the fluorescent light passes.

[0033] [Fig.8] shows the result of the comparison between the sensitivity to 2.2 MeV gamma rays and the thickness of a light transmission layer for each thickness of phosphor layers, in the scintillator used for the neutron detector according to the first embodiment.

[0034] [Fig.9] shows a result of the comparison between the sensitivity to 5.0 MeV gamma rays and the thickness of a light transmission layer for each thickness of phosphorus layers, in the scintillator used for the neutron detector according to the first embodiment.

[0035] [Fig. 10] 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.

[0036] [Fig. 11] is a diagram showing a configuration of a neutron detector according to the second embodiment.

[0037] [Fig. 12] shows the result of the comparison between the sensitivity to 2.2 MeV gamma rays and the thickness of a light transmission layer for each thickness of phosphor layers, in the scintillator used for the neutron detector according to the second embodiment.

[0038] [Fig. 13] is a diagram showing a configuration of a modification of the neutron detector according to the second embodiment.

[0039] [Fig. 14] shows the result of the comparison between the sensitivity to 5.0 MeV gamma rays and the thickness of a light transmission layer for each thickness of phosphor layers, in the scintillator used for the neutron detector according to the second embodiment. Description of the embodiments

[0040] A neutron detector according to each embodiment of the present invention is a scintillation detector for detecting neutrons. Therefore, similarly to the neutron detector described in patent literature 2, for example, it uses a scintillator absorbing neutrons and thus emitting fluorescent light and a photodetector detecting the fluorescent light in combination. The present invention is characterized by a structure of the scintillator or a form of combining the scintillator with the photodetector. Hereinafter, two embodiments distinguished by the basic structure of the scintillator will be described. First embodiment

[0041] [Fig.lA] shows a plan view showing a structure of a scintillator 10 used in a neutron detector according to a first embodiment, and [Fig.lB] shows a cross-sectional view showing this. Here, neutrons to be detected enter the scintillator 10 from the negative side in the z-direction in the diagrams. The scintillator 10 has a laminated structure in which a phosphor layer 11 and a light-transmitting layer 12 are alternately laminated in the z-direction. The phosphor layer 11 and the light-transmitting layer 12 each have a thin-film shape extending in the x- and y-directions in the diagrams. Although five phosphor layers 11 and four light-transmitting layers 12 are light transmission 12 are provided in Figures 1A, 1B, the numbers of layers are set appropriately.

[0042] The phosphor layer 11 is made of a phosphor that emits fluorescent light by absorbing the energy of charged particles, and the charged particles can be detected by detecting the fluorescent light. Here, in order for the phosphor to be particularly sensitive to charge-free neutrons, a phosphor to which a neutron-absorbing isotope is added is used, for example. Examples of such a phosphor are a scintillation glass already known for neutron detection such as 6Li-glass:Ce3+. Specifically, GS20, KG2 (manufactured by Scintacor) or the like are used. To achieve sufficient n / y discrimination ability, the amount of luminescence caused by neutron absorption is preferably 1.5 MeVee or more in terms of electron equivalent energy (MeVee).The light-transmitting layer 12 is made of a material having a high transmittance for fluorescent light emitted by the phosphor and only weakly absorbing neutrons. In addition, it is also preferable that the light-transmitting layer 12 has a refractive index for fluorescent light close to that of the phosphor layer 11, as will be described later, and is transparent to fluorescent light, and the materials preferably used to form the light-transmitting layer 12 include synthetic quartz, lead glass slightly containing lead oxide, or the like.

[0043] With such a configuration, the scintillator 10 has its n / y discrimination capability increased in neutron detection, and highly efficient neutron detection can be achieved due to this increased n / y discrimination capability. This point will be described below. In general, a phosphor forming a scintillator emits fluorescent light by absorbing the energy of charged particles. When the fluorescent light is detected by a photodetector (photomultiplier tube) or the like with high time resolution, photoelectrons are generated on the photocathode by the fluorescent light and are amplified, thereby producing a pulsed electrical output according to the time distribution of the luminescence. The number of photoelectrons corresponds to the intensity of the luminescence, which corresponds to the output pulse, called the pulse height or integrated charge value.

[0044] In this case, although there is a slight difference between neutrons and gamma rays in the waveform of the output pulse, caused by a difference in the form of energy transfer to the phosphor from them, it is generally difficult to distinguish between the output pulse produced by fluorescence at a time of neutron absorption and the output pulse produced by fluorescence at a time of energy transfer from gamma rays, in a case where the amount of light (total number of photons) is approximately the same for both types of fluorescence. For example, even when the emission energy at the time of neutron absorption in the phosphor is 4.78 MeV, as described later, the equivalent electron energy for the corresponding amount of luminescence is about 1.6 MeVee in the GS20 described above. On the other hand, for example, 2.2 MeV gamma rays mainly undergo Compton scattering in the phosphor, where they transfer continuous energy of about 2.0 MeV and below to the electrons in the phosphor. Therefore, in an area where the pulse heights due to the two types of fluorescence overlap, it is theoretically impossible to distinguish the two types of fluorescence from the pulse heights. It is required to detect neutrons as distinct from gamma rays even in such a situation.

[0045] Therefore, the scintillator 10 of the present invention is configured to have a structure in which, when neutrons and gamma-ray photons are incident, the luminescence intensity (pulse height) is largely different between the neutrons and the gamma-ray photons. Thus, it becomes possible to easily distinguish them from the pulse heights.

[0046] First, a description will be given of the output (output pulse height distribution) in a case where a general type of scintillator is used and both neutrons and gamma rays are present. [Fig. 2] is a diagram schematically illustrating the condition. The horizontal axis represents the pulse height of the output pulses, which corresponds to the energy absorbed by the phosphor from a single neutron or gamma ray photon. The vertical axis represents the detection frequency of neutrons or gamma ray photons when they are detected in large numbers.

[0047] In [Fig.2], DI is a distribution of the height of the output pulses due to neutrons in the case of using a usual scintillator whose entire body is made of a thick phosphor. Here, the peak energy of the distribution corresponds to a constant emission energy due to the nuclear reaction between 6Li and neutrons, which will be described later (the equivalent electron energy of the luminescence quantity is 1.6 MeVee in GS20, as described above). On the other hand, D2 is a pulse height distribution for gamma rays (2.2 MeV) when using the same scintillator. Here, unlike the DI distribution due to neutrons, which has a single peak as described above, the distribution due to gamma rays is broad, because gamma rays generate a continuous spectrum caused by Compton scattering, which extends from near a gamma ray energy maximum to the low energy side.As stated here, it is difficult, in the region. overlapping DI and D2, to distinguish neutrons and gamma rays from pulse heights alone.

[0048] In general, a phosphor forming the scintillator emits fluorescent light by absorbing the energy of charged particles having an energy sufficiently greater than that required to cause the electrons of the phosphor to transition from the ground state to the excited state. The wavelength of luminescence corresponds to the energy difference between the excited state and the ground state, and photons having the wavelength of luminescence are generated in numbers corresponding to the energy absorbed by the phosphor and the amount of energy absorbed per unit length or the like.

[0049] Here, since the photons of the gamma rays transfer their energy to the electrons of the phosphor by undergoing electromagnetic interaction with the electrons, the charged particles described above are the electrons, and fluorescent light is emitted by the electrons transferring their kinetic energy to the phosphor. At this time, the electrons ejected by the gamma rays tend to advance in the forward direction according to the law of conservation of momentum, and in particular, the advance direction of the electrons having received a large amount of energy being closer to the pulse height due to the neutrons becomes closer to the initial incidence direction of the gamma rays.

[0050] On the other hand, neutrons do not undergo electromagnetic interaction and, therefore, the probability of neutron absorption of general substances is low. However, when the phosphor contains a neutron-absorbing isotope that emits high-energy secondary charged particles by absorbing neutrons, high-energy secondary charged particles are generated by the neutron-absorbing isotope when it absorbs neutrons. For example, when 6Li, a well-known neutron-absorbing isotope, is used, the reaction is expressed by equation (1).

[0051] [Math.l] o + 6Li -* « + 3R + 4.78 MeV * • • (1)

[0052] That is, the secondary charged particles in this case are α-particles (4He nuclei) and tritium (3H nuclei), which are not direction-dependent in their emission distribution, unlike electrons in the case of the gamma ray described above, and are emitted in opposite directions while having kinetic energies of 2.05 MeV and 2.73 MeV respectively and in total about 4.78 MeV, according to the law of conservation of momentum. Subsequently, the electrons in the phosphorus are excited by the secondary charged particles, and fluorescent light is emitted in the same way. The same is true for other neutron-absorbing isotopes (e.g., 10B), where energy is transferred from secondary particles, but these may be of a different type and energy than those described above.

[0053] Here, since the mass of the secondary charged particle (nucleus) > mass of the electron (e.g., mass of the α-particle : mass of the electron = 7300 : 1), when they have almost the same energy, the equation velocity of the secondary charged particle < velocity of the electron holds. Therefore, there is a difference between the situation where electrons generated by gamma photons cause the phosphor to luminesce and the situation where secondary charged particles generated by neutrons cause the phosphor to luminesce. [Fig.3] is a diagram showing these states schematically. In this diagram, it is assumed that gamma photons and neutrons are incident from the left side of the diagram.

[0054] In [Fig. 3], (A) in the top row, schematically shows a situation in which high-energy electrons generated by gamma-ray photons transfer their energy and thus cause luminescence in a scintillator 100 made of a thick phosphor. On the other hand (B) in the middle row and (C) in the bottom row show similar situations for secondary charged particles generated by neutrons, in comparison. Here, the diagram is one that illustrates a region or the like of luminescence, but not one that illustrates the total amount of luminescence. In addition to the fact that the speed of electrons is higher than that of secondary charged particles, as described above, the amount of charge of electrons is half that of α-particles (secondary charged particles), for example.Since the transfer energy per unit transit distance (dE / dx) of charged particles (electrons, secondary charged particles) is proportional to the product of the transit time (inversely proportional to the velocity) and the square of the charge quantity, the equation becomes dE / dx of secondary charged particles > dE / dx of electrons. Therefore, in scintillator 100, the equation becomes range of secondary charged particles < range of electrons and for example, the maximum range of 1.5 MeV electrons in the GS20 phosphor described above is about 2.5 mm. However, since electrons are light (or have the same mass as the electrons in the phosphor), they are consequently easily scattered, have complicated paths, as shown in [Fig.3] (A), and have a shorter range.When the scintillator 100 is thinner than the range of the electrons, the probability that the electrons cannot transfer all the energy to the scintillator 100 becomes high, and the pulse height is reduced compared to the case when the scintillator 100 is sufficiently thick.

[0055] On the other hand, since high-energy secondary charged particles generated by neutron absorption transfer a large amount of energy per unit transit distance, they only need a short distance to transfer all the energy, for example, several pm for a-particles and several tens of pm for 3H, in phosphorus GS20.

[0056] Therefore, when a neutron-absorbing isotope is contained in a phosphor, the thickness of the phosphor can be set at a level allowing the transfer of almost all the kinetic energy of the secondary charged particles in the absorption of neutrons, and allowing the immediate departure of high-energy electrons generated in the interaction with gamma-ray photons. When the thickness is thus set, in [Fig. 2], contrary to the distribution of pulse height due to neutrons DI being ideally maintained without change, the distribution of pulse height due to gamma rays changes from D2 to D3 present on the lower energy side. By setting a threshold value T between the distributions DI and D3, it is possible to determine that neutrons have been detected when a detected pulse height is equal to or greater than T, and that gamma rays have been detected when the pulse height is less than T. 。

[0057] As described above, the direction of progression of high-energy electrons is substantially equal to the direction of incidence (progression) of gamma rays, it is preferable, in order to improve the effect described above, that the direction of incidence of gamma rays is equal to the direction of the thickness of the phosphor when the phosphor is in the form of a thin film. In many cases where both neutrons and gamma rays are present, the neutron source and the gamma ray source overlap, and thus the condition is satisfied.

[0058] Since the probability of neutron absorption is not high, there may be a case, such as that shown in (B) in [Fig. 3], where neutrons are absorbed near the surface and secondary charged particles are generated there, and there may also be a case, such as that shown in (C), where neutrons are absorbed at a position deep in the phosphor and secondary charged particles are generated there. When the phosphor is thin and composed of a single layer, neutrons can be detected in case (B), but neutrons cannot be detected in case (C) corresponding to a case where neutrons pass through the phosphor without undergoing interaction, where the neutron detection efficiency may therefore be reduced. In contrast, by employing a multi-layer structure composed of phosphor layers 11 and light transmission layers 12, as illustrated in [Fig.lB], to maintain a total thickness of the phosphorus 11 layers, it becomes possible to detect neutrons even in case (C), and thus to maintain a high neutron detection efficiency.

[0059] Hereinafter, a description of the result of a specific investigation carried out will be given to clarify the question described above. [Fig.4] shows the result of calculating the probability that most of the energy of charged particles secondary particles (α-particles, 3H nuclei) generated by neutrons are absorbed by the phosphor. Here, it is assumed that the neutron-absorbing isotope is 6Li as described previously, therefore the secondary charged particles are α-particles and 3H nuclei, and the phosphor is the already described GS20. Although it is preferable that the phosphor is thick in order to sufficiently increase the absorption probability (up to nearly 1), it is recognized from the result that the absorption ratio is 0.98 when the phosphor thickness is 1.0 mm, about 0.94 even when the thickness is 0.25 mm, but it is drastically reduced for smaller thicknesses.

[0060] On the other hand, as described above, the equivalent energy of electrons corresponding to the amount of luminescence due to neutrons (secondary charged particles described above) when using the GS20 is 1.6 MeVee. That is, the output pulse height becomes substantially the same for electrons having this energy and for secondary charged particles, so it is impossible to distinguish between gamma rays and neutrons (or between high-energy electrons and secondary charged particles) by the pulse height, as indicated by DI and D2 in [Fig. 2].

[0061] To enable the distinction between neutrons and gamma ray photons generating electrons having the energy level described above by the pulse height of the output pulses, the distribution of D3 in [Fig. 2] can be achieved by reducing the energy transferred from the gamma rays (electrons generated by them) to the phosphor so that it is sufficiently lower than 1.6 MeV. For example, the case considered here is that of the use of a general detector composed of a photomultiplier tube and the GS20 phosphor (scintillator) already described, whose energy resolution (value obtained by dividing the peak value of a pulse height distribution by the full width of the half-maximum of the pulse height distribution) is about 16% for secondary charged particles and about 24% for 1.2 MeV electrons. In this case, by fixing T, in the pulse height distribution of [Fig.2], so that 99% of the DI distribution for neutrons falls within a range equal to or greater than T, and by making the energy transfer caused by gamma rays in the phosphorus layer 1.2 MeV or less, almost no pulse height values ​​for gamma rays fall within the range equal to or greater than the threshold T, and therefore the n / y discrimination ability by the pulse height is increased. Therefore, the phosphorus layer 11 can be made thin so that the energy transferred from electrons to the phosphorus becomes 1.2 MeV or less.

[0062] As described above, the probability of absorption of neutrons having an energy equal to or greater than that of epithermal neutrons, the total thickness of the phosphorus layer 11 must be equal to or greater than a certain value in order to detect neutrons with high efficiency, for this, such a multilayer structure as shown in [Fig.lB] is effective for this purpose. However, by adopting the multilayer structure, the probability of interaction with gamma rays is also increased, and even if their pulse height distribution is shifted to the low energy side as described above, it may also happen that the frequency itself increases to cause an increase in sensitivity to gamma rays.Therefore, a study was carried out on the gamma-ray sensitivity (probability of transferring energy above 1.2 MeV to phosphorus) when the number of phosphorus-11 layers (and the thickness of each layer) is varied, while keeping the total thickness constant. [Fig. 5] shows the result of calculating the relative values ​​of gamma-ray sensitivity when the total thickness of the phosphorus-11 layers is set at 5 mm, and is distributed over a single layer (with a thickness of 5 mm), three layers (each individual phosphorus-11 layer having a thickness of 1.67 mm), five layers (with a thickness of 1.67 mm), three layers (with a thickness of 1.67 mm), three layers (with a thickness of 1.67 mm), three layers (with a thickness of 1.67 mm), five layers (with a thickness of 1.67 mm).67 mm), between 5 layers (each individual layer 11 having a thickness of 1 mm), between 10 layers (each individual layer 11 having a thickness of 0.5 mm) and between 20 layers (each individual layer 11 having a thickness of 0.25 mm), where, in the multi-layer cases, the light-transmitting layer 12 made of synthetic quartz 2 mm thick is inserted between the phosphor layers 11, thus forming the multi-layer structure of Figures 1A and 1B. In this case, it is assumed that the gamma-ray energy was 2.2 MeV, which corresponds to that emitted by the reaction between moderated neutrons and hydrogen in the neutron moderating material, as described above. Such 2.2 MeV gamma rays can be a significant background event under various measurement conditions, and are therefore one of the gamma rays to which the most attention should be paid in practice. .

[0063] In [Fig.5], the case of a thickness of 5 mm (single layer) corresponds to that of a usual scintillator entirely and uniformly constituted by a phosphor without having a light transmission layer 12. From [Fig.5], it is evident that by adopting the multi-layer structure in which the light transmission layers 12 are inserted, the intensity of the luminescence due to the photons of the gamma rays can be considerably reduced compared to the usual scintillator.

[0064] The amount of electron energy absorption depends greatly on the product of the density and the thickness of the phosphor (called the density length). Therefore, to discuss the result more generally, it is preferable to use the density length obtained by multiplying the abscissa of [Fig.5] by the density of GS20 (2.5 g / cm3) as an index, and in this way, it is considered that even when a phosphor other than GS20 is used, a result similar to that of GS20 can be obtained by using a phosphor having an equivalent density length.For example, compared to a case of using a single phosphorus 11 layer with a density length of 1.25 g / cm2 (equivalent to 5 mm thickness of GS20), the energy absorption is reduced by adopting the above-described layered structure comprising five phosphorus 11 layers each having a density length of 0.25 g / cm2 (equivalent to 1 mm thickness of GS20), where the energy absorption rate of 1.2 MeV or more becomes about 1 / 10. When the density length is used as an index, the preferred range of the density length of the phosphorus 11 layer for achieving the above-described effect is 0.0625 to 0.5 g / cm2.

[0065] Therefore, even in cases where the total thickness of the phosphor layers 11 is the same, when the thickness of each individual phosphor layer 11 is smaller and the total number of layers is larger, a difference between the pulse height of the output pulses due to neutrons and that due to gamma ray photons is increased.

[0066] However, if the neutrons are absorbed in the light-transmitting layer 12, the neutrons are not detected, because the energy due to this reaction does not contribute to the luminescence. Therefore, in the scintillator 10 having the structure of FIGS. 1A, 1B, the neutron detection efficiency is reduced by an amount corresponding to the absorption of the neutrons in the light-transmitting layer 12. Therefore, it is desirable that a material having a low probability of neutron absorption be used for the light-transmitting layer 12.

[0067] It is preferable that the light-transmitting layer 12 be made of a material not containing the neutron-absorbing isotope described above and being transparent to fluorescent light. However, in a neutron detector to be described later, the fluorescent light extracted from the scintillator 10 is detected externally, where the reflection at the interface between the phosphor layer 11 and the light-transmitting layer 12 becomes an obstacle to the extraction of the fluorescent light to the outside of the scintillator 10. To suppress this reflection at the interface, it is preferable that the refractive index of the phosphor layer 11 and that of the light-transmitting layer 12 be close to each other. for fluorescent light. Specifically, it is preferable that the ratio of the refractive index of the light-transmitting layer 12 to that of the phosphor layer 11 be between 0.90 and 1.10 at the wavelength of fluorescent light. When, for example, the GS20 described above is used for the phosphor layer 11, synthetic quartz can be used as a material satisfying the requirement described above. That is, by using materials whose main component is silicon dioxide (SiO2) to form the phosphor layer 11 and the light-transmitting layer 12, reflection at the interface can be suppressed.

[0068] As described above, when using the scintillator 10 of FIGS. 1A, 1B, it is possible to significantly reduce only the pulse height of the pulse output due to gamma photons, without reducing that due to neutrons and the neutron detection efficiency. Therefore, it becomes possible to distinguish neutrons from gamma photons only by the pulse height. Here, since the decay time constant of the output pulses is determined by the material constituting the phosphor layer 11, and the decay time constant is not increased at least by employing the configuration described above, the use of a material having a small value for the decay time constant, for the phosphor layer 11, enables the measurement under a high dose condition, similarly to usual scintillators.

[0069] Next, a description will be given of one aspect of the practical use of the scintillator 10 in a detector. Figures 6A and 6B schematically show two types of neutron detectors 1 and 2 in which the scintillator 10 described above is used. In the diagrams, an arrow A represents the direction of incidence of the neutrons to be detected or gamma rays which constitute an obstacle to the detection of the neutrons, and arrows B, C and D represent the progressive directions of the respective portions of the fluorescent light emitted by the neutrons or gamma rays in the scintillator 10 which are to be detected by a photodetector.

[0070] The photodetector used herein is one which has a high temporal resolution and which is capable of emitting output pulses by receiving the above-described fluorescent light emitted by the phosphor layer 11, and more particularly a photomultiplier tube or the like.

[0071] In the neutron detector 1 of [Fig.6A], the direction of incidence of the neutrons (arrow A) and that of the light to be detected by the photodetector 21 (arrow B) are set to be in the superposition direction of [Fig.1B] (z-axis direction). Therefore, when the scintillator 10 is of a planar shape extending in the xy plane in Figs. 1A, 1B, the configuration can be implemented particularly easily.

[0072] Furthermore, although the single photodetector 21 is used in [Fig. 6A], a multi-channel photodetector or the like may be used with respect to the xy plane of Figs. 1A, 1B. In this case, the neutron absorption positions (luminescence positions) in the xy plane of the scintillator 10 may be recognized with a resolution almost equal to the channel interval of the photodetector. In this case, just like the photodetector, the scintillator 10 may be similarly arranged in a split form.

[0073] As shown in points (B) and (C) of [Fig. 3], the positions at which neutron absorption occurs vary in depth due to a non-high probability of neutron absorption, and therefore, in the present case, the phosphor layers 11 and the light-transmitting layers 12 are configured in the multi-layer structure as described above. In [Fig. 6A], the fluorescent light emitted from the phosphor layer 11 on the most superficial side (the leftmost side in [Fig. 6A]) reaches the photodetector 21 after passing through all the layers located closer to the photodetector 21 than the phosphor layer 11 (four light-transmitting layers 12 and four phosphor layers 11), as indicated by a path RI.In contrast, the fluorescent light emitted from the phosphor layer 11 on the side closest to the photodetector 21 (the rightmost side) directly reaches the photodetector 21 at a short distance, as indicated by a path R2. Therefore, when absorption, decay, and reflection between the fluorescent light layers cannot be neglected in the phosphor layers 11 and the light-transmitting layers 12, the former fluorescent light is detected as having a lower intensity (lower pulse height) than the latter in the photodetector 21. Therefore, even when the same energy is transferred, the luminescence intensity to be detected (pulse height) may differ depending on which phosphor layer 11 in [Fig.lB] the energy is transferred to. This broadens DI in [Fig.2] in the lateral direction.

[0074] [Fig.7] shows a result of the practical measurement of the influence of the layer of light transmission 12 in such fluorescent light detection. This is the result of measuring the pulse height distributions in the output of the photodetector 21 corresponding to DI in [Fig.2], using the configuration of [Fig.6A], where, for the purpose of investigating the influence, only the leftmost of the phosphor layers 11 shown in [Fig.1B] was provided, without any other of the phosphor layers 11 being provided, and the number of light transmission layers 12 provided on the right side of the single phosphor layer 11 was changed. The difference in the number of light transmission layers 12 results in a difference in the influence of the total thickness of the light transmission layers of the light through which the fluorescent light must pass, and reflection between the layers. In this case, the light-transmitting layers 12 were made of synthetic quartz, and the thickness of each individual layer was set at 2.5 mm. In [Fig. 7], the case represented by "0 glass layers" corresponds to the one where the fluorescent light emitted by the phosphor layer 11 reaches the photodetector 21 without passing through any light-transmitting layers 12, the case represented by "4 glass layers" corresponds to the one where four light-transmitting layers 12 (with a total thickness of 10 mm) are provided, and the case represented by "12 glass layers" corresponds to the one where twelve light-transmitting layers 12 (with a total thickness of 30 mm) are provided.From this result, even when using a material having a sufficiently high light transmission such as synthetic quartz, a decrease in the detected pulse height is not negligible. When the scintillator 10 having the structure of [Fig. 1B] is used in [Fig. 6A], the phosphor layer 11 from which the fluorescent light detected by the photodetector 21 has been emitted is variable, resulting in a variation in the number (total thickness) of layers through which the fluorescent light passes, and therefore, a pulse height distribution practically obtained by the photodetector 21 becomes the sum of the pulse distributions shown in [Fig. 7]. In this case, even though the broadening of the pulse distribution (full width of half-maximum) shown in [Fig. 7] is small for each individual phosphor layer, the broadening of the summed pulse height distribution is increased.This is not desirable to make the DI and D3 distributions in [Fig.2] distinct from each other.

[0075] Therefore, in the case of the configuration of [Fig.6A], it is preferable that the light transmission is high for the phosphor layer 11 and the light transmission layer 12 and that the reflectance at the interface of these layers is low (the difference in refractive index is small between the layers). In addition, it is preferable not to increase the total number of laminated layers more than necessary.

[0076] [Fig.7] shows the variation of the pulse height distribution in neutron detection as a function of the number of light transmission layers 12 through which the fluorescent light has to pass, concerning the influence of the light transmission layer 12 on the detection of fluorescent light by the photodetector 21. The sensitivity to gamma rays when the thickness of the light transmission layer 12 is varied is then described.

[0077] Figures 8 and 9 each show a result of the calculation of the gamma ray sensitivity when varying the thickness of the light transmission layer 12, where, similarly to the case of [Fig.5], the total thickness of the layers of phosphorus 11 is set to 5 mm, and is distributed in a single layer (with a thickness of 5 mm), among three layers (each individual layer having a thickness of 1.67 mm), among 5 layers (each individual layer having a thickness of 1 mm), among 10 layers (each individual layer having a thickness of 0.5 mm), and among 20 layers (each individual layer having a thickness of 0.5 mm). 67 mm), among 5 layers (each individual layer having a thickness of 1 mm), among 10 layers (each individual layer having a thickness of 0.5 mm), and among 20 layers (each individual layer having a thickness of 0.25 mm), and Figures 8 and 9 show the results for gamma ray energy cases of 2.2 MeV and 5 MeV, respectively. The light transmission layer 12 is assumed to be made of synthetic quartz. Unlike [Fig.5], the vertical axis is represented on a logarithmic scale.

[0078] From the results of Figures 8 and 9, it is found that, in the multilayer structure, the gamma ray sensitivity (luminescence intensity) decreases with increasing total thickness of the light transmission layers 12 not contributing to luminescence. Therefore, when neutron absorption by the light transmission layer 12 can be neglected, it is preferable that the light transmission layer 12 is thick in order to reduce the luminescence intensity due to gamma ray photons. However, when neutron absorption by the light transmission layer 12 is low, the neutron detection efficiency gradually decreases with increasing total thickness of the light transmission layers 12. Moreover, particularly in one aspect shown in [Fig.6B], which will be described later, a photodetection area of ​​the photodetector must be enlarged when the light-transmitting layer 12 is thick, which results in another disadvantage in terms of cost. Therefore, it is not desirable to increase the thickness of the light-transmitting layer 12 more than necessary, and specifically, a thickness of the light-transmitting layer 12 of about 6 mm (1.3 g / cm2 in terms of density length) or less is preferable. As shown in [Fig. 8], when the phosphor layer 11 is set to 0.25 mm thick (0.0625 g / cm2 in terms of density length), a sufficient effect of suppressing the sensitivity to 2.2 MeV gamma rays is expected even when the light-transmitting layer 12 has a thickness of 1 mm (0.2 g / cm2 in terms of density length).However, when the phosphorus-11 layer is set to 0.25 mm thick, the total number of phosphorus-11 layers must be large to ensure the efficiency of neutron detection, which causes a broadening of the pulse height distribution in the configuration of [Fig.6A], as described above, and is therefore undesirable. Even in the case of . using the configuration of [Fig.6B], when the light transmission layer 12 is too thin, the efficiency of light propagation in the in-plane direction is decreased, which also causes broadening of the pulse height distribution and, therefore, is undesirable with respect to the separation of the DI and D3 distributions of [Fig.2]. Therefore, when expressed in terms of density length as in the above-described case of the phosphor layer 11, the preferred range for the light transmission layer 12 is 0.2 g / cm2 to 1.3 g / cm2.

[0079] Meanwhile, in the neutron detector 2 of [Fig.6B], the directions of incidence (arrows C and D) of the fluorescent light to be detected by the photodetectors are different from those in the case of [Fig.6A] by 90 degrees, where two photodetectors, a first photodetector 31A and a second photodetector 31B, opposite each other are used, with the scintillator 10 inserted between them in the y direction. Since the luminescence that occurs when neutrons are absorbed in the phosphor layer 11 has no specific directionality, and the light is emitted in all directions, the emitted light can be detected equally by the photodetectors 31A and 31B.

[0080] In this case, the light emitted from the leftmost phosphor layer 11 in [Fig. 6B] and the light emitted from the rightmost phosphor layer 11, mentioned above, each pass through the phosphor layer 11 having emitted the light and the light-transmitting layers 12 neighboring the phosphor layer 11, along the y direction, and there is no difference between them in the path length between their emission and their arrival at the photodetectors 31A and 31B. Therefore, in the present case, unlike the case in [Fig. 6A], the pulse height does not differ depending on which of the five phosphor layers 11 emitted the light (absorbed the neutrons), and therefore, there is no broadening of the pulse height distribution due to the use of the multilayer structure.

[0081] On the other hand, when such absorption of light or the like in the phosphor layer 11 and the light transmission layer 12 as described above cannot be neglected, it affects the pulse height of the output pulses obtained by the photodetectors 31A and 31B. In [Fig.6B], when neutrons are absorbed on the side of the photodetector 31A (upper side in the diagram) in a phosphor layer 11, the light thus emitted reaches the photodetector 31A via a short path R3 and reaches the photodetector 31B via a long path R4. On the other hand, when neutrons are absorbed on the side of the photodetector 31B (lower side in the diagram) in a phosphor layer 11, the light thus emitted reaches the photodetector 31A via a long path R5 and the photodetector 31B via a short path R6. Therefore, in a case where light absorption cannot be neglected, when neutrons are absorbed on the photodetector 31A side, the pulse height in the photodetector 31A becomes higher and that in the photodetector 31B becomes lower, relative to each other, and they have an inverse relationship when neutrons are absorbed on the photodetector 31B side. That is, there is a distribution of the output pulse height in each of the photodetectors 31A and 31B depending on the neutron incidence position in the y direction.

[0082] On the other hand, in the neutron detector 2, an output pulse of the photodetector 31A (first output pulse) PA and that of the photodetector 31B (second output pulse) PB are input to a coincidence counting circuit (coincidence counting unit) 32. The coincidence counting circuit 32 outputs their sum PA+PB when it simultaneously recognizes the output pulse PA and the output pulse PB. The pulse height of PA+PB is almost independent of the neutron incidence position in the y direction, and corresponds to the energy absorbed by neutron absorption in the phosphorus layer 11.Therefore, by using such a coincidence counting circuit 32 and thereby obtaining output pulses independent of the neutron incidence position in the y direction, even in the case where light absorption occurs in the phosphor layer 11, it is possible to suppress the broadening of the pulse height distribution in neutron detection. Therefore, it is easy to distinguish the outputs of the two types of radiation. However, since the decay of light due to absorption is nonlinearly related to the distance between the incident position and the photodetector, for example, a simple sum of PA and PB is not exactly an incident position-independent quantity. More exactly, it is preferable to use an incident position-independent pulse height calculated appropriately using PA and PB taking into account the point described above.Here, the coincidence counting circuit 32 may be configured as an electrical circuit, or may be configured using a computer or the like that performs processing on digitized output pulses. In particular, in the case of using a computer, the processing need not be performed in real time at the time of detection, and may be performed by the computer collectively on a series of output pulse data after storing the data for a certain period of time, for example. In this case, the processing may be performed in an offline state separate from the measurement environment by arranging the coincidence counting circuit 32 away from photodetectors and the like.

[0083] Furthermore, even in the configuration of [Fig.6B], by providing photodetectors 31A in the form of an array along the x direction, similarly providing photodetectors 31B, and providing coincidence counting circuits 32 according to the photodetectors, for example, neutron absorption positions (luminescence positions) in the x direction can be recognized.

[0084] In the configuration of [Fig.6B], since fluorescent light generated by neutron absorption is theoretically always detected by the photodetectors 31A and 31B at the same time, a noise component unrelated to the detection of radiation such as neutrons is eliminated from the outputs of the photodetectors 31A and 31B and is not included in the output of the coincidence counting circuit 32, unless its simultaneous output accidentally occurs. Here, with respect to the synchronization of the outputs of the photodetectors 31A and 31B in the coincidence counting circuit 32, those which both occur within a suitably determined short period of time are recognized as being synchronous with each other.

[0085] In the case illustrated in [Fig. 6B] where light emitted in the y direction is detected by the photodetectors, when absorption of light in the phosphor layer 11 can be neglected, a configuration not including the coincidence counting circuit 32 in [Fig. 6B] and including only one or the other of the photodetectors 31A and 31B can be used. Furthermore, it is particularly effective to use a configuration as described above, the phosphor layers 11 are formed to be thin in the z direction, and the light transmissive layers 12 thicker than the phosphor layers 11 are provided adjacent to the respective phosphor layers 11, thereby enabling detection of not only light propagating in the phosphor layers 11, but also light propagating in the light transmissive layers 12, along the y direction. Second embodiment

[0086] Next, a description of a second embodiment that uses a scintillator having a structure different from that of the scintillator 10 shown in FIGS. 1A, 1B will be detailed. [Fig. 10] is a cross-sectional view showing the structure of such a scintillator 50 used in the present embodiment, which corresponds to [Fig. 1B]. In the scintillator 50, as in the scintillator 10, five phosphor layers 11 and five light-transmitting layers 12 are provided, and the phosphor layers 11 emit fluorescent light by interacting with neutrons (or gamma rays).

[0087] However, in the present case, a thin light-shielding layer 13 that does not transmit, but reflects fluorescent light is formed on the right side of each of the light-transmitting layers 12 in the diagram. It is preferable that the light-shielding layer 13 is made of a material that does not transmit fluorescent light, but negligibly absorbs neutrons (e.g., aluminum). In general, it is difficult to absorb neutrons, but easy to shield visible light and ultraviolet light by using a thin metal, and therefore it is easy to provide such a light-shielding layer 13. In addition, the energy of gamma rays or high-energy electrons generated by gamma rays can be absorbed by the light-shielding layer 13.Meanwhile, in the scintillator 50, the light detection is carried out in terms of each of the segments, as will be described later, where the light shielding layer 13 serves as a boundary between the segments. The thickness of the light shielding layer 13 can be adjusted so as to facilitate the detection of light at each segment. This adjustment is also easy, since the absorption of neutrons by aluminum or the like, for example, is low.

[0088] In [Fig. 10], light emitted from a phosphor layer 11 enters the adjacent light-transmitting layer 12, but its incidence on a phosphor layer 11 neighboring the adjacent light-transmitting layer 12 is prevented by the light-shielding layer 13. Therefore, the scintillator 50 is divided into five segments S1 to S5 in the z-direction, with the light-shielding layer 13 being each boundary, with respect to fluorescent light. In [Fig. 10], a phosphor layer 11 and a light-transmitting layer 12 are provided in each segment, and light emitted from one of the phosphor layers 11 progresses only within the segment including the phosphor layer 11, particularly along the y-direction.Furthermore, due to the existence of the light shielding layer 13, the light emitted from the phosphor layer 11 is sent only in the directions of arrows C and D shown in [Fig. 6B], but not in the direction of an arrow B in [Fig. 6A].

[0089] Here, as in the case of [Fig. 1B], the number of laminated layers including the phosphor layers 11 and the like is set appropriately in practice. Moreover, while each segment is composed of a phosphor layer 11 and a light-transmitting layer 12 in the example of [Fig. 10], a plurality of phosphor layers 11 and a plurality of light-transmitting layers 12 may be provided in each segment (each of the regions separated by the light-shielding layers 13).

[0090] As described above, unlike the fact that the absorption of a single neutron causes luminescence only in a single layer of phosphorus 11, a single gamma ray photon can cause luminescence in more than one layer of phosphorus 11. In this case, when the segments S1 to S5 are arranged as in the structure of [Fig. 10] and the detection of the luminescence is thus carried out according to each of the segments, the influence of the luminescence in the other segments is suppressed with respect to the luminescence due to the gamma photons, which allows a further reduction in the intensity of the luminescence due to the gamma photons.

[0091] In the present case, scintillator 50 cannot be used instead of scintillator 10 of [Fig. 6A], because light cannot be extracted in the direction of arrow B of [Fig. 6A], but it can be used instead of scintillator 10 of [Fig. 6B]. Furthermore, scintillator 50 can be used in a different aspect than in [Fig. 6B]. [Fig. 11] shows a configuration of such a neutron detector 3 in a manner corresponding to [Fig. 6B]. Scintillator 50 is shown there in a simplified manner where it is represented only by segments S1 to S5.

[0092] In the configuration, so as to correspond to the photodetectors 31A and 31B of [Fig. 6B], the photodetectors 61A and 61B are provided in the segment S1, the photodetectors 62A and 62B in the segment S2, the photodetectors 63A and 63B in the segment S3, the photodetectors 64A and 64B in the segment S4, and the photodetectors 65A and 65B in the segment S5. In addition, in a manner corresponding to the coincidence counting circuit 32 of [Fig. 6B], the coincidence counting unit outputs P1A and PIB of the coincidence counting unit of [Fig. 6B].6B, the outputs PiA and PiB of the photodetectors 61A and 61B are input to a coincidence counting circuit (coincidence counting unit) 71, the outputs P2A and P2B of the photodetectors 62A and 62B to a coincidence counting circuit (coincidence counting unit) 72, the outputs P3A and P3B of the photodetectors 63A and 63B to a coincidence counting circuit (coincidence counting unit) 73, the outputs P4A and P4B of the photodetectors 64A and 64B to a coincidence counting circuit (coincidence counting unit) 74, and the outputs P5A and P5B of the photodetectors 65A and 65B to a coincidence counting circuit (coincidence counting unit) 75.

[0093] Therefore, the coincidence counting circuit 71 outputs PI which is a sum of the output pulses PiA and PiB recognized as being synchronous in the photodetectors 61A and 61B, and similarly, the coincidence counting circuits 72 to 75 respectively output P2 to P5, each being a sum of the output pulses of two photodetectors connected to the corresponding circuit of the coincidence counting circuits. That is, in neutron detector 3, the configuration of neutron detector 2 of [Fig.6B] is implemented in each of the segments, and Pi to P5 are obtained as outputs of the respective channels of the different channels. Here, Pi (and the like) need not necessarily be determined to be a sum of P1A and PiB (and the like) as described above, but a value of Pi (and the like) can be conveniently calculated from P[A and Pib (and the like) by performing a correction on a nonlinear component independent of the incident positions and then be used.

[0094] Thus, in the case of using the scintillator 50 of [Fig. 10], the output pulses Pi to P5 corresponding respectively to the segments S1 to S5 are extracted from the respective channels of the different channels CH1 to CH5, where PA+PB extracted as output in [Fig.6B] corresponds to each of the Pi to P5 in the present case.

[0095] [Fig. 12] shows a calculation result made concerning the fact that the luminescence intensity due to gamma ray photons can be particularly reduced in the configuration of [Fig. 11], in a manner corresponding to [Fig. 5]. The sensitivity to gamma rays is shown when the number of phosphor 11 layers (or the thickness of each individual layer) is varied, while keeping the total thickness of the phosphor 11 layers constant, with or without segmentation.

[0096] In [Fig. 12], concerning the result of [Fig.5] described above on the scintillator 10 (gamma-ray sensitivity of 2.2 MeV), a similar calculation result on scintillator 50 obtained by applying segmentation to scintillator 10 using light-shielding layers 13 (with segmentation) is shown, as well as the result of [Fig. 5] (without segmentation). Here, the case of the 5 mm thickness corresponds to that of using a single phosphorus 11 layer, and is therefore practically identical for both cases with and without segmentation. For the cases of the multi-layer structure, a calculation result on the first of the phosphorus 11 layers (segment SI), whose luminescence intensity (energy absorption) due to gamma photons is the highest, is shown. The result shows that when segmentation is applied, the luminescence intensity due to gamma photons can be significantly reduced, compared to cases without segmentation, especially in cases of a large number of thinner phosphorus-11 layers.On the other hand, when the absorption of neutron energy by the light shielding layer 13 can be neglected, the luminescence intensity due to neutron absorption is the same as that of the scintillator 10. Therefore, the n / y discrimination ability can be particularly increased by using the configuration of [Fig.11].

[0097] In the configuration of [Fig. 11], since the outputs are extracted from the five channels (CH1 to CH5), the distinction and detection of neutrons can be performed in terms of each of the channels, where the distinction is particularly easy to perform. Moreover, when the outputs are thus extracted separately from the five channels, for example, the counting rate of each segment is reduced, and therefore the configuration is also effective under a high dose condition. Moreover, the configuration is also effective in a case where an accurate measurement of the neutron velocity is required, such as in the measurement of the TOF of neutrons, since it is possible to recognize in which phosphorus layer (segment) the neutrons have undergone a reaction, thus reducing the uncertainty in the distance.

[0098] [Fig. 13] shows a configuration of a neutron detector 4 corresponding to a modification of the neutron detector 3 of [Fig. 11]. Unlike the case of the neutron detector 3 of [Fig.l 1] where the outputs are extracted for the respective channels of the five channels, an anti-coincidence counting circuit (anti-coincidence counting unit) 81 is used to generate a single output in the neutron detector 4.

[0099] The anti-coincidence counting circuit 81 accepts CH1 (Pi) to CH5 (P5) and outputs only one CH1 (Pi) to CH5 (P5) whose synchronicity has not been recognized, unlike the coincidence counting circuit 32 described above. Therefore, one of the elements Pi to P5 is output from the anti-coincidence counting circuit 81. The fact that the synchronization of this output has not been recognized means that none of the segments other than the segment corresponding to the output pulse to be delivered (Pi to P5) has emitted light simultaneously with the corresponding segment. While the distinction between neutrons and gamma photons is made on the basis of pulse height, as already described, gamma photons can cause simultaneous luminescence in more than one of the segments, and therefore the use of the anti-coincidence counting circuit 81 also makes it possible to suppress the detection of gamma rays and further increase the n / y discrimination capability.As in the case of the coincidence counting circuit 32 described previously, the anti-coincidence counting circuit 81 can be configured using a computer, and the computer processing does not necessarily have to be performed in real time, but can be performed offline. Especially under a high dose condition, the probability of more than one signal being accidentally counted at the same time in each segment increases, which should be paid attention to when using the anti-coincidence counting circuit 81.

[0100] The effect of the segmentation described above varies depending on the energy of the gamma rays. [Fig. 14] shows the result of a calculation for gamma rays of 5.0 MeV carried out in a similar manner to that carried out to obtain the result for 2.2 MeV gamma rays shown in [Fig. 12]. When the incident gamma rays are of high energy, there is a high probability that high-energy electrons scattered in a phosphor layer 11 will penetrate through the phosphor layer 11, then also penetrate through a light-transmitting layer 12, and then enter a phosphor layer 11 located next to the light-transmitting layer 12, thus contributing to the luminescence thereof. In addition, there is a very high probability that high-energy electrons scattered in a light-transmitting layer 12 will not be completely absorbed in the light-transmitting layer 12 and will also be absorbed in a phosphor layer located next to the light-transmitting layer 12. For this reason, in the case without segmentation of [Fig.14], the sensitivity to gamma rays is higher when the thickness of the phosphor layer 11 is 1 mm (0.25 g / cm2 in terms of density length) or 1.67 mm (0.42 g / cm2 in terms of density length) than when it is 5 mm (1.25 g / cm2 in terms of density length) (prior art). However, when segmentation is adopted, the luminescence occurring in a plurality of phosphor layers 11 separately generates the output of each segment, and therefore the output of each segment decreases and the sensitivity is greatly reduced. Therefore, the effect of segmentation is greater when the energy of the gamma rays is higher. Moreover, for these high-energy gamma rays, the anti-coincidence counting circuit 81 described above is particularly effective.

[0101] In the second embodiment, a preferred range of the thickness (density length) of the phosphor layer 11 is the same as that of the first embodiment. On the other hand, in the case of the second embodiment, since fluorescent light does not propagate between segments, the restriction on the thickness (density length) of the light-transmitting layer 12 is relaxed. However, in the second embodiment, when each segment is thick, the photodetection area of ​​the photodetectors must be large, resulting in a disadvantage in terms of cost. Therefore, it is not desirable to increase the thickness of the light-transmitting layer 12 more than necessary, and a thickness of the light-transmitting layer 12 of about 6 mm (1.3 g / cm2 in terms of density length) or less is preferable.

[0102] In the second embodiment, while the light transmission layer 12 has the function of propagating fluorescent light toward the photodetectors, the phosphor layer 11 can also similarly guide the fluorescent light toward the photodetectors, and therefore, especially when segmentation is adopted, the light transmission layer 12 is not necessarily provided in the segments. However, since the phosphor layer 11 is set to be thin as described above, its light propagation efficiency (fluorescent light) to the photodetectors is not high in the plane direction. Also, it is preferable to also provide the light transmission layer 12 in the segments.

[0103] In the technology described in patent literature 2, inorganic phosphorus particles and a resin material are used, and it is possible to consider that the inorganic phosphorus particles correspond to the phosphorus layer 11 and a layer made of the resin material corresponds to the light-transmitting layer 12. However, unlike the light-transmitting layer 12 described above, the probability that neutrons are scattered and thus thermalized or absorbed in the hydrogen-containing resin material is high, and therefore, the neutron detection efficiency in the prior technology is lower than that of the present invention. In addition, while the inorganic phosphorus particles and the resin material are generally made of completely different materials, their densities must be close to each other in order to mix them uniformly.Under these conditions, in order for their refractive indices to be close to each other in order to suppress reflection at an interface between them, as in the case of the phosphor layer 11 and the light-transmitting layer 12 in the present invention, the restriction on the material of the resin or the material of the inorganic phosphor particles becomes strict, therefore, it is difficult to select and use these materials in practice. In contrast, in the present invention, such a restriction is not placed on the densities of the phosphor layer 11 and the light-transmitting layer 12, and therefore the degree of freedom in selecting the materials is high.

[0104] Furthermore, in the case of the present invention, by forming the phosphor layer 11 to have a thin film shape with a small thickness in the direction of incidence of gamma rays (neutrons) and using the layered structure, the sensitivity to gamma rays can be greatly reduced without reducing that to neutrons, as described above, but in contrast, in the technology described in the patent literature 2, since the size of the inorganic phosphor particles is isotropic and independent of the direction of incidence, the effect of reducing the sensitivity to gamma rays without reducing that to neutrons is small.

[0105] As long as the same operation can be performed, a specific configuration of the neutron detector is optional. For example, if the same functions as described above are possible, any combination can be used for the phosphor layer and the light transmission layer, and a main component can be different between layers. In addition, the configuration of the photodetectors can be set appropriately.

[0106] [List of reference signs] 1 to 4: Neutron detector 10, 50, 100: Scintillator 11: Phosphorus layer 12: Light transmission layer 13: Light protection layer 21: Photodetector 31 A, 61 A, 62A, 63A, 64A, 65A: Photodetector (first photodetector) 31B, 61B, 62B, 63B, 64B, 65B: Photodetector (second photodetector) 32, 71 to 75: Coincidence counting circuit (coincidence counting unit) 81: Anti-coincidence counting circuit (anti-coincidence counting unit) SI to S5: Segment

Claims

1.

2. Claims A neutron detector (3, 4) configured to detect neutrons by means of fluorescent light emitted when the neutrons are absorbed by a phosphor, the phosphor containing a neutron-absorbing isotope that emits secondary charged particles upon absorbing the neutrons, the neutron detector comprising a scintillator (50) configured to have a layered structure comprising, along a neutron incidence direction, a plurality of combinations of a thin-film phosphor layer (11) made of the phosphor and a thin-film light-transmitting layer (12) made of a light-transmitting material transmitting fluorescent light and being adjacent to the phosphor layer in the thickness direction, and a photodetector (61A-65A, 61B-65B) configured to emit an output pulse as an output generated upon detection of the fluorescent light, and the photodetector (61A-65A, 61B-65B) detects the fluorescent light emitted by the scintillator (50) along the direction of incidence, and, in the scintillator (50), a plurality of segments, each composed of a combination of the phosphor layer (11) and the light-transmitting layer (12) neighboring the phosphor layer (11) and transmitting the fluorescent light emitted from the phosphor layer (11), are formed along the incident direction, and, in the layered structure, a light-shielding layer (13) for shielding the fluorescent light is provided between the segments neighboring each other in the incident direction so that the fluorescent light in one segment propagates in the plane direction, but does not propagate between the neighboring segments, and the photodetectors (61A-65A, 61B-65B) are provided according to each of the segments. A neutron detector (3, 4) according to claim 1, comprising as a photodetector, a first photodetector (61A-65A) and a second photodetector (61B-65B) opposite each other along the in-plane direction, the scintillator (50) being inserted therebetween, and a coincidence counting unit (71-75) configured to output, as a new output pulse, an output based on a first output pulse (P1A-P5A) corresponding to the output pulse (PA) of the first photodetector (61A-65A) and a second output pulse (P1B-P5B) corresponding to the output pulse (PB) of the second photodetector (61B-65B) when synchronization is recognized between the first and second output pulses.

3. A neutron detector (4) according to claim 1 or 2, comprising an anti-coincidence counting unit (81) configured to output, from among the output pulses of the respective segments, the output pulse whose synchronism with other output pulses has not been recognized.

4. A neutron detector according to any one of claims 1 to 3, wherein a ratio of the light transmission layer (12) to the phosphor layer (11) in refractive index for fluorescent light is in a range of 0.90 to 1.

10.

5. A neutron detector according to any one of claims 1 to 4, wherein a main component of the phosphor layer (11) and that of the light-transmitting layer (12) are silicon dioxide (SiO2), and the neutron-absorbing isotope has been added into the phosphor layer (11).

6. A neutron detector according to claim 5, wherein the neutron-absorbing isotope is 6Li or 10B.

7. A neutron detector according to any one of claims 1 to 6, wherein a density length, corresponding to the product of the density and the thickness, is set in a range of 0.0625 to 0.5 g / cm2 and in a range of 0.2 to 1.3 g / cm2, respectively, for the phosphor layer (11) and the light transmission layer (12).

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