JP6900697B2 - Neutron scintillator, neutron detector and neutron detection method - Google Patents

Description

The present invention relates to a neutron scintillator, a neutron detector and a method for detecting neutrons.

Recently, the development of a neutron detector using a neutron scintillator is in progress. A neutron scintillator is a substance that fluoresces due to the action of neutrons when they are incident. The neutron scintillator can be used as a neutron detector by combining it with a photodetector such as a photomultiplier tube.

A neutron detector using a neutron scintillator has an advantage of high detection efficiency for neutrons, but has a problem that it is also sensitive to γ-rays and has a low n / γ discrimination ability. The neutron detection efficiency is the ratio of the number of neutrons counted by the detector to the number of neutrons incident on the detector. When the detection efficiency is low, the absolute number of neutrons to be measured decreases, and the measurement accuracy Decreases. In addition to existing as natural radiation, γ-rays are also generated when neutrons hit the components of the detection system for detecting neutrons or the object to be inspected, so the n / γ discrimination ability is low. If γ-rays are counted as neutrons, the neutron counting accuracy will decrease.

In Patent Document 1, the wavelength conversion fiber is included in the resin composition section, and the wavelength conversion fiber is arranged at a specific position with respect to the outer wall surface of the resin composition section to enhance the n / γ discrimination ability. The scintillator is disclosed.

Japanese Unexamined Patent Publication No. 2016-3854

In the decommissioning work at the Fukushima Daiichi Nuclear Power Station, it is necessary to grasp the position of fuel debris accumulated in the reactor containment vessel. When investigating the internal space of the reactor containment vessel, since workers cannot enter the reactor containment vessel where fuel debris is accumulated, a remote controllable investigation device is used. A neutron scintillator is attached to such a survey device, and the position of fuel debris is grasped based on the detection result of neutrons by the neutron scintillator. However, gamma ray count around the fuel debris predominance against neutrons (e.g. 10 ⁸ times). Therefore, in order to accurately grasp the position of fuel debris, it is required to develop a neutron scintillator with further improved n / γ discrimination ability.

The present invention has been made in view of the above-mentioned problems, and is a neutron scintillator, a neutron detector, and a neutron detection method capable of accurately measuring neutrons even in a field where the dose of γ-rays, which is background noise, is high. The purpose is to provide.

The neutron scintillator of the present invention contains a resin composition portion containing inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10, and at least a part thereof contained in the resin composition portion. A neutron scintillator comprising the wavelength conversion fiber and the resin composition portion, the first tubular region including the outer periphery of the wavelength conversion fiber and the first region outside the first region. has a second region, wherein the density of the inorganic phosphor particles in the first region, wherein the density from rather low inorganic phosphor particles in the second region, the resin composition part, the first region and the The materials in the second region are the same .

According to this configuration, the resin composition portion covering the wavelength conversion fiber has inorganic phosphor particles containing neutron capture isotopes. When neutrons enter the neutron capture isotope, secondary particles (α-rays and tritium or α-rays and lithium-7) are ejected. In addition, gamma rays eject the orbital electrons of atoms in a substance as secondary electrons. Since the secondary particles caused by the incident of neutrons have a short range, most of the secondary particles consume energy in the inorganic phosphor particles and generate light having a substantially constant peak value. On the other hand, secondary electrons caused by the incident of γ-rays have a long range. By making the inorganic phosphor into particles, secondary electrons rapidly deviate from the inorganic phosphor particles, and the energy given to the inorganic phosphor is reduced. That is, it is possible to reduce the peak value of light emission associated with γ-ray incident. Therefore, it can be determined from the peak value that the light is caused by the incident of neutrons, and the n / γ discrimination ability can be enhanced.

Further, according to this configuration, by lowering the density (that is, the content per unit volume) of the inorganic phosphor particles in the vicinity of the wavelength conversion fiber (first region), the inorganic phosphor particles in the vicinity of the wavelength conversion fiber It is possible to reduce the light emission caused by the γ-rays emitted from. The peak value of light reaching the wavelength conversion fiber depends on the distance from the emitting inorganic phosphor particles. Therefore, by reducing the light emission from the vicinity of the wavelength conversion fiber, it is possible to reduce the component having a large peak value among the light detected due to the γ-ray. As described above, this neutron scintillator reduces the crest value of light caused by γ-rays and distinguishes light caused by neutrons from light caused by γ-rays by the difference in crest value. By reducing the light caused by γ-rays from the vicinity of the wavelength conversion fiber, it shows a high peak value of the light caused by γ-rays, and emits light in a region close to the peak value of light caused by neutrons. Can be suppressed, and the n / γ discrimination ability can be enhanced. By lowering the density of the inorganic phosphor particles in the vicinity of the wavelength conversion fiber, the peak value of light caused by neutrons emitted from the inorganic phosphor particles in the vicinity of the wavelength conversion fiber is also reduced. However, the peak value of light caused by neutrons shows a specific peak value. Therefore, the effect of lowering the density of the inorganic phosphor particles in the vicinity of the wavelength conversion fiber extends to a region where the peak value is slightly higher than the peak value, and the effect on the peak value is small. Therefore, even when the density of the inorganic phosphor particles in the vicinity of the wavelength conversion fiber is reduced, sufficient neutron detection efficiency can be obtained.

In the above neutron scintillator, the first region of the resin composition portion may not contain the inorganic phosphor particles.

According to this configuration, since the inorganic phosphor particles are not contained in the vicinity of the wavelength conversion fiber, the above-mentioned effect can be made more remarkable and the n / γ discrimination ability can be further enhanced.

In the above neutron scintillator, the ratio of the distance between the central axis of the wavelength conversion fiber and the outer wall surface of the resin composition portion to the diameter of the wavelength conversion fiber may be in the range of 1 to 10.

In the above neutron scintillator, the ratio of the refractive index of the inorganic phosphor particles to the refractive index of the resin may be in the range of 0.95 to 1.05 at the emission wavelength of the inorganic phosphor particles.

According to this configuration, by setting the ratio of the refractive indexes of the transparent resin to such a range, it is possible to suppress light scattering at the interface between the inorganic phosphor particles and the resin, and to improve the transparency of the resin composition portion. Can be done. As a result, it becomes possible to detect the light emission of the inorganic phosphor particles with high efficiency, and the neutron detection efficiency can be improved.

A neutron detector comprising the above-mentioned neutron scintillator and a photodetector in the above-mentioned neutron scintillator.

According to this configuration, it is possible to provide a neutron detector having a high n / γ discrimination ability.

Further, the neutron detector as one embodiment of the present invention includes a resin composition portion containing inorganic phosphor particles containing neutron capture isotopes, and a wavelength conversion fiber in which at least a part thereof is contained in the resin composition portion. A neutron scintillator having the above-mentioned components, a light detector for detecting light emitted from the neutron scintillator, and a determination unit for determining the presence or absence of neutron rays are provided. The logarithm of the detection frequency with respect to the peak value of the detected light is approximated to a linear function, and when the detection frequency of the peak value near 4.8 MeV or 2.3 MeV deviates from the linear function, the presence of a neutron beam is detected. Judging , the resin composition portion has a tubular first region including the outer periphery of the wavelength conversion fiber and a second region located outside the first region, and the resin composition portion in the first region. The density of the inorganic scintillator particles is lower than the density of the inorganic scintillator particles in the second region, and the resin composition portion is made of the same material in the first region and the second region .

According to this configuration, it is possible to provide a neutron detector capable of easily determining the presence of neutrons when it deviates from a linear function approximated based on the detection result.

Further, the method for detecting neutrons as one embodiment of the present invention includes a resin composition portion containing inorganic phosphor particles containing neutron capture isotopes and a wavelength conversion in which at least a part thereof is contained in the resin composition portion. Using a neutron scintillator having a fiber as a component and a light detector for detecting the light emitted from the neutron scintillator, the logarithm of the detection frequency with respect to the peak value of the light detected by the light detector. Is approximated to a linear function, and when the detection frequency of the peak value near 4.8 MeV or 2.3 MeV deviates from the linear function, the presence of neutron rays is determined, and the resin composition section determines the presence of the wavelength conversion fiber. It has a tubular first region that includes the outer periphery of the surface and a second region that is located outside the first region, and the density of the inorganic phosphor particles in the first region is in the second region. The material of the resin composition portion is the same as that of the first region and the second region, which is lower than the density of the inorganic phosphor particles .

According to this configuration, the presence of neutrons can be easily determined when the linear function approximated based on the detection result deviates.

According to the present invention, it is possible to provide a neutron scintillator, a neutron detector, and a neutron detection method capable of accurately measuring neutrons even in a field where the dose of γ-rays that becomes background noise is high.

It is sectional drawing of the neutron scintillator of one Embodiment. There is a schematic vertical cross-sectional view of the neutron scintillator of one embodiment, and the schematic structure of the neutron detector including the neutron scintillator is also shown. It is a graph which shows an example of the detection result at the time of irradiating the neutron scintillator which has the resin composition part in which the inorganic phosphor particles are uniformly dispersed with γ-rays and neutron n, respectively. It is a graph which simplifies the detection result in the neutron detector of one embodiment. It is the schematic which shows the neutron detector of the modification. It is a cross-sectional view of the neutron scintillator modeled in an Example. In the model of the Example, it is a graph which plotted the number of photons incident on a wavelength conversion fiber due to the incident of a neutron, and the number of occurrences of this event. In the model of the example, a graph plotting the number of photons irradiated from the inorganic phosphor particles located away from the wavelength conversion fiber due to the incident of neutrons and incident on the wavelength conversion fiber, and the number of occurrences of this event. Is. In the model of the Example, it is a graph which plotted the number of photons incident on a wavelength conversion fiber due to the incident of γ-rays, and the number of occurrences of this event. In the model of the example, the number of photons irradiated from the inorganic phosphor particles located away from the wavelength conversion fiber due to the incident of γ-rays and incident on the wavelength conversion fiber and the number of occurrences of this event are plotted. It is a graph. It is a graph in which FIGS. 7 and 9 are superimposed, and is a graph in which a logarithm is taken as the number of event occurrences. It is a graph in which FIGS. 8 and 10 are superimposed, and is a graph in which a logarithm is taken as the number of event occurrences. 7 is a partially enlarged graph in which FIGS. 7 and 9 are superimposed. 8 is a partially enlarged graph in which FIGS. 8 and 10 are superimposed.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In the drawings used in the following description, in order to make the features easier to understand, the featured parts may be enlarged for convenience, and the dimensional ratios of each component may not be the same as the actual ones. Absent.

FIG. 1 is a schematic cross-sectional view of the neutron scintillator 1 of the present embodiment. Further, FIG. 2 is a schematic vertical cross-sectional view of the neutron scintillator 1. Note that FIG. 2 also shows the schematic structure of the neutron detector 2 including the neutron scintillator 1.

The neutron scintillator 1 includes a resin composition unit 30, a wavelength conversion fiber 20 included in the resin composition unit 30, and a reflector 35 surrounding the outer periphery of the resin composition unit 30. Further, the resin composition unit 30 includes an inorganic phosphor particle 31 containing at least one neutron capture isotope selected from lithium 6 and boron-10, and a resin unit 32 as constituent elements.

The neutron detector 2 shown in FIG. 2 includes a neutron scintillator 1, a photodetector 3 connected to the wavelength conversion fiber 20 of the neutron scintillator 1, and a determination unit 4 connected to the photodetector 3. The neutron detector 2 determines the presence of neutrons by incident the light generated by the neutrons incident on the inorganic phosphor particles 31 onto the wavelength conversion fiber 20 and further detecting the light by the photodetector 3.

In the present specification, the “inclusion” means a state in which the fiber surface along the body axis direction of the wavelength conversion fiber 20 is covered with the 360 ° resin composition portion 30. On the other hand, both ends or one end of the end face of the fiber protrudes from the resin composition portion 30. In order to extract the scintillation light, at least one end of the fiber needs to be outside the resin composition part 30, but the remaining end may be present in the resin composition part 30 or protrudes from the resin composition part 30. May be good.

<Resin part>
The resin portion 32 is provided around the inorganic phosphor particles 31 and is interposed between the plurality of inorganic phosphor particles 31. That is, in the resin composition section 30, the inorganic phosphor particles 31 are dispersed in the resin section 32. As described above, the inorganic phosphor particles 31 of the present embodiment are smaller in size than the generally used inorganic phosphor. Therefore, since the single inorganic phosphor particles 31 have poor neutron detection efficiency, the neutron detection efficiency can be improved by dispersing the plurality of inorganic phosphor particles 31 in the resin portion 32.

The resin portion 32 is preferably a transparent resin in order to efficiently guide the fluorescence emitted from the inorganic phosphor particles 31 to the wavelength conversion fiber 20. Specifically, the internal transmittance of the resin at the emission wavelength of the inorganic phosphor particles 31 is preferably 80% / cm or more, and particularly preferably 90% / cm or more. Specific examples of such resins include silicone resins and fluororesins. Examples thereof include poly (meth) acrylate, polycarbonate, polystyrene, polyvinyltoluene, and polyvinyl alcohol. Further, a plurality of resins may be mixed and used for the purpose of adjusting the refractive index, strength and the like. Further, among the transparent resins, it is preferable to use a transparent resin in which the refractive index of the inorganic phosphor particles 31 at the emission wavelength is close to the refractive index of the inorganic phosphor particles 31. Specifically, the ratio of the refractive index of the transparent resin to the refractive index of the inorganic phosphor particles 31 is preferably 0.95 to 1.05, and most preferably 0.98 to 1.02. By setting the ratio of the refractive indexes of the transparent resin to such a range, it is possible to suppress light scattering at the interface between the inorganic phosphor particles 31 and the resin, and it is possible to enhance the transparency of the resin composition portion 30. The refractive index is the refractive index in the temperature range in which the neutron scintillator 1 of the present embodiment is used. For example, when the neutron scintillator 1 of the present embodiment is used at 100 ° C., the refractive index ratio needs to be determined at 100 ° C.

<Inorganic phosphor particles>
The inorganic phosphor particles 31 contain at least one neutron capture isotope selected from lithium 6 and boron-10. In the inorganic phosphor particles 31, the neutron capture reaction of lithium 6 or boron 10 and neutron n produces α-rays and tritium or α-rays and lithium 7 (hereinafter, also referred to as secondary particles), respectively, and the secondary particles. Gives the inorganic phosphor particles 31 an energy of 4.8 MeV or 2.3 MeV. By applying the energy, the inorganic phosphor particles 31 are excited and emit fluorescence.

The neutron scintillator 1 using the inorganic phosphor particles 31 is excellent in neutron detection efficiency because the efficiency of the neutron capture reaction by lithium 6 and boron 10 is high, and is applied to the inorganic phosphor particles 31 after the neutron capture reaction. Since the energy generated is high, the intensity of fluorescence emitted when neutron n is detected is excellent.

In the present embodiment, the inorganic phosphor particles 31 are particles composed of an inorganic substance containing a neutron capture isotope and emitting fluorescence, and the inorganic substance itself is grasped as one chemical substance. Therefore, the mixture particles obtained by mixing the non-fluorescent particles containing the neutron capture isotope and the phosphor particles not containing the neutron capture isotope are not included. More specifically, for example, it is preferable not to contain mixed particles such as a mixture of non-fluorescent LiF containing a neutron capture isotope and ZnS: Ag, which is a phosphor that does not contain a neutron capture isotope. In such mixture particles, some of the energy of the secondary particles generated by the particles containing the neutron capture isotopes is lost before reaching the fluorescing particles. The energy lost at this time varies depending on the range from the generation point of the secondary particles to the particles that emit fluorescence, and as a result, the fluorescence intensity of the particles that emit fluorescence varies greatly. Therefore, it is preferable that such mixture particles are not adopted in the present embodiment because the desired n / γ discrimination ability cannot be obtained.

In the present embodiment, the contents of lithium 6 and boron-10 in the inorganic phosphor particles 31 (hereinafter, also referred to as neutron capture isotope contents) shall be 1 atom / nm ³ and 0.3 atom / nm ³ or more, respectively. Is preferable, ^{and 6 atom / nm 3} and 2 atom / nm ³ or more are particularly preferable, respectively. The neutron capture isotope content refers to the number of neutron capture isotopes contained ^{in 1 nm 3} of the inorganic phosphor particles 31. By setting the neutron capture isotope content in the above range, the probability that the incident neutron n causes a neutron capture reaction is increased, and the neutron detection efficiency is improved.

For the neutron capture isotope content, the chemical composition of the inorganic phosphor particles 31 is selected, and lithium in lithium fluoride (LiF) or boron oxide (B ₂ O ₃ ) used as a raw material for the inorganic phosphor particles 31 is used. It can be appropriately adjusted by adjusting the isotope ratio of 6 and boron-10. Here, the isotope ratio is an element ratio of lithium 6 isotope to total lithium element and an element ratio of boron 10 isotope to total boron element, and is about 7.6% and about 7.6%, respectively, for natural lithium and boron, respectively. It is 19.9%.

On the other hand, the upper limit of the neutron capture isotope content is not particularly limited, but it is desirable that the content is as high as possible.

The content of lithium 6 (C _{Li, P} ) and the content of boron 10 (CB _{, P} ) in the inorganic phosphor particles 31 are determined in advance by the density of the inorganic phosphor particles 31 and in the inorganic phosphor particles 31. The mass fraction of lithium and boron and the isotope ratio of lithium 6 and boron 10 in the raw material can be obtained and substituted into the following formulas (1) and (2), respectively.

C _{Li, P} = ρ × W _Li × R _Li / (700-R _Li ) × A × ^10-23 (1)
_{C B, P = ρ × W} B × R B / (1100-R B) × A × 10 -23 (2)
(In the formula, C _{Li, P} and C _{B, P} are the content of lithium 6 and the content of boron 10 in the inorganic phosphor particles 31, respectively, and ρ is the density of the neutron scintillator [g / cm ³ ], W _Li and W _B is lithium and the mass fraction of boron for each inorganic phosphor particles 31 _[wt%], isotopic ratio of lithium 6 and boron 10 in each _{R Li} and _{R B} material [%], a is Avogadro's number [ 6.02 × 10 ²³ ] is shown)

The inorganic phosphor particles 31 are not particularly limited, and conventionally known inorganic fluorescent particles in the form of particles can be used. Specific examples thereof include Eu: LiCaAlF ₆ , Eu, Na: LiCaAlF _6. , Eu: LiSrAlF ₆ , Ce: LiCaAlF ₆ , Ce, Na: LiCaAlF ₆ , Ce: LiSrAlF ₆ , Ce: LiYF ₄ , Tb: LiYF ₄ , Eu: LiI, Ce: Li ₆ Gd (BO ₃ ) ₃ Inorganic fluorescent particles 31 composed of crystals such as LiCs ₂ YCl ₆ , Ce: LiCs ₂ YBr ₆ , Ce: LiCs ₂ LaCl ₆ , Ce: LiCs ₂ LaBr ₆ , Ce: LiCs ₂ CeCl ₆ , Ce: LiRb ₂ LaBr _{6. and,} inorganic phosphor particles 31, and the like consisting of _{Li 2 O-MgO-Al 2} O 3 -SiO 2 -Ce 2 O 3 system glass.

The emission wavelength of the inorganic phosphor particles 31 is preferably in the near-ultraviolet region to the visible light region, and particularly preferably in the visible light region, in that transparency can be easily obtained when mixed with the resin portion 32 described later. preferable.

It is preferable that lithium 6 is the only neutron capture isotope contained in the inorganic phosphor particles 31. By using only lithium 6 as the neutron capture isotope that contributes to the neutron capture reaction, a constant energy can always be imparted to the inorganic phosphor particles 31, and an extremely high energy of 4.8 MeV can be imparted. it can. Therefore, it is possible to obtain a neutron scintillator 1 having little variation in fluorescence intensity and particularly excellent fluorescence intensity.

Among the inorganic phosphor particles 31 containing only lithium 6 as a neutron capture isotope, the chemical formula LiM ¹ M ² X ₆ (where M ¹ is at least one alkaline earth metal selected from Mg, Ca, Sr and Ba). It is an element, M ² is at least one metal element selected from Al, Ga and Sc, and X is at least one halogen element selected from F, Cl, Br and I). Corkyrite-type crystals containing at least one lanthanoid element, and corquilite-type crystals that further contain at least one alkali metal element are preferable.
To give a specific example of the corkyrite type crystal, the _{inorganic phosphor particles 31 composed of Eu: LiCaAlF 6} , Eu, Na: LiCaAlF ₆ , Eu: LiSrAlF ₆ and Eu, Na: LiSrAlF ₆ have a high emission amount and also have a high emission amount. Most preferable because it is not deliquescent and is chemically stable.

In the neutron scintillator 1 of the present embodiment, the n / γ discrimination ability is improved by using the inorganic phosphor particles 31 in which the inorganic phosphor is in the form of particles. Hereinafter, the mechanism of action in which the n / γ discrimination ability is improved by using the inorganic fluorescent particle 31 will be described.

Generally, when γ-rays enter the neutron scintillator 1, secondary electrons e are generated inside the neutron scintillator 1, and the inorganic phosphor emits light by giving energy when the secondary electrons pass through the inorganic phosphor. To do. If the crest value output by such light emission is as high as the crest value due to the incident of neutron n and the two cannot be discriminated, γ-rays are counted as neutron n and an error occurs in the neutron count. In particular, when the dose of γ-rays is high, the error due to γ-rays increases, which becomes a serious problem.

Since the peak value output from the neutron detector due to the incident of γ-rays depends on the energy given by the secondary electrons e, by reducing the energy, it is output when the γ-rays are incident on the neutron scintillator 1. The peak value to be generated can be reduced.

Here, the range of secondary electrons e generated when γ-rays are incident on the neutron scintillator 1 travels in the neutron scintillator 1 while giving energy to the neutron scintillator 1 is relatively long, about several mm.

On the other hand, when the neutron n is incident on the neutron scintillator 1, as described above, it is generated by the neutron capture reaction between the lithium 6 and boron 10 contained in the inorganic phosphor in the neutron scintillator 1 and the neutron n. When the secondary particles give energy to the inorganic phosphor, the inorganic phosphor emits light, but the flight distance of the secondary particles is several μm to several tens of μm, which is shorter than that of the secondary electrons.

According to the present embodiment, by forming the inorganic phosphor into particles, the secondary electrons e are rapidly deviated from the inorganic phosphor particles 31, and the energy given to the inorganic phosphor by the secondary electrons e is reduced. Can be done. Since the secondary particles caused by the incident of neutrons have a short range, most of the secondary particles consume energy in the inorganic phosphor particles 31 and generate light having a substantially constant peak value. On the other hand, the secondary electrons e caused by the incident of γ-rays jump out of the inorganic phosphor particles 31 due to the long range, and do not consume all the energy in the inorganic phosphor particles 31. The peak value associated with light emission can be reduced. Therefore, it can be determined from the peak value that the light is caused by the incident of neutron n, and the n / γ discrimination ability can be enhanced.

In the present embodiment, the size of the inorganic phosphor particles 31 is such that almost all the energy of the secondary particles generated by the incident of neutrons n is given to the inorganic phosphor, and the incident of γ-rays is as much as possible. Therefore, it is preferable that the secondary electrons e are small enough to deviate.

According to the study by the present inventors, the shape of the inorganic phosphor particles 31 ^{is preferably a shape having a specific surface area of 50 cm 2} / cm ³ or more, and a shape having a specific surface area of 100 cm ² / cm ³ or more. Is particularly preferable. In the present specification, the specific surface area of the inorganic phosphor particles 31 refers to the surface area per unit volume of the inorganic phosphor particles 31.

Since the specific surface area is the surface area per unit volume, (1) the smaller the absolute volume of the inorganic phosphor particles 31, the larger the specific surface area tends to be, and (2) the specific surface area is the largest when the shape is spherical. The smaller the size, and conversely, the larger the specific surface area of the inorganic phosphor particles 31, the farther the inorganic phosphor particles 31 are from the spherical shape. For example, when considering a cube having sides extending in the X-axis, Y-axis, and Z-axis directions, the specific surface area is the smallest in the case of a regular hexahedron with X = Y = Z, and the length in one of the axial directions is shortened. However, the specific surface area of a product having a longer side in the other axial direction is larger even if the volume is the same.

More specifically, a regular hexahedron with a side of 0.5 cm has a specific surface area of 12 cm ² / cm 3, but a regular hexahedron with a side of 0.1 cm (0.001 cm ³ ) ^{has a specific surface area of 12 cm 2 / cm 3.} The surface area is 60 cm ² / cm ³ . Further, assuming that the volume is the same (0.001 cm ³ ) and the thickness is 0.025 cm, the size is 0.2 cm × 0.2 cm in length and width, and therefore the specific surface area is 100 cm ² / cm ³ .

In other words, a large specific surface area indicates that at least one of them has a portion having an extremely small axial length. Then, since the secondary electrons e generated by the small axial direction and the γ-rays traveling in the direction close to the axial direction rapidly deviate from the crystal as described above, the energy given to the inorganic phosphor particles 31 from the secondary electrons e Can be reduced.

The suitable shape of the inorganic phosphor particles 31 based on the specific surface area has been found by the above findings and considerations, and the specific surface area is imparted to the inorganic phosphor particles 31 from the secondary electrons e. In consideration of energy, it can be used as an index of the shape in consideration of the fact that the inorganic phosphor particles 31 have various particle morphologies. Practically, by setting the specific surface area to preferably 50 cm ² / cm ³ or more, more preferably 100 cm ² / cm ³ or more, a neutron detector having particularly excellent n / γ discrimination ability can be obtained. it can.

In the present embodiment, the upper limit of the specific surface area is not particularly limited, but is preferably ^{1000 cm 2} / cm ^{3 or less.} When the specific surface area exceeds 1000 cm ² / cm ³ , that is, when the axial length of at least one of the inorganic phosphor particles 31 is excessively small, the neutrons of the lithium 6 and the boron 10 and the neutron n There is a possibility that the secondary particles generated in the capture reaction may deviate from the inorganic phosphor particles 31 before the total energy is applied to the inorganic phosphor particles 31. In such an event, the energy given to the inorganic phosphor particles 31 due to the incident of the neutron n decreases, so that the intensity of light emission of the inorganic phosphor decreases. In order to ensure that the total energy of the secondary particles is given to the inorganic phosphor particles 31 and to increase the emission intensity of the inorganic phosphors, it is particularly necessary ^{to set the specific surface area of the inorganic phosphor particles 31 to 500 cm 2} / cm ^{3 or less.} preferable.

Although the term axis has been used in the above description, it is only used for convenience to indicate the spatial coordinate positions of X, Y, and Z, and the inorganic phosphor particles 31 used in the present embodiment are oriented in these specific axial directions. Of course, it is not limited to cubes with sides.

When the inorganic phosphor particles 31 have an irregular shape, the specific surface area can be easily obtained from the density obtained by using a density meter and a BET specific surface area meter and the specific surface area based on mass.

A specific example of the shape of the inorganic phosphor particles 31 preferably used in the present embodiment is a flat plate-like, prismatic, columnar, spherical, or amorphous particle form, which is equivalent to an equal specific surface area sphere. Examples thereof include shapes having a diameter of 50 to 1500 μm, particularly preferably 100 to 1000 μm.

The method for producing the inorganic phosphor particles 31 used in the present embodiment is not particularly limited, and a method for obtaining particles having a desired shape by pulverizing and classifying a bulk body having a shape larger than the particles having a suitable shape. Alternatively, a method of directly obtaining inorganic phosphor particles 31 having a suitable shape by a particle formation reaction using a solution as a starting material can be mentioned.

<Resin composition part>
As described above, the resin composition unit 30 has a resin unit 32 and inorganic phosphor particles 31 dispersed in the resin unit 32.
As shown in FIG. 1, the resin composition unit 30 has a tubular first region A1 that includes the outer circumference of the wavelength conversion fiber 20 and a second region A2 that is located outside the first region A1. The first region A1 does not contain the inorganic phosphor particles 31, and the second region A2 contains the inorganic fluorescent particles 31.

Here, in order to explain the effect of forming the first region A1 that does not contain the inorganic phosphor particles 31, the detection result when the inorganic phosphor particles are uniformly dispersed in the resin composition section will be described.
FIG. 3 is a graph showing an example of detection results when a neutron scintillator having a resin composition portion in which inorganic phosphor particles are uniformly dispersed is irradiated with γ-rays and neutrons n, respectively.

As shown in FIG. 3, the detection result of the γ-ray shows that the neutron ray extends beyond the peak value indicating the peak value P at the point X. There are two main reasons why the peak value of light caused by γ-rays takes a wide range.
The first factor is that the range of secondary electrons e due to the incident of γ-rays is large, and the amount of energy consumed in the inorganic phosphor particles 31 depends on the position of the reaction and the emission direction of the secondary electrons e. Due to various changes.
The second factor is that the peak value of the light output by the light emission caused by the incident of γ-rays and reaching the wavelength conversion fiber 20 depends on the distance between the light emitting inorganic phosphor particles 31 and the wavelength conversion fiber 20. by. The peak value of the light reaching the wavelength conversion fiber 20 (which may be replaced with the number of photons) becomes larger when emitted from the vicinity.

When the above two factors overlap, that is, when the secondary electrons e fly a long distance in the inorganic phosphor particles 31 located near the wavelength conversion fiber 20, the crest value due to γ-rays is high. It is considered that the measurement is performed and the detection result of γ-rays overlaps with the peak value P of neutrons as shown at point X.

On the other hand, when the neutron n is incident on the inorganic phosphor particles 31, the inorganic phosphor is excited by the secondary particles generated by the neutron capture reaction between the lithium 6 and boron 10 contained in the inorganic phosphor particles 31 and the neutron n. And emits light L4. Since the secondary particles (α rays and tritium or α rays and lithium 7) generated when the neutron n is incident have a short range, they consume energy in the inorganic phosphor particles 31 and cause the inorganic phosphor particles 31 to be generated. Make it emit light. Therefore, the intensity (peak value) of the light emission of the inorganic phosphor particles 31 due to the incident of the neutron n is almost constant. As for the secondary particles when the neutron n is incident, only a small part of the secondary particles are ejected from the inorganic phosphor particles 31. Therefore, in reality, a part of the light emission is weakened (region Y in FIG. 3).

FIG. 3 shows the detection results when the number of photons of γ-rays is significantly larger than the number of neutrons. In the vicinity of radioactive material such as fuel debris, the number of photons γ-rays are also said to be 10 ⁸ times the number of neutrons.

In the present embodiment, the light L3 caused by γ-rays and the light L4 caused by neutron rays are determined from the difference in their peak values. However, as described above, when the inorganic phosphor particles 31 are located in the vicinity of the wavelength conversion fiber 20, the peak value of the light L3 caused by γ-rays is detected high, and the light L4 caused by neutrons is detected. There is a risk of being mistaken for. According to the present embodiment, by not including the inorganic phosphor particles 31 in the first region A1 surrounding the wavelength conversion fiber 20, it is possible to prevent the peak value of the light L4 caused by γ-rays from being measured high. can do. As a result, the light L4 caused by neutrons can be detected separately from the light L3 caused by γ-rays, and n / γ discrimination can be improved.

Since the first region A1 does not include the inorganic phosphor particles 31, the neutron measurement result shown in FIG. 3 shows that the peak value P is smaller than the peak value P on the side where the peak value is large. The detection frequency of the value P is unlikely to decrease. Therefore, according to the neutron scintillator, it is possible to improve n / γ discrimination while suppressing a decrease in neutron detection efficiency due to the absence of the inorganic phosphor particles 31 in the first region A1.

In the present embodiment, the case where the first region A1 does not contain the inorganic phosphor particles 31 is illustrated. However, if the density of the inorganic phosphor particles 31 in the first region A1 (that is, the content per unit volume) is lower than the density of the inorganic phosphor particles in the second region A2, a certain effect can be obtained. The larger the difference in density of the inorganic phosphor particles 31 between the first region A1 and the second region A2, the easier it is to exert the above-mentioned effect. Therefore, as shown in the above-described embodiment, the effect is most likely to be exhibited when the first region A1 does not contain the inorganic phosphor particles 31.

In the present embodiment, the outer shape of the cross section of the first region A1 is a circular shape surrounding the wavelength conversion fiber 20. The cross-sectional outer shape of the first region A1 is concentric with the cross-sectional outer shape of the wavelength conversion fiber 20. However, the outer shape of the cross section of the first region A1 is not limited to this, and may be a rectangular shape or another polygonal shape.
Further, in the present embodiment, the first region A1 and the second region A2 preferably have the resin portion 32 of the same material, but the material constituting the first region A1 is not necessarily the same material as the second region A2. It does not have to be. In addition, the first region A1 may be a void that is not filled with material.

The neutron detection efficiency of the neutron scintillator 1 of the present embodiment depends on the content (that is, the density) of the neutron capture isotope derived from the inorganic phosphor particles 31 in the second region A2 of the resin composition section 30, and the content thereof. Can be improved by increasing. The neutron capture isotope content refers to the number of neutron capture isotopes derived from the inorganic phosphor particles 31 contained on average per ^{1 nm 3} in the second region A2 of the resin composition section 30, and refers to the number of neutron capture isotopes of lithium 6. the content _{(C Li, C)} and the content of boron 10 _{(C B, C)} for each of the content of the content of lithium 6 of inorganic phosphor particles 31 _{(C Li, P)} and boron 10 ( It can be obtained from the following equations (3) and (4) by using the body integration rate (V) of the inorganic phosphor particles 31 in the second region A2 as well as CB and _P).

C _{Li, C} = C _{Li, P} × (V / 100) (3)
C _{B, C} = CB _{, P} × (V / 100) (4)
(In the formula, C _{Li, C} and C _{B, C} are the content of lithium 6 and the content of boron 10 in the second region A2 of the resin composition part 30, respectively, and C _{Li, P} and C _{B, P} are inorganic, respectively. The content of lithium 6 and the content of boron 10 in the phosphor particles 31, and V indicate the body integration ratio (V) [volume%] of the inorganic phosphor particles 31 in the resin composition portion 30).

In the present embodiment, the content of the inorganic phosphor particles 31 in the resin composition unit 30 (first region A1 and second region A2) is not particularly limited, but as is clear from the above formula, the resin composition unit The neutron detection efficiency can be improved by increasing the volume fraction of the inorganic phosphor particles 31 in 30. Therefore, the volume fraction of the inorganic phosphor particles 31 in the resin composition section 30 is preferably 5% by volume or more, particularly preferably 10% by volume or more, and most preferably 20% by volume or more. The upper limit of the volume fraction of the inorganic phosphor particles 31 with respect to the entire resin composition section 30 is not particularly limited, but is preferably less than 80% by volume and less than 50% by volume in consideration of the viscosity and the like when the resin composition section 30 is manufactured. Is more preferable.

On the other hand, in the resin composition portion 30 of the present embodiment, it is desirable that the inorganic phosphor particles 31 are uniformly dispersed in the resin in the second region A2, and the distance between the adjacent inorganic phosphor particles 31 is wide. By widening the distance between the particles, secondary electrons deviating from a certain inorganic phosphor particle 31 enter into another adjacent inorganic phosphor particle 31 and give energy to the inorganic phosphor particle 31 to emit light. It is possible to prevent the total sum of the particles from increasing.

However, in general, since the specific gravity of the inorganic phosphor particles 31 is larger than the specific gravity of the resin, when the liquid or viscous resin and the inorganic phosphor particles 31 are mixed, the particles settle and aggregate at the bottom, and the particles are aggregated with each other. It can be difficult to maintain the spacing. In such a case, a method of increasing the viscosity of the resin to impart thixotropic properties to the resin composition portion 30 and suppressing the precipitation of the particles, further blending particles different from the inorganic phosphor particles 31, and making the particles inorganic fluorescent. A method of maintaining the spacing between the inorganic phosphor particles 31 by filling between the body particles, or a layer in which the resin composition portion 30 is thinner than the desired thickness and the particles are settled and aggregated at the lower part. A method of maintaining the distance between the macroscopic particles by preparing a plurality of resin composition portions 30 having a layer composed of only resin on the upper portion and laminating them can be preferably adopted.

The manufacturing method of the resin composition section 30 is not particularly limited, but a specific manufacturing method will be illustrated below.

First, the first region A1 is formed on the outer circumference of the wavelength conversion fiber 20. An uncured resin material to be used as a material for the resin portion 32 is prepared. Further, the resin material is poured into a mold, the wavelength conversion fiber 20 is positioned substantially in the center, the resin material is cured, and then the resin material is removed from the mold. As a result, the wavelength conversion fiber 20 is held in the center of the resin material, and the first region A1 is formed around the wavelength conversion fiber 20.

Next, the second region A2 is formed outside the first region. When the second region A2 of the resin composition part 30 is a slurry-like or paste-like resin composition part 30, first, the inorganic phosphor particles 31 and a liquid or viscous resin are mixed. In such a mixing operation, a known mixer such as a propeller mixer, a planetary mixer, or a butterfly mixer can be used without particular limitation.

Next, it is preferable to defoam the bubbles generated in the resin composition portion 30 in the mixing operation. In such a defoaming operation, a defoaming machine such as a vacuum defoaming machine or a centrifugal defoaming machine can be used without particular limitation. By performing such a defoaming operation, it is possible to suppress light scattering due to bubbles and increase the internal transmittance of the resin composition unit 30.

In the mixing operation and defoaming operation, an organic solvent may be added to the resin composition unit 30 for the purpose of reducing the viscosity of the resin composition unit 30 and efficiently mixing and defoaming.

Further, when the second region A2 of the resin composition portion 30 is made into a solid state, it can be easily produced by the following method. That is, the mixing operation and the defoaming operation are performed in the same manner as described above using the liquid or viscous resin precursor. Then, the obtained mixture of the inorganic phosphor particles 31 and the resin precursor is injected into a mold having a desired shape to cure the resin precursor. The method of curing is not particularly limited, but a method of polymerizing the resin precursor by heating, irradiation with ultraviolet rays, or addition of a catalyst is preferable.

Here, the resin composition portion 30 of the present embodiment can be used in the form of a slurry or a paste, and even when used in the form of a solid, it can be molded by a mold having a desired shape, so that the resin composition portion 30 can have an arbitrary shape. It's easy. Therefore, according to the present embodiment, it is possible to provide a rod-shaped, hollow tube-shaped, or large-area neutron scintillator 1 depending on the application.

<Wavelength conversion fiber>
The wavelength conversion fiber 20 acts as a light guide for guiding the light emission from the inorganic phosphor particles 31 in the resin composition unit 30 to the photodetector 3. The mechanism of action of the wavelength conversion fiber 20 will be described with reference to FIG. When the light L1 emitted from the inorganic phosphor particles 31 reaches the wavelength conversion fiber 20, it is absorbed by the wavelength conversion fiber 20. Further, the wavelength conversion fiber 20 re-emits light at a wavelength different from the original wavelength to emit light L2. The light L2 is generated isotropically, but the light emitted at a certain angle with respect to the outer surface of the wavelength conversion fiber 20 propagates while being totally reflected inside the wavelength conversion fiber 20, and the end portion of the wavelength conversion fiber 20. To reach. Then, by installing the photodetector 3 at the end of the wavelength conversion fiber 20, the light L2 generated from the inorganic phosphor particles 31 can be collected via the wavelength conversion fiber 20.

In the wavelength conversion fiber 20 of the present embodiment, the ratio of the refractive index of the wavelength conversion fiber 20 to the refractive index of the resin portion 32 in the resin composition portion 30 at the emission wavelength of the inorganic phosphor particles 31 is 0.95 or more. Those are preferable. The light emitted from the inorganic phosphor particles 31 is incident on the wavelength conversion fiber 20 via the resin portion 32, but the light L1 that is about to be incident on the wavelength conversion fiber 20 at an incident angle exceeding a certain critical angle is the resin portion 32. It is totally reflected at the interface between the wavelength conversion fiber 20 and the wavelength conversion fiber 20, and cannot be incident. By setting the refractive index ratio to 0.95 or more, the critical angle can be set to about 70 degrees or more, and the efficiency of incident from the resin composition section 30 to the wavelength conversion fiber 20 can be improved. By setting the ratio of the refractive indexes to 1 or more, total reflection of the light L1 does not occur at the interface between the resin portion 32 and the wavelength conversion fiber 20, which is particularly preferable. Further, by setting the refractive index ratio to 1.05 or more, the light L2 re-emitted by the wavelength conversion fiber 20 is totally reflected at the interface between the wavelength conversion fiber 20 and the resin composition portion 30 while being totally reflected by the wavelength conversion fiber 20. It is most preferable because it can increase the efficiency of propagating inside.

The material of the wavelength conversion fiber 20 is not particularly limited, but generally, a plastic base material such as polystyrene, polymethylmethacrylate, and polyvinyltoluene containing an organic phosphor having various absorption wavelengths is commercially available. , The refractive index (n _D ) of these sodium D lines is about 1.5 to 1.6. Therefore, as the resin portion 32 included in the resin composition portion 30, it is preferable to use a low refractive index resin having a refractive index lower than these in order to satisfy the above-mentioned requirements for the refractive index ratio. Such a low refractive index resin, silicone resin, fluorinated silicone resins, phenyl silicone resin and fluorine resin can be preferably used, n _D is commercially available in about 1.3 to 1.5.

Further, as described above, the refractive index of the inorganic phosphor particles 31 is preferably as low as that of the low refractive index resin. The inorganic phosphor particles 31 are represented by the chemical formula LiM ¹ AlF ₆ (where M ¹ is at least one alkaline earth metal element selected from Mg, Ca, Sr and Ba), and at least one kind. A corkyrite-type crystal containing the lanthanoid element of the above, and a particle composed of the corquilite-type crystal, which further contains at least one alkali metal element, is preferable. N _D of the Korukiraito type crystals was about 1.4 before and after, it is easy to meet the specific requirements of the refractive index. Although the refractive index of the sodium D line has been described here as an example for convenience, the same applies to the emission wavelength of the inorganic phosphor.

In the present embodiment, it is preferable that the wavelength conversion fiber 20 is included in the resin composition section 30.
Generally, when light emitted from the resin composition unit 30 is collected by the wavelength conversion fiber 20, there is a problem that the light collection efficiency is poor. That is, (1) a part of the light emitted from the resin composition unit 30 is extinguished without reaching the wavelength conversion fiber 20, and (2) the wavelength conversion fiber 20 absorbs the light emitted from the resin composition unit 30. It is installed at the end of the wavelength conversion fiber 20 because there is a loss caused by re-emission, (3) a loss due to the light emission of the wavelength conversion fiber 20 being dissipated without propagating inside the wavelength conversion fiber 20. The amount of light reaching the light detector 3 is generally only a few percent of the amount of light emitted from the inorganic phosphor particles 31.

As described above, when the amount of light reaching the photodetector 3 is small, the variation in the amount of light inevitably becomes large. Therefore, as described above, a threshold value is set for the peak value and neutrons and γ-rays are discriminated by the threshold value. It becomes difficult to do so, and the counting error due to γ-rays becomes remarkable.

In the present embodiment, by including the wavelength conversion fiber 20 in the resin composition section 30, a part of the light emitted from the resin composition section 30 is extinguished without reaching the wavelength conversion fiber 20. Loss can be significantly reduced and light collection efficiency is improved. That is, when the wavelength conversion fiber 20 is arranged outside the resin composition unit 30, the light emission generated in the resin composition unit 30 is once emitted from the resin composition unit 30, and then again via the intermediate phase. Need to be incident on. Conventionally, various studies have been made to reduce the loss in this process, but there is still room for improvement. On the other hand, when the wavelength conversion fiber 20 is included in the resin composition unit 30, the light emitted from the resin composition unit 30 is incident on the wavelength conversion fiber 20 without passing through the intermediate phase, so that the loss is significantly reduced. it can.

Further, by including the wavelength conversion fiber 20 in the resin composition unit 30, the amount of the wavelength conversion fiber 20 used can be reduced, and the structure of the resin-based composite can be simplified.
In the case of a structure in which the wavelength conversion fiber is arranged outside the resin composition section, in order for the light emitted from the resin composition section to be efficiently incident on the wavelength conversion fiber, the surface on which the light is emitted from the resin composition section is the wavelength conversion fiber. It is necessary to cover the light without gaps or to collect the light emitted from the resin composition portion on the wavelength conversion fiber by using a reflective material having a complicated shape.
As shown in the present embodiment, when the wavelength conversion fiber 20 is included in the resin composition section 30, light is emitted from the resin composition section by a conventional simple method such as covering the outer periphery of the resin composition section 30 with a reflective material 35. Since the light traveling vertically and horizontally inside the resin composition section 30 will eventually reach the wavelength conversion section simply by confining the light inside the resin composition section 30, the light emitted from the resin composition section 30 can be efficiently incident on the wavelength conversion fiber 20. it can. In order to confine the light inside the resin composition part 30, the surface of the resin composition part 30 may be covered with a reflective material 35, and the reflective material 35 is an unbaked polytetrafluoroethylene or a white pigment such as barium sulfate. The one consisting of is preferable.

The neutron scintillator 1 of the present embodiment is characterized in that when the wavelength conversion fiber 20 is included in the resin composition section 30, the wavelength conversion fiber 20 is arranged at a specific position with respect to the outer wall surface of the resin composition section 30. Let it be one. According to the study by the present inventors, the wavelength conversion fiber 20 is arranged at a specific position with respect to the outer wall surface of the resin composition section 30, and the wavelength conversion fiber 20 is included in the resin composition section 30 in as narrow a region as possible. Improves n / γ discrimination. Specifically, the ratio (D / d) of the distance (D) between the central axis of the fiber and the outer wall surface of the resin composition portion 30 with respect to the diameter (d) of the wavelength conversion fiber 20 is in the range of 1 to 10. The wavelength conversion fiber 20 is installed in.

When D / d is in the range of 1 to 10, the position of the outer wall surface closest to the central axis of the wavelength conversion fiber 20 is at least 1 times the diameter of the wavelength conversion fiber 20 and the farthest outer wall surface. This means that the position is at most 10 times the diameter of the wavelength conversion fiber 20 or less. D / d is preferably 2-5.

When a plurality of wavelength conversion fibers 20 are included in the resin composition unit 30, the D / d that can be taken by each wavelength conversion fiber 20 is calculated, and the maximum value and the minimum value among them are calculated for the plurality of fibers. It is a value of D / d that can be taken by the included resin composition portion 30.

1 ≦ D / d ≦ 10 (5)
By including the wavelength conversion fiber 20 in the resin composition portion 30 having a region as narrow as possible so as to satisfy such an index, the distance that the secondary electrons e generated by the γ-rays travel in the inorganic phosphor particles 31 is reduced. As a result, the peak value output when γ-rays are incident on the neutron scintillator 1 can be reduced.

In this embodiment, the shape of the cross section of the wavelength conversion fiber 20 is not particularly limited. With respect to the diameter (d) of the wavelength conversion fiber 20, when the cross section of the fiber is not a circle, the diameter equivalent to the equal cross-sectional area circle of the wavelength conversion fiber 20 is regarded as the diameter of the wavelength conversion fiber 20.

Further, if the diameter of the wavelength conversion fiber 20 is too large, the optical path length of the light transmitted through the fiber may be extended and cause a decrease in the amount of light. Therefore, in this embodiment, a fiber having a diameter of 0.1 to 5 mm is used. It is preferable to do so.

According to the present embodiment, the amount of the wavelength conversion fiber 20 used can be reduced by the above-mentioned effect, but if the amount used is excessively small, the loss of the amount of light may increase, which is appropriate. It is preferable to use the amount. The appropriate amount to be used can be defined by the ratio of the cross-sectional area of the wavelength conversion fiber 20 to the cross-sectional area of the resin composition portion 30 in the cross section perpendicular to the body axis of the wavelength conversion fiber 20. In order for the light emitted from the resin composition section 30 to be efficiently incident on the resin composition section 30, the cross-sectional area ratio is preferably 0.0025 or more, and particularly preferably 0.01 or more. On the other hand, in order to reduce the amount of the wavelength conversion fiber 20 used and to simplify the structure of the resin complex and reduce the manufacturing cost, it is preferable that the cross-sectional area ratio is 1 or less. It is particularly preferable that the value is 1 or less.

<Neutron detector>
As shown in FIG. 2, the neutron detector 2 can be obtained by combining the neutron scintillator 1 with the photodetector 3 and the determination unit 4.
The light emitted from the resin composition unit 30 due to the incident of neutrons is guided to the photodetector 3 via the wavelength conversion fiber 20. The light that reaches the photodetector 3 is converted into an electric signal by the photodetector 3, and the incident of neutrons is measured as an electric signal, so that it can be used for neutron counting and the like. The photodetector 3 is not particularly limited, and conventionally known photodetectors 3 such as a photomultiplier tube, a photodiode, an avalanche photodiode, a Geiger mode avalanche photodiode, and a microPMT can be used without any limitation.
In the wavelength conversion fiber 20, it is preferable that the end surface facing the photodetector 3 is a light emitting surface and the light emitting surface is a smooth surface. By having such a light emitting surface, the light generated by the neutron scintillator 1 can be efficiently incident on the photodetector 3.

The method of combining the neutron scintillator 1 and the photodetector 3 is not particularly limited. For example, the light emitting surface of the wavelength conversion fiber 20 is optically bonded to the photodetector surface of the photodetector 3 with optical grease or optical cement. , A neutron detector can be manufactured by connecting a power supply and a signal readout circuit to the photodetector 3. The signal readout circuit is generally composed of a preamplifier, a shaping amplifier, a pulse height analyzer, and the like.

FIG. 4 is a graph illustrating the detection results of the neutron detector 2 of the present embodiment. In FIG. 4, the peak value of the light detected by the photodetector 3 is taken on the horizontal axis, and the logarithm of the number of detections (frequency) is shown on the vertical axis.
In the photodetector 3, the light caused by γ-rays and the light caused by neutrons are detected in an overlapping state. Therefore, in the measurement in an environment where γ-rays and neutrons coexist, it is difficult to confirm the existence of neutrons by simply observing the number of detections.

According to the study by the present inventors, as shown in FIG. 4, in the detection result by the photodetector 3, the logarithm of the detection frequency with respect to the crest value is approximated to a linear function (linear function LF).

Focusing only on the detection results of light caused by γ-rays, the logarithm of the detection frequency converges to a linear function of the crest value. When γ-rays are incident, secondary electrons e are generated. Since the secondary electrons have a long range, they are less likely to consume energy only in the inorganic phosphor particles 31. Therefore, the emission of the inorganic phosphor particles 31 due to the incident of γ-rays is not constant in intensity and stochastically converges to a linear function.

On the other hand, the secondary particles (α rays and tritium in the case of lithium 6 and α rays and lithium 7 in the case of boron-10) generated when neutrons enter the neutron capture isotope of inorganic phosphor particles have a flight distance. The distance is short. Therefore, the secondary particles consume energy in the inorganic phosphor particles 31 to cause the inorganic phosphor particles 31 to emit light with a specific intensity, and exhibit a specific peak value. Therefore, in the light detection result by the neutron beam, a peak is observed at the peak value depending on the neutron capture isotope contained in the inorganic phosphor particle 31. More specifically, when lithium 6 is contained as a neutron capture isotope, a peak is observed at a peak value corresponding to 4.8 MeV, and when boron 10 is contained as a neutron capture isotope, it is 2.3 MeV. A peak is observed at the corresponding peak.

As shown in FIG. 4, by taking a logarithm as the detection frequency, the detection result caused by γ-rays can be determined as a linear function LF, and the detection result caused by neutrons is detected as a peak. Therefore, the peak of the detection result caused by the neutron can be observed as a value E deviating from the linear function LF, and the existence of the neutron can be determined.
According to the determination method of the present embodiment, even when the irradiation amount of neutrons incident on the neutron scintillator 1 is smaller than the irradiation amount of γ-rays, the detection result caused by γ-rays is linear (linear function). By approximating to LF), the peak of the detection result caused by neutrons can be easily confirmed, and an accurate judgment can be made.

In the present embodiment, the determination unit 4 determines the presence of neutrons based on the determination method described above. That is, the determination unit 4 approximates the logarithm of the detection frequency with respect to the peak value of the light detected by the photodetector 3 to the linear function LF, and is close to 4.8 MeV or 2.3 MeV with respect to the approximated linear function LF. The presence of neutrons is determined when the detection frequency of the peak value of is different. More specifically, at the peak value near 4.8 MeV or 2.3 MeV, the ratio (E / σ) of the measured detection frequency (E) to the standard deviation (σ) of the detection frequency obtained from the linear function LF. ) Is the degree of deviation, and the presence of neutrons is determined based on the degree of deviation. The criteria for determining the presence of neutrons are not particularly limited and may be determined according to the required determination accuracy, but it is preferable to determine the presence of neutrons when the degree of divergence exceeds 1, and the degree of divergence exceeds 2. In some cases, it is more preferable to determine the presence of neutrons, and it is most preferable to determine the presence of neutrons when the degree of divergence exceeds 3. It has been confirmed that 99.7% of the measured values are within 3σ of the linear function LF in an environment where neutrons do not exist.

<Modification example of neutron detector>
FIG. 5 is a schematic view showing a modified example of the neutron detector 2A.
The neutron detector 2A of the modified example includes a plurality of arranged neutron scintillators 1, a plurality of photodetectors 3 connected to the wavelength conversion fibers 20 of each neutron scintillator 1, and a determination unit connected to the photodetectors 3. 4A and. The plurality of arranged neutron scintillators 1 are referred to as neutron detection units 8.
The neutron detector 2A provides a neutron detector having good neutron detection efficiency and good n / γ discrimination by arranging a plurality of neutron scintillators 1 in the normal direction of the spherical surface of the sphere centered on the neutron source. be able to.
Further, the neutron detector 2A has a sensitive area by arranging a plurality of neutron scintillators 1 so that the neutron sensitive portion of the neutron detection unit 8 has a large solid angle extending over the sphere centered on the neutron source. Can provide a large neutron detector.

In the modified example shown in FIG. 5, in the neutron detector 2A, a plurality of photodetectors 3 are combined with each of the neutron scintillators 1 constituting the neutron detection unit 8. However, a configuration in which one photodetector 3 is combined with the neutron detection unit 8 may be adopted.

<Other variants>
Hereinafter, additional aspects for carrying out the invention will be described.
In addition to the inorganic phosphor particles 31 and the resin, a phosphor containing no neutron capture isotope (hereinafter, also referred to as a neutron insensitive phosphor) is further mixed in the resin composition section 30 constituting the neutron scintillator 1 in the above embodiment. It is also possible to do.

In such an embodiment, the secondary electrons generated by the incident of γ-rays deviate from the inorganic phosphor particles 31 and then reach the neutron insensitive phosphor to give energy, and the neutron insensitive phosphor emits fluorescence. That is, when γ-rays are incident, both the inorganic phosphor particles 31 and the neutron insensitive phosphor are given energy and emit fluorescence. On the other hand, when neutrons are incident, the secondary particles generated by the inorganic phosphor particles 31 do not deviate from the inorganic phosphor particles 31, so that only the inorganic phosphor particles 31 fluoresce.

Here, if a neutron insensitive phosphor having different fluorescence characteristics such as fluorescence lifetime and emission wavelength from that of the inorganic phosphor particles 31 is used, neutrons and γ-rays can be discriminated by utilizing the difference in fluorescence characteristics. That is, a mechanism capable of discriminating the difference in fluorescence characteristics is provided in the neutron detector, and when both the fluorescence derived from the inorganic phosphor particle 31 and the fluorescence derived from the neutron insensitive phosphor are detected, γ-rays are incident. If only the fluorescence derived from the inorganic phosphor particles 31 is detected, it can be treated as an event in which neutrons are incident. By taking such a process, a neutron detector having particularly excellent n / γ discrimination ability can be obtained.

To give a concrete example of a mechanism capable of discriminating the difference in fluorescence characteristics, a waveform analysis mechanism capable of discriminating the difference in fluorescence lifetime between the inorganic phosphor particles 31 and the neutron insensitive phosphor, and a waveform analysis mechanism capable of discriminating the difference in fluorescence lifetime between the inorganic phosphor particles 31 and the neutron insensitive fluorescence. Examples thereof include a wavelength analysis mechanism capable of identifying the emission wavelength of the body.

Hereinafter, the waveform analysis mechanism will be more specifically exemplified. The waveform analysis mechanism includes a preamplifier, a main amplifier, a waveform analyzer, and a time-amplitude converter.

In the neutron detector of the present embodiment in which the neutron scintillator 1 and the photodetector 3 are combined, the signal output from the photodetector 3 is input to the main amplifier via the preamplifier, and is amplified and shaped. Here, the intensity of the signal amplified and shaped by the main amplifier and output from the main amplifier increases with time, and the time required for such increase (hereinafter, also referred to as rise time) is the inorganic phosphor particles 31. Alternatively, it reflects the fluorescence lifetime of the neutron insensitive phosphor, and the shorter the fluorescence lifetime, the shorter the rise time.

In order to analyze the rise time, the signal amplified and shaped by the main amplifier is input to the waveform analyzer. The waveform analyzer time-integrates the signal input from the main amplifier, and outputs a logic signal when the time-integrated signal strength exceeds a predetermined threshold value. Here, the waveform analyzer is provided with a two-step threshold value, and the first logic signal and the second logic signal are output at a certain time interval.

Next, the two logic signals output from the waveform analyzer are input to the time-amplitude converter (TAC), and the time difference between the two logic signals output from the waveform analyzer is converted into pulse amplitude. Output. The pulse amplitude reflects the time interval between the first logic signal and the second logic signal output from the waveform analyzer, that is, the rise time.

As can be understood from the above description, the smaller the pulse amplitude output from the time amplitude converter, the shorter the rise time, and therefore the shorter the fluorescence lifetime of the inorganic phosphor particles 31 or the neutron insensitive phosphor is identified. ..

Hereinafter, the wavelength analysis mechanism will be more specifically exemplified. The wavelength analysis mechanism is composed of an optical filter, a second photodetector connected to the neutron scintillator 1 via the optical filter, and a discrimination circuit.

In this embodiment, a part of the light emitted from the neutron scintillator 1 is guided to the first photodetector without passing through the optical filter, and the other part is guided to the second photodetector through the optical filter. Be guided.

Here, it is assumed that the inorganic phosphor particles 31 emit light at a wavelength of Anm, and the neutron insensitive phosphor emits light at a wavelength of Bnm different from Anm. Then, as described above, when γ-rays are incident, both the inorganic phosphor particles 31 and the neutron-insensitive phosphor emit fluorescence, so that the neutron scintillator 1 emits light of Anm and Bnm, and neutrons are emitted. When incident, only the inorganic phosphor particles 31 fluoresce, so only Anm light is emitted.

In this embodiment, the optical filter is a filter that blocks light having a wavelength of Anm and transmits light having a wavelength of Bnm. Therefore, the Anm light emitted from the neutron scintillator 1 when irradiated with neutrons reaches the first photodetector, but does not reach the second photodetector because it is blocked by the optical filter. On the other hand, among the light emitted from the scintillator when γ-rays are irradiated, the light of Anm is the same as the case of irradiating the neutrons, but the light of Bnm reaches the first photodetector. It also reaches a second photodetector to pass through the optical filter.

Therefore, when light of Anm is incident on the first photodetector and a signal is output from the photodetector, if no signal is output from the second photodetector, it is regarded as an event due to neutrons and light of Bnm. Is incident on the second photodetector and a signal is output from the photodetector, it can be identified as an event by γ-rays.

In this embodiment, a discrimination circuit is provided to discriminate between neutrons and γ-rays as described above. The discrimination circuit operates in synchronization with the signal from the first photodetector, and when the signal from the photodetector is output, determines the presence or absence of the signal from the second photodetector. It is a circuit. Specific examples of the discrimination circuit include an anticoincidence circuit, a gate circuit, and the like.

Specific examples of such organic phosphors include 2,5-Diphenyloxyzole, 1,4-Biz (5-phenyl-2-oxazole) benzene, 1,4-Biz (2-methylstyly) benzene, anthracene, stilbene and the like. Examples thereof include naphthalene and organic phosphors such as derivatives thereof. Since such an organic phosphor generally has a shorter fluorescence lifetime than the inorganic phosphor particles 31, it can be suitably used to improve the n / γ discrimination ability by utilizing the difference in the fluorescence lifetime.

The content of the neutron insensitive phosphor can be appropriately set within a range in which the effect of the present embodiment is exhibited, but is preferably 0.01% by mass or more, preferably 0.1% by mass or more with respect to the resin. Is particularly preferable. By setting the content to 0.01% by mass or more, the neutron insensitive phosphor is efficiently excited by the energy applied from the secondary electrons, and the intensity of light emission from the neutron insensitive phosphor is increased. The upper limit of the content of the neutron insensitive phosphor is not particularly limited, but it should be 5% by mass or less with respect to the resin in order to prevent the emission intensity of the neutron insensitive phosphor from being attenuated by concentration quenching. Is preferable, and it is particularly preferable that the content is 2% by mass or less. By setting the content of the neutron insensitive phosphor in such a range, the intensity of light emission from the neutron insensitive phosphor is increased, and the difference in fluorescence characteristics from the inorganic phosphor particles 31 is utilized to increase neutron and γ-ray. It becomes easy to discriminate.

Hereinafter, the present invention will be specifically described with reference to examples of the present invention, but the present invention is not limited to these examples. Also, not all combinations of features described in the examples are essential to the solution of the present invention.

In this embodiment, the effect of not including the inorganic phosphor particles 31 in the tubular first region including the outer circumference of the wavelength conversion fiber 20 is confirmed by assuming the modeled neutron scintillator 1S.

FIG. 6 is a cross-sectional view of the modeled neutron scintillator 1S. In this model, the optical path of light in the neutron scintillator 1S will be described with respect to the cross section seen from the longitudinal direction of the neutron scintillator 1S, but the longitudinal direction of the neutron scintillator 1S in FIG. The same applies even when the optical path of (direction) is added.

The neutron scintillator 1S includes a wavelength conversion fiber 20S and a resin composition unit 30S surrounding the wavelength conversion fiber 20S. The neutron scintillator 1S has a uniform cross section and is formed in a rod shape. The neutron scintillator 1S has a circular cross section with a diameter of 5 mm.

The wavelength conversion fiber 20S has a circular cross section with a diameter of 1 mm. The resin composition unit 30S includes a resin unit 32S, one first inorganic phosphor particle 31Sa, and 20 second inorganic phosphor particles 31Sb. The first inorganic phosphor particles 31Sa are located on the surface 20a of the wavelength conversion fiber 20S. Further, the second inorganic phosphor particles 31Sb are arranged at a position at a distance of 1 mm from the surface 20a of the wavelength conversion fiber.

The first and second inorganic phosphor particles 31Sa and 31Sb emit light isotropically by the incident of γ-rays or neutrons.
The first inorganic phosphor particles 31Sa are located on the surface 20a of the wavelength conversion fiber 20S. Therefore, of the light emitted from the first inorganic phosphor particles 31Sa in all directions of 360 ° in the cross section, 180 ° of the light is incident on the wavelength conversion fiber 20S.
On the other hand, since the second inorganic phosphor particles 31Sb are separated from the surface 20a of the wavelength conversion fiber 20S by a distance equal to the radius of the wavelength conversion fiber 20S, among the light emitted in all directions of 360 ° in the cross section. , Geometrically 60 ° light is incident on the wavelength conversion fiber 20S.

A diffused reflective material 35S is provided on the outer peripheral surface 30a of the resin composition portion 30S. The diffuse reflection material 35S shall reflect the light emitted from the first and second inorganic phosphor particles 31Sa and 31Sb and reaching the outer peripheral surface 30a of the resin composition unit 30S with a reflectance of 90%. Further, the outer peripheral surface 30a is separated from the surface 20a of the wavelength conversion fiber 20S by a distance four times the radius of the wavelength conversion fiber 20S. Therefore, of the light reflected by the diffusely reflecting material 35S and diffused in the direction of 180 ° and emitted, the light of 22 ° geometrically enters the wavelength conversion fiber 20S.

In the model of FIG. 6, the ratio of the light incident on the wavelength conversion fiber 20S to the emitted light of the first and second inorganic phosphor particles 31Sa and 31Sb is calculated.
(Equation M1) is an equation for calculating the ratio Ra of the light incident on the wavelength conversion fiber 20S among the light emitted from the first inorganic phosphor particles 31Sa.
Similarly, (Equation M2) is an equation for calculating the ratio Rb of the light incident on the wavelength conversion fiber 20S among the light emitted from the second inorganic phosphor particles 31Sb.

The first term on the right side of (Equation M1) and (Equation M2) is the ratio of the light emitted from the first or second inorganic phosphor particles 31Sa, 31Sb to the light directly incident on the wavelength conversion fiber 20S. Represents.
Further, the second term on the right side of (Equation M1) and (Equation M2) is the light emitted from the first or second inorganic phosphor particles 31Sa, 31Sb, after being reflected only once by the diffuse reflection material 35S. , Represents the proportion of light incident on the wavelength conversion fiber 20S. Further, the third and subsequent terms on the right side are the ratio of light incident on the wavelength conversion fiber 20S after being reflected by the diffuse reflection material 35S, such as twice, three times, and so on.
(Equation M1) and (Equation M2) can be calculated by converting them into the following (Equation M3) and (Equation M4), respectively.

From (Equation M3), in this model, the ratio Ra of the light incident on the wavelength conversion fiber 20S among the light emitted from the first inorganic phosphor particles 31Sa is 0.76 (that is, 76%). Further, from (Equation M4), the ratio Rb of the light incident on the wavelength conversion fiber 20S among the light emitted from the second inorganic phosphor particles 31Sb is 0.60 (that is, 60%).

Next, in the model of FIG. 6, when neutrons N and γ rays were incident on the neutron scintillator 1S, they were emitted from the first and second inorganic phosphor particles 31Sa and 31Sb due to the neutrons and γ rays. The probability that m photons are incident on the wavelength conversion fiber 20S is calculated.

In the present model, the neutron N one inorganic phosphor particles 31sa, when incident on 31SB, a number of reactions with 1 × 10 ^3. The number of reactions here means the number of excitations of the inorganic phosphor by the secondary particles generated by the neutron N incident on the inorganic phosphor particles 31Sa and 31Sb. Therefore, when the neutron N is incident on one inorganic phosphor particle 31Sa, 31Sb, excitation occurs ^{1 × 10 3 times.}
Further, in this model, the number of reactions when γ-rays are incident on one inorganic phosphor particle 31Sa, 31Sb is 1 × 10 ¹¹ . The number of reactions here means the number of times the inorganic phosphor is excited by secondary electrons generated by the γ-rays incident on the inorganic phosphor particles 31Sa and 31Sb. Therefore, when γ-rays are incident on one inorganic phosphor particle 31Sa, 31Sb, ^{excitation of 1 × 10 11} times occurs.

Further, in this model, the number of photons emitted from the inorganic phosphor particles 31Sa and 32Sb per reaction caused by the incident of neutrons is 200photon (actually, it is said to be 20,000photon). To do. Similarly, in this model, the number of photons emitted from the inorganic phosphor particles 31Sa and 32Sb per reaction caused by the incident of γ-rays is said to be 125photon (actually, 0 to 15,000photon). ).
In an actual neutron scintillator, it is said that the value is in parentheses, but in this model, the above value is used for simplification of calculation.

First, when neutrons are incident on the inorganic phosphor particles 31Sa and 32Sb, the number of photons incident on the wavelength conversion fiber 20S will be examined.
When a neutron is incident on the first inorganic phosphor particle 31Sa, the probability Pna that only m photons emitted due to the incident of the neutron are incident on the wavelength conversion fiber is as follows (Equation M5). expressed. In (Equation M5), Ra is 0.76 as obtained above.

Similarly, when a neutron is incident on the second inorganic phosphor particle 31Sb, the probability Pnb that only m photons emitted due to the incident of the neutron are incident on the wavelength conversion fiber 20S is as follows ( It is represented by the formula M6). In (Equation M6), Rb is 0.60 as obtained above.

Further, in one inorganic phosphor particle 31Sa, 31Sb, a ^{point where the reaction occurs 1 × 10 3} times due to the incident of neutron N, and one first inorganic phosphor particle 31Sa exists, and the second inorganic fluorescence Taking into account the fact that there are 20 body particles 31Sb, in the neutron scintillator of FIG. 6, photons incident on the wavelength conversion fiber 20S when the neutron n is incident on the first and second inorganic phosphor particles 31Sa and 31Sb. The number of occurrences of events Cnab in which the number is m is expressed by the following (Equation M7).
Cnab = (1 × 10 ³ × 1 × Pna) + (1 × 10 ³ × 20 × Pnb)
(Equation M7)

Based on the above (Equation M7), FIG. 7 shows a graph in which the number of photons m incident on the wavelength conversion fiber 20S and the number of occurrences of this event Cnab are plotted.

In the above (Equation M7), the first term on the right side corresponds to the number of photons emitted from the first inorganic phosphor particle 31Sa, and the second term is emitted from the second inorganic phosphor particle 31Sb. Corresponds to the number of photons to be produced.

Here, as shown in the present embodiment, when the inorganic phosphor particles are not contained in the vicinity of the wavelength conversion fiber 20S (that is, in FIG. 1, the inorganic phosphor particles 31 are not contained in the first region A1 of the resin composition portion 30). In the case), when the neutron N is incident on the second inorganic phosphor particle 31Sb, the number of occurrences Cnb of the event in which the number of photons incident on the wavelength conversion fiber 20S is m is expressed by the following (Equation M8). Will be done.
Cnb = (1 × 10 ³ × 20 × Pnb) (Equation M8)

Based on the above (Equation M8), FIG. 8 shows a graph in which the number of photons m incident on the wavelength conversion fiber 20S and the number of occurrences of this event Cnb are plotted.

Next, the number of photons incident on the wavelength conversion fiber 20S when γ-rays are incident on the inorganic phosphor particles 31Sa and 32Sb will be examined.
When a γ-ray is incident on the first inorganic phosphor particle 31Sa, the probability Pγa that only m photons emitted due to the incident of the γ-ray are incident on the wavelength conversion fiber is as follows (Equation M9). ).

Similarly, when γ-rays are incident on the second inorganic phosphor particles 31Sb, the probability Pγb that only m photons emitted due to the incident of the γ-rays are incident on the wavelength conversion fiber 20S is as follows. It is represented by (Equation M10).

Further, in one inorganic phosphor particle 31Sa, 31Sb, a ^{point where 1 × 10 11} reactions occur due to the incident of γ-ray, and one first inorganic phosphor particle 31Sa exists, and the second inorganic fluorescence Taking into account the fact that there are 20 body particles 31Sb, in the neutron scintillator of FIG. 6, the number of photons incident on the wavelength conversion fiber 20S is m with respect to the incident of γ-rays on the inorganic phosphor particles 31S. The number of occurrences of Cγab is represented by the following (Equation M11).
Cγab = (1 × 10 ¹¹ × 1 × Pγa) + (1 × 10 ¹¹ × 20 × Pγb)
(Equation M11)

Based on the above (Equation M11), when γ-rays are incident on the first and second inorganic phosphor particles 31Sa and 31Sb, the number of photons m incident on the wavelength conversion fiber 20S and the number of occurrences of this event Cγab A graph plotting the above is shown in FIG.

Further, when the inorganic phosphor particles are not contained in the vicinity of the wavelength conversion fiber 20S (that is, when the first region A1 of the resin composition portion 30 does not contain the inorganic phosphor particles 31 in FIG. 1), the γ-rays are the first. The number of occurrences of an event in which the number of photons incident on the wavelength conversion fiber 20S is m when incident on the inorganic phosphor particles 31Sb of No. 2 Cγb is represented by the following (Equation M12).
Cγb = (1 × 10 ¹¹ × 20 × Pγb) (Equation M12)

Based on the above (Equation M12), the number of photons m incident on the wavelength conversion fiber 20S when γ-rays are incident on the second inorganic phosphor particles 31Sb and the number of occurrences of this event Cγb are plotted. The graph is shown in FIG.

FIG. 11 shows the number of photons m incident on the wavelength conversion fiber 20S when the neutrons N and γ rays are incident on the first and second inorganic phosphor particles 31Sa and 31Sb, respectively, and the phenomenon of this event. It is a graph which shows the relationship of the number of occurrences. That is, FIG. 11 is a graph in which the graph shown in FIG. 7 and the graph shown in FIG. 9 are superimposed.
Further, FIG. 13 shows the number of photons m incident on the wavelength conversion fiber 20S due to the neutrons N and γ rays in the model having the first and second inorganic phosphor particles 31Sa and 31Sb, respectively, and the occurrence of this event. It is a graph which shows the sum of the relation of the number of times. That is, FIG. 13 is a graph obtained by adding the number of occurrences shown in FIGS. 7 and 9 to each other.
Note that FIG. 11 has a logarithm on the vertical axis (number of occurrences), and FIG. 13 has a linear vertical axis.

In FIG. 12, when the first inorganic phosphor particles 31Sa of this model are omitted (when the inorganic phosphor particles are not contained in the vicinity of the wavelength conversion fiber 20S), the neutrons N and γ rays are the first and the first, respectively. 3 is a graph showing the relationship between the number of photons m incident on the wavelength conversion fiber 20S when incident on the inorganic phosphor particles 31Sa and 31Sb of No. 2 and the number of occurrences of this event. That is, FIG. 12 is a graph in which the graph shown in FIG. 8 and the graph shown in FIG. 10 are superimposed.
Further, FIG. 14 shows the sum of the relationships between the number of photons m incident on the wavelength conversion fiber 20S due to the neutrons N and γ-rays and the number of occurrences of this event in the model having only the second inorganic phosphor particles 31Sb. It is a graph which shows. That is, FIG. 14 is a graph obtained by adding the number of occurrences shown in FIGS. 8 and 10 to each other.
Note that FIG. 12 has a logarithm on the vertical axis (number of occurrences), and FIG. 14 has a linear vertical axis.

As shown in FIG. 11, when the inorganic phosphor particles are present in the vicinity of the wavelength conversion fiber 20S (that is, when the first inorganic phosphor particles 31Sa are present), the output result derived from γ-rays and the output result The output results, which depend on neutron N, overlap significantly. Further, one of the peaks of the output result depending on the neutron N overlaps with the output result derived from the γ-ray.
FIG. 13 can be seen as the detection result of the neutron scintillator 1S when the inorganic phosphor particles are present in the vicinity of the wavelength conversion fiber 20S. As shown in FIG. 13, when the inorganic phosphor particles are present in the vicinity of the wavelength conversion fiber 20S, one of the peaks of the output result depending on the neutron N overlaps with the output result derived from the γ-ray. , The detection result of neutrons is difficult to confirm due to the presence of γ-rays.

On the other hand, as shown in FIG. 12, when the inorganic phosphor particles do not exist in the vicinity of the wavelength conversion fiber 20S (that is, when the first inorganic phosphor particles 31Sa do not exist), the detection depends on the neutron N. The resulting peak is not buried in the graph due to gamma rays.
FIG. 14 can be seen as the detection result of the neutron scintillator 1S when the inorganic phosphor particles are not present in the vicinity of the wavelength conversion fiber 20S. As shown in FIG. 14, in this detection result, since the peak of the detection result depending on the neutron N is not buried in the γ-ray, the number of events of a specific number of photons (converted to the peak value) is confirmed. can do.

That is, when FIG. 13 and FIG. 14 are compared, when the inorganic phosphor particles are not arranged in the vicinity of the wavelength conversion fiber 20S (in the case of FIG. 14), the peak of the detection result depending on the neutron N is easily recognized. be able to. Therefore, in the actual measurement, it is possible to easily identify only the peak caused by the neutron in the measurement result measured by the neutron scintillator. As a result, it was confirmed that in the modeled neutron scintillator 1S, the n / γ discrimination ability can be enhanced by not arranging the inorganic phosphor particles in the vicinity of the wavelength conversion fiber 20S.

1 ... Neutron scintillator, 2 ... Neutron detector, 3 ... Photodetector, 4 ... Judgment unit, 20 ... Wavelength conversion fiber, 30 ... Resin composition part, 31 ... Inorganic phosphor particles, 32 ... Resin part, A1 ... 1st Area, A2 ... Second area

Claims (7)

A resin composition portion containing inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron-10, and a wavelength conversion fiber having at least a part contained in the resin composition portion. A neutron scintillator that is a component
The resin composition portion has a tubular first region including an outer circumference of the wavelength conversion fiber and a second region located outside the first region, and the inorganic phosphor in the first region. The density of the particles is lower than the density of the inorganic phosphor particles in the second region.
The resin composition section is a neutron scintillator in which the materials in the first region and the second region are the same. The neutron scintillator according to claim 1, wherein the first region of the resin composition section does not contain the inorganic phosphor particles. The ratio of the distance between the central axis of the wavelength conversion fiber and the outer wall surface of the resin composition portion to the diameter of the wavelength conversion fiber is in the range of 1 to 10.
The neutron scintillator according to claim 1 or 2. The ratio of the refractive index of the inorganic phosphor particles to the refractive index of the resin in the emission wavelength of the inorganic phosphor particles is in the range of 0.95 to 1.05.
The neutron scintillator according to any one of claims 1 to 3. The neutron scintillator according to any one of claims 1 to 4 and a photodetector are provided.
Neutron detector. A neutron scintillator comprising a resin composition portion containing inorganic phosphor particles containing a neutron capture isotope and a wavelength conversion fiber having at least a part contained in the resin composition portion.
A photodetector that detects the light emitted from the neutron scintillator,
Equipped with a determination unit that determines the presence or absence of neutron rays,
The determination unit approximates the logarithm of the detection frequency with respect to the peak value of the light detected by the photodetector to a linear function, and the detection frequency of the peak value near 4.8 MeV or 2.3 MeV is obtained from the linear function. When diverging, determine the presence of neutron rays and
The resin composition portion has a tubular first region including an outer circumference of the wavelength conversion fiber and a second region located outside the first region, and the inorganic phosphor in the first region. The density of the particles is lower than the density of the inorganic phosphor particles in the second region.
The resin composition section is a neutron detector in which the materials in the first region and the second region are the same. A neutron scintillator comprising a resin composition portion containing inorganic phosphor particles containing a neutron capture isotope and a wavelength conversion fiber having at least a part contained in the resin composition portion.
Using a photodetector that detects the light emitted from the neutron scintillator,
When the logarithm of the detection frequency with respect to the peak value of light detected by the optical detector is approximated to a linear function and the detection frequency of the peak value near 4.8 MeV or 2.3 MeV deviates from the linear function, Determine the presence of neutron rays and
The resin composition portion has a tubular first region including an outer circumference of the wavelength conversion fiber and a second region located outside the first region, and the inorganic phosphor in the first region. The density of the particles is lower than the density of the inorganic phosphor particles in the second region.
The resin composition section is a method for detecting neutrons, wherein the materials in the first region and the second region are the same.

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