Colquiriite-type crystal, scintillator for neutron detection, and neutron radiation detector

The present invention relates to a corkyrite-type crystal and a scintillator for neutron detection composed of the corquilite-type crystal. Specifically, it contains an alkali metal element and a lanthanoid element, and⁶It relates to a corkyrite type crystal having an isotope ratio of Li of 20% or more.

A scintillator is a substance that absorbs radiation such as α-rays, β-rays, γ-rays, X-rays, and neutron rays and emits fluorescence. Used for radiation detection in combination with photodetectors such as photomultiplier tubes, medical fields such as tomography, industrial fields such as non-destructive inspection, safety fields such as personal belongings inspection, academic fields such as high energy physics. It has various application fields such as.

As this scintillator, there are various types of scintillators depending on the type of radiation and the purpose of use. Specifically, bismuth germanium oxide (Bi)₄Ge₃O₁₂), Cerium-containing gadolinium silicon oxide (Gd)₂SiO₅There are inorganic crystals such as: Ce), organic crystals such as anthracene, polymer substances such as polystyrene and polyvinyltoluene containing an organic phosphor, and liquid scintillators and gas scintillators.

For conventional neutron detection,³Neutron beam detectors using He gas have been used, but they are rare.³Due to the soaring price of He gas, replacement with alternative technology is required. A neutron beam detector using a scintillator for solid neutron detection is one of the promising candidates as an alternative technology. Typical characteristics required of a scintillator include high light emission, high radiation stopping power, and fast fluorescence attenuation. Especially in scintillators that detect neutron rays, neutrons and Fe (iron), Pb (lead), Cd (cadmium), C (carbon), N (nitrogen), etc. can be used as detector materials and substances to be inspected. Since a radiation capture reaction occurs with the contained absorbing substance and γ-rays are likely to be generated, it is necessary to have a discrimination ability from these γ-rays.

As a solid scintillator for neutron detection, as a material that has no deliquescent property and has high-speed response.⁶Li glass scintillators have been used. However, since the manufacturing process is complicated, it is expensive and there is a limit to the increase in size. On the other hand, a scintillator for neutron detection made of fluoride crystals has an advantage that a large scintillator can be manufactured at low cost. For example, LiBaF₃A scintillator consisting of crystals has been proposed. However, since the scintillator is highly sensitive to γ-rays and has a large background noise derived from γ-rays, it is necessary to take special measures when using it as a scintillator for neutron detection (see Non-Patent Document 1).

In view of this problem, the present inventors have irradiated various fluoride crystals with neutron rays and evaluated their characteristics as a scintillator for neutron detection. As a result, LiCaAlF containing a lanthanoid element₆In crystals, 1.1 to 20 atoms per unit volume (atom / nm)³)of⁶It was found that the inclusion of Li has particularly good characteristics as a scintillator for neutron detection (see Patent Document 1). LiCaAlF containing the lanthanoid element₆Although the crystal has high detection efficiency for neutron rays and excellent discrimination ability between neutron rays and γ-rays, the above-mentioned³When used as a scintillator for solid neutron detection for the purpose of substituting a neutron beam detector using He gas, there was room for improvement in discrimination ability. However, there is no generally accepted theory for predicting the discrimination ability, and therefore it has been difficult to predict the discrimination ability between neutron rays and γ-rays in advance for various materials.

<p num="0007"><patcit num="1"><text>International Publication 2009/119 378 Pamphlet</text></patcit></p>

<p num="0008"><nplcit num="1"><text>CWE van Eijk et al LiBaF3, a thermal neutron scintillator with optimal n-γ discrimination Nuclar Instruments and Methods in Physics Research A 374 (1996) 197-201.</text></nplcit></p>

<p num="0009"> An object of the present invention is to provide a corquilite-type crystal suitable for a neutron detection scintillator having high detection efficiency for neutron rays and less background noise derived from γ-rays, and a neutron detection scintillator composed of the crystals. To do.</p>

<p num="0010"> The present inventors have described the LiCaAlF.₆Evaluate the detection efficiency for neutron rays and the ability to discriminate between neutron rays and γ-rays when various corquilite-type crystals having the same crystal structure as the crystal are prepared and the corquilite-type crystal is used as a neutron detection scintillator. did. as a result,⁶We have found that good discrimination ability can be obtained by incorporating a specific alkali metal element and a specific lanthanoid element into a corkyrite-type crystal having an increased Li isotope ratio, and have completed the present invention.</p><p num="0011"> That is, according to the present invention, the chemical formula LiM¹M²X₆, (However, M¹Is at least one alkaline earth metal element selected from the group consisting of Mg, Ca, Sr and Ba.²Is at least one metal element selected from the group consisting of Al, Ga and Sc, and X is at least one halogen element selected from the group consisting of F, Cl, Br and I) It is a corkyrite type crystal represented by It contains at least one alkali metal element selected from the group consisting of Na, K, Rb and Cs, and at least one lanthanoid element selected from the group consisting of Ce, Pr and Nd.<u style="single">The contents of the alkali metal element and the lanthanoid element are 0.001 to 10 mol% and 0.01 to 0.5 mol%, respectively, with respect to the corkyrite type crystal.</u>and⁶A scintillator for neutron detection is provided, which comprises a corkyrite-type crystal having an isotope ratio of Li of 20% or more. In the invention of the neutron scintillator,<u style="single">1)</u>Corkyrite type crystals have the chemical formula LiCa_1-xSr_xAlF₆(However, x is a number from 0 to 1),<u style="single">2)</u>M¹Is Ca and M²Is Al, the alkali metal element is Na, the lanthanoid element is Ce,⁶The isotope ratio of Li is preferably 50% or more. The present invention further provides a neutron beam detector comprising the scintillator and a photodetector.</p>

<p num="0012"> The corkyrite-type crystal of the present invention can be suitably used as a scintillator for neutron detection, which has less background noise derived from γ-rays than before. By combining with a photodetector, the scintillator can be suitably used as a neutron beam detector used for purposes such as determining the presence or absence of neutron rays in the environment. Further, since the corkyrite type crystal of the present invention has an excellent amount of light emission, it is particularly easy to detect the light emission of the scintillator with a photodetector.</p>

<figref num="1">This figure is a schematic view of a Czochralski method manufacturing apparatus for manufacturing the corkyrite type crystal of the present invention.</figref><figref num="2">This figure is a wave height distribution spectrum obtained by irradiating the scintillator for neutron detection of Example 1 with neutron rays and γ-rays.</figref><figref num="3">This figure is a wave height distribution spectrum obtained by irradiating the scintillator for neutron detection of Comparative Example 1 with neutron rays and γ-rays.</figref><figref num="4">This figure is a wave height distribution spectrum obtained by irradiating a neutron detection scintillator of Example 1 and Comparative Example 1 with a neutron beam.</figref><figref num="5">This figure is a schematic view of a micro-pulling method manufacturing apparatus for manufacturing the corkyrite type crystal of the present invention.</figref><figref num="6">This figure is a wave height distribution spectrum obtained by irradiating the scintillator for neutron detection of Example 2 with neutron rays and γ-rays.</figref><figref num="7">This figure is a wave height distribution spectrum obtained by irradiating the scintillator for neutron detection of Example 3 with neutron rays and γ-rays.</figref>

The corkyrite type crystal of the present invention has the chemical formula LiM.¹M²X₆(However, M¹Is at least one alkaline earth metal element selected from the group consisting of Mg, Ca, Sr and Ba.²Is at least one metal element selected from the group consisting of Al, Ga and Sc, and X is at least one halogen element selected from the group consisting of F, Cl, Br and I). It consists of a compound. The corkyrite type crystal is a hexagonal crystal belonging to the space group P31c, and can be easily identified by a powder X-ray diffraction method.

Among the corkylite-type crystals, the corquilite-type crystal in which the halogen element is F is most preferable because it has no deliquescent property and is excellent in chemical stability. In a corkyrite type crystal in which the halogen element is F, a part of F may be replaced with Cl, Br or I for the purpose of improving the characteristics as a scintillator such as the amount of light emitted. Among the corkyrite-type crystals in which the halogen element is F, the chemical formula LiCa_1-xSr_xAlF₆A corkyrite-type crystal represented by (where x is 0 to 1) is preferable because it facilitates the production of large crystals and can increase the amount of light emitted when used as a scintillator. Furthermore, LiCaAlF in which x in the chemical formula is 0₆Is most preferable because it has a small effective atomic number and can reduce the sensitivity to γ-rays. In the present invention, the effective atomic number is an index defined by the following equation. Effective atomic number = (ΣW_iZ_i⁴)^1/4 In the formula, Wi and Zi are mass fractions and atomic numbers of the i-th element among the elements constituting the scintillator, respectively.

The corkyrite type crystal of the present invention⁶It is characterized by an isotope ratio of Li of 20% or more. The relevant⁶Li isotope ratio accounts for total lithium elements⁶It is the ratio of Li isotopes and affects the detection efficiency for neutron rays. That is, the neutrons incident on the corkyrite crystal are the relevant⁶Because it is detected by causing a nuclear reaction with the Li isotope⁶The higher the isotope ratio of Li, the higher the detection efficiency for neutron rays when used as a scintillator for neutron detection. Since the incident neutron beam can be detected without exception by improving the detection efficiency, such a neutron detection scintillator can be particularly preferably used in an application requiring high sensitivity.⁶By setting the isotope ratio of Li to 20% or more, the detection efficiency of the obtained corkyrite crystal for neutron rays can be sufficiently increased, but it is preferably 50% or more for the purpose of further improving the detection efficiency. , 90% or more is most preferable.

Take⁶The isotope ratio of Li is determined in lithium halide (hereinafter referred to as LiX) such as LiF used as a raw material.⁶It can be adjusted as appropriate by adjusting the isotope ratio of Li. In naturally occurring Li⁶The isotope ratio of Li is only about 7.6%,⁶Concentrate Li isotope⁶Raw materials with an increased Li isotope ratio are commercially available and can be easily obtained. In the present invention⁶As a method of adjusting the isotope ratio of Li,⁶Li isotope is expected⁶A method using raw materials concentrated to the isotope ratio of Li, or in advance⁶Li is the intended⁶Examples thereof include a method in which a raw material concentrated to an isotope ratio of Li or higher is prepared, and the concentrated raw material and a general-purpose raw material having a natural isotope ratio are mixed and adjusted.

The corkyrite type crystal of the present invention is characterized by containing at least one lanthanoid element selected from Ce, Pr and Nd. The lanthanoid element acts as an activator to emit light when neutrons are incident. The Ce, Pr, and Nd all exhibit light emission due to electronic transition from the 5d level to the 4f level, and since such light emission has a short fluorescence lifetime, a scintillator having excellent high-speed response can be obtained. Among the lanthanoid elements, Ce is preferable because it emits a large amount of light. In the present invention, the amount of light emitted is the number of photons of scintillation light generated by the incident of radiation, and is represented by the number of photons per incident neutron (photons / neutron) for neutron rays and incident on γ rays. It is expressed by the number of photons per γ-ray energy (photons / MeV). The content of the lanthanoid element is preferably 0.01 to 0.5 mol% with respect to the corkyrite type crystal. When the content is 0.01 mol% or more, a corkyrite-type crystal having good characteristics as a scintillator can be obtained, and when the content is 0.5 mol% or less, problems such as cloudiness of the crystal in the production of the crystal can be avoided. .. The content of the lanthanoid element can be appropriately adjusted by the mixing ratio of the halide of the lanthanoid element added to the mixed raw material in the production of the corkyrite type crystal, as will be described later.

The corkyrite type crystal of the present invention is characterized by containing at least one alkali metal element selected from Na, K, Rb and Cs. By containing the alkali metal element, not only the amount of light emitted when the corkyrite type crystal is used as a scintillator is increased, but also the ability to discriminate between neutron rays and γ rays is improved. In general, when the amount of light emitted by the incident of neutrons increases, the amount of light emitted by the incident of γ-rays also increases. On the other hand, the corkyrite type crystal of the present invention contains the alkali metal element, so that the amount of light emitted by the incident of neutrons increases, but the amount of light emitted by the incident of gamma rays does not increase. Therefore, the difference between the two emission amounts is extremely large, and if such a difference in emission amount is used, a threshold value is set for the peak value of the emission pulse generated from the scintillator, and only the emission pulse exceeding the threshold value is used as a signal due to the incident of neutron rays. It becomes easy to handle, that is, to discriminate between neutron rays and γ rays. The corkyrite-type crystal of the present invention specifically exhibits the ability to discriminate between neutron rays and γ-rays as described above, but the mechanism of action thereof is not clear. Among the alkali metal elements, Na is the most preferable. That is, Na has a small atomic number, so that it has a low probability of interacting with γ-rays, and a corkyrite-type crystal having no deliquescent property can be obtained, which is preferable.

The content of the alkali metal element is preferably 0.001 to 10 mol% and particularly preferably 0.01 to 0.5 mol% with respect to the corkyrite type crystal for the same reason as in the case of the lanthanoid element. As will be described later, the content of the alkali metal element can be appropriately adjusted by the mixing ratio of the halide of the lanthanoid element added to the mixed raw material in the production of the corkyrite type crystal.

The corkyrite-type crystal of the present invention may be in either a single crystal or polycrystal form. However, a single crystal is preferable in order to obtain a scintillator for neutrons having a high emission amount without causing loss due to non-radiative transition due to lattice defects or dissipation of scintillation light at grain boundaries.

The corkyrite-type crystal of the present invention is a colorless or slightly colored transparent crystal, and has excellent transparency of scintillation light. In addition, it has good chemical stability, and no deterioration in performance is observed in a short period of time in normal use. Further, it has good mechanical strength and workability, and it is easy to process it into a desired shape and use it.

The method for producing the corkyrite type crystal of the present invention is not particularly limited and can be produced by a known crystal production method, but it is preferably produced by the Czochralski method or the micro-pulling method. By producing by the Czochralski method or the micro-pulling method, a corkyrite type crystal having excellent quality such as transparency can be produced. According to the micro-pulling method, the crystal can be directly produced in a specific shape, and can be produced in a short time. On the other hand, according to the Czochralski method, it is possible to inexpensively produce a large crystal having a diameter of several inches.

Hereinafter, a general method for producing a corkyrite type crystal by the Czochralski method will be described. First, a predetermined amount of raw material is filled in crucible 1. The purity of the raw material is not particularly limited, but is preferably 99.99% or more. By using such a high-purity raw material, the purity of the obtained crystal can be increased, so that the characteristics such as the amount of light emitted are improved. As the raw material, a powdery or granular raw material may be used, or the raw material may be sintered or melt-solidified in advance before use.

As a raw material, LiX, a halide of the alkaline earth metal (for example, MgF), depending on the target corkyrite type crystal.₂, CaF₂, SrF₂And BaF₂Etc.), halides of the metal element (eg, AlF)₃, GaF₃And ScF₃Etc.), halides of the alkali metal element (eg, NaF, KF, RbF and CsF, etc.) and halides of the lanthanoid element (eg, CeF).₃, PrF₃And NdF₃Etc.) are appropriately mixed to use a mixed raw material.

The mixing ratio of LiX, the halide of the alkaline earth metal and the halide of the metal element in the mixed raw material is adjusted to be a molar ratio of 1: 1: 1. However, when a corkyrite type crystal is produced by a melt growth method such as the Czochralski method or the micro-pulling method, the halides of LiX and the metal element are easily volatilized, so that each of them is excessive by about 1 to 10%. It may be. Since the volatilization amount is completely different depending on the crystal production conditions (temperature, atmosphere, process), it is desirable to check the volatilization amount of LiX and the halide of the metal element in advance to determine the mixing ratio of the raw materials.

The amount of the alkali metal element and the lanthanoid element contained in the corkyrite type crystal can be appropriately adjusted by the mixing ratio of the alkali metal element halide and the lanthanoid element halide added to the mixed raw material. The amount of the alkali metal added is preferably 0.001 to 10 mol%, more preferably 0.01 to 5 mol%, based on the corkyrite type crystal, in consideration of the change in the amount of the alkali metal element contained in the crystal due to segregation. The amount of the lanthanoid element added is preferably 0.01 to 0.5 mol% with respect to the corkyrite type crystal.

Next, the crucible 1, the heater 2, the heat insulating material 3, and the movable stage 4 filled with the above raw materials are set as shown in FIG. A double crucible structure may be formed by installing another crucible with a hole at the bottom on the crucible 1 and fixing it to a heater 2 or the like to hang it. The seed crystal 5 is attached to the tip of the automatic diameter control device 6. A metal such as platinum, which has excellent corrosion resistance at high temperatures, may be used instead of the seed crystal, but it is better to use a corkyrite-type crystal to be produced or a single crystal having a crystal structure close to that of the corkyrite-type crystal to be produced. Can be avoided and is preferable. The automatic diameter control device is composed of a load cell for measuring the weight of a crystal and a circuit system for feeding back the measured weight to a heater output. By using the automatic diameter control device, a crystal having a desired diameter is used. Can be manufactured accurately and stably.

Next, using a vacuum exhaust device, 1.0 x 10 inside the chamber 7.^-3After vacuum exhausting to Pa or less, an inert gas such as high-purity argon is introduced into the chamber to perform gas replacement. The pressure in the chamber after gas replacement is not particularly limited, but atmospheric pressure is common. By this gas replacement operation, the moisture adhering to the raw material or the chamber can be removed, and the deterioration of the crystals due to the moisture can be prevented. It is preferable to use a scavenger having high reactivity with water in order to avoid adverse effects due to water that cannot be removed by the gas replacement operation. As the scavenger, methane tetrafluoride or the like can be preferably used, and the scavenger is mixed with the above-mentioned inert gas and introduced into the chamber.

After performing the gas replacement operation, the raw material is heated and melted by the high frequency coil 8 and the heater 2. The heating method is not particularly limited, and for example, a resistance heating method using a carbon heater or the like can be appropriately used instead of the induction heating method using the high frequency coil and the heater. The melted raw material melt is then brought into contact with the seed crystal. After adjusting the heater output so that the part in contact with the seed crystal becomes the temperature at which it solidifies, the crystal is pulled up while automatically adjusting the pulling speed under the control of the automatic diameter control device 6. The movable stage 4 may be appropriately moved in the vertical direction in order to adjust the liquid level. A corkyrite type crystal is obtained by continuously pulling up the output of the high frequency coil while adjusting it appropriately, separating it from the liquid surface when it reaches the desired length, and cooling it for a sufficient time so that the crystal does not crack. be able to.

The produced crystal may be annealed for the purpose of removing crystal defects caused by defects in fluorine atoms or thermal strain. The obtained corkyrite-type crystal has good processability and can be processed into a desired shape and used as a scintillator for neutron detection. For processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing machine can be used without any limitation.

The shape of the neutron detection scintillator of the present invention is not particularly limited, but it is preferable that the scintillator has a light emitting surface facing a photodetector described later, and the light emitting surface is optically polished. By having such a light emitting surface, the light generated by the scintillator can be efficiently incident on the photodetector. The shape of the light emitting surface is not limited, and a shape suitable for the intended use, such as a quadrangle having a side length of several mm to several hundred mm square or a circular shape having a diameter of several mm to several hundred mm, is appropriately selected and used. be able to. The thickness of the scintillator with respect to the neutron beam incident direction varies depending on the energy of the neutron beam to be detected, but is generally several hundred μm to several hundred mm. It is preferable to apply an anti-reflective film made of aluminum, Teflon (registered trademark), or the like on a surface that does not face the photodetector, because it is possible to prevent the dissipation of light generated by the scintillator.

The scintillator for neutron detection of the present invention can be combined with a photodetector to form a neutron beam detector. That is, by converting the light emitted from the neutron detection scintillator by the irradiation of the neutron beam into an electric signal by the photodetector, the presence / absence and intensity of the neutron beam can be grasped as an electric signal. In the present invention, the photodetector is not particularly limited, and conventionally known photodetectors such as a photomultiplier tube and a photodiode can be used without any limitation.

The method for manufacturing a neutron beam detector by combining the neutron detection scintillator and the photodetector of the present invention is not particularly limited. For example, the light emitting surface of the neutron detection scintillator is set on the light detection surface of the photodetector with optical grease. A neutron beam detector can be manufactured by connecting a power supply and a signal readout circuit to the photodetector by adhering them with a photodetector or the like. The signal readout circuit is generally composed of a preamplifier, a shaping amplifier, a pulse height analyzer, an oscilloscope, and the like. Further, by arranging a large number of scintillators coated with the light reflecting film and using a position-sensitive photodetector as a photodetector, it is possible to impart position resolution to the neutron detector.

<p num="0035"> Hereinafter, the present invention will be specifically described with reference to examples of the present invention, but the present invention is not limited to these examples. Also, not all combinations of features described in the examples are essential to the solution of the present invention.</p><p num="0036"> Example 1 [Manufacture of corkyrite crystals and scintillators for neutron detection] Chemical formula LiCaAlF₆A corkyrite-type crystal represented by and containing Na and Ce as an alkali metal element and a lanthanoid element, respectively, was produced. In addition, it should be noted⁶The isotope ratio of Li was 95%. The corkyrite type crystal was produced using the crystal production apparatus by the Czochralski method shown in FIG. As raw materials, LiF and CaF with a purity of 99.99% or more₂, AlF₃, NaF and CeF₃High-purity fluoride powder was used. Also, as LiF,⁶A commercially available product having an isotope ratio of Li of 95% was used. The crucible 1, the heater 2, and the heat insulating material 3 were made of high-purity carbon.</p><p num="0037"> First, LiF, CaF₂, AlF₃, NaF and CeF₃Weighed so that the mixing ratio (molar ratio) of the above was 1.01: 1: 1.03: 0.02: 0.02, and mixed well to prepare a mixed raw material. The total weight of the mixed raw materials was 3 kg. The obtained mixed raw material was filled in the crucible 1, the crucible 1 was placed on the movable stage 4, and the heater 2 and the heat insulating material 3 were sequentially set around the crucible 1. Next, as seed crystal 5, LiCaAlF₆Single crystal 6 × 6 × 30 mm³The one processed into a rectangular parallelepiped shape was attached to the tip of the automatic diameter control device. 5.0 × 10 inside chamber 6 using a vacuum exhaust device consisting of an oil rotary pump and an oil diffusion pump.^-4After vacuum exhausting to Pa, a methane-argon tetrafluoride mixed gas was introduced into the chamber 7 to atmospheric pressure to replace the gas.</p><p num="0038"> A high-frequency current was applied to the high-frequency coil 8 and the raw material was heated and melted by induction heating. Move the seed crystal 5 and 6 x 6 mm of the seed crystal 5²The surface of the material was brought into contact with the liquid surface of the molten raw material. After adjusting the heater output to a temperature at which the part in contact with the seed crystal solidifies, the diameter of the crystal is gradually increased to 55 mm under the control of the automatic diameter control device 6, and then the diameter is kept constant at 55 mm. While pulling up the crystal. During growing, the movable stage 4 is moved appropriately to adjust the liquid level to be constant, and the output of the high-frequency coil is continuously adjusted while being appropriately pulled up. When the length reaches about 80 mm, it is separated from the liquid level and about. By cooling for 48 hours, a single crystal having a diameter of 55 mm and a length of about 80 mm was obtained. As a result of crushing a part of the single crystal and performing X-ray diffraction measurement on the obtained powder, the single crystal is LiCaAlF which is a kind of corkyrite type crystal.₆It turned out to be a single crystal.</p><p num="0039"> Next, a disk-shaped sample having a thickness of 1 mm and having both sides optically polished was prepared using a part of the single crystal. As a result of measuring the sample by SEM-EDS, Na was detected. In addition, as a result of measuring the absorption spectrum of the sample, characteristic absorption derived from Ce was observed in the wavelength region of about 270 nm. When the content of various ions in the crystal was examined by ICP analysis, the content of Ce in the crystal was 0.074 mol% and the content of Na was 0.85 mol%. From these, the single crystal produced in this example has the chemical formula LiCaAlF.₆Represented by, containing Na and Ce, and⁶It is a corkyrite type crystal with an isotope ratio of Li of 95%. The obtained corkyrite type crystals were cut with a wire saw equipped with a diamond wire. Next, the entire surface of the crystal is ground and optically polished by a polishing machine to obtain 10 × 10 × 4 mm.³The scintillator for neutron detection of the present invention was obtained by processing into the shape of.</p><p num="0040">[Manufacturing of neutron detector] First, the 10 x 10 mm of the scintillator for neutron detection.²Was used as a light emitting surface, and a tape-shaped Teflon (registered trademark) was wrapped around a surface other than the light emitting surface to form a light reflecting film. Next, a photomultiplier tube (R7600U manufactured by Hamamatsu Photonics Co., Ltd.) was prepared as a photodetector, and the light emitting surface of the scintillator was adhered to the light detection surface of the photomultiplier tube with optical grease. A neutron detector was manufactured by connecting a power supply and a signal readout circuit to the photomultiplier tube. As the signal readout circuit, a preamplifier, a shaping amplifier, and a pulse-height analyzer were connected from the photomultiplier tube side.</p><p num="0041">[Characteristic evaluation of neutron detector] After covering the neutron detector with a black sheet for shading, the radioactivity of about 1 MBq²⁵²The neutron beam from the Cf sealed radiation source was decelerated and irradiated with a polyethylene block having a thickness of 40 mm. A high voltage of 650V was applied to the photomultiplier tube using a power source connected to the photomultiplier tube. The emission pulse generated by the scintillator due to the incident of neutrons was converted into a pulsed electric signal by a photomultiplier tube, and the electric signal was input to the pulse height analyzer via a preamplifier and a shaping amplifier. The wave height distribution spectrum was created by analyzing the electrical signal input to the pulse height analyzer. Next, instead of neutron rays, the radioactivity of about 1 kBq⁶⁰A crest height distribution spectrum was prepared in the same manner as described above except that γ-rays from a Co-sealed radiation source were irradiated.</p><p num="0042"> The obtained wave height distribution spectrum is shown in Fig. 2. The solid and dotted lines in FIG. 2 are the peak height distribution spectra under neutron and γ-ray irradiation, respectively. The horizontal axis of the crest height distribution spectrum represents the crest value of the pulsed electric signal, that is, the amount of light emitted by the scintillator. Here, the crest value of the peak of the crest height distribution spectrum under neutron irradiation is set to 1. It is shown as a relative value. The vertical axis represents the frequency of the electric signal indicating each peak value, and here, the frequency of the electric signal measured (counts / sec) is shown. From Fig. 2, a clear peak generated as a result of detecting the neutron beam can be confirmed, and it can be seen that the neutron beam detector is operating. Further, it can be seen that the peak value of the electric signal generated by the incident of γ-rays is much lower than the peak value of the electric signal generated by the incident of neutron rays, and the γ-ray and the neutron ray can be easily distinguished.</p><p num="0043"> Comparative example 1 Chemical formula LiCaAlF₆A corkyrite-type crystal represented by the above-mentioned alkali metal element and containing Ce as a lanthanoid element was produced. In addition, it should be noted⁶The isotope ratio of Li was 95%. Single crystals were obtained in the same manner as in Example 1 except that NaF was not added to the mixed raw material. As a result of performing the same measurement as in Example 1 on the obtained single crystal, the single crystal produced in this comparative example has the chemical formula LiCaAlF.₆Represented by, contains Ce, and⁶It was a corkyrite-type crystal with an isotope ratio of Li of 95%. In the same manner as in Example 1, the obtained single crystal was processed into a scintillator, and then a neutron beam detector was manufactured. Using the obtained neutron beam detector, a wave height distribution spectrum under neutron beam and γ-ray irradiation was created. The obtained wave height distribution spectrum is shown in Fig. 3. The solid and dotted lines in FIG. 3 are the peak height distribution spectra under neutron and γ-ray irradiation, respectively. The horizontal axis and vertical axis are the same as in FIG. From Fig. 3, a clear peak derived from neutron rays can be confirmed, and it operates as a neutron beam detector. However, although the crest value derived from γ-rays is lower than the crest value derived from neutron rays, the maximum value reaches about 0.6 with respect to the crest value of neutrons. On the other hand, in the neutron beam detector of Example 1, the peak value derived from γ-rays is only about 0.4, and the ability to discriminate between neutron rays and γ-rays is inferior.</p><p num="0044"> FIG. 4 shows the wave height distribution spectra obtained by irradiating the neutron detection scintillators of Example 1 and Comparative Example 1 with neutron rays. Here, it is shown as a relative value with the peak value of the wave height distribution spectrum obtained by the scintillator for neutron detection of Comparative Example 1 as 1. From FIG. 4, the neutron detection scintillator of Example 1 has reached about 1.2 times the amount of light emitted by the incident of neutron rays as compared with the conventional one, and the scintillator for neutron detection of the present invention is also excellent in the amount of light emission. I understand. From the above description, the neutron detection scintillator made of the corkyrite type crystal of the present invention is excellent in the ability to discriminate between neutron rays and γ rays and the amount of light emitted when the neutron rays are incident, and the neutron detection scintillator. It can be seen that the neutron beam detector using is effective.</p><p num="0045"> Example 2 Crystals of lithium calcium fluoride aluminum containing Ce and Na were produced using the production apparatus shown in FIG. As a raw material, lithium fluoride with a purity of 99.99% (⁶Li isotope ratio 95%), and calcium fluoride, aluminum fluoride, cerium fluoride, and sodium fluoride were used. The after-heater 10, heater 11, heat insulating material 12, stage 13, and crucible 14 are made of high-purity carbon, and the shape of the holes provided at the bottom of the crucible is a columnar shape with a diameter of 2.0 mm and a length of 0.5 mm. did. First, 0.53 g of lithium fluoride, 1.66 g of calcium fluoride, 1.87 g of aluminum fluoride, 84 mg of cerium fluoride, and 18 mg of sodium fluoride were weighed, mixed well, and then filled in crucible 14. The crucible 14 filled with the raw material was set on the upper part of the afterheater 10, and the heater 11 and the heat insulating material 12 were sequentially set around the crucible 14. Next, using a vacuum exhaust device consisting of an oil rotary pump and an oil diffusion pump, the inside of the chamber 15 is 5.0 × 10^-4Vacuum exhaust was performed up to Pa. At the same time, heating was performed using the high frequency coil 16 so that the temperature inside the crucible at the time of vacuum exhaust was 570K.</p><p num="0046"> Argon 95 vol.%-Methane tetrafluoride 5 vol.% Mixed gas is introduced into the chamber 15, and while measuring the temperature at the bottom of the crucible using the high frequency coil 16, the output of the high frequency heating coil is set so that the heating temperature is 790K. Was adjusted. The pressure in the chamber 15 after the replacement of the mixed gas was set to atmospheric pressure, and heating was continued for 30 minutes in this state. Next, while continuing the overheating by the high-frequency heating coil, vacuum exhaust was performed, and argon gas was further introduced into the chamber 15 to perform gas replacement. The pressure in the chamber 15 after the argon gas replacement was set to atmospheric pressure. The same operation was performed twice. Using the high-frequency heating coil 16, the raw material was heated to the melting point of lithium calcium fluoride aluminum and melted. Next, while adjusting the high frequency output and gradually raising the temperature of the raw material melt, a metal wire made of W-Re alloy provided at the tip of the pulling rod 17 is inserted into the hole at the bottom of the crucible 14 and pulled down. Was repeated, and the raw material melt was pulled out from the above hole. The high frequency output was fixed so that the temperature at this point was maintained, the melt of the raw material was lowered, and the growth of crystals was started. It was continuously lowered at a rate of 1 mm / hr for 14 hours to finally obtain a lithium fluoride calcium aluminum single crystal containing Ce and Na having a diameter of 2.1 mm and a length of 40 mm. When the content of various ions in the crystal was examined by ICP analysis, the content of Ce in the crystal was 0.082 mol% and the content of Na was 0.91 mol%.</p><p num="0047"> In the same manner as in Example 1, the obtained single crystal was processed into a scintillator, and then a neutron beam detector was manufactured. Using the obtained neutron beam detector, a wave height distribution spectrum under neutron beam and γ-ray irradiation was created. The obtained wave height distribution spectrum is shown in FIG. The solid and dotted lines in FIG. 6 are the peak height distribution spectra under neutron and γ-ray irradiation, respectively. The horizontal axis and vertical axis are the same as in FIG. From FIG. 6, a clear peak derived from neutron rays can be confirmed, and it can be seen that the device operates as a neutron beam detector and that γ-rays and neutron rays can be easily discriminated from each other.</p><p num="0048"> Example 3 Crystal growth was carried out in the same manner as in Example 1 except that lithium fluoride 0.53 g, calcium fluoride 1.66 g, aluminum fluoride 1.88 g, cerium fluoride 84 mg, and sodium fluoride 2.7 mg were used as raw materials, and finally. A lithium fluoride calcium aluminum fluoride crystal containing Eu having a diameter of 2.1 mm and a length of 40 mm was obtained. When the content of various ions in the crystal was examined by ICP analysis, the content of Ce in the crystal was 0.081 mol% and the content of Na was 0.27 mol%. In the same manner as in Example 1, the obtained single crystal was processed into a scintillator, and then a neutron beam detector was manufactured. Using the obtained neutron beam detector, a wave height distribution spectrum under neutron beam and γ-ray irradiation was created. The obtained wave height distribution spectrum is shown in FIG. The solid and dotted lines in FIG. 7 are the peak height distribution spectra under neutron and γ-ray irradiation, respectively. The horizontal axis and vertical axis are the same as in FIG. From FIG. 7, a clear peak derived from neutron rays can be confirmed, and it can be seen that the device operates as a neutron beam detector and that γ-rays and neutron rays can be easily discriminated from each other.</p>

1 Crucible 2 heater 3 Insulation 4 Movable stage 5 seed crystals 6 Automatic diameter controller 7 chamber 8 high frequency coil 9 Gas inlet 10 After heater 11 heater 12 insulation 13 stages 14 Crucible 15 chambers 16 high frequency coil 17 Pulling rod