JP6204763B2 - Neutron scintillator and neutron detector - Google Patents

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 the 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.

International Publication No. 2009/119378 Pamphlet

  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.

  The present invention has been made to solve the above problems, and provides a neutron detector excellent in neutron counting accuracy capable of accurately measuring neutrons even in a field where the dose of γ-rays that become background noise is high. The purpose is to provide.

  The present inventors use various inorganic phosphors containing at least one neutron capture isotope selected from lithium 6 and boron 10 in order to obtain various neutron scintillators with excellent neutron detection efficiency and n / γ discrimination ability. investigated. As a result, the inventors have conceived that the shape of the inorganic phosphor is particulate, and a resin composition obtained by mixing the inorganic phosphor particles and a resin is used as a neutron scintillator. Then, by further mixing filler particles different from the inorganic phosphor particles, the dispersion state of the inorganic phosphor particles in the resin composition is made uniform, and the inorganic phosphor particles and the fluorescent material are used as the filler particles. It has been found that a better n / γ discrimination ability can be obtained by employing particles having different characteristics or that do not emit fluorescence. The present invention has been completed.

That is, according to the present invention, inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 and filling that does not emit fluorescence or have fluorescence characteristics different from those of the inorganic phosphor particles Ri Do a resin composition comprising agent particles, the specific surface area of the inorganic phosphor particles 50 cm ² / cm ³ or more, the neutron scintillator is provided Ru der 1000 cm ² / cm ³ or less. In the neutron scintillator, the amount of neutron capture isotope derived from the filler particles is preferably ½ or less of the amount of neutron capture isotope derived from the inorganic phosphor particles. In the neutron scintillator, the volume fraction of the filler particles with respect to the inorganic phosphor particles is preferably 10 to 800% by volume.

  According to the present invention, there is provided a neutron detector comprising the neutron scintillator and a photodetector.

  According to the present invention, it is possible to provide a neutron scintillator excellent in neutron detection efficiency and n / γ discrimination ability and a neutron detector using the neutron scintillator. The neutron detector can accurately measure neutrons even in high gamma-ray doses that cause background noise, so it can be used for security inspections such as cargo inspection, academic research such as structural analysis by neutron diffraction, nondestructive It can be suitably used in the examination field or the medical field such as boron neutron capture therapy.

This figure is the figure which showed typically the measuring method of the internal transmittance in an Example. This figure is a pulse height distribution spectrum obtained in Example 1. This figure is a pulse height distribution spectrum obtained in Example 2. This figure is a pulse height distribution spectrum obtained in Example 3. This figure is a pulse height distribution spectrum obtained in Comparative Example 1. This figure is a diagram showing a signal waveform when the neutron detector obtained in Example 4 is irradiated with neutrons. This figure is a diagram showing a signal waveform when the neutron detector obtained in Example 4 is irradiated with γ rays. This figure is a schematic diagram showing an operation mechanism of the present invention.

  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.

In addition, the content of lithium 6 (C _{Li, P} ) and the content of boron 10 (C _{B, P} ) in the inorganic phosphor particles are determined in advance according to the density of the inorganic phosphor particles, the lithium in the inorganic phosphor particles, and The mass fraction of boron and the isotope ratios of lithium 6 and boron 10 in the raw material can be obtained and can be obtained by substituting 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 contents of lithium 6 and boron 10 in the inorganic phosphor particles, respectively, ρ is the density [g / cm ³ ] of the scintillator, W _Li and W _B lithium and boron mass fraction of each in the inorganic phosphor particles _[wt%], isotopic ratio of lithium 6 and boron 10 in each _{R Li} and _{R B} material [%], a is Avogadro's number [6.02 X10 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 , Tb: LiYF ₄ , Eu: LiI, Ce: Li ₆ Gd (BO ₃ ) ₃ , C Inorganic phosphor particles 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 the like consists _{Li 2 O-MgO-Al 2} O 3 -SiO 2 -Ce 2 O 3 system glass Aircraft phosphor particles, and the like.

  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 order to improve the n / γ discrimination ability of a neutron scintillator, the present invention is characterized by using inorganic phosphor particles as a constituent element of a neutron scintillator instead of a conventional bulk material of an inorganic phosphor. . 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 an inorganic phosphor, fast electrons are generated inside the inorganic phosphor, and the fast electrons impart energy to the inorganic phosphor, whereby the inorganic phosphor emits 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 distance traveled by the high-speed electrons generated when γ rays are incident on the scintillator while moving the scintillator while energizing the scintillator is as long as several millimeters.

  On the other hand, when neutrons are incident on the scintillator, as described above, the secondary particles generated by the neutron capture reaction of lithium 6 and boron 10 contained in the inorganic phosphor in the scintillator with neutrons are inorganic. By applying energy to the phosphor, the inorganic phosphor emits light, but the range of the secondary particles is several μm to several tens μm, which is shorter than the high-speed electrons.

  As the first feature of the present invention, by making the inorganic phosphor into a particulate form, the above-mentioned fast electrons will be quickly deviated from the inorganic phosphor particles, and the energy given to the inorganic phosphor by the fast electrons will be reduced. It is what.

  In the present invention, the size of the inorganic phosphor particles is such that almost all the energy of the secondary particles generated by the incidence of neutrons is imparted to the inorganic phosphor, and fast electrons are preferably produced by the incidence of γ rays. Is preferably small enough to deviate.

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 generated 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 reduced. It is something that can be done.

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 in the axial direction of at least one of the inorganic phosphor particles is excessively small, a neutron capture reaction between the lithium 6 and boron 10 and neutrons. There is a possibility that an event may occur in which the secondary particles generated in (1) deviate from the inorganic phosphor particles before giving the total energy to the inorganic phosphor particles. 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.

  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. From the viewpoint of ease of production and availability, an irregular shape obtained by pulverizing a bulk material is preferable.

  The inorganic phosphor particles having the shape described above can be suitably obtained by sieving. The sieving means particles that are substantially free of particles that pass through the lower screen through the upper screen using an upper screen with a predetermined mesh and a lower screen with a smaller mesh size than the upper screen. This is a classification method. In such sieving, at least one of the particles having a small axial length with respect to the sieve opening tends to pass through the sieve. Accordingly, the inorganic fluorescent particles having a shape in which the fast electrons excited by the γ-rays easily depart from the upper screen are separated, and the inorganic fluorescent particles having a shape in which the fast electrons generated by the γ-rays are easily deviated by the lower screen. It is easy to sort the body particles and remove the inorganic phosphor particles having such a shape that the secondary particles generated by the neutrons deviate.

  Specifically, by screening, inorganic phosphor particles that pass through a sieve having an aperture of 1000 μm and pass through a sieve having an aperture of 100 μm are substantially classified, and the inorganic phosphor particles are converted into a neutron scintillator. It is preferable to use it. Furthermore, it is particularly preferable to classify inorganic phosphor particles that pass through a sieve having an opening of 500 μm and pass through a sieve having an opening of 100 μm, and to use the inorganic phosphor particles in a neutron scintillator.

  The neutron scintillator of the present invention is characterized by comprising a resin composition (hereinafter also simply referred to as a resin composition) containing the inorganic phosphor particles. As understood from the above description, since the inorganic phosphor particles of the present invention are small in size as compared with generally used inorganic phosphors, there is a problem that the single inorganic phosphor particles have poor neutron detection efficiency. Such a problem can be solved by mixing a plurality of inorganic phosphor particles with a resin and dispersing them in the resin. The filler particles have excellent n / γ discrimination ability and yet have high neutron detection. An efficient neutron scintillator can be obtained.

That is, the neutron detection efficiency of the neutron scintillator of the present invention depends on the content of neutron capture isotopes derived from the inorganic phosphor particles in the resin composition, and can be improved by increasing the content. The neutron capture isotope content refers to the number of neutron capture isotopes derived from inorganic phosphor particles contained on average per 1 nm ^{3 of} the resin composition, and the content of lithium 6 (C _{Li, C} ) and For each of the content of boron 10 (C _{B, C} ), the content of lithium 6 (C _{Li, P} ) and the content of boron 10 (C _{B, P} ) in the inorganic phosphor particles, and the resin composition It can obtain | require from the following formula | equation (3) and Formula (4) using the volume fraction (V) of the inside inorganic fluorescent substance particle.

C _{Li, C} = C _{Li, P} × (V / 100) (3)
C _{B, C} = C _{B, P} × (V / 100) (4)
(In the formula, C _{Li, C} and C _{B, C} are the contents of lithium 6 and boron 10 in the resin composition, respectively, C _{Li, P} and C _{B, P} are the lithium in the inorganic phosphor particles, respectively. 6 content and boron 10 content, V represents the volume fraction of inorganic phosphor particles in the resin composition (V) [volume%])
In the present invention, the content of the inorganic phosphor particles in the resin composition is not particularly limited, but as apparent from the above formula, by increasing the volume fraction of the inorganic phosphor particles in the resin composition. The neutron detection efficiency can be improved. Therefore, the volume fraction of the inorganic phosphor particles in the resin composition 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 with respect to the entire resin composition is not particularly limited, but is preferably less than 80% by volume and more preferably less than 50% by volume in consideration of the viscosity when the resin composition is produced. .

  Here, it is desirable that the inorganic phosphor particles are dispersed as uniformly as possible in the resin, and if there is some spacing between the particles, high-speed electrons deviating from certain inorganic phosphor particles are present. It is preferable in that it is incident on other adjacent inorganic phosphor particles and energy is given to the inorganic phosphor particles to increase the total light emission. Therefore, in the present invention, filler particles different from the inorganic phosphor particles are further blended, and the filler particles are filled between the inorganic phosphor particles to maintain the spacing between the inorganic phosphor particles. (See FIG. 8). Further, in this embodiment, since the high-speed electrons deviating from the inorganic phosphor impart a part of the energy to the filler particles, the energy imparted to the inorganic phosphor is further reduced, and the n / γ discrimination ability is reduced. Will improve.

  The filler particles are preferably 10% by volume or more, and particularly preferably 30% by volume or more with respect to 100% by volume of the inorganic phosphor particles, from the viewpoint of easily obtaining the effects of the present invention. Further, in order to obtain the content of the inorganic phosphor particles in the above range, it is preferably 800% by volume or less, more preferably 300% by volume or less, and particularly preferably 150% by volume or less.

  In the present invention, the filler particles are particles having fluorescence characteristics different from those of the inorganic phosphor particles or not emitting fluorescence. By adopting such particles, the n / γ discrimination ability can be improved.

  As described above, in the neutron scintillator of the present invention, a part of the energy of fast electrons generated by the incidence of γ rays is applied to the inorganic phosphor particles, and a part is applied to the filler particles.

  When the filler particles are particles that do not emit fluorescence, the energy imparted to the filler particles does not contribute to light emission, so the intensity of the pulsed signal that is output when γ rays are incident, that is, the peak value However, it is significantly reduced compared to the case where neutrons are incident. Therefore, 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, and an event showing a peak value less than the threshold value can be easily treated as noise caused by γ rays. Thus, the n / γ discrimination ability is improved.

  On the other hand, when the filler particles are particles having a fluorescence characteristic different from that of the inorganic phosphor particles, when the γ-rays are incident on the neutron scintillator, both the inorganic phosphor particles and the filler particles are given energy and emit fluorescence. To emit. On the other hand, when neutrons are incident, secondary particles generated in the inorganic phosphor particles do not deviate from the inorganic phosphor particles, so that only the inorganic phosphor particles emit fluorescence.

  Here, since the inorganic phosphor particles and the filler particles have a difference in fluorescence characteristics such as fluorescence lifetime and emission wavelength, neutrons and γ rays can be discriminated using the difference in fluorescence characteristics. In other words, a mechanism is provided in the neutron detector that can distinguish the difference in fluorescence characteristics, and when both fluorescence derived from inorganic phosphor particles and fluorescence derived from filler particles are detected, an event in which γ rays are incident When only fluorescence derived from inorganic phosphor particles is detected, 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 filler particles, and light emission of the inorganic phosphor particles and the filler particles. There is a wavelength analysis mechanism that can identify the wavelength.

  Hereinafter, the waveform analysis mechanism will be illustrated more specifically by taking as an example a case where the fluorescence lifetime of the filler particles is shorter than the fluorescence lifetime of the inorganic phosphor particles. 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 invention, which is a combination of the neutron scintillator and the photodetector, the signal output from the photodetector 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. The time required for such increase (hereinafter also referred to as rise time) is the inorganic phosphor particles or Reflecting the fluorescence lifetime of the filler particles, 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 explanation, in the event that γ rays are incident, the fluorescence of the filler particles having a short fluorescence lifetime is observed, so that the rise time is relatively short compared to the event that neutrons are incident. 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 filler particles emit light at a wavelength of Bnm different from Anm. Then, as described above, when the γ-rays are incident, both the inorganic phosphor particles and the filler particles emit fluorescence. Therefore, when the neutron is incident, the neutron scintillator emits light of Anm and Bnm. Since only the inorganic phosphor particles emit fluorescence, only the light of An nm 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 another aspect of the present invention, a substance that does not emit light is used as a filler particle, and an organic phosphor (hereinafter, simply referred to as an organic phosphor) having a fluorescence characteristic such as a fluorescence lifetime or a light emission wavelength that is different from that of an inorganic phosphor is resin. A mode of mixing with can also be suitably employed in the present invention. In such an embodiment, fast electrons generated by the incidence of γ-rays deviate from the inorganic phosphor particles, reach the organic phosphor and give energy, and the organic phosphor emits fluorescence. That is, when γ rays are incident on the neutron scintillator, both the inorganic phosphor particles and the organic phosphor give energy and emit fluorescence. On the other hand, when neutrons are incident, secondary particles generated in the inorganic phosphor particles do not deviate from the inorganic phosphor particles, so that only the inorganic phosphor particles emit fluorescence.

  Here, since the inorganic phosphor particles and the organic phosphor have a difference in fluorescence characteristics such as fluorescence lifetime and emission wavelength, neutrons and γ rays can be discriminated using the difference in fluorescence characteristics. In other words, a mechanism is provided in the neutron detector that can identify the difference in fluorescence characteristics, and when both fluorescence derived from inorganic phosphor particles and fluorescence derived from organic phosphors are detected, an event in which γ-rays are incident When only fluorescence derived from inorganic phosphor particles is detected, 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 mechanism similar to the waveform analysis mechanism and the wavelength analysis mechanism can be adopted as a mechanism capable of identifying the difference in fluorescence characteristics.

  Specific examples of such organic phosphors include 2,5-Diphenyloxazole, 1,4-Bis (5-phenyl-2-oxazole) benzene, 1,4-Bis (2-methylstyryl) benzene, anthracene, stilbene and Organic phosphors such as naphthalene and derivatives thereof may be mentioned. Since such organic phosphors generally have a shorter fluorescence lifetime than the inorganic phosphor particles, they can be suitably used to improve n / γ discrimination using the difference in fluorescence lifetime.

  In the present invention, the filler particles contain a neutron capture isotope similarly to the inorganic phosphor particles, and the content of the neutron capture isotope derived from the filler particles in the resin composition is derived from the inorganic phosphor. When compared with the content of neutron capture isotopes, the neutron detection efficiency may decrease. That is, the neutron capture isotope contained in the filler particle competes with the neutron capture isotope contained in the inorganic phosphor particle, and the neutron capture isotope in the filler particle is neutron capture in some neutron incident events. Cause a reaction. In such an event, light emission having a fluorescence characteristic different from that of the inorganic phosphor particles or fluorescence does not occur, and in any case, it is identified as a γ-ray incident event, and thus a neutron counting loss occurs.

Therefore, in order to obtain particularly excellent neutron detection efficiency, the content of neutron capture isotopes derived from the filler particles in the resin composition is reduced using filler particles that are substantially free of neutron capture isotopes. It is preferable to reduce. Specifically, the content of the neutron capture isotope derived from the filler particles in the resin composition is preferably ½ or less of the content of the neutron capture isotope derived from the inorganic phosphor particles, It is more preferably 1/5 or less, and particularly preferably 1/10 or less. The content of lithium 6 (C ′ _{Li, C} ) and the content of boron 10 (C ′ _{B, C} ) derived from the filler particles in the resin composition are the same as in the case of the inorganic phosphor particles. The content of lithium 6 (C ′ _{Li, P} ) and the content of boron 10 (C ′ _{B, P} ) in the agent particles are determined from the formulas (1) and (2), respectively, and these values and the resin composition The volume fraction (V ′) of the filler particles can be obtained from equations (3) and (4), respectively.

  As the filler particles, particles that are the same kind of inorganic substance as the inorganic phosphor particles and that have different fluorescence characteristics from the inorganic phosphor particles or do not emit fluorescence are most preferable. That is, the filler particles made of the same kind of inorganic substance as the inorganic phosphor particles have a specific gravity that matches the specific gravity of the inorganic phosphor particles, so that when mixed with the resin, the gaps between the inorganic phosphor particles are uniformly filled. Is easy. Further, 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 is close. Therefore, as described later, the internal transmittance of the resin composition can be easily increased by bringing the refractive index of the inorganic phosphor particles and the refractive index of the filler particles close to the refractive index of the resin.

  Examples of other materials that can be used as filler particles include silica, titanium oxide, barium sulfate, calcium fluoride, magnesium fluoride, lithium fluoride, magnesium carbonate, strontium fluoride, mica (mica), and various glasses. Inorganic particles, or organic particles such as silicone resin, fluororesin, poly (meth) acrylate, polycarbonate, polystyrene, polyvinyl toluene, polyvinyl alcohol, polyethylene, and styrene butadiene.

  Since the filler particles are filled in the gaps between the inorganic phosphor particles, the shape thereof is preferably the same as that of the inorganic phosphor particles or smaller than that of the inorganic phosphor particles. Specifically, the particles are smaller than the inorganic phosphor particles by classifying them with a filler particle whose particle size is adjusted in the same manner as the inorganic phosphor particles, or with a sieve having a smaller opening than in the case of the inorganic phosphor particles. Each of the filler particles whose particle size is adjusted can be suitably used.

  A method for producing the inorganic phosphor particles and the filler particles is not particularly limited, and a method of obtaining particles having a desired shape by pulverizing and classifying a bulk body having a shape larger than the particles having the suitable shape, or And a method of directly obtaining inorganic phosphor particles and filler particles having a suitable shape by a particle generation reaction using a solution as a starting material.

Among these production methods, a method of pulverizing and classifying a bulk body is preferable because production efficiency is high and desired particles can be obtained at low cost. The method for pulverizing the bulk body 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. When producing inorganic phosphor particles, the generation of so-called fine powder having an extremely large specific surface area is suppressed, and as described above, preferably 1000 cm ² / cm ³ or less, particularly preferably 500 cm ² / cm ³ or less. In order to obtain inorganic phosphor particles having a specific surface area, it is particularly preferable to use a hammer mill and a roller mill.

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

  Among the classification methods, as described above, the method of classification by sieving is preferable because particles having a suitable specific surface area can be easily obtained. Moreover, according to this sieving, it can classify | categorize efficiently using an inexpensive installation.

  In the present invention, in order to efficiently guide the fluorescence emitted from the inorganic phosphor particles in the resin composition to a subsequent photodetector, the resin composition is preferably a transparent body. According to the study by the present inventors, the resin composition emits from the inorganic phosphor particles by setting the internal transmittance per 1 cm of the optical path length to 30% / cm or more at the emission wavelength of the inorganic phosphor particles. The emitted fluorescence can be efficiently guided to the subsequent photodetector. Thereby, the peak value of the signal output from the photodetector is increased, and the signal / noise ratio of the neutron detector is improved. Further, since the variation of the peak value is reduced, it becomes easy to discriminate neutrons and γ rays by providing the threshold value. The internal transmittance is preferably 50% / cm or more. By setting the internal transmittance to 50% / cm or more, a neutron detector particularly excellent in signal / noise ratio and n / γ discrimination ability can be obtained.

In the present invention, the internal transmittance is a transmittance excluding surface reflection loss generated on the incident side and the outgoing side surface of the resin composition when light is transmitted through the resin composition, It is expressed as a value per 1 cm of optical path length. For the internal transmittance (τ ₁₀ ) per 1 cm of the optical path length, the transmittance including the respective surface reflection losses is measured for a pair of resin compositions having different thicknesses, and is substituted into the following equation (5). Can be obtained.

log (τ ₁₀ ) = {log (T ₂ ) −log (T ₁ )} / (d ₂ −d ₁ ) (5)
(In the formula, d ₁ and d ₂ indicate the thickness in cm of the pair of resin compositions, and d ₂ > d _1. Also, T ₁ and T ₂ have thicknesses d ₁ and d ₂ shows the transmittance including the surface reflection loss of the resin composition of d ₂ )
The third essential component of the resin composition in the present invention is an organic resin. The resin preferably has an internal transmittance of 80% / cm or more, particularly preferably 90% or more, at the emission wavelength of the inorganic phosphor particles. Specific examples of such organic resins include silicone resins and fluororesins. Examples include 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.

  Among the transparent resins, it is preferable to use a transparent resin having a refractive index at the emission wavelength of the inorganic phosphor particles close to both the refractive index of the inorganic phosphor particles and the refractive index of the filler particles. Specifically, the ratio of the refractive index of the transparent resin to the refractive index of the inorganic phosphor particles and the refractive index of the filler particles is preferably 0.90 to 1.10, preferably 0.95 to 1.05. It is particularly preferred that it is 0.98 to 1.02. By making the ratio of the refractive index of the transparent resin within such a range, light scattering at the interface between the inorganic phosphor particles and the resin and the interface between the filler particles and the resin can be suppressed, and the inside of the resin composition The transmittance can be increased. In addition, the said refractive index is a refractive index in the temperature range which uses the scintillator of this invention. For example, when the scintillator of the present invention is used near 100 ° C., the refractive index ratio needs to be obtained at 100 ° C.

The refractive index at the emission wavelength of the inorganic phosphor particles can be measured using a refractometer. In general, as a light source for a refractometer, He line d line (587.6 nm), r line (706.5 nm), H ₂ lamp F line (486.1 nm), C line (656.3 nm), The i-line (365.0 nm), h-line (404.7 nm), g-line (435.8 nm), and e-line (546.1 nm) of the Hg lamp can be used. Among these light sources, a light source on the short wavelength side and a light source on the long wavelength side with respect to the emission wavelength of the inorganic phosphor particles are appropriately selected, and the wavelength of each light source and the refractive index measured at the wavelength are respectively determined as follows. After substituting into the equation (6) to obtain the constants A and B, the desired refractive index can be obtained by substituting the emission wavelength of the inorganic phosphor particles into the equation. In addition, what is necessary is just to obtain | require a refractive index using the said light source, when the light emission wavelength of inorganic fluorescent substance particle corresponds with the wavelength of one of the said light sources. In the measurement of the refractive index, a bulk body of an inorganic phosphor having a shape suitable for the measurement, a bulk body of the same material as the filler particles, and a bulk body of resin may be used.

n ² −1 = Aλ ² / (λ ² −B) (6)
(Where n represents the refractive index at wavelength λ, and A and B are constants)
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 filler particles are mixed with a liquid or viscous resin, or inorganic phosphor particles and filler particles. And a liquid or viscous resin precursor may be mixed and then used as a solid resin composition obtained by curing the resin precursor.

  Although the manufacturing method in particular of the said resin composition is not restrict | limited, A specific manufacturing method is illustrated below.

  When the resin composition in the present invention is used as a slurry or paste resin composition, first, inorganic phosphor particles and filler particles are mixed with a liquid or viscous resin. 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, bubbles generated in the resin composition in the mixing operation are defoamed. 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 defoaming operation, light scattering due to bubbles can be suppressed, and the internal transmittance of the resin composition can be increased.

  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, filler particles and resin precursor is injected into a mold having a desired shape, and the resin precursor is cured. The curing method is not particularly limited, but a method of polymerizing the resin precursor by heating, ultraviolet irradiation, or addition of a catalyst is preferable.

  Here, the neutron scintillator made of the resin composition of the present invention can be used in a slurry form or a paste form, and can be molded with a mold having a desired shape even when used in a solid form. Is easy. 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 photomultiplier tube, a photodiode, an avalanche photodiode, or a Geiger mode avalanche photodiode can be used without any limitation.

  The neutron scintillator 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 is generally composed of a preamplifier, a shaping amplifier, a multiple wave height analyzer, and the like.

  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, a neutron detector was manufactured using inorganic phosphor particles made of Eu: LiCaAlF ₆ crystal doped with 0.04 mol% of Eu.

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 (C _{Li, P} ) is 9.1 atom / nm ^{3 based} on 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 ²⁴¹ Am 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 and not emitting fluorescence 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 (C ′ _{Li, P} ) 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.

Filler particles consisting of the LiCaAlF ₆ crystal, similar to the inorganic phosphor particles, pulverizing the bulk of LiCaAlF ₆ crystal, mesh opening used was passed through a sieve of 100 [mu] m.

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 of the Eu: LiCaAlF ₆ crystal, LiCaAlF ₆ crystal, and the silicone resin at 370 nm 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 obtained by substituting the wavelength of each light source and the refractive index measured at the wavelength into the Cermeier equation (6), and then the refractive index at 370 nm was obtained 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.

10.0 g of inorganic phosphor particles made of Eu: LiCaAlF ₆ crystals, 10.0 g of filler particles made of LiCaAlF ₆ crystals, and 10.0 mL of a silicone resin resin precursor in which equal amounts of liquid A and liquid B are mixed in advance. Was added to a mixing vessel, and 1 mL of toluene was added for the purpose of reducing the viscosity of the mixture.

  These were mixed well using a stir bar, and then bubbles generated in the mixture in the mixing operation were defoamed using a vacuum defoamer.

  Next, each was poured into a mold made of polytetrafluoroethylene 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 to distill off toluene, and then heated at 100 ° C. for 24 hours. Thus, the resin precursor was cured to obtain a neutron scintillator of the present invention comprising a resin composition containing inorganic phosphor particles and filler particles.

The resin composition had a diameter of 2 cm, a thickness of 0.3 cm, and a volume of 0.94 mL. In addition, 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 the filler particles. Therefore, the volume fraction (V) of the inorganic phosphor particles and the volume fraction (V ′) of the filler particles relative to the resin composition were both 20% by volume. Therefore, the content of neutron capture isotopes derived from inorganic phosphor particles in the resin composition (C _{Li, C} ) and the content of neutron capture isotopes derived from filler particles (C ′ _{Li, C} ) are: Each of them was 1.8 atom / nm ³ and 0.15 atom / nm ³ , and C ′ _{Li and C} were 1/12 of C _{Li and C} , respectively. The volume fraction of the filler particles relative to the inorganic phosphor particles was 100% by volume.

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 ) The values of d ₁ , d ₂ , T ₁ and T ₂ were substituted into the equation (5), and the internal transmittance per 1 cm of the optical path length was determined. As a result, it was 67% / 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 a preamplifier, a shaping amplifier, and a multi-wave height analyzer were connected as a signal readout circuit from the photomultiplier tube side 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. Cf-252 with 2.4 MBq of radioactivity is installed in the center of a 20 cm square cubic high-density polyethylene, and a neutron detector is installed so that a scintillator is arranged at a position close to the high-density polyethylene, Neutrons from Cf-252 were irradiated at a reduced speed with high density polyethylene.

  A high voltage of -1300 V was applied to the photomultiplier using a power source connected to the photomultiplier. The light emitted from the neutron scintillator was converted into a pulsed electric signal by a photomultiplier by the incidence of neutrons, and the electric signal was input to the multi-wave height analyzer via a preamplifier and a shaping amplifier. A pulse height distribution spectrum was created by analyzing the electrical signal input to the multi-wave height analyzer.

  Next, instead of neutrons, 0.86 MBq of radioactive Co-60 was installed at a distance of 5 cm from the neutron scintillator, and γ-rays from the Co-60 were irradiated. A spectrum was created. 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.

  The obtained wave height distribution spectrum is shown in FIG. The solid line and the dotted line in FIG. 2 are wave 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. 2, a clear neutron peak can be confirmed, and the crest value of the electric signal generated by the incidence of γ rays is much lower than the crest value of the neutron peak, and gamma rays and neutrons can be easily distinguished. I understand.

  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 broken line in FIG. The frequency of signals exceeding the threshold was counted for 300 seconds for each of neutron irradiation and γ-ray irradiation, and the count rate (counts / sec) was determined. The results are shown in Table 1. As can be seen from Table 1, the neutron detector of the present invention has a γ-ray count rate of only 0.043 Counts / sec even in the presence of extremely high doses of γ-rays. It can be seen that the error is minor.

Example 2
Eu: LiCaAlF ₆ inorganic phosphor particles 12.5g of crystalline, filler particles 7.5g consisting LiCaAlF ₆ crystal, and advance equal amounts of A liquid and B liquid resin precursor 10.0mL of the mixed silicone resin A neutron scintillator made of the resin composition of the present invention was produced in the same manner as in Example 1 except for mixing. For the resin composition, V and V 'are 25 vol% and 15 vol% _{respectively, C Li, C} and C' _{Li, C} are each 2.3atom / nm ³ and 0.11atom / nm ^3, _{C 'Li,} C was 1/21 of _{C Li, C.} The volume fraction of the filler particles relative to the inorganic phosphor particles was 60% by volume.

  With respect to the resin composition, the internal transmittance per 1 cm of optical path length at the emission wavelength of the inorganic phosphor particles was measured in the same manner as in Example 1. As a result, it was 71% / cm.

  In the same manner as in Example 1, a neutron detector was manufactured using the resin composition, creation of a pulse height distribution spectrum, setting of a threshold value, and counting of signals exceeding the threshold value under neutron irradiation and γ-ray irradiation. A rate measurement was performed to evaluate the performance of the neutron detector. The obtained wave height distribution spectrum is shown in FIG. 3, and the results of counting rate measurement are shown in Table 1. From FIG. 3, a clear neutron peak can be confirmed, and the crest value of the electric signal generated by the incidence of γ-ray is much lower than that of the neutron peak, and γ-ray and neutron can be easily distinguished. I understand.

Example 3
Eu: LiCaAlF ₆ inorganic phosphor particles 14.9g of crystalline, filler particles 5.0g consisting LiCaAlF ₆ crystal, and advance equal amounts of A liquid and B liquid resin precursor 10.0mL of the mixed silicone resin A neutron scintillator of the present invention comprising a resin composition was produced in the same manner as in Example 1 except for mixing. For the resin composition, V and V ′ are 30% by volume and 10% by volume, respectively, C _{Li, C} and C ′ _{Li, C} are 2.7 atom / nm ³ and 0.073 atom / nm ³ , respectively. C ′ _{Li, C} was 1/37 of C _{Li, C.} The volume fraction of the filler particles relative to the inorganic phosphor particles was 33% by volume.

  With respect to the resin composition, the internal transmittance per 1 cm of the optical path length at the emission wavelength of the inorganic phosphor particles was measured in the same manner as in Example 1. As a result, it was 70% / cm.

  In the same manner as in Example 1, a neutron detector was manufactured using the resin composition, creation of a pulse height distribution spectrum, setting of a threshold value, and counting of signals exceeding the threshold value under neutron irradiation and γ-ray irradiation. A rate measurement was performed to evaluate the performance of the neutron detector. The obtained pulse height distribution spectrum is shown in FIG. 4, and the results of counting rate measurement are shown in Table 1. From FIG. 4, a clear neutron peak can be confirmed, and the crest value of the electrical signal generated by the incidence of γ rays is much lower than the crest value of the neutron peak, so that γ rays and neutrons can be easily distinguished. I understand.

  Moreover, it turns out that n / gamma discrimination ability can be improved by raising the volume fraction of the filler particle | grains with respect to an inorganic fluorescent substance particle from the comparison of Examples 1-3. Moreover, it turns out that the neutron detection efficiency per unit volume of a resin composition can be improved by raising the volume fraction of the inorganic fluorescent substance particle in a resin composition.

Comparative Example 1
Resin composition in the same manner as in Example 1 except that 20 g of inorganic phosphor particles composed of Eu: LiCaAlF ₆ crystals and 10.0 mL of a silicone resin resin precursor in which equal amounts of liquid A and liquid B were mixed in advance were mixed. A neutron scintillator of the present invention consisting of a product was produced. The resin composition does not use filler particles. As for the resin composition, V is 50 _{vol%, C Li, C} were, respectively, 4.6atom / nm ^3.

  With respect to the resin composition, the internal transmittance per 1 cm of the optical path length at the emission wavelength of the inorganic phosphor particles was measured in the same manner as in Example 1. As a result, it was 70% / cm.

  In the same manner as in Example 1, a neutron detector was manufactured using the resin composition, creation of a pulse height distribution spectrum, setting of a threshold value, and counting of signals exceeding the threshold value under neutron irradiation and γ-ray irradiation. A rate measurement was performed to evaluate the performance of the neutron detector. FIG. 5 shows the obtained pulse height distribution spectrum, and Table 1 shows the results of counting rate measurement. In FIG. 5, although a clear neutron peak can be confirmed, it can be seen that the crest value of the electric signal generated by the incidence of γ rays is high and the n / γ discrimination ability is inferior compared with Examples 1-3. Also, in the count rates of Table 1, the count rate of γ rays is as large as 9.6 Counts / sec, and in the presence of such a very high dose of γ rays, errors derived from γ rays with respect to the neutron count become a significant problem. There is a fear.

Example 4
In Example 1, when mixing the inorganic phosphor particles, the filler particle particles, and the resin precursor of the silicone resin, instead of 1 mL of toluene, 1 mL of a toluene solution in which 5 wt% of 2,5-Dipheniloxazole was dissolved was added. A resin composition was prepared in the same manner as in Example 1 except for adding. That is, in this example, a neutron scintillator made of a resin composition obtained by mixing an organic phosphor with a resin was produced. In the resin composition, V and V ′ and C _{Li, C} and C ′ _{Li, C} are the same as in Example 1.

  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 68% / cm.

  In the same manner as in Example 1, a neutron detector was manufactured using the resin composition. However, in order to observe the signal waveform output from the photomultiplier tube, an oscilloscope was connected instead of the signal readout circuit composed of the preamplifier, the shaping amplifier and the multiple wave height analyzer.

  In the same manner as in Example 1, the neutron detector was irradiated with neutrons and γ rays, and the signal waveform output from the photomultiplier tube was recorded using an oscilloscope. Signal waveforms obtained under neutron irradiation and γ-ray irradiation are shown in FIGS. 6 and 7, respectively. The inset in each figure is an enlarged view around time 0. 6 and 7, when neutrons are incident, only inorganic phosphor particles emit fluorescence, whereas when γ rays are incident, both inorganic phosphor particles and organic phosphors give energy. It can be seen that fluorescence is emitted when applied, and that there is a clear difference in fluorescence lifetime between the inorganic phosphor particles and the organic phosphor. Therefore, by providing the neutron detector in the present embodiment with the waveform analysis mechanism described above, a neutron detector that is particularly excellent in n / γ discrimination can be obtained.

1 Neutron scintillator 2 Photodetector

Claims (6)

Resin comprising inorganic phosphor particles containing at least one neutron capture isotope selected from lithium 6 and boron 10 and filler particles having fluorescence characteristics different from those of the inorganic phosphor particles or not emitting fluorescence Ri Do from the composition, the specific surface area of the inorganic phosphor particles 50 cm ² / cm ³ or more, 1000 cm ² / cm ³ or less der Ru neutron scintillator. The inorganic specific surface area of the phosphor particles 100 cm ² / cm ³ or more, 500cm ² / cm ³ or less der Ru claim 1 neutron scintillator according. The neutron scintillator according to claim 1 or 2, wherein the filler particles do not emit fluorescence . The neutron scintillator according to any one of claims 1 to 3 , wherein the filler particles are inorganic particles . Inorganic phosphor particles, a particle of Zentsu the sieve mesh opening 200 [mu] m, eye particles passing through the sieve opening 100μm the claims 1 to 4, wherein any one is a particle that is substantially free Neutron scintillator.   A neutron detector comprising the neutron scintillator according to any one of claims 1 to 5 and a photodetector.

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