Sintered particles for radiation shielding material, method of manufacturing the same, and method of using the same

Abstract

To enable stable and inexpensive provision of highly dense, highly heterogeneous sintered particles for radiation shielding materials.SOLUTION: Sintered particles for radiation shielding material are provided for radiation shielding tools, the sintered particles being produced by using LiF, or LiF mixed with one or more fluorides selected from MgF2, CaF2, NaF, AlF3, and / or BaF2, to form spheres with a particle size ratio of 1.2 or less and sintering the spheres.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to sintered particles for use in a radiation shielding material, a method for producing the same, and a method for using the same. More specifically, the present invention relates to a method for improving the positioning accuracy of a patient's affected area in radiation therapy using radiation including neutrons, and reducing the amount of radiation leaking from a gap between a radiation irradiation port and the patient's affected area. The present invention relates to sintered particles for a radiation shielding material used in a radiation shielding device that can reduce or eliminate the amount of radiation irradiated to healthy areas other than the affected area and reduce exposure to radiation of the human body and / or peripheral devices of this treatment device, as well as a method for producing the same and a method for using the same. [Background technology]

[0002] In the field of radiotherapy, new applications utilizing the radiation shielding effect of specific elements are being developed. Neutrons, a type of radiation, have no electrical charge and are easily absorbed when they collide with atomic nuclei. This absorption of neutrons is called "neutron capture," and one example of a medical application that takes advantage of this property is "Boron Neutron Capture Therapy (BNCT)." In recent years, Japan and other major countries have been actively developing and commercializing this cutting-edge treatment for intractable cancers.

[0003] Neutrons have a short half-life of about 15 minutes, and decay within a short time, releasing an electron and a neutrino and turning into a proton. Neutrons have no electric charge, and when they collide with the nuclei of relatively light elements such as hydrogen (H), lithium (Li), boron (B), carbon (C), and nitrogen (N), they release and absorb all or most of their energy. On the other hand, when a neutron collides with another type of atomic nucleus, specifically, iron (Fe), magnesium (Mg), calcium (Ca), aluminum (Al), or fluorine (F), it releases some of its energy and becomes a neutron with a lower energy level. This phenomenon is called "neutron moderation," and materials that contain this "other type of atomic nucleus" can be used as "neutron moderators." The energy of a neutron depends on its flight speed.

[0004] In BNCT, boron isotopes are first injected into the body via injection or intravenous drip. 10 A boron agent containing B is reacted with tumor cells such as malignant cancer cells, and a reaction product of the boron compound is formed in the tumor area. This reaction product is irradiated over a suitable area with neutrons of an energy level that has little effect on the healthy human body (preferably composed of neutrons of a medium energy level, such as epithermal neutrons), and a nuclear reaction occurs between the boron compound that has been segregated in high concentration in the tumor area in advance, within a diameter of approximately 10 micrometers, i.e., an area equivalent to one human cell. 7 Lithium ions and alpha rays kill only tumor cells.

[0005] Cancer cells naturally take up boron easily during their rapid proliferation, and BNCT utilizes this property to effectively destroy only the tumor. An irradiation beam, primarily consisting of neutrons at an energy level that has little effect on healthy tissue, is irradiated in a planar shape large enough to encompass the tumor. This dramatically reduces irradiation time compared to the pinpoint irradiation used in conventional radiation therapy, and also eliminates unirradiated areas (irradiation leaks).

[0006] Neutrons have a wide range of energy levels, from high energies exceeding 100 MeV to low energies below 0.002 eV, and are called "fast neutrons," "epithermal neutrons," "thermal neutrons," etc., in order of increasing energy. Of these, "epithermal neutrons (with an energy level of 0.025 eV to 10 keV)" are desirable for BNCT, but it is difficult to control all to be epithermal neutrons, so the irradiation beam contains a mixture of fast neutrons with an energy level of 10 keV or higher and thermal neutrons with an energy level of less than 0.025 eV.

[0007] In the early stages of development of the BNCT system, test reactors were mainly used as the neutron generation source, but since it is not practical to place a "nuclear reactor" as a treatment device in a hospital, development of a small "accelerator" as a neutron generation source began around 1990, and in recent years accelerators for BNCT have approached completion.

[0008] BNCT devices consist of an "accelerator system," a "deceleration system," and a "treatment device system," and currently there are three types of accelerators: "cyclotron type," "linear type," and "electrostatic type." Protons accelerated by this accelerator are collided with a "target" made of lithium (Li) or beryllium (Be), and the high-energy neutrons generated, which are mainly fast neutrons, are passed through various "moderators" where their energy is appropriately reduced (decelerated), and these decelerated neutrons are irradiated as a treatment beam onto the patient's affected area from an irradiation beam port (i.e., the opening of the collimator) located at the edge of the device's periphery.

[0009] [Problem to be solved by the invention] The problem here is, (a) Radiation such as neutrons that penetrate the structure of the BNCT device leaks from the outer periphery, exposing patients, medical staff, and peripheral equipment of the device. (b) Part of the beam irradiating the affected area of ​​the patient leaks through the gap between the irradiation beam port of the BNCT device and the affected area of ​​the patient, exposing healthy parts of the patient, medical staff, and peripheral equipment of the device to radiation. is.

[0010] Therefore, as a solution to the above problem (i), the inventors have conducted research and development into a radiation shielding material that has excellent shielding performance for neutrons with energy levels mainly below medium that are used in this treatment, and have filed a patent application (Patent Document 1). Furthermore, as a solution to the above problem (b), research results have been reported on a "shielding jig" in which a mixture of polyethylene (hereinafter referred to as "PE") particles and LiF particles is packed inside a resin container (in the document, this container is called a "suction bag") between the irradiation beam port and the patient's affected area, as shown in Non-Patent Document 1. In this document, it is said that "neutron and gamma ray shielding performance was improved" by packing "bead-shaped shielding material" inside the container and appropriately changing the blending ratio of PE particles and LiF particles.

[0011] However, the shielding jig of Non-Patent Document 1 has the following drawbacks. (A) This shielding material is in the shape of beads, i.e., it is a rod-shaped material cut into short pieces with angular ends. When packed into a weighing bag, the shielding material has poor fluidity and poor deformability when applied to the patient's affected area, easily creating gaps between the bead and the affected area. Furthermore, because the edges are sharp, the particles are prone to chipping or cracking when they come into contact with each other, easily generating fine powder. This fine powder easily reacts with moisture in the air or in the surrounding area to generate small amounts of hydrogen fluoride, which may have adverse effects on the patient and surrounding equipment. (B) The energy range of neutrons is extremely wide, from 0.025 eV to several tens of MeV. Neutrons are divided into three main categories: thermal neutrons, epithermal neutrons, and fast neutrons. Neutrons have such a wide range of energies that it is not possible to shield them across the entire energy range with a single type of shielding material. The main shielding effects of PE and LiF used in the shielding jig in Non-Patent Document 1 against each neutron species are as follows: PE is mainly effective against fast neutrons, has a small effect against epithermal neutrons, and has almost no effect against thermal neutrons. On the other hand, LiF is mainly effective against thermal neutrons, has little effect against epithermal neutrons, and has almost no effect against fast neutrons. Furthermore, the energy of neutrons gradually decreases with each passing moment, and the composition of neutron species changes, so even within the beam flow, i.e., within the layers of the shielding jig, there are also temporary changes. Although the ratio of PE particles to LiF particles can be set variably, there is naturally a limit to how much shielding performance can be improved with the same ratio in an integrated structure. The most important issue in developing this countermeasure is how to effectively shield the various types of neutrons that change over time within the beam flow. (C) The true density of PE particles is approximately 0.9 g / cm 3 , the true density of LiF is 2.64 g / cm 3 Therefore, if these two particles with such a large density difference are mixed and packed into a monolithic container, the heavier LiF particles will move downwards due to the density difference inside the container during use, while the PE particles will tend to remain where they are, resulting in segregation between the two particles.

[0012] Therefore, as a solution to the above problem (b), i.e., the problem of neutrons leaking from the irradiation beam port, the inventors have invented and filed a patent application for a radiation shielding jig in which the sintered body for radiation shielding invented in Patent Document 1 above is crushed or sliced ​​into small pieces, and then ground to form spherical particles called "sintered body particles," and "resin particles" are filled in a hollow, three-dimensional resin container with a divided structure to prevent a gap from occurring between the irradiation beam port and the affected area of ​​the patient (Patent Document 2).

[0013] When "sintered particles" were produced using the method described in Patent Document 2, the shielding performance of the "radiation shielding jig" was extremely high, and the effect of reducing leakage neutrons was significantly excellent, but it also had the following additional problems. (1) The method of first producing a large sintered body, crushing or slicing it into small pieces, and then grinding it into spheroids requires many steps in the manufacturing process, resulting in low productivity, low yield, and high cost. This leaves a challenge in the stable and inexpensive mass production of sintered particles, a prerequisite for the widespread use of new cancer treatments such as the BNCT system. (2) The “sintered particles” produced by the method of Patent Document 2 above were “spherical” or “approximately spherical” in shape. However, when used repeatedly for a long period of time, contact between the particles or with the container would cause a small amount of fine powder to be generated (dust generation), making them unsuitable as medical materials that come into direct contact with patients. (3) As with the above problem (2), in a shielding jig in which the "sintered particles" produced by the method of Patent Document 2 are filled into a resin bag, the sintered body has a particulate shape, so voids form between the sintered particles in the bag, and needless to say, the voids are devoid of shielding material, resulting in a corresponding decrease in shielding performance. Moreover, when particles of similar particle size (i.e., "single particle size") are used, the void ratio is high at approximately 45 to 50 vol.%, as will be described later, which results in a corresponding decrease in shielding performance. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Patent application No. 2021-115328 [Patent Document 2] Patent application No. 2022-164259 [Non-patent literature]

[0015] [Non-Patent Document 1] Hiroki Tanaka and Yoshinori Sakurai, Grant-in-Aid for Scientific Research, Research Report, "Development of freely deformable neutron and gamma ray shielding materials for neutron capture therapy," September 7, 2018 Summary of the Invention Means to solve the problem and their effects

[0016] [The process leading up to the invention] The present invention eliminates the shaping process or minimizes the number of steps required. Specifically, spherical or nearly spherical green compact particles (hereinafter referred to as "compact particles") are directly produced, and the compact particles are then fired to produce spherical or nearly spherical sintered particles for a radiation shielding material. Furthermore, by coating the surfaces of the sintered particles, the sintered particles for a radiation shielding material have excellent abrasion resistance and reduced dust generation. Furthermore, before coating the fired sintered particles, it is possible to add a grinding step to adjust the particle size (hereinafter referred to as "particle size") or to increase the particle size ratio of the sintered particles. The definition of particle size ratio will be described later.

[0017] Therefore, the present invention was made based on the following basic idea. (a) To produce spherical or nearly spherical pre-sintered compact particles with as few steps as possible, thereby significantly reducing the total number of steps, thereby improving mass productivity and reducing costs. (b) By increasing the density of the green body particles as much as possible, problems during the firing process such as cracking and chipping of the green body particles and sintered body particles are reduced, thereby increasing the density of the sintered body particles, improving yield, and reducing costs. (c) To minimize the occurrence of burrs in the compact particles and the sintered particles obtained by firing the compact particles, and to eliminate cracks and chips in the sintered particles, thereby preventing the generation of fine powder. (d) The density of the sintered particles is increased as much as possible, and the increased density improves the shielding performance. (e) Even if the density of the sintered particles is the same, adjusting the particle size reduces the gaps between particles in the packed bed. Specifically, by combining multiple different particle sizes, it is possible to create a packed bed with an ideal high packing state, that is, approaching what is known in the field of powder engineering as "closest packing," and improve the shielding performance of a shielding jig filled with these particles.

[0018] Based on the above basic concept, the present inventors have researched and developed a molding method for forming spherical or nearly spherical molded bodies, and have completed the present invention. The first method we investigated involves fluidizing the raw material powder while heating it on a rotating inclined disk. This method involves agglomerating the powder to form compact particles. It has been found that this method requires the raw material powder to have an appropriate viscosity, and that it is not easy to control the level of agglomeration, requiring a fairly sophisticated control mechanism.

[0019] In the second method, raw material powder is supplied between two parallel rotating disks, and is heated to agglomerate the powder into compact particles. As with the first method, this method also requires the raw material powder to have an appropriate viscosity. Furthermore, it was found that controlling the agglomeration level is not easy and requires a fairly sophisticated control mechanism.

[0020] Furthermore, as a third method, we adopted a "tablet press" used in the manufacturing process of medical drugs and attempted to use this "tablet press" to create "molded particles." Tableting machines used in pharmaceutical manufacturing processes stably mass-produce so-called "tablet-shaped drugs," which are thin cylinders with thinly sliced ​​portions of a sphere attached to the top and bottom surfaces, while preventing defects such as burrs, cracks, and breaks.To do this, the molding mold has a structure as shown in Figure 1. The forming mold consists of two parts: the lower receiving mold 1 has a straight cylindrical opening in the center of a block-shaped metal fitting, and the bottom of the opening is countersunk into a dome shape facing downward. On the other hand, the upper mold 2 has a cylindrical pushing part, the outer diameter of which matches the inner diameter of the opening of the lower receiving mold 1. The lower surface of the pushing part (i.e., the side that comes into contact with the raw material powder) is countersunk into a dome shape facing upward, corresponding to the dome-shaped countersunk bottom of the receiving opening.

[0021] With this mold structure, the filling amount of the work (i.e., in the case of manufacturing pharmaceutical tablets, powdered pharmaceutical raw materials) is empirically set so that the volume of the compact after tableting is larger than the volume of the dome-shaped spaces facing each other in the upper and lower molds. Therefore, the compact has a so-called "tablet shape," like a thin cylindrical shape with dome-shaped objects attached to the top and bottom surfaces. The reasons for using such a "tablet shape" are, first, to prevent the tablet from rolling away, and second, to make it easier to release the tablet-shaped compact from the mold after pressing and tableting. In other words, if the compact were hemispherical, it would crack or break when released from the mold, so the dome shape was used to prevent this.

[0022] On the other hand, the shape required for the "sintered particles for radiation shielding material" according to the present application is preferably "spherical" or "approximately spherical," and it was clear that the above-mentioned "tablet press for pharmaceuticals" could not form molded bodies having the desired spherical particle shape. Therefore, the inventors conducted research and development with the aim of modifying this "tablet press" to produce the desired "true spherical" or "approximately spherical" molded bodies, and after much effort, they have completed the present invention. Furthermore, since the minimum practical size of the opening of the above-mentioned molding mold is approximately 2 mm, it is currently impossible to produce small compact particles with a diameter of less than 2 mm using this "tablet press," and it was necessary to find a new method of producing finer particles. Therefore, the inventors decided to sinter compact particles having a diameter of 2 mm or more, as described below, to form sintered particles, and then grind these particles in a grinder to obtain the desired fine sintered particles.

[0023] [Means for solving the problem and their effects] The present invention has been made in consideration of the above-mentioned problems, and aims to provide sintered particles for a radiation-shielding material to be used in a radiation-shielding device that can improve the positioning accuracy of the affected area of ​​a patient, reduce the amount of radiation leaking from the gap between the radiation irradiation port and the affected area of ​​the patient, and further reduce or eliminate the amount of radiation irradiated to healthy areas other than the affected area, in radiation therapy using radiation including neutrons, for example, "Boron Neutron Capture Therapy (BNCT)," which is attracting attention as a next-generation treatment for intractable cancers, and can reduce exposure of the human body, including patients and medical staff, and / or peripheral equipment of this treatment device to radiation, as well as a method for producing the same and a method for using the same.

[0024] In order to achieve the above object, the sintered particle (1) for a radiation-shielding material according to the present invention is characterized in that it is LiF, or LiF mixed with one or more fluorides selected from MgF2, CaF2, NaF, AlF3, and / or BaF2, and is formed into a spherical shape with a particle size ratio of 1.2 or less, sintered, or is formed, sintered, and then ground. An example of a standard for the "sphericity" of a spherical or nearly spherical shape is the standard "JIS B 1501:2009" for "rolling bearings - steel balls," which defines "sphericity" as "the average difference between the smallest circumscribed sphere and the largest inscribed sphere, with the center of the least squares mean sphere of the steel ball surface as the center." This standard is intended for nearly spherical objects with high "sphericity," such as rolling bearing steel balls. In contrast, the present application is intended for "spherical particle groups including non-spherical particles," and, moreover, "particle groups of different particle sizes within a wide particle size range are sometimes mixed together," and because of these special usage conditions, an appropriate evaluation standard is required to replace the detailed evaluation standard of "sphericity." Therefore, in this application, the ratio of the "maximum diameter" to the "minimum diameter" of the particles to be measured, obtained using the measurement method specified in the JIS standard, i.e., the value of "maximum diameter" / "minimum diameter," will be referred to as the "particle size ratio," and the shape of the particles will be evaluated using this "particle size ratio." A particle size ratio of 1 is considered to be "spherical," and the smaller the value closer to 1, the more spherical the particle can be said to be with an "approximately spherical shape." The number of particles measured was 10 for each assumed particle size.

[0025] LiF sintered compacts contain a high concentration of Li, which has a large neutron absorption cross section, and have excellent shielding properties against radiation, especially low-energy neutrons such as thermal neutrons. Since LiF is a typical raw material that is difficult to sinter, in order to sinter it stably and uniformly, it is desirable to use not only LiF but also one or more other fluorides, such as MgF2, CaF2, AlF3, KF, NaF, and / or YF3, mixed with LiF to form a multi-component fluoride sintered body.

[0026] The reason why we chose other fluorides to be mixed with LiF is that: Fluorides similar to LiF tend to form solid solutions during the sintering process, creating a eutectic point that allows for lower sintering temperatures. As a result, the decomposition and vaporization (i.e., sublimation) of fluorides such as LiF is suppressed, foaming is prevented, and a dense sintered body is more easily obtained.

[0027] The sintered particles for radiation shielding material (1) have a large absorption cross section for neutrons. It contains a high concentration of Li, and can be made to have excellent shielding performance against radiation, especially against low-energy neutrons such as thermal neutrons. Furthermore, in a method in which a large sintered body is first produced, crushed or sliced ​​into small pieces, and then ground into spheroids, the production requires many steps, resulting in low productivity, low yield, and high cost. This method solves the problems, and enables stable and inexpensive production of high-density, highly homogeneous sintered particles for a radiation-shielding material. Furthermore, because the sintered particles are formed into a spherical shape with a particle size ratio of 1.2 or less and then fired, they have excellent fluidity, and a radiation shielding jig in which the sintered particles are filled into a container can ensure flexibility. The particle size ratio is set to 1.2 or less because, when used as particles for a shielding material, a suitable angle of repose is obtained as described below, and flexibility of the packed layer of the jig container is ensured.

[0028] The sintered particles (2) for a radiation-shielding material according to the present invention are LiF or a compound containing LiF. A multi-element fluoride containing one or more fluorides selected from MgF2, CaF2, NaF, AlF3, and BaF2, and a boron compound selected from B2O3, B(OH)3, LiB3O5, or Li2B4O7, containing a boron isotope 10 The material is characterized in that B is added in an amount of 0.1 to 5 wt.% as an outer coating, and is formed into a spherical shape with a particle size ratio of 1.2 or less, fired, or is formed, fired, and then ground.

[0029] According to the sintered particle (2) for a radiation shielding material, the sintered particle contains B (isotope) which has a larger absorption cross section for neutrons than Li. 10 Since the sintered particles contain B) at a high concentration, the sintered particles can have even better neutron shielding performance.

[0030] The sintered particles (3) for a radiation-shielding material according to the present invention are LiF or a compound containing LiF. A multi-element fluoride containing one or more fluorides selected from MgF2, CaF2, NaF, AlF3, and BaF2, and a gadolinium compound selected from Gd2O3, Gd(OH)3, or GdF3, is added as a gadolinium isotope. 157 The material is characterized by being added with Gd at a ratio of 0.1 to 2 wt.% as an outer percentage, and being formed into a spherical shape with a particle size ratio of 1.2 or less, and then fired, or being formed, fired, and then ground.

[0031] According to the sintered particle for radiation shielding material (3), Gd (which has a larger absorption cross section for neutrons than Li) is used.157 Since the sintered particles contain a high concentration of ZnO (Gd), the sintered particles can be made to have even better shielding performance against neutrons.

[0032] The sintered particles (4) for a radiation-shielding material according to the present invention are LiF or a compound containing LiF. A multi-element fluoride containing one or more fluorides selected from MgF2, CaF2, NaF, AlF3, and BaF2, and a boron compound selected from B2O3, B(OH)3, LiB3O5, or Li2B4O7, containing a boron isotope 10 B is added in an amount of 0.1 to 5 wt.% by weight, and a gadolinium compound selected from Gd2O3, Gd(OH)3, or GdF3 is added in an amount of 0.1 to 5 wt.% by weight ... 157 The material is characterized by being added with Gd at a ratio of 0.1 to 2 wt.% as an outer percentage, and being formed into a spherical shape with a particle size ratio of 1.2 or less, and then fired, or being formed, fired, and then ground.

[0033] According to the sintered particle for radiation shielding material (4), B (isotope) has a larger absorption cross section for neutrons than Li. 10 B) and Gd (isotope 157 Since the sintered particles contain a high concentration of ZnO (Gd), the sintered particles can be made to have even better shielding performance against neutrons.

[0034] The sintered particles (5) for a radiation-shielding material according to the present invention are any of the sintered particles (1) to (4) for a radiation-shielding material, characterized in that the relative density of the sintered particles before firing, i.e., the particulate compact, is 40% or more and 80% or less.

[0035] Tableting tests under various conditions showed that compacts with a relative density of less than 40% are prone to cracks, breaks, and chips, while a compact with a relative density of 40% or higher can produce a compact free of cracks, breaks, and chips. However, due to limitations in the device structure, current tablet presses are unable to perform extremely high-pressure press molding, so the upper limit for the relative density of the compact is estimated to be 80%, and it was found that the appropriate relative density for such a compact is 40% to 80%. Furthermore, it was found that if the compact particles are in this state, defects such as cracks, breaks, and chips will not occur in the sintered particles after sintering. However, if the device structure is further improved and higher pressure pressing than the current level becomes possible, it may be possible to increase the relative density of the compact to over 80%. The sintered particles (5) for a radiation-shielding material can be easily formed into compacts free from cracks, breaks, chips, etc. using an existing improved tablet press.

[0036] The sintered particles (6) for a radiation-shielding material according to the present invention are any of the sintered particles (1) to (4) for a radiation-shielding material, characterized in that the particle diameter is 0.5 mm or more and 7 mm or less.

[0037] The inside of a radiation shielding jig was filled with sintered particles for shielding material of various particle sizes (hereinafter referred to as "particle size"), and the "flexibility of the jig," which indicates its fluidity, was examined by sensory testing. Particle sizes less than 0.5 mm resulted in poor flexibility, and the reason for limiting the size of the coarse particles is that, first, the thickness of the jig's inside the jig, i.e., its height, is expected to be about 10 mm to 30 mm, and for the smaller height (i.e., 10 mm), the maximum particle size required to fill the jig and provide "flexibility" was expected to be about 7 mm; second, coarse particles with a particle size greater than 7 mm caused a rough, uncomfortable sensation on the patient's face when they came into contact with the jig, so the appropriate range of "particle size" was set at 0.5 to 7 mm. The sintered particles for a radiation-shielding material (6) can provide sintered particles for a radiation-shielding jig that are highly flexible and do not cause discomfort to patients.

[0038] The sintered particles for a radiation-shielding material (7) according to the present invention are characterized in that the sintered particles for a radiation-shielding material according to any one of the above sintered particles for a radiation-shielding material (1) to (4) are coated with a coating agent selected from fluorine-based coating agents such as fluorocarbon coating, polyimide varnish, TFE coating, and silicone varnish, resin-based coating agents, polyimide-based coating agents, and silicone-based coating agents.

[0039] The sintered particles for radiation shielding material (7) can be endowed with excellent abrasion resistance, and even when used repeatedly for a long period of time as a shielding jig material, the particles do not generate fine powder due to contact with each other or with a container, making them desirable as a medical jig material that comes into direct contact with patients.

[0040] The sintered particles for a radiation-shielding material (8) according to the present invention are characterized in that the sintered particles for a radiation-shielding material according to any one of the sintered particles for a radiation-shielding material (1) to (4) are for use as a neutron shielding material.

[0041] The sintered particles for a radiation shielding material (8) can provide sintered particles for a radiation shielding jig having excellent neutron shielding performance.

[0042] The method (1) for producing sintered particles for a radiation-shielding material according to the present invention is also a method for producing sintered particles for a radiation-shielding material by adding high-purity LiF raw material or high-purity MgF2, CaF2, NaF, AlF3, A multi-element fluoride containing one or more fluorides selected from BaF2, Each was individually crushed to a median particle size of 8 μm or less. Then, these primary crushed individual raw materials are mixed in a predetermined ratio, Further, secondary crushing is carried out to reduce the particle size to 6 μm or less. Then, the blended raw materials are mixed (a raw material blending step). a step of forming spherical or approximately spherical molded particles with an inner diameter of 2 to 8 mm and a particle size ratio of 1.2 or less using a tablet press (molding step); Spherical compact particles with a particle size ratio of 1.2 or less are heated from 350°C to 180°C in an atmospheric pressure atmosphere. A process of heating and sintering at a temperature range of 1000°C (sintering process); It is characterized by having the following.

[0043] According to the above-mentioned method (1) for producing sintered particles for a radiation-shielding material, it is possible to produce sintered particles that contain a high concentration of Li, which has a large absorption cross section for neutrons, and have excellent shielding performance against radiation, particularly against low-energy neutrons such as thermal neutrons. Furthermore, if the particle size of the sintered particles before firing, i.e., the particulate compact, is set to 2 mm or more and 8 mm or less, the sintered particles obtained by firing the compact particles can be milled as required in the manufacturing method (2) described below, thereby making it possible to reduce the diameter of the sintered particles, i.e., the sintered particles for a shielding material, to 0.5 mm or more and 7 mm or less, and it is possible to provide sintered particles for a radiation shielding jig that are appropriate for filling a jig and do not cause discomfort to patients. Furthermore, spherical sintered particles with a particle size ratio of 1.2 or less can be produced with a small number of steps, and the produced sintered particles have excellent fluidity. Therefore, when these sintered particles are used in a radiation shielding jig, the radiation shielding jig can be made to have excellent flexibility. Furthermore, compared to a method in which a large sintered body is first produced, and then the large sintered body is crushed or sliced ​​into small pieces, which are then ground into spheroids, the complicated manufacturing process can be significantly reduced, improving productivity, improving yield, and significantly reducing costs. Moreover, high-density, highly uniform sintered particles for radiation shielding materials can be stably provided at low cost.

[0044] The method (2) for producing sintered particles for a radiation-shielding material according to the present invention is characterized in that the method (1) for producing sintered particles for a radiation-shielding material further includes a step of grinding the sintered particles using a pot mill (grinding step).

[0045] According to the method (2) for producing sintered particles for a radiation shielding material, the particle size of the sintered particles before firing, i.e., the particle size of the particulate compact, is set to 2 mm or more and 8 mm or less, and then the fired sintered particles are milled as needed to reduce the diameter of the sintered particles for a radiation shielding material to: It can be easily made to be between 0.5mm and 7mm. Therefore, it is possible to eliminate waste of raw materials and achieve significant cost reductions by making effective use of raw materials.

[0046] The method (3) for producing sintered particles for a radiation-shielding material according to the present invention is characterized in that it further comprises, in addition to the method (1) or (2) for producing sintered particles for a radiation-shielding material, a step (coating step) of applying a coating agent selected from a fluorine-based, resin-based, polyimide-based, or silicon-based coating agent, such as fluoroplastic coating, polyimide varnish, TFE coating, or silicon varnish, to the ground sintered particles, drying the coating agent at room temperature in the atmosphere, and baking the coating agent.

[0047] According to the above-mentioned manufacturing method (3) of sintered particles for radiation shielding materials, sintered particles with excellent abrasion resistance can be easily manufactured. Even if these sintered particles are repeatedly used for a long period of time as a shielding jig material and frequently come into contact with each other or with a container, the generation of fine powder from the sintered particles can be prevented. Therefore, it is possible to stably and inexpensively provide a shielding jig material that is desirable as a material for medical jigs that come into direct contact with patients.

[0048] The method (4) for producing sintered particles for a radiation-shielding material according to the present invention further comprises the step of blending a high-purity LiF raw material or a multi-component fluoride containing one or more fluorides selected from high-purity MgF2, CaF2, NaF, AlF3, and BaF2 in the high-purity LiF raw material, and further comprising adding B2O3, A boron compound selected from B(OH)3, LiB3O5, or Li2B4O7 is 10 The characteristic feature of this method is that B is added at a rate of 0.1 to 5 wt.% as an outer coating.

[0049] According to the method for producing sintered particles for a radiation shielding material (4), B (isotope) having a larger absorption cross section for neutrons than Li is used. 10 B) at a high concentration, and sintered particles having even better neutron shielding performance can be produced stably and inexpensively.

[0050] The method (5) for producing sintered particles for a radiation-shielding material according to the present invention further comprises the step of blending raw materials in the method (1) for producing sintered particles for a radiation-shielding material, wherein the raw materials are a high-purity LiF raw material or a multi-component fluoride containing high-purity LiF raw material and one or more fluorides selected from high-purity MgF2, CaF2, NaF, AlF3, and BaF2, and a gadolinium compound selected from Gd2O3, Gd(OH)3, or GdF3, and a gadolinium isotope 157 The characteristic feature of this alloy is that Gd is added at a rate of 0.1 to 2 wt.% on an outer percentage basis.

[0051] According to the method for producing sintered particles for a radiation shielding material (5), Gd (which has a larger absorption cross section for neutrons than Li) is used. 157 This makes it possible to stably and inexpensively produce sintered particles containing a high concentration of ZnO (Gd) and having even better neutron shielding performance.

[0052] The method (6) for producing sintered particles for a radiation-shielding material according to the present invention further comprises the step of blending a high-purity LiF raw material or a multi-component fluoride containing one or more fluorides selected from high-purity MgF2, CaF2, NaF, AlF3, and BaF2 in the high-purity LiF raw material, and further comprising adding B2O3, A boron compound selected from B(OH)3, LiB3O5, or Li2B4O7 is 10 B is added in an amount of 0.1 to 5 wt.% by weight, and Gd2O3, A gadolinium compound selected from Gd(OH)3 or GdF3 is 157The characteristic feature of this alloy is that Gd is added at a rate of 0.1 to 2 wt.% on an outer percentage basis. According to the method for producing sintered particles for a radiation shielding material (6), B (isotope) having a larger absorption cross section for neutrons than Li is used. 10 B) and Gd (isotope 157 This makes it possible to stably and inexpensively produce sintered particles containing a high concentration of ZnO (Gd) and having even better neutron shielding performance.

[0053] Furthermore, the method for using sintered particles for a radiation-shielding material (1) according to the present invention is a method for using sintered particles for a radiation-shielding material according to any one of the above sintered particles for a radiation-shielding material (5) to (8), characterized in that two or more types of particle groups having different particle sizes are mixed, the void ratio in a packed bed filled with the mixed particle groups is reduced, and the radiation-shielding performance is improved. According to the above-mentioned method (1) of using sintered particles for a radiation-shielding material, the porosity of particles with similar particle sizes is high, approximately 45 to 50 vol.%, which has the problem of reducing the shielding performance accordingly. However, by mixing two or more types of particle groups with different particle sizes, the porosity in the packed bed filled with the mixed particle groups can be reduced, thereby improving the radiation-shielding performance. [Brief explanation of the drawings]

[0054] [Figure 1] FIG. 1 is a schematic cross-sectional side view showing the molding mold portion of a tablet press used for pharmaceutical manufacturing. [Figure 2] FIG. 1 is a schematic side cross-sectional view showing the molding mold portion of an improved tablet press developed for the present invention. [Figure 3] 10 is a table showing the results of a resin coating test (wear resistance test results) on sintered particles. [Figure 4] 1 is a table showing evaluations of the properties of compact particles and sintered particles. DETAILED DESCRIPTION OF THE INVENTION

[0055] Hereinafter, embodiments of the sintered particles for a radiation-shielding material and the method for producing the same according to the present invention will be described with reference to the drawings.

[0056] [Preparation of raw powder for sintered particles for radiation shielding material] The present inventors have already invented a shielding material that far exceeds the shielding performance of conventional shielding materials (Patent Document 1, Patent Application No. 2021-115328), and in the preparation of the raw material powder in the embodiment of this application, Basically, the composition and blend of raw material powders in Patent Application No. 2021-115328 is adopted. In order to manufacture the sintered particles for a radiation shielding material according to the embodiment, LiF or a multi-element fluoride containing LiF is used as a raw material, and / or the multi-element fluoride is 10 The raw material is a boron compound containing B, and / or the same multi-element fluoride 157 The raw material is a compound containing Gd and gadolinium.

[0057] To produce the sintered particles for a radiation-shielding material according to the embodiment, first, predetermined amounts of high-purity (98.5 wt.% or more) LiF powder and one or more fluoride powders selected from MgF, CaF, AlF, KF, NaF, and / or YF, each of which has a high purity (99.9 wt.% or more), are measured and taken as raw materials.

[0058] The raw materials are ground by placing alumina balls (φ5mm: 1800g, φ10mm: 1700g, φ20mm: 3000g, φ30mm: 2800g) in an alumina rotary ball mill (inner diameter 280mm, length 400mm), adding 3000g of the raw material to be ground, and rotating it for a specified time to grind it. Other grinding methods may also be used, such as "media agitation type fine grinding methods" called "bead mill grinding method" or "dynamic mill grinding method," in which media made of alumina or the like are agitated together with the raw material to be ground to produce fine grinding.

[0059] When a boron compound is added to this multi-element fluoride, the boron compound is selected from high-purity (purity of 99.5 wt.% or more) B2O3, B(OH)3, LiB3O5, or Li2B4O7. The boron source is natural boron and / or an isotope of natural boron. 10 B is concentrated. The above multi-element fluoride raw material and the boron compound raw material to be added are separately pulverized for two weeks (primary pulverization) using the pulverization method described below, and then a predetermined amount of each is measured out and mixed for one day and night using a V-type mixer. Furthermore, the mixed raw materials are crushed for one week (secondary crushing) using the crushing method described below. After being individually ground for two weeks, the median particle size was 8 μm or less for all materials, regardless of the type of raw material. After mixing and grinding for another week, the median particle size was 6 μm or less.

[0060] When a gadolinium compound is added to the multi-element fluoride, the raw materials are adjusted in the same manner as in the case of adding the boron compound. The gadolinium compound to be used is selected from high-purity (purity of 99.9 wt.% or more) Gd2O3, Gd(OH)3, or GdF3, and the gadolinium source is natural gadolinium. As in the case of adding a boron compound, the above multi-element fluoride raw material and the gadolinium compound raw material to be added were separately crushed for two weeks (primary crushing) using the crushing method described below, and then a predetermined amount of each was measured out and mixed overnight using a V-type mixer. Furthermore, the mixed raw materials were crushed for one week (secondary crushing) using the crushing method described below.

[0061] These finely pulverized raw materials were kneaded for 12 hours using a kneader to prepare a starting material (raw material blending step). In order to maintain the shape, sintering aids are generally used, but if the sintering aids remain after sintering, they become impurities and may have a significant impact on the neutron shielding performance. Therefore, we decided not to use a sintering aid here.

[0062] [Formation of molded body] The improved tablet press shown in Figure 2 was used to form the compacts. The shape required for the "sintered particles for radiation shielding material" according to the present invention is preferably "spherical" or "approximately spherical," and the above-mentioned "pharmaceutical tablet press" was unable to form compacts having the desired spherical particle shape. The molding mold in the improved tablet press was improved to have the structure shown in Figure 2. The forming mold consists of two parts. The lower receiving mold 3 is a block-shaped metal piece with a straight cylindrical opening in the center, the bottom of which is countersunk downward into a hemispherical shape. On the other hand, the upper mold 4 has a cylindrical pushing part whose outer diameter matches the inner diameter of the opening of the lower receiving mold 3. The lower surface of the pushing part (i.e., the side that comes into contact with the raw material powder) is countersunk upward into a hemispherical shape, corresponding to the hemispherical countersunk bottom of the receiving opening.

[0063] In order to facilitate the "release after tableting" described above, the amount of compounded raw materials to be filled into the mold was set according to the calculation conditions shown below, and the resulting mixture was then molded.

[0064] First, as a result of tableting tests under various conditions, it was found that when the relative density of the compact was less than 40%, cracks, breaks, chips, etc. were likely to occur, whereas when the relative density was 40% or more, compacts free from cracks, breaks, chips, etc. were produced.

[0065] Furthermore, due to limitations in the device structure, current tablet presses are unable to perform extremely high-pressure press molding, so the upper limit of the relative density of the compact is estimated to be 80%, and the appropriate relative density of the compact is considered to be 40% to 80%.

[0066] The amount of work W (g) supplied to the mold of the tablet press should be within the appropriate range of relative density M (%) when a desirable molded body without cracks, breaks, or chips is produced, i.e., 40% to 80% and the volume V (cm) of the spherical cavity consisting of the hemispherical cavities of the upper and lower molds. 3 ) and set it.

[0067] First, assuming that the target relative density M of the compact is, for example, M=60%, the weight Wt of the compact can be calculated using the following formula. Wt=Mo×M×(V / 100)(g)···(1) formula However, Mo is the true density of the workpiece (g / cm 3 )

[0068] Based on the tableting test results, it was estimated that the optimum amount of work W (g) to be supplied to the mold of the tablet press was the weight Wt of the compact calculated by equation (1) multiplied by 1.1 to 1.2. The reason for this is that the relative density of the compact may not be formed according to the set value, and if the relative density becomes greater than the set value, the amount of workpiece filled will be insufficient, resulting in voids in the compact. On the other hand, if the relative density of the compact is smaller than the set value, the bulk volume of the compact increases, which can result in the formation of a thin, cylindrical straight section like a tablet, or the remaining work becoming burrs.

[0069] In this way, the optimum amount of workpiece W (g) to be supplied to the mold of the tablet press was set taking into account fluctuations from the set value of the relative density of the compact. By filling the mold with the appropriate amount of blended raw materials calculated based on the above formula (1) and adjusting the molding pressure, molding speed, etc., it was possible to form a molded body in excellent condition without crumbling or chipping. The minimum practical diameter of the opening of a molding mold is currently 2 mm, and the diameter of the compact particles is also limited to a similar size. Therefore, the inventors adopted a method in which compact particles of the minimum diameter that can be produced using current equipment are sintered, as described below, and then ground using a pot mill to obtain the required fine sintered particles.

[0070] Next, the compact particles were sintered by placing them individually on a substrate in the soaking zone of a normal pressure sintering furnace so that the particles did not come into contact with each other, heating them in the air at temperatures ranging from 350°C to 1000°C depending on the particle composition, and then cooling them to room temperature.

[0071] The particle size ratio of the sintered particles was measured in accordance with JIS B1501:2009, as described above. However, the "particle size ratio" used in this application is the ratio of the minimum diameter to the maximum diameter of the measured "target particle," i.e., the value of minimum diameter / maximum diameter, in order to serve as an index that universally represents the difference in particle shape across a wide particle size range and with large particle size differences. If the particle size ratio was less than 1.2, the particles were used as sintered particles for the shielding material as they were, whereas if the particle size ratio was 1.2 or more, the particles were ground using a pot mill to a particle size ratio of 1.2 or less, and used as sintered particles for the shielding material.

[0072] The particle size ratio of the sintered particles was set to less than 1.2 because if the particle size ratio was 1.2 or more, the angle of repose (the angle of inclination of the particle layer with respect to the horizontal plane when the particles were piled up on a flat plate) would be too large, resulting in insufficient fluidity when the particles were filled into the container of the shielding jig, and poor adhesion between the shielding jig and the affected area of ​​the patient. For these reasons, the particle size ratio of the sintered particles was set to less than 1.2.

[0073] After firing, the sintered particles were visually inspected to confirm that there were no abnormalities. After that, they were weighed, their density was measured (to calculate the relative density), and then they were subjected to the coating process described below. In order to prevent the generation of fine powder (dust) from the surface or surface layer of the sintered particles produced by the above method during use, a coating agent was applied to the particle surface, dried, and fired, and the abrasion resistance of the coated particles was also evaluated.

[0074] The coating agents used in the coating process were selected from fluorine-based, resin-based, polyimide-based, and silicone-based materials. Specifically, "Fussopla Coat," "Polyimide Varnish," "TFE Coat," and "Silicone Varnish" were used.

[0075] The sintered particles were coated with the above-mentioned coating agent and heated to bake the coating agent. For example, if the median diameter of the particles was 1 mm or more, 100 particles were collected and weighed. Alternatively, if the median diameter of the particles was less than 1 mm, an amount equivalent to 500 particles was weighed and collected. These were then packed into a 120 cc polypropylene cylindrical bottle and rotated on a rotating roller at 120 rpm for 2 hours. Thereafter, the bottle was sieved, and the particles on the sieve were weighed. The difference between the weighed value before and after packing, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance.

[0076] The results are shown in Figure 3. The wear rate was lowest for "Fussopla Coat," "Polyimide Varnish," "Silicon Varnish," and "TFE Coat," in that order, demonstrating that fluorine-based and resin-based materials have superior wear resistance.

[0077] In this way, to address the above-mentioned problem (2) "dust generation," various coating agents were applied to the sintered particles, ensuring excellent abrasion resistance and thereby solving the problem.

[0078] The present sintered particles for radiation shielding material are not limited to use in the above-mentioned radiation shielding jig for preventing a gap between the beam irradiation port and the affected area of ​​the patient, but can also be applied to prevent radiation leakage from other locations, for example, locations where radiation leaks from a radiation generating device.

[0079] In addition, sintered particles of various "particle sizes" were filled into a weighing container, and the "flexibility of the jig," which indicates its fluidity, was examined by sensory testing. Particle sizes of less than 0.5 mm were found to be poor in flexibility, while coarse particles of more than 7 mm were limited for the following reasons: first, the thickness of the jig inside the weighing container, i.e., the height, is expected to be about 10 mm to 30 mm, and in the case of the smaller height (i.e., 10 mm), the maximum particle size required to fill the jig and provide its "flexibility" is expected to be about 7 mm; second, coarse particles of more than 7 mm in size caused a rough, uncomfortable sensation on the patient's face when they came into contact with the jig, so the appropriate range of "particle size" was determined. It was set to 0.5 to 7 mm.

[0080] The methods for measuring the "particle size" and "angle of repose", which are the main properties of sintered particles, will be described below. The "particle size" is measured by sieving using a JIS sieve and showing the median diameter, while the "angle of repose" is measured by (1) If the particle size of the particles to be measured is less than 10 mm or more than 6 mm, place a pair of transparent PVC plates vertically on the surface plate with a 12 mm gap between them. (2) If the particle size of the particles to be measured is less than 6 mm or more than 3 mm, use the same transparent PVC plate. Stand them vertically with a 7.5mm gap between them. (3) If the particle size of the particles to be measured is less than 3 mm or more than 1 mm, use the same transparent PVC plate. Stand them vertically with a 4.5mm gap between them. (4) If the particle size of the particles to be measured is less than 1 mm, stand the same transparent PVC board vertically with a gap of 2 mm between them. A predetermined amount of each particle (75 g if the particle size was 6 mm or more, 30 g if the particle size was less than 6 mm and 3 mm or more, 3 g if the particle size was less than 3 mm and 1 mm or more, and 0.5 g if the particle size was less than 1 mm) was dropped into the gap from one point in the upper center of the gap, and the angle of inclination of the mountain shape of the piled particles was measured.

[0081] The appropriate range of the "angle of repose" was determined by preparing a weighing container in advance, filling it with sintered particles of various particle size distributions and particle shapes, i.e., sintered particles exhibiting various "angles of repose," setting a simulated affected area on a patient, and conducting a sensory test on the "flexibility" of the container, i.e., the "fluidity" of the particles. As a result, the range from 8 degrees to 45 degrees was determined as the appropriate range, and the range from 10 degrees to 35 degrees, with the median being 20 degrees, which felt extremely good, was determined as the best range. It was confirmed that the larger the particle size and the closer the shape to a sphere, the better the "fluidity," i.e., the smaller the angle of repose, and conversely, the smaller the particle size and the more angular the shape, the worse the "fluidity," i.e., the larger the angle of repose.

[0082] In order to simulate the measurement of the "void ratio" between particles in the packed layer when the shielding material particles are packed into the tare of the shielding jig, the particles to be measured were packed into a square container with an open top shown below, known as a "masu" in Japan, until it overflowed, and the top side of the container was smoothed with a spatula to level off the overflowing particles. The amount of packed particles was weighed, and the "void ratio" was calculated using the apparent density of the same particles, which was measured separately. The internal dimensions of the container are (1) If the particle size is less than 1 mm, the dimensions are 5 mm x 5 mm x 5 mm in height, and the internal volume is 0.125 cm 3 , (2) If the particle size is 1 mm or more but less than 2 mm, the dimensions are 10 mm x 10 mm x 10 mm in height, and the internal volume is 1 cm 3 , (3) If the container contains coarse particles with a particle size of 2 mm or more, the dimensions are 30 mm x 30 mm x 30 mm in height, and the internal volume is 27 cm 3 , and so on.

[0083] Specific examples of the present invention will be described below. Figure 4 (Table 2) shows various data relating to the properties of the compact particles and sintered particles in the examples. [Example 1] The raw materials were 21.0 wt.% high-purity LiF (using natural Li), 49.8 wt.% MgF2, and 29.2 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0084] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 5 mm. The average relative density of the compact particles was 50.1%. These compacts were placed in a sintering furnace and sintered at 620°C for 1 hour in an atmospheric air atmosphere at normal pressure to produce sintered particles with a particle size of 4.3 mm and a relative density of 78.8%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 20 degrees.

[0085] These sintered particles with a particle size ratio of less than 1.2 were coated with Fluoroplacoat. The sintered particles were coated with the coating agent, heated to bake the coating agent, and then sampled to measure the angle of repose, which was found to be 18 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, as described above, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0086] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The abrasion rate of the sintered particles according to Example 1 using Fluoroplastic Coat was 0.000 wt.%.

[0087] [Example 2] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0088] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 5 mm. The average relative density of the compact particles was 48.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 4.3 mm and a relative density of 75.8%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 20 degrees.

[0089] These sintered particles with a particle size ratio of less than 1.2 were coated with polyimide varnish. The sintered particles were coated with the coating agent, heated to bake the coating agent, and then sampled to measure the angle of repose, which was found to be 18 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0090] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The degree of abrasion of the sintered particles of Example 2 using polyimide varnish was 0.000 wt.%.

[0091] [Example 3] The raw materials are high-purity LiF (using natural Li): 70.0 wt.%, MgF2: 18.9 wt.%, CaF2: 11.1 wt.%, and as a boron compound, B(OH)3: 8.7 wt.%, boron isotopes 10 B was added in an amount of 0.5 wt. % by weight, and the mixture was pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw material was kneaded to prepare the starting material.

[0092] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 5 mm. The average relative density of the compact particles was 47.3%. These compacts were placed in a sintering furnace and sintered at 560°C for 3 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 4.3 mm and a relative density of 74.4%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 21 degrees.

[0093] These sintered particles with a particle size ratio of less than 1.2 were coated with a TFE coat. The sintered particles were coated with the coating agent, heated to bake the coating agent, and sampled to measure the angle of repose, which was 19 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0094] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The abrasion rate of the sintered particles according to Example 3, which were coated with TFE, was 0.042 wt.%.

[0095] [Example 4] The raw materials are high-purity LiF (naturally produced Li): 70.0 wt.%, MgF2: 18.9 wt.%, CaF2: 11.1 wt.%, and Gd2O3 as a gadolinium compound. 3.84 wt.% gadolinium isotopes 157Gd was added at a rate of 0.52 wt.% in terms of outer percentage, and the mixture was pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw material was kneaded to form the starting material.

[0096] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 5 mm. The average relative density of the compact particles was 46.7%. These compacts were placed in a sintering furnace and sintered at 600°C for 2 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 4.3 mm and a relative density of 73.4%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 20 degrees.

[0097] These sintered particles with a particle size ratio of less than 1.2 were coated with silicon varnish. The sintered particles were coated with the coating agent, heated to bake the coating agent, and then sampled to measure the angle of repose, which was found to be 18 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0098] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The degree of abrasion of the sintered particles of Example 4 using silicon varnish was 0.012 wt.%.

[0099] [Example 5] The raw materials are high-purity LiF (using natural Li): 70.0 wt.%, MgF2: 18.9 wt.%, CaF2: 11.1 wt.%, and as a boron compound, B(OH)3: 8.7 wt.%, boron isotopes 10 B was added at a rate of 0.5 wt.% by outer percentage, and 3.84 wt.% of Gd2O3 was added as a gadolinium compound, and 1.0 gadolinium isotope was added. 157 Gd as outer The starting material was prepared by adding 0.52 wt. % of the cellulose acylate and grinding it using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and kneading the ground material.

[0100] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 5 mm. The average relative density of the compact particles was 46.0%. These compacts were placed in a sintering furnace and sintered at 620°C for 2 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 4.2 mm and a relative density of 77.6%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 19 degrees. These sintered particles with a particle size ratio of less than 1.2 were coated with silicon varnish. The sintered particles were coated with the coating agent, heated to bake the coating agent, and then sampled to measure the angle of repose, which was found to be 18 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0101] Furthermore, 100 sintered particles that had not been coated with a coating agent were collected, weighed, and filled into a cylindrical polypropylene bottle with an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The degree of wear of the sintered particles of Example 5, which were not coated with a coating agent, was 0.125 wt.%.

[0102] [Example 6] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0103] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 7.5 mm. The average relative density of the compact particles was 47.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 6.55 mm and a relative density of 70.9%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 11 degrees.

[0104] These sintered particles with a particle size ratio of less than 1.2 were coated with polyimide varnish. The above coating agent was applied, heated to bake the coating agent, and 100 sintered particles were sampled and the angle of repose was measured, which was 9 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 45 vol.%.

[0105] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The abrasion rate of the sintered particles according to Example 6, in which polyimide varnish was used, was 0.000 wt.%.

[0106] [Example 7] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0107] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 2 mm. The average relative density of the compact particles was 48.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric air atmosphere at normal pressure, resulting in sintered particles with a particle size of 1.7 mm and a relative density of 78.5%. The particle size ratio of these sintered particles was measured and found to be less than 1.2. The sintered particles were then ground to a particle size of 1 mm using a pot mill. The particle size ratio of the ground sintered particles was measured and found to be less than 1.2. The angle of repose of these sintered particles was measured and found to be 39 degrees.

[0108] These sintered particles were coated with polyimide varnish. The sintered particles were then coated with the coating agent, heated to bake the coating agent, and sampled. The angle of repose was measured and found to be 36 degrees. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 47 vol.%.

[0109] Furthermore, an amount equivalent to 500 sintered particles onto which the coating agent had been baked was weighed and collected, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The degree of abrasion of the sintered particles of Example 7, in which polyimide varnish was used, was 0.000 wt.%.

[0110] [Example 8] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0111] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 2 mm. The average relative density of the compact particles was 48.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere to produce sintered particles with a particle size of 1.7 mm and a relative density of 78.5%. The particle size ratio of these sintered particles was measured and found to be less than 1.2. These sintered particles were then milled using a pot mill to produce sintered particles with a particle size of 0.6 mm. The particle size ratio of the sintered particles after grinding was less than 1.2, and the angle of repose was measured to be 45 degrees.

[0112] These sintered particles were coated with polyimide varnish. The sintered particles were coated with the coating agent, heated to bake the coating agent, and the angle of repose was measured, which was 41 degrees. The coated sintered particles were packed into a container used as a tare for the shielding jig, and the porosity of the packed layer was measured. It was 52 vol.%.

[0113] Furthermore, an amount equivalent to 500 sintered particles onto which the coating agent had been baked was weighed and collected, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The abrasion rate of the sintered particles of Example 8, which used polyimide varnish, was 0.000 wt.%.

[0114] [Example 9] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0115] This starting material was pressed into a tablet with a modified tablet press to form compacts with a particle size of 5 mm. The average relative density of the compact particles was 47%. This starting material was then compressed into compacts with a particle size of 2 mm using an improved tablet press, and the average relative density of the compact particles was 48%. These compacts were individually placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere to obtain sintered particles. The particle size ratio of the sintered particles was 1.7 mm and the relative density was 78.5%. When the particle size ratio of the sintered particles was measured, both were less than 1.2. Furthermore, 500 of the sintered particles obtained by firing the compact with a particle size of 2 mm were ground to a particle size of 1 mm using a pot mill. The particle size ratio of these ground sintered particles was measured, and the results were as follows: It was less than 1.2. The above three types of sintered particles were mixed in blending ratios of 50 wt.%, 40 wt.%, and 10 wt.% from the coarse particle side, and the angle of repose was measured and found to be 28 degrees.

[0116] The sintered particles were coated with polyimide varnish. The sintered particles were coated with the coating agent, heated, and the angle of repose was measured, which was 26 degrees. The coated sintered particles were packed into a container used as a tare for the shielding jig, and the porosity of the packed layer was measured, which was 40 vol.%. The degree of abrasion of the sintered particles of Example 9, in which polyimide varnish was used, was 0.000 wt.%.

[0117] [Example 10] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0118] This starting material was pressed into a tablet with a modified tablet press to form compacts with a particle size of 5 mm. The average relative density of the compact particles was 47%. This starting material was then compressed into compacts with a particle size of 2 mm using an improved tablet press, and the average relative density of the compact particles was 48%. These compacts were individually placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere to obtain sintered particles. The latter sintered particles had a particle size of 1.7 mm and a relative density of 78.5%. When the particle size ratios of these sintered particles were measured, they were all less than 1.2. Furthermore, 500 of the 1.7 mm sintered particles obtained by firing a 2 mm compact were ground to a particle size of 1 mm using a pot mill. The particle size ratio of these ground sintered particles was measured and found to be less than 1.2. The above three types of sintered particles were mixed in blending ratios of 70 wt.%, 20 wt.%, and 10 wt.% from the coarse particle side, and the angle of repose was measured and found to be 26 degrees.

[0119] The sintered particles were coated with polyimide varnish. The sintered particles were coated with the coating agent, heated, and the angle of repose was measured, which was 24 degrees. The coated sintered particles were packed into a container used as a shielding jig, and the porosity of the packed layer was measured, which was 38 vol.%. The degree of abrasion of the sintered particles of Example 10, in which polyimide varnish was used, was 0.000 wt.%.

[0120] [Comparative Example 1] The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0121] This starting material was pressed into a tablet with a modified tablet press to form compacts with a particle size of 9 mm. The average relative density of the compact particles was 46.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric air atmosphere at normal pressure, resulting in sintered particles with a particle size of 7.85 mm and an average relative density of 69.6%. The particle size ratio of these sintered particles was measured, and those with a particle size ratio of less than 1.2 were used as sintered particles for a radiation shielding material as they were. On the other hand, those with a particle size ratio of 1.2 or more were milled using a pot mill to obtain sintered particles with a particle size ratio of less than 1.2. The angle of repose of these sintered particles having a particle size ratio of less than 1.2 was measured and found to be 7 degrees.

[0122] These sintered particles with a particle size ratio of less than 1.2 were coated with polyimide varnish. The sintered particles were coated with the above coating agent, heated to bake the coating agent, and then sampled to measure the angle of repose, which was 6 degrees.

[0123] Furthermore, 100 sintered particles onto which the coating agent was baked were collected, weighed, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The abrasion rate of the sintered particles in Comparative Example 1, which used polyimide varnish, was 0.000 wt.%. The coated sintered particles were packed into a container that served as a tare for the shielding jig, and the porosity of the packed layer was measured, which was 45 vol.%.

[0124] Comparative Example 2 The raw materials were 70.0 wt.% high-purity LiF (using natural Li), 18.9 wt.% MgF2, and 11.1 wt.% CaF2, which were pulverized using the ball mill method described in the above section [Embodiment for Carrying Out the Invention], and the pulverized raw materials were kneaded to form the starting material.

[0125] This starting material was pressed into a tablet with a modified tablet press to form a compact with a particle size of 2 mm. The average relative density of the compact particles was 48.2%. These compacts were placed in a sintering furnace and sintered at 580°C for 2 hours in an atmospheric pressure atmosphere. The sintered particles were milled in a pot mill to obtain sintered particles with a particle size of 0.4 mm. The particle size ratio of the sintered particles after milling was also less than 1.2. The angle of repose was measured to be 48 degrees.

[0126] These sintered particles were coated with polyimide varnish. The sintered particles were then coated with the coating agent, heated to bake the coating agent, and sampled to measure the angle of repose, which was 46 degrees.

[0127] Furthermore, an amount equivalent to 500 sintered particles onto which the coating agent had been baked was weighed and collected, and filled into a cylindrical polypropylene bottle having an internal volume of 120 cc. After rotating the bottle at 120 rpm for 2 hours, the bottle was sieved and the particles on the sieve were weighed. The difference between the weighed value before and after filling, i.e., the amount of wear during bottle rotation, was measured to evaluate the wear resistance. The degree of abrasion of the sintered particles according to Comparative Example 2, in which polyimide varnish was used, was 0.000 wt.%. The coated sintered particles were packed into a container used as a weighing container for the shielding jig, and the porosity of the packed layer was measured, which was found to be 58 vol.%. [Explanation of symbols]

[0128] 1 Lower receiving mold (for pharmaceutical manufacturing) 2 Upper mold (for pharmaceutical manufacturing) 3 Lower receiving mold (improved tablet press) 4 Upper mold (improved tablet press)

Claims

1. LiF or LiF with MgF 2 , CaF 2 , NaF, AlF 3 , and / or BaF 2 and one or more fluorides selected from the group consisting of (a) and (b), and the sintered particles for use in a radiation shielding material are mixed with the fluoride, and the sintered particles are molded into a spherical shape having a particle size ratio of 1.2 or less, fired, or are molded, fired, and then ground.

2. LiF or LiF with MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the following: ２ O ３ , B(OH) ３ , LiB ３ O ５ or Li ２ B ４ O ７ The boron compounds selected from the above are 10 % of B in an outer percentage, and the particles are molded into a spherical shape having a particle size ratio of 1.2 or less, fired, or molded, fired, and then ground.

3. LiF or LiF with MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the above, further containing Gd 2 O 3 , Gd(OH) 3 or GdF 3 The gadolinium compounds selected from the gadolinium isotopes 157 1. A sintered particle for a radiation shielding material, which is characterized in that Gd is added in an amount of 0.1 to 2 wt. % by outer percentage, and the particle size ratio is 1.2 or less ...

4. LiF or LiF with MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the following: 2 O 3 , B(OH) 3 , LiB 3 O 5 or Li 2 B 4 O 7 The boron compounds selected from the above are 10 B is added in an amount of 0.1 to 5 wt.% by weight, and Gd 2 O 3 , Gd(OH) 3 or GdF 3 The gadolinium compounds selected from the gadolinium isotopes 157 1. A sintered particle for a radiation shielding material, which is characterized in that Gd is added in an amount of 0.1 to 2 wt. % by outer percentage, and the particle size ratio is 1.2 or less ...

5. 5. The sintered particles for a radiation-shielding material according to claim 1, wherein the relative density of the sintered particles before sintering, i.e., the granular compact, is 40% or more and 80% or less.

6. 5. The sintered particles for a radiation-shielding material according to claim 1, wherein the particle diameter is 0.5 mm or more and 7 mm or less.

7. 5. The sintered particle for a radiation-shielding material according to claim 1, wherein the sintered particle for a radiation-shielding material is coated with a coating agent selected from a fluorine-based coating agent, such as fluoroplastic coating, polyimide varnish, TFE coating, or silicone varnish, a resin-based coating agent, a polyimide-based coating agent, or a silicone-based coating agent.

8. 5. The sintered particle for a radiation-shielding material according to claim 1, wherein the sintered particle for a radiation-shielding material is used as a neutron shielding material.

9. High-purity LiF raw material, or high-purity LiF raw material plus high-purity MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the above is individually subjected to primary pulverization to make each particle size 8 μm or less in median diameter, Then, these primary crushed individual raw materials are mixed in a predetermined ratio, Further, a step of subjecting the mixture to secondary pulverization to reduce the particle size to 6 μm or less and kneading the mixture (raw material blending step); a step of forming spherical or nearly spherical molded particles having a mold inner diameter of 0.6 to 8 mm and a particle size ratio of 1.2 or less using a tablet press (molding step); Spherical compact particles with a particle size ratio of 1.2 or less are heated to 350°C in an atmospheric pressure atmosphere. a step of heating and sintering at a temperature range of 1000°C (sintering step); 1. A method for producing sintered particles for a radiation shielding material, comprising:

10. The method for producing sintered particles for a radiation-shielding material according to claim 9, Further, a step of grinding the sintered particles using a pot mill (grinding step); 1. A method for producing sintered particles for a radiation shielding material, comprising:

11. The method for producing sintered particles for a radiation-shielding material according to claim 9 or 10, Further, a coating process is carried out in which the ground sintered particles are coated with a coating agent selected from fluorine-based, resin-based, polyimide-based, or silicone-based coating agents such as fluoroplastic coating, polyimide varnish, TFE coating, or silicone varnish, and then dried at room temperature in the atmosphere and baked.

1. A method for producing sintered particles for a radiation shielding material, comprising:

12. 10. The method for producing sintered particles for a radiation shielding material according to claim 9, wherein the raw materials in the raw material blending step are a high-purity LiF raw material, or a high-purity LiF raw material and a high-purity MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the following: 2 O 3 , B(OH) 3 , LiB 3 O 5 or Li 2 B 4 O 7 The boron compounds selected from the above are 10 1. A method for producing sintered particles for a radiation shielding material, wherein B is added in an amount of 0.1 to 5 wt. % by weight.

13. 10. The method for producing sintered particles for a radiation shielding material according to claim 9, wherein the raw materials in the raw material blending step are a high-purity LiF raw material, or a high-purity LiF raw material and a high-purity MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the above, further containing Gd 2 O 3 , Gd(OH) 3 or GdF 3 The gadolinium compounds selected from the gadolinium isotopes 157 A method for producing sintered particles for a radiation shielding material, characterized in that Gd is added in an amount of 0.1 to 2 wt. % by weight.

14. 10. The method for producing sintered particles for a radiation shielding material according to claim 9, wherein the raw materials in the raw material blending step are a high-purity LiF raw material, or a high-purity LiF raw material and a high-purity MgF 2 , CaF 2 , NaF, AlF 3 , BaF 2 A multi-component fluoride containing one or more fluorides selected from the following: 2 O 3 , B(OH) 3 , LiB 3 O 5 or Li 2 B 4 O 7 The boron compounds selected from the above are 10 B is added in an amount of 0.1 to 5 wt.% by weight, and Gd 2 O 3 , Gd(OH) 3 or GdF 3 The gadolinium compounds selected from the gadolinium isotopes 157 A method for producing sintered particles for a radiation shielding material, characterized in that Gd is added in an amount of 0.1 to 2 wt. % by weight.

15. 9. A method for using the sintered particles for a radiation-shielding material according to claim 5, comprising mixing two or more types of particle groups having different particle sizes, reducing the void ratio in a packed bed filled with the mixed particle groups, and improving radiation-shielding performance.

Images