JP2017198691A - Neutron scintillator and neutron detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a neutron scintillator that includes a resin composition containing inorganic phosphor particles and exhibits a high discrimination performance between neutrons and γ rays even when the light permeability of the resin composition is reduced by inconsistency or the like of refractive index and the fluorescence caused by neutrons cannot be taken sufficiently.SOLUTION: The neutron scintillator includes a resin composition of a resin and inorganic phosphor particles, such as Eu:LiCaAlF, containing at least one neutron capture isotope selected from lithium 6 and boron 10. The resin composition further contains neutron dead phosphors of different fluorescence characteristics. The neutron scintillator can discriminate between neutrons and γ rays using the difference in fluorescence characteristic such as fluorescence lifetime, emission wavelength, or the like between the incidence time of neutrons and the incidence time of γ rays. When internal transmittance of the resin composition is less than 30%/cm, it is particularly effective.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a neutron scintillator and a neutron detector using the neutron scintillator. Specifically, the present invention relates to a novel neutron scintillator and a neutron detector capable of accurately measuring neutrons even in a field with high neutron detection efficiency and a high dose of γ-rays that become background noise.

  The neutron detector is an elemental technology that supports neutron utilization technology. In the security field such as cargo inspection, academic research field such as structural analysis by neutron diffraction, non-destructive inspection field, or medical field such as boron neutron capture therapy. With the development of neutron utilization technology, higher performance neutron detectors are required.

  Important characteristics required for such a neutron detector include neutron detection efficiency and discrimination ability between neutrons and γ rays (hereinafter also referred to as n / γ discrimination ability). 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, γ-rays exist as natural radiation, and are also generated when a neutron hits a component of a detection system for detecting neutrons or an object to be inspected, so the n / γ discrimination ability is low, If γ rays are counted as neutrons, the accuracy of neutron counting decreases.

  When detecting neutrons, neutrons are generally detected using a neutron capture reaction because neutrons have a strong force to penetrate without any interaction in the material. For example, a helium 3 detector that detects using protons and tritium generated by a neutron capture reaction between helium 3 and neutrons has been conventionally known. This detector is a proportional counter filled with helium 3 gas, and has high detection efficiency and excellent n / γ discrimination. However, helium 3 is an expensive substance, and its resource amount is limited. There was a problem that there was.

  Recently, a neutron detector using a neutron scintillator is being developed in place of the helium 3 detector. A neutron scintillator refers to a substance that emits fluorescence when neutrons are incident. The neutron scintillator is combined with a photodetector such as a photomultiplier tube to form a neutron detector. it can. Note that the various performances of the neutron detector using the neutron scintillator depend on the substances constituting the neutron scintillator. For example, if a large amount of isotopes having high neutron capture reaction efficiency are contained, the detection efficiency for neutrons is improved. Examples of such isotopes include lithium 6 and boron 10 (see, for example, Patent Document 1).

  In the neutron detector, the photodetector detects the light emitted from the neutron scintillator, and a pulse signal is output from the photodetector. In general, the number of neutrons is measured by the intensity of the pulsed signal, so-called peak value. That is, a predetermined threshold value is provided for the peak value, an event that indicates a peak value that exceeds the threshold value is counted as a neutron incident event, and an event that indicates a peak value that is less than the threshold value is treated as noise.

  Although the neutron detector using the neutron scintillator has the advantage of high detection efficiency for neutrons, it is sensitive to γ rays and has a problem of poor n / γ discrimination.

  In order to solve such a problem, we comprise inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 and have transparency of scintillation light. A neutron scintillator made of a high resin composition has been proposed (Japanese Patent Application No. 2012-271318).

International Publication No. 2009/119378 Pamphlet

  The neutron scintillator made of the resin composition has high neutron detection efficiency and excellent discrimination ability between neutrons and γ rays, but it is necessary to make the resin composition itself highly transparent.

  In order to increase the transparency of the resin composition, theoretically, the refractive index of the resin and the refractive index of the inorganic phosphor particles should be matched.

  However, there is a limit to the control of the refractive index of a resin that is an organic substance, and there is a limit to providing a resin having a refractive index that matches all of the various inorganic phosphor particles. Further, when the resin composition is adjusted for the purpose of controlling physical properties other than the refractive index of the resin composition, it is often difficult to make the refractive indexes coincide.

  Furthermore, the refractive index is temperature dependent, which varies from material to material. Therefore, even if the refractive indices are matched at room temperature, when the operating temperature is other than room temperature, a difference between the refractive index of the resin and the refractive index of the inorganic phosphor particles occurs, resulting in loss of transparency. Of course, if the operating temperature is constant, the refractive index at the operating temperature should be matched, but for neutron detectors used outdoors, the operating temperature range must be several tens of degrees or more. There are many cases where it cannot be obtained.

  Alternatively, even when the refractive index of the resin and the refractive index of the inorganic phosphor particles coincide with each other, if the air bubbles are mixed in the resin composition, the transparency tends to decrease.

  And if the transparency of the resin composition becomes insufficient due to these factors, there is a problem that the fluorescence caused by neutrons is not sufficiently guided to the photodetector, and thus the discrimination ability between neutrons and γ rays becomes insufficient. there were.

  Accordingly, the present invention provides a neutron having excellent discrimination ability between neutrons and γ-rays even when the light transmittance of the resin composition is poor due to factors such as inconsistency in refractive index and fluorescence due to neutrons cannot be sufficiently extracted. The object is to provide a scintillator.

  In view of the above problems, the present inventors have conducted intensive studies. In addition to the inorganic phosphor particles, a phosphor having no neutron capture ability is contained in the resin composition, and by analyzing the difference in fluorescence characteristics between the two phosphors, neutrons and γ rays can be distinguished. The inventors have found that it is easy to complete the present invention.

  That is, the present invention is different from the inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 and the inorganic phosphor particles not containing a neutron capture isotope and having fluorescence characteristics. A neutron scintillator comprising a resin composition comprising a phosphor, wherein the resin composition has an internal transmittance of less than 30% / cm.

  According to the present invention, even when the light transmittance of the resin composition is not sufficient, sufficient discrimination ability between neutrons and γ rays can be obtained. Therefore, there is no need to forcibly match the refractive index of the resin with the refractive index of the inorganic phosphor particles, or to completely remove bubbles.

  Thereby, it becomes possible to manufacture a high-performance neutron scintillator more easily than before.

This figure is the figure which showed typically the measuring method of the internal transmittance in an Example. This figure is a diagram showing a signal waveform when the neutron detector obtained in Example 1 is irradiated with neutrons. This figure is a diagram showing a signal waveform when the neutron detector obtained in Example 1 is irradiated with γ rays. This figure is a wave height distribution spectrum obtained in Example 1 without using the waveform analysis mechanism. This figure is a wave height distribution spectrum obtained by using the waveform analysis mechanism in the first embodiment. This figure is a wave height distribution spectrum obtained in Example 2 without using the waveform analysis mechanism. This figure is a wave height distribution spectrum obtained by using the waveform analysis mechanism in the second embodiment.

  The neutron scintillator of the present invention has inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 (hereinafter also simply referred to as inorganic phosphor particles) as a first constituent element.

  In the inorganic phosphor particles, α-rays and tritium or α-rays and lithium 7 (hereinafter also referred to as secondary particles) are generated by a neutron capture reaction between lithium 6 or boron 10 and neutrons, respectively. The energy of 4.8 MeV or 2.3 MeV is applied to the inorganic phosphor particles. By applying the energy, the inorganic phosphor particles are excited and emit fluorescence.

  A neutron scintillator using such inorganic phosphor particles has high neutron detection efficiency due to the high efficiency of the neutron capture reaction by lithium 6 and boron 10, and is applied to the inorganic phosphor particles after the neutron capture reaction. Because of its high energy, it excels in the intensity of fluorescence emitted when neutrons are detected.

In the present invention, the content of lithium 6 and boron 10 in the inorganic phosphor particles (hereinafter also referred to as neutron capture isotope content) is preferably 1 atom / nm ³ and 0.3 atom / nm ³ or more, respectively. Are particularly preferably 6 atom / nm ³ and 2 atom / nm ³ or more, respectively. The neutron capture isotope content refers to the number of neutron capture isotopes contained per 1 nm ^{3 of} inorganic phosphor particles. By setting the neutron capture isotope content in the above range, the probability that incident neutrons cause a neutron capture reaction is increased, and the neutron detection efficiency is improved.

Such neutron capture isotope content selects the chemical composition of the inorganic phosphor particles, and lithium 6 in lithium fluoride (LiF) or boron oxide (B ₂ O ₃ ) used as a raw material for the inorganic phosphor particles and It can adjust suitably by adjusting the isotope ratio of boron-10. Here, the isotope ratio is an element ratio of the lithium 6 isotope to the total lithium element and an element ratio of the boron 10 isotope to the total boron element. For natural lithium and boron, about 7.6% and about 19.9%. As a method of adjusting the neutron capture isotope content, a method of adjusting the isotope ratio of lithium 6 and boron 10 to a desired isotope ratio using a general-purpose material having a natural isotope ratio as a starting material Alternatively, there is a method in which a concentrated raw material in which the isotopic ratio of lithium 6 and boron 10 is previously concentrated to a desired isotope ratio or more is prepared, and the concentrated raw material and the general-purpose raw material are mixed and adjusted.

On the other hand, the upper limit of the neutron capture isotope content is not particularly limited, but is preferably 60 atom / nm ³ or less. In order to achieve a neutron capture isotope content exceeding 60 atoms / nm ³ , it is necessary to use a large amount of special raw materials in which neutron capture isotopes are concentrated at a high concentration in advance, so that the production cost is extremely high. In addition, the selection of the type of inorganic phosphor particles is significantly limited.

  The contents of lithium 6 and boron 10 in the inorganic phosphor particles are determined in advance according to the density of the inorganic phosphor particles, the mass fraction of lithium and boron in the inorganic phosphor particles, and the lithium 6 and boron 10 in the raw material. The isotope ratios can be obtained by substituting them into the following formulas (1) and (2), respectively.

Content of lithium 6 = ρ × W _Li × C _Li / (700-C _Li ) × A × 10 ⁻²³ (1)
Content of boron 10 = ρ × W _B × C _B / (1100−C _B ) × A × 10 ⁻²³ (2)
(Wherein, [rho is the density of the scintillator _{^{[g / cm 3], W}} Li and _{W B} is lithium and the mass fraction of boron respectively in the inorganic phosphor particles _[wt%], in each _{C Li} and _{C B} material Lithium 6 and boron 10 isotope ratio [%], A indicates Avogadro number [6.02 × 10 ²³ ]
The inorganic phosphor particles are not particularly limited, and a conventionally known inorganic phosphor in the form of particles can be used. Specific examples are Eu: LiCaAlF ₆ , Eu, Na: LiCaAlF _6. , Eu: LiSrAlF ₆ , Ce: LiCaAlF ₆ , Ce, Na: LiCaAlF ₆ , Ce: LiSrAlF ₆ , Ce: LiYF ₄ , Eu: LiI, Ce: Li ₆ Gd (BO ₃ ) ₃ , Ce: LiCs ₂ YCl ₆ Inorganic phosphor particles composed of crystals such as 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 Li ₂ O— inorganic phosphor particles and the like elevation consisting of _{MgO-Al 2 O 3 -SiO 2} -Ce 2 O 3 system glass It is.

  In the present invention, the wavelength of light emitted from the inorganic phosphor particles is preferably in the near-ultraviolet region to the visible light region, and is preferably in the visible light region in that it is easy to obtain transparency when mixed with a resin described later. Particularly preferred.

  In the present invention, it is preferable that the neutron capture isotope contained in the inorganic phosphor particles is lithium 6 only. 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, and an extremely high energy of 4.8 MeV can be imparted. . Therefore, it is possible to obtain a neutron scintillator with little variation in fluorescence intensity and particularly excellent in fluorescence intensity.

Among inorganic phosphor particles containing only lithium 6 as a neutron capture isotope, chemical formula LiM ¹ M ² X ₆ (where M ¹ is at least one alkaline earth metal element selected from Mg, Ca, Sr and Ba) 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), and at least A cordierite type crystal containing one kind of lanthanoid element and a cordierite type crystal containing at least one kind of alkali metal element are preferred.

If the corkrite type crystal is specifically exemplified, inorganic phosphor particles composed of Eu: LiCaAlF ₆ , Eu, Na: LiCaAlF ₆ , Eu: LiSrAlF ₆ and Eu, Na: LiSrAlF ₆ have a high light emission amount, and deliquescence. It is most preferable because it is not stable and chemically stable.

  In the present invention, as a first means for improving the n / γ discrimination ability of the neutron scintillator, one of the features is that the shape of the inorganic phosphor is particulate. Hereinafter, the action mechanism in which the n / γ discrimination ability is improved by using such inorganic phosphor particles will be described.

  In general, when γ rays are incident on the scintillator, fast electrons are generated inside the scintillator, and the fast electrons impart energy to the inorganic phosphor particles, whereby the inorganic phosphor particles emit light. When the wave height value output by such light emission is as high as the wave height value due to the incidence of neutrons and both cannot be distinguished, γ rays are counted as neutrons, and an error occurs in neutron counting. In particular, when the dose of γ-rays is high, errors due to the γ-rays increase and become a significant problem.

  The peak value output from the neutron detector by the incidence of γ-rays depends on the energy imparted by the fast electrons, so by reducing the energy, it is output when the γ-rays are incident on the neutron scintillator. The peak value can be reduced.

  Here, the range in which high-speed electrons generated when γ rays are incident on the scintillator moves through the scintillator while imparting energy to the scintillator is relatively long, about several millimeters. In the neutron scintillator of the present invention, since inorganic phosphor particles are used, fast electrons generated when γ rays are incident on the inorganic phosphor particles deviate from the inorganic phosphor particles before imparting all the energy. Therefore, the energy imparted to the inorganic phosphor particles from the fast electrons is reduced, and the peak value output when γ rays are incident is lower than the peak value when neutrons are incident. Discrimination ability is improved.

  In the present invention, as described above, the shape of the inorganic phosphor particles is not particularly limited as long as fast electrons generated by the incidence of γ rays deviate from the inorganic phosphor particles and the n / γ discrimination ability is improved. . In general, the smaller the size, the faster the fast electrons deviate from the inorganic phosphor particles and the n / γ discrimination ability is improved. However, the inorganic phosphor particles have various particle forms such as a flat plate shape, a prismatic shape, a cylindrical shape, or a spherical shape depending on the type and production method, and the ease of departure of the fast electrons depends on the particle shape. Because of the difference, it is generally difficult to know a suitable shape for quickly deviating the fast electrons from the inorganic phosphor particles.

According to the study by the present inventors, the shape of the inorganic phosphor particles 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. It is particularly preferable to do this. In the present invention, the specific surface area of the inorganic phosphor particles refers to the surface area per unit volume of the inorganic phosphor particles.

  Here, since the specific surface area in the present invention is a surface area per unit volume, (1) it tends to increase as the absolute volume of the inorganic phosphor particles decreases, and (2) the shape is spherical. The specific surface area is the smallest, and conversely, the larger the specific surface area of the inorganic phosphor particles is, the more the inorganic phosphor particles have a shape far from the true sphere. 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 either axial direction is shortened. As a result, the specific surface area becomes larger even if the other axial side is made longer in the same volume.

More specifically, the specific surface area of a regular hexahedron having a side of 0.5 cm is 12 cm ² / cm ³ , but in the case of the regular hexahedron of 0.1 cm (0.001 cm ³ ), the ratio is The surface area is 60 cm ² / cm ³ . Furthermore, when the thickness is 0.025 cm with the same volume (0.001 cm ³ ), the vertical and horizontal dimensions are 0.2 cm × 0.2 cm, and the specific surface area is 100 cm ² / cm ³ .

  In other words, a large specific surface area indicates that at least one of the axial lengths has a very small portion. And since the fast electrons excited by the γ rays running in the small axial direction and the direction close to the axial direction deviate from the crystal as described above, the energy imparted to the inorganic phosphor particles from the fast electrons is increased. It can be reduced.

The shape of a suitable inorganic phosphor particle based on the specific surface area has been found by the above knowledge and consideration, and the specific surface area is taken into account the energy imparted from the high-speed electrons to the inorganic phosphor particle. Above, it can use as a parameter | index of the shape which considered that the inorganic fluorescent substance particle has various particle | grain forms. In practice, the specific surface area is preferably 50 cm ² / cm ³ or more, and more preferably, the specific surface area is 100 cm ² / cm ³ or more, thereby obtaining a neutron detector particularly excellent in n / γ discrimination. it can.

In the present invention, 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 length of all the inorganic phosphor particles in the axial direction is extremely small, 2 generated by the neutron capture reaction of the lithium 6 and boron 10 with neutrons. There is a possibility that an event in which the secondary particles deviate from the inorganic phosphor particles before giving the total energy to the inorganic phosphor particles may occur. In such an event, the energy imparted to the inorganic phosphor particles by the incidence of neutrons decreases, so the intensity of light emitted from the inorganic phosphor decreases. In order to reliably give the total energy of the secondary particles to the inorganic phosphor particles and increase the intensity of light emission of the inorganic phosphor, it is particularly preferable that the specific surface area of the inorganic phosphor particles is 500 cm ² / cm ³ or less.

  Although the term “axis” is used in the above description, it is merely used for convenience to indicate the spatial coordinate position of X, Y, and Z, and the inorganic phosphor particles used in the present invention have sides in these specific axial directions. Of course, it is not limited to the cube that has.

  When the inorganic phosphor particles are indefinite, the specific surface area can be easily determined from the density and mass-based specific surface area obtained using a density meter and a BET specific surface area meter.

  Further, when the size of the inorganic phosphor particles used in the present invention is defined from another viewpoint, the particles may be substantially free of particles that pass through a sieve having an opening of 2 mm and pass through a sieve having an opening of 100 μm. Preferably, it is more preferable that the particles passing through the sieve having an aperture of 1000 μm are substantially not included in the particles passing through the sieve having an aperture of 100 μm. Here, the phrase “substantially free of particles passing through a sieve having an aperture of 100 μm” means that the mass change upon shaking for 5 minutes with a sieve with the same aperture is 5% or less.

  In the present invention, specific examples of the shape of the inorganic phosphor particles that are preferably used are flat, prismatic, cylindrical, spherical, or irregular particle forms, and have an equivalent surface area sphere equivalent diameter. The shape which is 50-1500 micrometers, Most preferably, it is 100-1000 micrometers is mentioned.

  The method for producing the inorganic phosphor particles is not particularly limited, and an inorganic phosphor having a desired shape is obtained by pulverizing and classifying a bulk body of the inorganic phosphor having a shape larger than that of the inorganic phosphor particles having a suitable shape. Examples thereof include a method for obtaining particles, and a method for directly obtaining inorganic phosphor particles having a suitable shape by a particle formation reaction using an inorganic phosphor solution as a starting material.

Among these production methods, the method of pulverizing and classifying the bulk material of the inorganic phosphor is preferable because the production efficiency is high and the initial inorganic phosphor particles can be obtained at a low cost. A method for pulverizing the bulk of the inorganic phosphor is not particularly limited, and a known pulverizer such as a hammer mill, a roller mill, a rotary mill, a ball mill, or a bead mill can be used without any limitation. Among them, the generation of so-called fine powder having an extremely large specific surface area is suppressed, and as described above, inorganic phosphor particles having a specific surface area of preferably 1000 cm ² / cm ³ or less, particularly preferably 500 cm ² / cm ³ or less. In particular, it is preferable to use a hammer mill and a roller mill.

  In addition, as a method of classifying the inorganic phosphor particles obtained by pulverizing the bulk of the inorganic phosphor, a known method such as dry sieving, wet sieving, or air classification can be applied without particular limitation.

  The neutron scintillator of the present invention is a resin composition (hereinafter also simply referred to as a resin composition) comprising the inorganic phosphor particles. As understood from the above description, the inorganic phosphor particles of the present invention are small in size compared to generally used inorganic phosphors having a neutron capturing ability, so that the single inorganic phosphor particles have poor neutron detection efficiency. The problem arises. Such a problem can be solved by mixing a plurality of inorganic phosphor particles with a resin and dispersing it in the resin, and has a high neutron detection efficiency while having the excellent n / γ discrimination ability. A neutron scintillator can be obtained.

  The resin constituting the resin composition may be liquid or solid, but is preferably a solid resin from room temperature to use temperature from the viewpoint of ease of handling.

  Specific examples of the resin that can be used in the present invention include silicone resin, fluororesin, poly (meth) acrylate, polycarbonate, polystyrene, polyvinyl toluene, and polyvinyl alcohol. A plurality of resins may be mixed and used for the purpose of adjusting the refractive index, strength, and the like.

  In the present invention, the proportion of the inorganic phosphor particles in the resin composition is not particularly limited, but the volume fraction of the inorganic phosphor particles in the resin composition is preferably 10% or more, and 20% or more. It is particularly preferable to do this. By setting the volume fraction of the inorganic phosphor particles in the resin composition within such a range, the neutron detection efficiency per unit volume of the resin composition can be improved. On the other hand, the volume fraction of the inorganic phosphor particles in the resin composition is preferably 50% or less, and particularly preferably 40% or less. By setting the volume fraction to be equal to or less than this amount, it becomes easy to prevent fast electrons deviating from the inorganic phosphor particles from being incident on another nearby inorganic phosphor particle and emitting light.

  The resin composition constituting the neutron scintillator of the present invention has a low light transmittance as described above, and thus has a good neutron / γ ray discrimination ability even when the extraction efficiency of scintillation light caused by neutrons is low. . However, at the emission wavelength of the inorganic phosphor particles, it preferably has an internal transmittance of at least 3%, more preferably 5% or more, and particularly preferably 10% or more.

Here, the internal transmissivity is a transmissivity excluding surface reflection loss generated on the incident side and the emissive surface of the organic resin when light is transmitted through the organic resin, and the per unit optical path length is 1 cm. Expressed by value. The internal transmittance (τ ₁₀ ) per 1 cm of the optical path length is obtained by measuring the transmittance including the respective surface reflection losses for a pair of organic resins having different thicknesses and substituting them into the following formula (3): Can be sought.

log (τ ₁₀ ) = {log (T ₂ ) −log (T ₁ )} / (d ₂ −d ₁ ) (3)
In the above formula, d ₁ and d ₂ indicate the thickness in cm of the pair of organic resins, and d ₂ > d ₁ . T ₁ and T ₂ indicate transmittances including surface reflection loss of organic resins having thicknesses d ₁ and d ₂ , respectively.

  The upper limit of the internal transmittance is not particularly limited, and may be as high as 90 to 100%, for example, but in the present invention, the effect is particularly effective when the internal transmittance of the resin composition is less than 50%, particularly less than 30%. Is expensive.

  In order to increase the internal transmittance of the resin composition in the present invention, it is preferable to use a transparent resin (hereinafter also simply referred to as a transparent resin) at the emission wavelength of the inorganic phosphor particles. More specifically, the internal transmittance of the resin at the emission wavelength of the inorganic phosphor particles is preferably 80% / cm or more, and particularly preferably 90% / cm or more. The internal transmittance of the resin can be obtained in the same manner as the internal transmittance of the resin composition described above, using a single resin as a sample. Specific examples of such transparent resins include silicone resin, fluororesin, poly (meth) acrylate, polycarbonate, polystyrene, polyvinyl toluene, and polyvinyl alcohol. A plurality of resins may be mixed and used for the purpose of adjusting the refractive index.

  As described above, the internal transmittance has a very strong tendency to depend on the refractive index ratio of each element (resin, inorganic phosphor particles, filler particles, etc.) constituting the resin composition. In other words, in the present invention, the refractive index of the resin and the inorganic phosphor particles and / or filler particles may differ by 10% or more at the emission wavelength of the inorganic phosphor particles, and further differ by 20% or more. Even if it is, sufficient performance can be obtained.

  The resin composition according to the present invention has, as a second feature, a phosphor that does not contain a neutron capture isotope and has a fluorescence characteristic different from that of the inorganic phosphor particles in addition to the inorganic phosphor particles and the resin (hereinafter referred to as neutrons). (Also called insensitive phosphor).

  The reason is as follows. Fast electrons generated by the incidence of γ rays on the scintillator not only cause the inorganic phosphor particles to emit light, but also reach the neutron-insensitive phosphor to impart energy, and the neutron-insensitive phosphor also emits fluorescence. That is, when γ rays are incident, both the inorganic phosphor particles and the neutron-insensitive phosphor give energy and emit fluorescence. On the other hand, when neutrons are incident, secondary particles generated in the inorganic phosphor particles hardly deviate from the inorganic phosphor particles, so that only the inorganic phosphor particles emit fluorescence.

  And since the said inorganic fluorescent substance particle and this neutron insensitive fluorescent substance have a difference in fluorescence characteristics, such as a fluorescence lifetime and light emission wavelength, a neutron and a gamma ray can be discriminated using the difference in this fluorescence characteristic. 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 particles and the fluorescence derived from the neutron-insensitive phosphor are detected, gamma rays are incident. When only fluorescence derived from inorganic phosphor particles is detected as an event, it can be treated as an event in which neutrons are incident. By taking such a process, a neutron detector that is particularly excellent in n / γ discrimination ability can be obtained.

  A specific example of a mechanism capable of identifying a difference in fluorescence characteristics is a waveform analysis mechanism capable of identifying a difference in fluorescence lifetime between the inorganic phosphor particles and the neutron-insensitive phosphor, and the inorganic phosphor particles and the neutron-insensitive phosphor. And a wavelength analysis mechanism that can identify the emission wavelength.

  Hereinafter, the waveform analysis mechanism will be described more specifically by taking as an example a case where the fluorescence lifetime of the neutron-insensitive phosphor is shorter than the fluorescence lifetime of the inorganic phosphor particles.

In the neutron detector of the present invention formed by combining the neutron scintillator and the photodetector, the signal output from the photodetector is a pulse-like signal reflecting the fluorescence lifetime of the neutron scintillator. With respect to the pulse-like signal, an integral value of signal intensity, that is, a so-called peak value is acquired in a time range from when the signal rises until a predetermined time elapses. At this time, the peak value is acquired in two different time ranges, and the peak value in the short time range is set as S ₁ and the peak value in the long time range is set as S ₂ . In the event of incident neutrons, only the fluorescence derived from the inorganic phosphor particles is observed, so the ratio of the crest values (S ₁ / S ₂ ) depends on the fluorescence lifetime of the inorganic phosphor particles, and is approximately It becomes a constant value. On the other hand, in the event of γ-ray incidence, in addition to the fluorescence derived from the inorganic phosphor particles, the fluorescence of the neutron-insensitive phosphor with a short fluorescence lifetime is observed. Thus, S ₁ becomes relatively large, and S ₁ / S ₂ also becomes large. Thus, may be provided a threshold S 1 _/ S _2, if S 1 _/ S ₂ is more than the threshold value is an event that the incident γ-rays, neutrons When S 1 _/ S ₂ is less than the threshold value is It can be processed as an incident event. Specifically, the signal output from the photodetector is digitized and recorded by a digital oscilloscope, a high-speed digitizer, etc., and S ₁ / S ₂ is calculated from the recorded digital signal, and neutron and γ-rays are discriminated. Just do it. Alternatively, the signal output from the photodetector may be input to two analog-digital converters having different integration time constants, S ₁ and S ₂ may be acquired, and S ₁ / S ₂ may be obtained sequentially.

  Different aspects of the waveform analysis mechanism will be described. First, a signal output from the photodetector is input to a main amplifier via a 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. The time required for such increase (hereinafter also referred to as rise time) is the inorganic phosphor particles or This reflects the fluorescence lifetime of the neutron-insensitive phosphor. 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 intensity exceeds a predetermined threshold. Here, the waveform analyzer is provided with two-stage threshold values, 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 a time amplitude converter (TAC), and the time difference between the two logic signals output from the waveform analyzer is converted into a 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 understood from the above description, in the event that the γ-ray is incident, the fluorescence of the neutron-insensitive phosphor having a short fluorescence lifetime is observed, so that the rise time is relatively short compared to the event in which the neutron is incident. Thus, the pulse amplitude output from the time amplitude converter is also reduced. Accordingly, a threshold value is provided for the pulse amplitude, and if the pulse amplitude is less than the threshold value, it is treated as an event in which γ rays are incident. it can.

  Hereinafter, the wavelength analysis mechanism will be illustrated more specifically. The wavelength analysis mechanism includes an optical filter, a second photodetector connected to the neutron scintillator through the optical filter, and a discrimination circuit.

  In this embodiment, part of the light emitted from the neutron scintillator 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. It is burned.

  Here, the inorganic phosphor particles 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 and the neutron-insensitive phosphor emit fluorescence, so that light of Anm and Bnm is emitted from the neutron scintillator, and neutrons are incident. In this case, only the inorganic phosphor particles emit fluorescence, so only the light of Annm is emitted.

  In this aspect, the optical filter is a filter that blocks light having a wavelength of Anm and transmits light having a wavelength of Bnm. Therefore, although the light of Anm emitted from the neutron scintillator when irradiated with neutrons reaches the first photodetector, it 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 irradiated with γ-rays, the light of Anm is the same as the case of irradiating the neutron, but the light of Bnm reaches the first photodetector. And also reaches the second photodetector for transmission through the optical filter.

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

  In this embodiment, a discrimination circuit is provided to discriminate neutrons and γ rays as described above. The discrimination circuit operates in synchronization with the signal from the first photodetector, and determines whether or not there is a signal from the second photodetector when the signal from the photodetector is output. Circuit. Specific examples of the discrimination circuit include an anti coincidence circuit and a gate circuit.

  As the neutron-insensitive phosphor used in the present invention, a known phosphor can be used without particular limitation, but a particulate inorganic substance or an organic substance can be preferably used.

  Neutron-insensitive phosphors made of particulate inorganic substances (hereinafter referred to as neutron-insensitive phosphor particles) are preferable in that various fluorescence lifetimes and emission wavelengths can be obtained depending on the type. Further, the neutron-insensitive phosphor particles are most preferably made of the same inorganic substance as the inorganic phosphor particles, and the fluorescence lifetime or emission wavelength is changed by changing the kind and concentration of the dopant. That is, since the refractive index of such neutron-insensitive phosphor particles matches that of the inorganic phosphor particles, it is easy to match the refractive index with the resin, and a resin composition having a particularly high internal transmittance is obtained. be able to. The neutron-insensitive phosphor particles can also have a function as filler particles described later.

  On the other hand, a neutron-insensitive phosphor made of an organic substance is preferable in that it can be easily dissolved and uniformly present in the resin composition, and high light transmittance can be easily obtained. Furthermore, since a phosphor made of an organic substance generally has a shorter fluorescence lifetime than the inorganic phosphor particles, it can be suitably used to improve n / γ discrimination using the difference in fluorescence lifetime.

  Illustrative examples of neutron-insensitive phosphors made of organic materials include 2,5-Diphenyloxazole, 1,4-Bis (5-phenyl-2-oxazole) benzene, 1,4-Bis (2-methylstyryl) benzene, Organic phosphors such as anthracene, stilbene and naphthalene, and derivatives thereof may be mentioned.

  The content of the neutron-insensitive phosphor can be appropriately set within a range in which the effect of the present invention is exerted, but is preferably 0.01% by mass or more with respect to 100% by mass of the resin, and 0.1% by mass or more. It is particularly preferable to do this. By setting the content to 0.01% by mass or more, the neutron-insensitive phosphor is efficiently excited by the energy applied from the fast electrons, and the intensity of light emitted from the neutron-insensitive phosphor increases. Moreover, the upper limit of the content of the neutron-insensitive phosphor is not particularly limited, but should be 10% by mass or less based on the resin for the purpose of preventing the intensity of light emitted from the neutron-insensitive phosphor from being attenuated by concentration quenching. Is preferably 5% by mass or less, and particularly preferably 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 increases, and neutrons and γ-rays are converted using the difference in fluorescence characteristics with the inorganic phosphor particles. It becomes easy to discriminate.

  In the neutron scintillator comprising the resin composition of the present invention, particles that are not phosphors (for adjusting the existence interval of inorganic phosphor particles, adjusting the viscosity of a liquid resin or a solid resin before curing, and other various physical properties) Hereinafter, filler particles) may be included.

  Specific examples of such filler particles include inorganic particles such as silica, titanium oxide, barium sulfate, calcium fluoride, magnesium fluoride, lithium fluoride, magnesium carbonate, strontium fluoride, mica (mica), and various glasses. Or organic particles such as silicone resin, fluororesin, poly (meth) acrylate, polycarbonate, polystyrene, polyvinyl toluene, polyvinyl alcohol, polyethylene, and styrene butadiene.

Further, the filler particles are most preferably filler particles made of an inorganic material that is the same kind as the inorganic phosphor particles and has no doping element. That is, the filler particles made of the same kind of inorganic material as the inorganic phosphor particles have a specific gravity that matches the specific gravity of the inorganic phosphor particles, so that the inorganic phosphor particles can be uniformly dispersed in the resin composition. it can. Furthermore, since the refractive index of the filler particles matches the refractive index of the inorganic phosphor particles, if a resin having a refractive index close to that of the inorganic phosphor particles is selected, the filler particles and the resin are refracted by themselves. The rate becomes close, and the internal transmittance of the resin composition can be easily increased. More preferably, the crystal is represented by the chemical formula LiM ¹ M ² X ₆ and is not doped with a lanthanoid element or the like.

  The content of the filler particles is preferably 20% or more, particularly preferably 50% or more, when the volume of the inorganic phosphor particles is 100%. By setting it as this ratio, the effect which suppresses isolation | separation of inorganic fluorescent substance particle is fully exhibited. In addition, the upper limit of the mixing ratio of the filler particles to the resin composition is not particularly limited. In particular, in order to dramatically reduce the volume fraction of inorganic phosphor particles for the purpose of dramatically improving the n / γ discrimination ability of the neutron scintillator, a large amount of filler particles are used to uniformly disperse the inorganic phosphor particles. Are preferably mixed. However, in consideration of the viscosity at the time of producing the resin composition, the volume fraction of the filler particles with respect to the entire resin composition is preferably less than 80% by volume, and more preferably less than 50% by volume.

  The shape of the filler particles is not particularly limited. However, in order to fill the gaps between the inorganic phosphor particles, it is preferable that the shape is equal to or smaller than that of the inorganic phosphor particles.

  In the present invention, the resin composition may be used as a slurry-like or paste-like resin composition in which inorganic phosphor particles and a liquid or viscous resin are mixed, or the inorganic phosphor particles and a liquid or viscous resin precursor. After mixing, the resin precursor may be used as a solid resin composition cured.

  Although the manufacturing method of the resin composition which comprises the neutron scintillator of this invention is not specifically limited, It can manufacture suitably as follows. That is, when the resin composition according to the present invention is used in the form of a slurry or paste resin composition, first, inorganic phosphor particles 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. At this time, the neutron-insensitive phosphor may be mixed in the resin in advance (dissolved if it is soluble), or may be mixed simultaneously with the inorganic phosphor particles or afterwards.

  Next, it is preferable to defoam bubbles generated in the resin composition in the mixing operation. In the defoaming operation, a defoamer such as a vacuum defoamer or a centrifugal defoamer can be used without any particular limitation. By performing such a defoaming operation, it is possible to suppress meaningless light scattering by the bubbles.

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

  When the resin composition is used as a solid in the present invention, a mixing operation and a defoaming operation are performed in the same manner as described above using a liquid or viscous resin precursor. Next, the obtained mixture of inorganic phosphor particles and resin precursor is injected into a mold having a desired shape, and the resin precursor is cured. The curing method can be appropriately selected according to the resin precursor used, and is not particularly limited, but a method of polymerizing the resin precursor by heating, ultraviolet irradiation, or addition of a catalyst is suitable.

  The neutron scintillator comprising the resin composition of the present invention can be used in the form of a slurry or paste, and can be molded into a desired shape even when used in a solid form, so it can be easily formed into an arbitrary shape. It is. Therefore, according to the present invention, a neutron scintillator having a fiber shape, a hollow tube shape, or a large area can be provided depending on the application.

  The neutron detector of the present invention is a combination of the neutron scintillator and a photodetector. That is, the light emitted from the neutron scintillator by the incidence of neutrons is converted into an electrical signal by a photodetector, and the incidence of neutrons is measured as an electrical signal, so that it can be used for neutron counting and the like. In the present invention, the photodetector is not particularly limited, and a conventionally known photodetector such as a general photomultiplier tube, a photodiode, an avalanche photodiode, or a Geiger mode avalanche photodiode can be used without any limitation. However, when the photodetector is used in a high temperature environment of 100 to 200 ° C., it is preferable to use a high temperature photomultiplier tube excellent in operating characteristics in the temperature range. Specific examples of such high-temperature photomultiplier tubes include R3991A, R1288A, R1288AH, and R5473-02 manufactured by Hamamatsu Photonics. A neutron detector that can be used in such a high temperature environment can be used particularly suitably in the field of resource exploration and the like.

  The neutron scintillator of the present invention preferably has a light exit surface facing the photodetector, and the light exit surface is preferably a smooth surface. By having such a light exit surface, light generated by the neutron scintillator can be efficiently incident on the photodetector. Further, it is preferable that a light reflecting film made of aluminum, polytetrafluoroethylene, or the like is provided on a surface that does not face the light detector, so that dissipation of light generated by the neutron scintillator can be prevented.

  The method of manufacturing the neutron detector by combining the neutron scintillator of the present invention and the photodetector is not particularly limited. For example, the light emission surface of the neutron scintillator is connected to the light detection surface of the photodetector with optical grease or optical cement. A neutron detector can be fabricated by optically bonding and connecting a power source and signal readout circuit to the photodetector. The signal readout circuit generally includes a preamplifier, a shaping amplifier, an analog-digital converter, and the like, and the waveform analysis mechanism or the wavelength analysis mechanism is added to these in the present invention.

  Further, by arranging a large number of neutron scintillators provided with the light reflecting film and using a position sensitive photodetector as a photodetector, it is possible to give a position resolution to the neutron detector.

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

Example 1
In this example, neutron detection was performed using a neutron scintillator made of a resin composition containing Eu: LiCaAlF ₆ crystal doped with 0.04 mol% of Eu as inorganic phosphor particles and 2,5-Dipheniloxazole as a neutron-insensitive phosphor. A vessel was made. Note that the fluorescence lifetimes of Eu: LiCaAlF ₆ and 2,5-Dipheniloxazole are 1600 nsec and 2 nsec, respectively.

The Eu: LiCaAlF ₆ crystal contains only lithium 6 as a neutron capture isotope. The density of the Eu: LiCaAlF ₆ crystal is 3.0 g / cm ³ , the mass fraction of lithium is 3.2% by mass, the isotope ratio of lithium 6 in the raw material is 95%, and therefore its neutron capture isotope content Is 9.1 atom / nm ³ from the formula (1).

In addition, the Eu: LiCaAlF ₆ crystal was irradiated with radiation, and the emission wavelength of the Eu: LiCaAlF ₆ crystal was measured with a fluorometer. The radiation was α-ray, which is one of secondary particles generated during neutron irradiation, and Am-241 was used as a radiation source.

When producing inorganic phosphor particles comprising Eu: LiCaAlF ₆ crystals, first, a bulk body of Eu: LiCAF ₆ crystals having an irregular shape of about 2 cm square is prepared, and the bulk body is pulverized by a hammer mill. Then, the particles were classified by dry classification, passed through an upper sieve of 200 μm, and those remaining on the lower sieve of 100 μm were collected to obtain amorphous inorganic phosphor particles.

It was 0.015 m < ² > / g when the specific surface area of the mass basis of this inorganic fluorescent substance particle was measured using the BET specific surface area meter. Therefore, the surface area per unit volume was 450 cm ² / cm ³ .

In this example, filler particles made of LiCaAlF ₆ crystals were used. The density of the LiCaAlF ₆ crystal is 3.0 g / cm ³ , the mass fraction of lithium is 3.7% by mass, the isotope ratio of lithium 6 in the raw material is 7.6%, and therefore its neutron capture isotope content Was 0.73 atom / nm ³ from the formula (1). Further, the specific gravity of the LiCaAlF ₆ crystals, the Eu: LiCaAlF ₆ crystal and the same was 3.0 g / cm ^3.

The filler particles made of the LiCaAlF ₆ crystals were obtained by pulverizing a bulk material in the same manner as the inorganic phosphor particles and passing through a sieve having an opening of 100 μm.

In this example, a silicone resin (manufactured by Shin-Etsu Chemical Co., Ltd., KER-7030) was used as the resin. The resin is composed of two liquids, liquid A and liquid B. After preparing an equal amount of two liquids to prepare a resin precursor, the resin precursor can be cured by heating and used. The resin is a transparent resin having an internal transmittance of 95% / cm at the emission wavelength of 370 nm of the Eu: LiCaAlF ₆ crystal.

The refractive index at room temperature at 370 nm of the inorganic phosphor made of Eu: LiCaAlF ₆ crystal, the filler made of LiCaAlF ₆ crystal, and the silicone resin was measured using a refractometer. In the measurement of the refractive index, a bulk body of Eu: LiCaAlF ₆ crystal, a bulk body of LiCaAlF ₆ crystal, and a bulk body of resin having a predetermined shape suitable for the measurement were used. As a light source for the refractometer, i-line (365.0 nm) and h-line (404.7 nm) of an Hg lamp were used. The constants A and B were determined by substituting the wavelength of each light source and the refractive index measured at the wavelength into the Cermeier equation (4), and then the refractive index at 370 nm was determined using the equation. As a result, the refractive indexes of Eu: LiCaAlF ₆ crystal, LiCaAlF ₆ crystal, and silicone resin at 370 nm are 1.40, 1.40, and 1.41, respectively, and the refractive index of the transparent resin with respect to the refractive index of the inorganic phosphor particles. The ratio of the refractive index and the ratio of the refractive index of the transparent resin to the refractive index of the filler particles were both 1.01.

  The neutron scintillator of the present invention was produced by the following method. First, 19.9 g of a resin precursor of a silicone resin in which equal amounts of liquid A and liquid B are mixed, 2.0 g of a toluene solution in which 5% by mass of 2,5-Dipheniloxazole is dissolved in advance is added, Stir well. Then, toluene was distilled off under vacuum to obtain a resin precursor of a silicone resin containing 0.5% by mass of 2,5-Dipheniloxazole.

10.0 g of inorganic phosphor particles composed of the Eu: LiCaAlF ₆ crystals, 10.0 g of filler particles composed of the LiCaAlF ₆ crystals, and 11.0 g of the resin precursor of the silicone resin are placed in a mixing container, and a stirring bar is used. After mixing well, bubbles generated in the mixture in the mixing operation were defoamed using a vacuum defoamer.

  Next, each was poured into a polytetrafluoroethylene mold having a diameter of 2 cm, a thickness of about 0.3 cm, and a thickness of about 1 cm, heated at 80 ° C. for 5 hours, and then heated at 100 ° C. for 24 hours to obtain a resin precursor. Curing was performed to obtain a neutron scintillator of the present invention comprising a resin composition containing inorganic phosphor particles and a neutron-insensitive phosphor.

  The resin composition had a diameter of 2 cm, a thickness of 0.3 cm, and a volume of 0.94 mL. Further, the resin composition contains 0.56 g of inorganic phosphor particles and filler particles, and the volume of both is 0.19 mL from the density of the inorganic phosphor particles and filler particles. Therefore, the volume fraction of the inorganic phosphor particles in the resin composition was 20% by volume, and the mixing ratio of the filler particles to the resin composition was 20% by volume with respect to the entire resin composition.

With respect to the resin composition, the internal transmittance per 1 cm of the optical path length at the emission wavelength (370 nm) of the inorganic phosphor particles was measured by the following method. First, as shown in the left diagram of FIG. 1, light having a wavelength of 370 nm was incident on the light detection surface of the photodetector, and the intensity (I ₀ ) of the incident light was measured. Next, a resin composition having a diameter of 2 cm and a thickness of 0.3 cm (d ₁ ) is placed on the light detection surface of the photodetector as shown in the right diagram of FIG. 1, and light having a wavelength of 370 nm is incident thereon. the thickness was measured 0.3 cm (d ₁₎ light intensity through the resin composition enters the light detector (I _1). By dividing the _{I 1} with _{I 0,} thickness was obtained 0.3cm transmittance including the surface reflection loss of the resin composition of _{(d 1) (T 1)} . Similarly, the light intensity through the resin composition is incident on the photodetector having a thickness of 1 cm (d ₂₎ a (I ₂₎ is measured, the transmittance including the surface reflection loss of the resin composition (T ₂ ) As a result of substituting these d ₁ , d ₂ , T _1, and T ₂ values into the equation (3) to determine the internal transmittance per 1 cm of the optical path length, it was 68% / cm.

  A neutron detector of the present invention was manufactured by connecting a neutron scintillator made of a resin composition having a diameter of 2 cm and a thickness of 0.3 cm to a photodetector. First, one surface of the neutron scintillator having a diameter of 2 cm was used as a light emitting surface, and tape-like polytetrafluoroethylene was wound around a surface other than the light emitting surface to form a light reflecting film. Next, a photomultiplier tube (H6521 manufactured by Hamamatsu Photonics Co., Ltd.) is prepared as a photodetector, and after optically bonding the light detection surface of the photomultiplier tube and the light emitting surface of the neutron scintillator with optical grease, The neutron scintillator and the photodetector were covered with a black sheet for shading.

  A power source was connected to the photomultiplier tube, and the signal output of the photomultiplier tube was connected to a digital oscilloscope to obtain a neutron detector of the present invention.

  The performance of the neutron detector of the present invention was evaluated by the following method. In addition, when irradiating with neutrons, 2.4 MBq of radioactive Cf-252 is installed at the center of a 20 cm square cubic high-density polyethylene, and a scintillator is arranged at a position close to the high-density polyethylene. A neutron detector was installed as described above, and the neutrons from the Cf-252 were irradiated with a low-density polyethylene that was decelerated. On the other hand, when γ-rays were irradiated, 0.83 MBq of radioactive Co-60 was placed at a distance of 5 cm from the neutron scintillator, and γ-rays from the Co-60 were irradiated. The dose of γ rays at a distance of 5 cm from Co-60 with a radioactivity of 0.83 MBq is a very high dose of 10 mR / h.

  A high voltage of -1300 V was applied to the photomultiplier using a power source connected to the photomultiplier. By irradiating neutrons or γ-rays individually, the light emitted from the neutron scintillator is converted into pulsed electrical signals with a photomultiplier tube, and the electrical signals are digitized and recorded with a digital oscilloscope. did. Pulse-shaped signals obtained under neutron irradiation and γ-ray irradiation are shown in FIGS. 2 and 3, respectively. The inset in each figure is an enlarged view around time 0. 2 and 3, when neutrons are incident, only inorganic phosphor particles having a long fluorescence lifetime emit fluorescence, and when γ rays are incident, the inorganic phosphor particles and neutron insensitivity having a short fluorescence lifetime are emitted. It can be seen that both phosphors emit fluorescence.

For the obtained digital signal, the signal intensity in the time range until 500 nsec elapsed from the rising edge of the signal was integrated to obtain a peak value (S ₂ ) in a long time range. The S ₂ horizontal axis, to create a pulse height distribution spectrum frequency of events showing the value of each S ₂ to the vertical axis.

  The obtained wave height distribution spectrum is shown in FIG. The solid line and dotted line in FIG. 4 are the pulse height distribution spectra under neutron and γ-ray irradiation, respectively. In the wave height distribution spectrum, the horizontal axis represents the relative value with the wave height value of the neutron peak being 1.

  From FIG. 4, it can be seen that the peak value of the electrical signal generated by the incidence of γ-rays is generally lower than the peak value of the neutron peak, but part of it has a peak value equivalent to that of the neutron peak. Therefore, in the same way as in the prior art, when a predetermined threshold value is set for the peak value, and an event showing a peak value exceeding the threshold value is counted as a neutron incident event, a γ-ray showing a peak value equivalent to the neutron peak An incident event may be erroneously counted as a neutron incident event.

  The dispersion (σ) was obtained by fitting the neutron peak with a normal distribution function, and a threshold value was set at a peak value 3σ lower than the peak value of the neutron peak. The threshold value is indicated by a chain line in FIG. As a result of counting the frequency of signals exceeding the threshold for 300 seconds and obtaining the count rate (counts / sec) in the case of irradiating γ-rays, the result is 0.21 Counts / sec. It turns out that it occurs slightly.

In order to remove such errors derived from γ rays, neutrons and γ rays were discriminated using a waveform analysis mechanism. For the digital signal, in addition to the S _2, it was obtained the peak value of the short time range from the rise of the signal until 5nsec elapses (S _1). When creating a pulse height distribution spectrum using the S ₂ , events in which S ₁ / S ₂ did not satisfy a predetermined threshold were removed as γ-ray incident events. The wave height distribution spectrum thus obtained is shown in FIG.

  From FIG. 5, it can be seen that the γ-ray incident event showing the peak value equivalent to the neutron peak can be removed.

  In the same manner as described above, when a threshold value (chain line in FIG. 5) is provided for the peak value and γ rays are irradiated, the frequency of signals exceeding the threshold value is counted for 300 seconds to obtain the count rate (counts / sec). As a result, it was 0 Counts / sec. From this, it can be seen that if a neutron scintillator comprising a resin composition containing the inorganic phosphor of the present invention and a neutron-insensitive phosphor is used, neutron detection particularly excellent in n / γ discrimination ability can be obtained.

Example 2
A neutron scintillator made of a resin composition in the same manner as in Example 1 except that a silicone resin having a refractive index at 370 nm of 1.62 and an internal transmittance of 92% is used instead of the silicone resin of Example 1. Was made. In this example, the ratio of the refractive index of the resin to the refractive index of the inorganic phosphor particles and the ratio of the refractive index of the resin to the refractive index of the filler particles were both 1.19. The volume fraction of the inorganic phosphor particles in the resin composition was 20% by volume, and the mixing ratio of the filler particles to the resin composition was 20% by volume with respect to the entire resin composition.

  The resin composition was opaque because the refractive index of the resin did not match that of the inorganic phosphor particles and filler particles. With respect to the resin composition, the internal transmittance per optical path length of 1 cm at the emission wavelength of the inorganic phosphor particles was measured in the same manner as in Example 1. As a result, it was 26% / cm.

  In the same manner as in Example 1, a neutron detector is manufactured using the resin composition, light emitted from the neutron scintillator is converted into a pulsed electric signal by a photomultiplier tube, and the electric signal is converted into a digital oscilloscope. Was digitized and recorded. Similar to Example 1, only the fluorescence of inorganic phosphor particles having a long fluorescence lifetime is observed under neutron irradiation, and in addition to the fluorescence of inorganic phosphor particles under the irradiation of γ rays, the neutron-insensitive phosphor having a short fluorescence lifetime is observed. Fluorescence was observed.

In the same manner as in Example 1, a peak value (S ₂ ) was obtained from the obtained digital signal, and a peak distribution spectrum was created using the S ₂ .

  The obtained wave height distribution spectrum is shown in FIG. The solid line and the dotted line in FIG. 6 are wave height distribution spectra under neutron and γ-ray irradiation, respectively.

  In FIG. 6, since the resin composition is opaque, a clear neutron peak is not seen, and it is difficult to set a threshold value for the peak value. Here, for convenience, a threshold value is provided at a position where the peak value is 0.6. In the case of γ-ray irradiation, the frequency of signals exceeding the threshold was counted for 300 seconds, and the count rate (counts / sec) was obtained. As a result, it was 0.84 Counts / sec.

  Next, in the same manner as in Example 1, a wave height distribution spectrum was created after discriminating neutrons and γ rays using a waveform analysis mechanism. The obtained wave height distribution spectrum is shown in FIG. In the same manner as described above, a threshold value (chain line in FIG. 7) was provided for the peak value, and when γ-rays were irradiated, the frequency of signals exceeding the threshold value was counted for 300 seconds to obtain a count rate (counts / sec). The result was 0.03 Counts / sec. Therefore, if a neutron scintillator comprising a resin composition containing the inorganic phosphor of the present invention and a neutron-insensitive phosphor is used, neutron detection with excellent n / γ discrimination ability can be obtained even when the scintillator has poor transparency. I understand that

1 Neutron scintillator 2 Photodetector

Claims (5)

  Inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 and phosphors not containing a neutron capture isotope and having fluorescence characteristics different from those of the inorganic phosphor particles A neutron scintillator characterized in that the internal transmittance of the resin composition is less than 30% / cm.   The neutron scintillator according to claim 1, wherein the ratio of the refractive index of the resin to the refractive index of the inorganic phosphor particles is in the range of 0.90 to 1.10 at the emission wavelength of the inorganic phosphor particles.   The neutron scintillator according to claim 1 or 2, wherein a phosphor not containing a neutron capture isotope is dissolved in a resin component constituting the resin composition.   The neutron scintillator according to any one of claims 1 to 3, wherein the inorganic phosphor particles are substantially free of particles that pass through a sieve having an aperture of 1000 µm and pass through the sieve having an aperture of 100 µm.   A neutron detector comprising the neutron scintillator according to any one of claims 1 to 4 and a photodetector.

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