JP7165339B2 - Sintered body for radiation shielding material, radiation shielding material and method for producing the same - Google Patents

Description

The present invention provides a sintered body for a radiation shielding material, which can reduce or eliminate low-energy radiation including thermal neutrons in a radiation field including thermal neutrons, thereby reducing exposure of the human body and/or equipment. The present invention relates to a shielding material and a manufacturing method thereof.
Examples of radiation fields containing thermal neutrons include, for example, therapeutic radiation fields in "boron neutron capture therapy (hereinafter referred to as BNCT)", which is a cancer treatment method that has attracted attention in recent years. can be done.

More specifically, the present invention is used, for example, to remove therapeutically unwanted or harmful radiation species, such as neutrons, in the therapeutic beam used in the BNCT and/or the therapeutic beam is To prevent leakage outside the treatment area, and/or to prevent equipment failure or activation due to beams leaking outside the treatment area, and/or similarly prevent medical treatment due to leaked beams or activation. The present invention relates to a sintered body for a radiation shielding material having a structure suitable for a radiation shielding material installed in order to prevent workers from being exposed to radiation, a radiation shielding material, and a method for manufacturing the same.

In the field of radiation medicine, new applications are being developed that utilize the radiation shielding and/or moderating effects of specific elements.

We will now discuss "radiation" as it relates to the application of the present invention. Types of "radiation" are broadly divided into alpha (α) rays, beta (β) rays, gamma (γ) rays, X (x) rays and neutron rays, and those listed here are described later. The higher the material, the greater the ability (penetration power) to penetrate substances.

Neutron beams with the highest penetrating power (hereinafter referred to as neutrons) are further classified according to the level of energy they retain, but there are various theories regarding this classification.
For example, there is a theory that neutrons on the high energy side are fast neutrons, neutrons on the medium energy level are epithermal neutrons, and neutrons on the low energy side are thermal neutrons. be.

In the "Japan Neutron Capture Therapy Society" related to the content of the present invention, the energy level of epithermal neutrons mainly used for neutron capture therapy described later is set to 0.5 eV or more and 10 keV or less, and fast neutrons on the high energy side are It is defined as 10 keV or more and 0.5 eV or less for thermal neutrons on the low energy side. Here, we will comply with the energy levels specified in the guidelines of this Society.

High-energy neutrons, that is, fast neutrons and γ-rays generated as secondary radiation hit normal cells, damaging DNA and causing side effects mainly inside the body, such as radioactive anemia and leukopenia.
Furthermore, long after irradiation, late effects may occur, such as rectal or bladder tumors and bleeding.

On the other hand, when exposed to a large amount of low-energy thermal neutrons, inflammation occurs mainly in the beam irradiation area and the skin in the vicinity, and after a while after irradiation, hair loss occurs in the same area, mainly in the skin and subcutaneous integument. may cause side effects in some parts of the body.
In recent years, therapeutic methods that use radiation effectively have been researched to minimize the occurrence of such side effects and late effects.

A characteristic of neutrons is that they have a short half-life of about 15 minutes and decay in a short period of time, emitting electrons and neutrinos to become protons.
Also, neutrons have no electric charge and are therefore easily absorbed when they collide with the nucleus.

Absorbing neutrons in this way is called "neutron capture" or "neutron capture", and an example of its application in the medical field using this property is BNCT. Law.

In this BNCT, first, tumor cells such as malignant cancer are reacted with a boron drug containing the boron isotope ^10B that is injected into the body by injection or infusion to form a boron compound reaction product in the tumor part. Keep
This reaction product is surface-irradiated with neutrons at an energy level that has little effect on healthy parts of the human body (preferably composed of neutrons at a medium energy level, such as epithermal neutrons). A boron compound that has been segregated in the tumor area at a high concentration in advance causes a nuclear reaction only within a very small range (a range equivalent to one cell), killing only the tumor cells. Let

Originally, cancer cells tend to incorporate boron into their tumor cells during the process of vigorous proliferation, and BNCT utilizes this property to effectively destroy only the tumor portion.
Using neutrons, which have little effect on the healthy part, the irradiation beam is irradiated in a planar shape with a size that includes the tumor part.

As a result, compared to pinpoint irradiation in conventional radiotherapy, irradiation time can be dramatically shortened, and non-irradiated portions (irradiation leaks) can be eliminated.
Furthermore, in normal radiation therapy, the number of irradiations (that is, the number of treatment days) is 20 or 30 days, and it takes more than a month even if irradiated every day. In addition, each irradiation time is about 30 to 60 minutes, and the burden on the patient during treatment can be remarkably reduced.

In this BNCT, the "irradiation beam" (neutrons) applied to the affected part of the patient is mainly epithermal neutrons of 40 keV or less, preferably 10 keV or less, and a small amount of low energy level neutrons below thermal neutrons (target It is desired that the value is (thermal neutron weight)/(epithermal neutron weight) ≤ (1/20)) and that fast neutrons are not mixed.
In addition, the number of neutrons in the neutron flux of the irradiation beam is required to be 1×10 ⁹ (n/cm ² ·s) or more.

The reason for this is that after irradiation, fast neutrons damage the DNA in the cells of the body while retaining the energy of the fast neutrons, and also damage the body fluids, which are the main constituents of the body (mainly water H ₂ O and nitrogen N ), they are rapidly absorbed and decelerated, gradually changing to epithermal neutrons, and then to low-energy neutrons below thermal neutrons. , to damage healthy cells in the same way as the fast neutrons described above.

On the other hand, the thermal neutrons generated in this absorption/moderation process react with ¹⁰ B in the boron compound administered to the affected area to destroy cancer cells, causing a so-called neutron capture reaction. If the secondary radiation is actively generated, it will cause side effects to healthy tissue inside.
On the other hand, the irradiated thermal neutrons react immediately with the outer skin tissue in the outer skin, causing side effects such as skin inflammation and hair loss.

As the latest BNCT method, for example, a group of Kyoto University and Sumitomo Heavy Industries Co., Ltd. is advancing (Non-Patent Document 1 and Non-Patent Document 2).
This system is not attached to existing nuclear reactors, and is composed of equipment dedicated to treatment with a dedicated cyclotron-type accelerator as a neutron generator.
Here, in order to safely reduce the generated neutrons (mainly fast neutrons) to an energy level that is easy to use, it is necessary to reduce the number of generated neutrons to an energy level that is easy to use without reducing the number of generated neutrons. It is necessary to have a deceleration system with deceleration performance. Specifically, it is necessary to configure the moderator with an appropriate chemical composition and to secure the shape of the moderator.

Furthermore, it is also important to prevent neutrons from leaking out of the moderator system by using shielding materials with appropriate shielding performance.
It is also important to remove unnecessary or harmful neutrons and gamma rays from the therapeutic beam.

Sumitomo Heavy Industries, Ltd., which is mainly responsible for the equipment development of this Kyoto University group, has applied for the following patents regarding "moderator system and moderator" for BNCT equipment (Patent Document 1: Patent No. 5112105).
According to this patent document 1, the moderators of the moderator system are, from the upstream side in the flow direction of the neutron beam, the moderator 1 is Pb, the moderator 2 is Fe, the moderator 3 is Al or AlF ₃ , and the moderator 4 is CaF. ₂ or CaF ₂ mixed with AlF ₃ .

On the other hand, the University of Tsukuba group, which is the second development group in Japan, uses moderator 1 and moderator 2 as moderators for its moderation system, as shown in Patent Document 2 (Patent No. 5813258) below. is the same as the Kyoto University group, the moderator 3 is Al, and the moderator 4 is _MgF2 sintered body, and the moderator 4 is greatly different from the Kyoto University group.

Patent document 1 also describes the prevention of leakage of neutrons to the outside of the moderation system.
Specifically, in paragraph [0022], "A shielding material 5 is provided on the output end side of the moderator system 1. The shielding material 5 is a first shielding layer 10 made of LiF-containing polyethylene or high-density polyethylene. , and a second shielding layer 11 made of Pb." At the exit end of the deceleration system 1, a device commonly called a "collimator" is arranged.

The first shielding layer 10 is mainly made of a substance with a large radiation absorption cross section, that is, LiF and/or polyethylene, and plays a role in preventing relatively low-energy neutrons such as thermal neutrons and epithermal neutrons from leaking out of the device system. is responsible for
As described in the same paragraph, the second shielding layer 11 is defined as "the second shielding layer 11 mainly plays a role of shielding gamma rays and the like generated from the target 2."

[Problems to be solved by the invention]
The problem here is the constituent material of the first shielding layer 10 .
LiF is a major constituent radiation of therapeutic beams, along with boron (B), cadmium (Cd), and cadmium (Gd) among the elements whose main constituent element lithium (Li) exists in nature. Among the epithermal neutrons and thermal neutrons, it has a large absorption cross section for thermal neutrons unnecessary for treatment.

Naturally occurring Li (hereinafter referred to as “natural Li”) has two isotopes, ⁶ Li and ⁷ Li, and their abundance ratio (i.e., “natural abundance ratio”) is 7.5 atoms for ⁶ Li. %, ⁷ Li is known to be 92.5 atom %.
Of these, ⁶ Li has a large absorption cross-section for thermal neutrons, which is the reason why natural Li and its compounds have high shielding performance against thermal neutrons.

Elements other than Li described above that have a large absorption cross section with respect to relatively low-energy neutrons such as thermal neutrons include B, Cd, and Gd described above.
In the case of Li, alpha (α) rays are generated as secondary radiation when absorbing relatively low energy level neutrons, such as the radiation beam in BNCT. This alpha ray does little harm to the human body.
In contrast, B, Cd, and Gd all have the potential to generate γ-rays that are harmful to the human body. The possibility and amount of gamma ray generation increases in the order of B, Cd, and Gd.

However, with regard to B, when it absorbs high-energy neutrons such as fast neutrons, it produces ⁷ Li and ⁴ He, and a gamma ray (0.478 MeV: this is called a "prompt gamma ray" as a secondary radiation). However, when neutrons with energy levels lower than epithermal neutrons are absorbed, γ-rays are generated as secondary radiation less, and ⁷ Li and ⁴ He have no effect on the human body. It is said to generate alpha rays.
This latter reaction (“Generate He and α-rays”) is called the neutron absorption (capture) reaction by the reaction product of drug 10B pre ^- administered to the affected area of the patient, which is the so-called “principle of the BNCT method”. It's becoming

Incidentally, in the treatment process of BNCT, as a method of grasping the transition of this neutron capture reaction, a method of measuring the aforementioned "prompt gamma ray (0.478 MeV)" for on-the-spot observation may be adopted.
In this case, the contamination of therapeutic neutron beams with “prompt gamma rays” as secondary radiation caused by shielding materials (e.g., B- or Gd-containing shielding materials) becomes a factor of field observation disturbance. must be avoided.

There are ¹⁰ B and ¹¹ B isotopes of naturally occurring boron, and the respective abundance ratios are 19.9 atom % and 80.1 atom % (some say "20 atom % and 80 atom %"). Of these, ^{10 B is mainly involved in neutron absorption reactions, and the absorption cross section of this 10 B} for thermal neutrons is as large as 3,837 barns.

On the other hand, the absorption cross section of ⁶ Li is 940 barns, which is about (1/4) that of ¹⁰ B, and the natural abundance ( ¹⁰ B is 19.9 atom %, ⁶ Li is 7.5 atom %). Taking this into account, it can be said that the neutron absorption capacity of Li is about one order of magnitude smaller than that of B among natural products.

The technology for enriching the B isotope ¹⁰ B has already been established for use in the same field of “medicine administered by the BNCT method”, and the concentrated ¹⁰ B (hereinafter referred to as “enriched type”) in Japan is It is commercially available. In addition, when the enriched ¹⁰ B is used for BNCT applications, it is subject to special measures, and it is easy to use without any legal restrictions.

Eight types of Cd isotopes, from ¹⁰⁶ Cd to ¹¹⁶ Cd, are known. Among them, ¹¹² Cd has a large absorption cross section of 2,450 barns for thermal neutrons, and is expected as a shielding material. there is The natural abundance of ¹¹² Cd is 24.13 atom %.

As for isotopes of Gd, ¹⁵² Gd to ¹⁶⁰ Gd are known. Among them, ¹⁵⁷ Gd has an extremely large absorption cross section of 254,000 barns for thermal neutrons and is expected as a shielding material. The natural abundance of ¹⁵⁷ Gd is 15.65 atom %.

There is a tendency to think that the shielding performance can be enhanced by using a material with a higher ^6Li content, ie, a concentrated ^6Li material.
For example, in paragraph [0052] of the specification of Patent Document 3 below (Re-Table 2018-181395 (Japanese Patent Application No. 2018-517656)), it states, "The abundance ratio of ⁷ Li in nature is 92.5 atom%. ⁶ Li is ^7.5 atom %, of which ⁶ Li contributes to the ^shielding of neutron beams. Therefore, the LiF sintered body is more preferably a ⁶ LiF sintered body. Hereinafter, the ⁶ LiF sintered body will be described."

However, this enriched ⁶ Li can be used as "the raw material for nuclear fusion fuel for hydrogen bombs", and tritium that can be used for the detonator of atomic bombs and "material for enhanced atomic bombs" is also produced. It becomes possible.
The major nuclear powers are expected to possess the enrichment technology, but in Japan, even for peaceful use, the technology development is subject to various regulations, and the enrichment of ^6Li has not been realized. .
For example, as described in Non-Patent Document 3, which describes "Li isotope separation and enrichment technology in relation to fuel for nuclear fusion reactors" posted by the Japan Atomic Energy Agency, " ⁶ Li At present, Japan has yet to establish a stable mass-production technology for enrichment.”

In addition, it is practically difficult to import raw materials and products in which ⁶ Li is concentrated from foreign countries.
For example, in the United States, which possesses concentration technology, export to other countries is subject to national regulation (classification number: 1C233 of the United States Department of Commerce Control List).
In addition, exporting ^6Li -enriched raw materials and products from Japan to other countries poses a security problem and is subject to regulation (Ministry of Economy, Trade and Industry: According to the "List Control" of the "Ministerial Ordinance on Goods, etc.") It has become.
It can be said that it is difficult to stably secure ^6LiF in which ^6Li is concentrated in this way.

Therefore, in order to obtain high shielding performance with naturally-derived LiF having a constant ⁶ Li content, it is inevitably necessary to obtain a high-density product while increasing the proportion of LiF.
However, when sintering LiF alone or a starting material in which LiF is mixed with other fluorides at a high concentration by conventional sintering technology, LiF is heated at a very low temperature compared to other fluorides, so-called " The sublimation phenomenon occurs actively, and it vaporizes violently and foams during the sintering process.
For this reason, it has been difficult to obtain a dense melt or a sintered body from a starting material in which LiF alone or other fluorides are mixed with LiF at a high concentration. In particular, it was impossible to obtain a large-sized, high-density sintered body.

Regarding the state of a LiF simple sintered body, for example, Li in LiF is ⁶ Li, that is, the density of a ⁶ Li enriched simple sintered body is described in the above-mentioned Patent Document 3.
Claim 3 of this patent document 3 states, "The ⁶ LiF sintered body is made of ⁶ LiF, has a relative density of 83% or more and 90% or less, and has a good structure in which cracks and blisters on the outer surface are suppressed. It has a nice appearance, … … ”.
However, when the relative density is 83% or more and 90% or less, it is actually very easy to crack during handling, for example, it is very similar to "a state in which the surface layer of concrete immediately after placement has just begun to solidify". , is in an extremely brittle sintered state.
Such a fragile sintered body cannot maintain its shape during handling and is easily crumbled or cracked, resulting in a quality level unsuitable for practical use.

The sintering method described in Patent Document 3 is generally called “Spark Plasma Sintering (abbreviated as SPS) method” and is suitable for sintering difficult-to-sinter powder materials. known as
For example, the sintered body obtained by the SPS method is described in the manual on the website of Fuji Denpakoki Co., Ltd., a manufacturer of SPS sintering equipment (Non-Patent Document 4: "[Spark Plasma Sintering (discharge plasma sintering) )] What's SPS"), ``As a phenomenon in the early stages of energization, the generation of discharge plasma accompanying pulse energization/discharge is due to the discharge impact pressure and the spattering action.'' In the figure of "Representative examples of target materials", "(Phenomenon) is marked with "Discharge impact pressure generation Local stress/spatter action" and "Joule heat generation Local high temperature generation". The principle of this sintering method is specified as stress generation and local high temperature generation.

More specifically, in the sintered body produced by the SPS method, a large stress is generated due to the impact caused by the spark discharge in the sintering process, which remains inside the sintered body after cooling, It contains a large residual stress.
Moreover, the sintered body has a non-uniform sintering progress. For this reason, the SPS method sintered body is remarkably inferior in impact resistance and has extremely large variations in density.

The following non-patent document 5 describes a commentary titled "Current status and future potential of ceramic sintering by spark plasma sintering (SPS) method". Among them, for example, in the right column on page 95, section 3.7 "Homogeneous SPS sintering technology for large ceramic materials",
“Because the SPS method is characterized by rapid heating and rapid sintering, 'larger size and homogeneous sintering' is a typical issue for developing processing technology and know-how. Since homogenization can be easily obtained, this kind of problem is difficult to manifest.
However, large materials of φ100 to 350 mm have variations. Due to the thermal conductivity of the material, the rearrangement of the grains, the uniform press pressure load contact area, the circumference, etc., uneven load and uneven heating tend to occur, and rapid and homogeneous sintering is difficult. … … ”
As in Non-Patent Document 3, it is explained that uniform sintering becomes extremely difficult in the SPS method as the compact size increases.

As described above, according to the SPS method, a high temperature is generated locally, and it is difficult to obtain a uniform sintered body. Impact resistance will decrease.

As a result, the flexural strength, which is an index of impact resistance, is remarkably low. Further, when the size is increased, the sintered body becomes non-homogeneous, resulting in variations in density, variations in machining strength, and portions with low strength tend to occur.

As described above, the SPS method has a major drawback that "there is a localized high temperature generation, resulting in a non-uniform sintering process". It can be said that it is a completely unsuitable method as a method.

Furthermore, in the case of the SPS method, the furnace material of the SPS furnace, the electrodes, the load press plate, etc. are all made of high-purity carbon (carbon: elemental symbol C), and the work (in this case, is in direct contact with enriched ⁶ LiF).
Since LiF is easily vaporized during the sintering process, the carbon naturally reacts with the vaporized F gas to form a CF-based compound. As a result, in the sintered body fired by the SPS method, a CF-based compound is formed in part of pure LiF.
This has a large adverse effect on shielding performance.

Furthermore, the lower the relative density of the sintered body, the lower the shielding performance of the sintered body. end up
In the conventional sintering technology as described in Patent Document 3, Li having large dimensions, high density that is easy to handle, and high mechanical strength, or a melt containing Li at a high concentration, or A mass such as a sintered body cannot be produced.

As described above, the sintered body described in Patent Document 3 has a major problem in securing a raw material source, and in addition, the relative density of the ⁶ LiF sintered body is remarkably low at 83% or more and 90% or less. A sintered body with such a low density is of a low quality level that is completely unsuitable for practical use, such as cracking, cracking, and sometimes collapsing during handling.

Moreover, in such a low-density sintered body, when a test piece to be subjected to a test required for a mechanical strength test is taken (specifically, a test piece is cut using a cutting device), cracks may occur. , the sample collapses, making it impossible to collect the test piece itself. Therefore, even a strength test cannot be performed, and a strength test value conforming to the JIS standard cannot be presented.

Furthermore, Patent Document 4 (Japanese Patent Application No. 2018-514325, Japanese Patent Application No. 2017-557373), which uses ⁶ Li as a moderator system material, describes the following.
Claim 1 of this Patent Document 4 states, "The neutrons from the target are moderated to the epithermal neutron energy region by the moderator (that is, the moderator"), and the material of the moderator is PbF ₄ , one or more of _Al2O3 , _AlF3 , _CaF2 , or _MgF2 , and one or _more of said _PbF4 , _{Al2O3 , AlF3} , _CaF2 , or _MgF2 ; A variety of combination materials are mixed with a material containing 0.1 to 5% by weight of ^6Li element, and the material of the slowdown section is powder sintered by powder sintering equipment. Blocks are formed from powders or green compacts, and are described as follows.

Furthermore, in claim 2 of Patent Document 4, "the neutrons from the target are moderated to the epithermal neutron energy region by the moderator (i.e., the "moderator"), and the moderator (i.e., the moderator) The material of the body") is made of _one or a combination of LiF, _Li2CO3 , _{Al2O3 , AlF3} , _CaF2 or _MgF2 , and the material of the moderator part is sintered powder. Depending on the equipment, the powder or green compact is turned into a block in the powder sintering process, … …”.
Thus, the material of the moderator described in claims ₁ and ₂ is ", AlF ₃ , CaF ₂ or MgF ₂ . . . ." It is not a system.

Also, ⁶ Li is used as a material for the moderator system, but as described above, ⁶ Li is inherently poor in moderation performance. They are good at absorbing (i.e. shielding) low-energy level neutrons such as thermal neutrons, but very poor at slowing down high-energy level neutrons, mostly fast neutrons, generated at the target.
It is slightly moderated by other mixed compounds other than ⁶ Li, and ⁶ Li is thought to exhibit only the effect of shielding epithermal neutrons generated in small quantities and thermal neutrons.

When this ⁶ Li is mixed in the moderator, the amount of neutrons for treatment is reduced, and the prescribed value of neutrons for treatment (in the IAEA guideline, the amount of epithermal neutrons is 1×10 ⁹ (n/cm ² s) The above is required) is no longer easy to secure, and each ^BNCT development team continues to struggle to secure the specified value. Must avoid.

Since this ⁶ Li exerts the above-mentioned actions and effects on neutrons, when this ⁶ Li is used in the moderator, it should be restricted to a very low concentration.
However, in the examples described in Patent Document 4, no clear mixing ratio of ⁶ Li is shown.
Furthermore, this Patent Document 4 does not clearly describe "sintering conditions (molded body, sintering temperature conditions, etc.)" and "quality of the sintered body", and the invention described in Patent Document 4 is The possibility of violating enablement requirements and clarity requirements is extremely high.

In addition, in Patent Document 4, regarding the sintering method, only "sintering method" is described in [Claims], but in the above [Mode for Carrying Out the Invention] section, " Two methods are described: spark plasma sintering and hot press sintering.
Both are methods of applying pressure during the heating and sintering process, and stress (that is, distortion) due to pressure remains inside the manufactured sintered body, which causes the sintered body, even if it has a high density. However, it is brittle and easily broken.

Various sizes and shapes are required for the "shielding material for BNCT", and machining of the manufactured sintered body is indispensable. Even if the sintered body has a low density or a high density, the sintered body has internal strain and is easily cracked during machining, making it an unsuitable material for this application.

A rare case in which LiF is sintered alone is described in the following Patent Document 5 (JP-A-51-94098, entitled "Sintered Lithium Fluoride Neutron Shielding Material").
In Patent Document 5, the dimensions of the LiF sintered body are not described and are unknown, but the inner diameter of the mold in the preheating step is 30 mm, and the subsequent spark discharge sintering under heating shrinks further. Therefore, the size after sintering becomes even smaller, and it can be said that this patent relates to a method of manufacturing a very small sintered body. In addition, this patent does not include any description regarding the mechanical strength of the fired sintered body, which is unknown.

In addition, in Patent Document 5, Li-based metals and compounds such as LiF, Li ₂ O, LiH, Li, and Li ₂ CO ₃ are used as raw materials and sintered using a sintering aid to achieve a theoretical density of 90% or more. It is the content of obtaining a sintered body of.

Although the size of the sintered body is not described, in the example, "Before firing, the starting material has an internal volume of 100 mm × 100 mm × 10 mm ("10 mm" is presumed to be the "height of the mold"). The mold is filled and molded at a pressure of about 500 kg/cm@2. ”, and since the molded body shrinks during firing, the size of the sintered body is much smaller than the inner dimension of the mold.

When the size is small, especially a thin sintered body with a thickness of several millimeters as in this case, it is easy to densify and obtain a high density.
When the thickness of the sintered body exceeds about 30 mm, the difference in sintering speed between the inner side and the surface layer of the sintered body becomes large, and uniform densification becomes difficult. It tends to be uniform, and it becomes difficult to increase the density.

On the other hand, Nikkato Co., Ltd. sells a small, thin tile-shaped shielding material for BNCT under the product name "Lithium Fluoride" (2 types of 50 mm × 50 mm × t10 mm and 100 mm × 100 mm × t10 mm in the catalog) for BNCT. It is
The patent on which these products are based is probably the Nippon Kagaku Togyo Co. It is presumed to be related to a LiF sintered body filed long ago by a company (now Nikkato Co., Ltd.).

The above-mentioned current product is recognized as a shielding material that can be used for test equipment using neutrons because of its small size and thin plate-like shape. The size of the main unit alone is barely enough to fit in a room of about 15m x 9m x 5m high, and the room that houses the downstream deceleration system and irradiation treatment system is about half the volume of the upstream room. It is a fairly large device.
Therefore, a shielding material with a large area is required. Unreasonable.

Shielding material around the perimeter of the moderator system and the downstream end of the moderator system, i.e., an opening for emitting therapeutic neutron beams (hereinafter referred to as "therapeutic beams") toward the patient's affected area (generally, " A device called a "collimator" is disposed at a portion forming a round-shaped opening with a diameter of about 100 mm to 250 mm, which is called an "exit port" or "irradiation port". As a constituent material of this "collimator", a LiF-containing polyethylene resin is often used.

Looking at the components of LiF-containing polyethylene resin, polyethylene consists of carbon C and hydrogen H, and has a shielding performance against high-energy fast neutrons and epithermal neutrons, especially against fast neutrons. However, it can be seen that it has almost no shielding performance against thermal neutrons.

On the other hand, the LiF in the LiF-containing polyethylene resin is in the form of powder, and the contained ⁶ Li has excellent shielding performance against thermal neutrons, but it can slightly shield epithermal neutrons. It is a degree and can hardly shield against fast neutrons.

LiF powder mainly exerts a shielding effect against low-energy thermal neutrons. , the performance cannot be fully exhibited.
Incident high-energy neutrons become thermal neutrons via epithermal neutrons due to the shielding and moderating effects of polyethylene resin, but these thermal neutrons can be shielded by LiF only after reaching the exit side.

In this way, the shielding effect of LiF in the LiF-containing polyethylene resin cannot be effectively exhibited because there are few thermal neutrons on the incident side, and there are many thermal neutrons generated by being moderated by polyethylene on the exit side. As much as possible, it can be effectively exhibited. However, if there are too many thermal neutrons, it may become unshielded.

Thus, the shielding effect of LiF depends on the amount of thermal neutrons, and tends to be localized and inefficient in the LiF-containing polyethylene resin.
On the incident side of this LiF-containing polyethylene resin, the amount of thermal neutrons in the incident beam is small, and the shielding of thermal neutrons by LiF is in time. and thermal neutrons are generated, the shielding of thermal neutrons by LiF tends to be insufficient on the output side.

Furthermore, the LiF-containing polyethylene resin is in a state in which LiF powder is kneaded into the polyethylene resin, and the distribution of LiF is likely to be uneven. It is in a mixed state, and it is easy to cause unevenness in the shielding performance.

As described above, the LiF-containing polyethylene resin has the disadvantage of inferior thermal neutron shielding performance mainly due to its structure. Therefore, the current situation is that the LiF-containing polyethylene resin is unavoidably used. This inevitably causes various problems due to neutron leakage.

Regarding the reactivity of LiF with neutrons, as described above, LiF originally has a large absorption cross section for thermal neutrons due to the ⁶ Li contained therein, but neutrons at other energy levels, such as high-speed The absorption cross section for neutrons and epithermal neutrons is small.
Therefore, when the BNCT therapeutic beam is shielded by LiF alone, thermal neutrons are shielded, but epithermal neutrons and fast neutrons are hardly shielded and pass through.

In other words, epithermal neutrons suitable for treatment can be transmitted without being attenuated, which is convenient for treatment, but harmful fast neutrons to be removed cannot be shielded and are transmitted.
Therefore, if the therapeutic beam contains many fast neutrons and thermal neutrons, LiF alone cannot ensure sufficient shielding performance, and another shielding material specialized for fast neutrons is also used. It becomes necessary to

Furthermore, the guidelines of the International Atomic Energy Agency (IAEA) on BNCT indicate that the epithermal neutron intensity (number of epithermal neutrons) in the therapeutic beam in BNCT is desirable to be 1×10 ⁹ (n/cm ² s) or more. It is
In this connection, in addition to the performance of neutron generation in accelerators and targets, there are problems such as beam leakage around the deceleration system and around the beam exit.

Further, when irradiating a therapeutic beam from a BNCT apparatus to an affected part of a patient, for example, the moderator 4 of the Kyoto University method of Patent Document 1 or the moderator 4 of the University of Tsukuba method of Patent Document 2 is used. There is a beam exit port on the downstream side of the beam flow, and if there is a gap between it and the affected area of the patient, part of the therapeutic beam will leak to the surroundings through this gap. Even a small gap causes a considerable amount of neutron leakage, which poses a serious problem.

In this way, if the therapeutic beam leaks from the shielding layer on the outer periphery of the deceleration system, insufficient shielding performance at the beam exit, or from the gap between the beam exit and the patient's affected area, the patient as well as the medical staff will be affected. There is a high risk of human and property damage, such as exposure to radiation, radiation of peripheral equipment and components, and radiation damage to measuring and transmitting equipment, resulting in reduced treatment accuracy.
Prevention of these leaks is an important issue for establishing a safe and stable treatment method.

In addition, in the BNCT device, in order to remove thermal neutrons in the treatment beam for the purpose of preventing side effects from occurring in the patient's outer skin during treatment, thermal neutrons are selectively absorbed so as to traverse the beam flow. Materials with the ability to shield may be installed.
As such materials, "polyethylene containing LiF powder", "cadmium (Cd)", and " ⁶ Li" are known.

As described above, the material "polyethylene containing LiF powder" has a low probability of being absorbed and shielded by colliding with the LiF powder because the LiF is in powder form, and most of the thermal neutrons have the ability to absorb thermal neutrons. Thermal neutrons pass through the polyethylene layer surrounding the LiF powder, which is almost non-existent, resulting in insufficient shielding of thermal neutrons.

In addition, "cadmium (Cd)" is a metal, and if a plate-shaped one is used, it is a high-density shielding material.In addition, as mentioned above, Cd has a large absorption coefficient for thermal neutrons and has high shielding performance. Shielding with Cd generates a large amount of secondary radiation, including gamma rays, and with the generation of a large amount of secondary radiation, the energy of the transmitted therapeutic beam is greatly attenuated, ensuring the neutron flux necessary for treatment. I can't do it.

Furthermore, even if a radiation shielding material such as Pb is installed, all of the gamma rays that are harmful to the human body generated by the shielding by Cd cannot be shielded, and secondary rays such as gamma rays in the therapeutic beam cannot be blocked. Radiation will be mixed, which is also a problem.

When removing thermal neutrons in therapeutic beams, thermal neutron shielding materials that do not generate secondary radiation such as gamma rays should be used. The development of the material is earnestly desired.

In addition, this type of shielding material has a wide range of uses, and various sizes and shapes are required. , it becomes necessary to form the shielding material by applying various types of machining.
For this reason, such a sintered body is required to have high mechanical working strength, that is, to be dense, homogeneous, and less distorted, in other words, to have high density, high Vickers hardness, and high bending strength.
Moreover, there is a demand for a shape having a large area and a sufficient thickness at the baked stage.

Here, once again, the applications of the shielding material used in the BNCT apparatus and the neutron irradiation apparatus for multi-purpose applications are subdivided and the problems in each application are listed.
(1) To prevent neutron leakage from the outer periphery of the moderator system (the outer periphery of the moderator system includes the lateral part of the moderator system and the periphery of the therapeutic beam exit (“collimator” part)).
The shape required for the shielding material used in this location is plate-like, block-like, or tube-like.
In addition, in the sintered body stage, the problem is that it is possible to provide a mass of a large shape so that a mass with a relatively large volume can be obtained.

(2) To prevent the therapeutic beam from leaking from the gap between the therapeutic beam outlet and the patient's affected area.
The exit aperture is generally a circular aperture with a diameter of about 100 mm to 250 mm (in most cases, the diameter is 100 to 150 mm).
On the other hand, the skin shape and size of the patient's affected area vary greatly depending on the location of the cancer lesion.
For example, the shape and size of the outer skin vary greatly depending on the head, back neck, face, and heels of the feet. Moreover, the skin shape of the affected area varies greatly among individuals.

In addition, the irradiation time of the beam in one treatment is about 30 to 60 minutes, and in addition, the preparation time before treatment is required before and after the irradiation time. For patients who are forced to do so, the irradiation time is a length that feels "extremely long".
It must be assumed that the patient's posture changes during irradiation, and the gap between the beam irradiation port and the patient's affected area changes accordingly. It is not easy to efficiently shield these changing and diversely shaped gaps with a fixed-shaped shielding material.
The shape of the sintered body, which is the original material of the shielding material used at this location, is expected to be ring-shaped or plate-shaped with a thickness of several tens of millimeters.

(3) For exposure prevention of control equipment around the BNCT device or other radiation generating devices (specifically, for example, nuclear reactors and accelerators other than BNCT devices).
It is known that peripheral control equipment malfunctions or stops when exposed to high-energy radiation such as leaked neutrons.
Although it is desired to prevent the occurrence of failures due to exposure to radiation in such control devices, the current situation is that no effective countermeasures against these failures have been taken.

Therefore, shielding materials are used to enclose the parts of the above equipment that are prone to failure, such as sensors, control equipment parts, wiring parts, and so-called "light electrical equipment, wiring parts" in the field of electrical technology, so that failures can occur. The challenge is to prevent the
Shielding materials for this application are required to have a large area and a moderate thickness in the shape of a plate, ring, or tube. The challenge is to be able to provide

(4) Use as a thermal neutron shield.
As described above, the irradiation beam of the BNCT apparatus moderates high-energy neutrons generated at the target using various moderators, and moderate- and low-energy neutrons, mainly epithermal neutrons suitable for treatment, and processes after the target and secondary radiation generated in various ways.
Both neutrons other than epithermal neutrons and secondary radiation may cause side effects, are not necessary for treatment, and should be removed.

Known thermal neutron shielding materials (thermal neutron filters) include, for example, those using the above Cd (metal).
Although this shielding material can remove thermal neutrons, it generates a large amount of harmful secondary radiation such as gamma rays.
We are trying to remove these radiations by using the Pb (metal) of the radiation shielding material together with the secondary radiation generated by this shielding material, but the Cd shielding material (thermal neutron filter) emits a large amount of secondary radiation. As a result, radiation shielding becomes insufficient and secondary radiation remains.

Another shielding material is shown in Patent Document 4, which uses ⁶ Li (metal) as the thermal neutron filter 15 and Pb as the radiation shield 16. However, as described above, ⁶ Li is used for security reasons. there is a problem.
In view of this situation, the development of a thermal neutron filter that has remarkable thermal neutron shielding performance and extremely low generation of secondary radiation such as gamma rays that are harmful to the human body has become an issue.

(5) Multi-purpose “accelerator-type” or “nuclear-reactor-type” neutron irradiation equipment that uses low-energy neutrons, mainly thermal neutrons.
In the accelerator method, the neutrons generated at the target (in most cases, the generated neutrons are "fast neutrons") are moderated by a moderator and controlled to the target energy level.
The device part including this target and moderator is called "moderator system", and this moderator system is surrounded by "thermal neutron shielding material" and "γ-ray shielding material" for leakage prevention. there is

However, the shielding by both shielding materials installed around the moderator system is often insufficient, and thermal neutrons and γ-rays are leaked.
"Shielding" against thermal neutrons leaking from both shielding materials installed around this moderator system is an issue.

In the above, the problems related to shielding materials have been described with a focus on the BNCT device, but the following can be mentioned as other facilities that have similar problems regarding neutron shielding materials other than the BNCT device.
(1) "Neutron Experimental Facility" that mainly uses low-energy neutrons (cold neutrons to thermal neutrons),
(2) “research reactors”;
(3) “J-PARC”,
(4) “RANS of RIKEN”,
(5) “Aomori Cyclotron Neutron Experimental Facility”

In view of the current state of the BNCT device development described above, the present inventors first used a fluoride-based raw material containing a high concentration of naturally occurring Li, which has excellent thermal neutron shielding performance and has no security problems, The development of a sintering method capable of producing large shape agglomerates with high density and high mechanical strength was investigated.
Furthermore, with the aim of improving the shielding performance, even if a raw material containing boron (B) and/or gadolinium (Gd) is added to the fluoride-based raw material, and a mixed raw material of them is used, high density and high machinability can be achieved. The development of a sintering method capable of producing strong, large-shaped agglomerates was also investigated.

As a result, we found that the main issues in these are the following three points.
(B) The most important performance for radiation shielding materials, especially for neutron shielding, is to ensure neutron shielding performance first. should be the sintering conditions (in particular, the density level of the sintered body)?
(b) How to secure the machining strength of the agglomerate (sintered body) necessary for obtaining the neutron shielding material.
(c) How to ensure the size and shape of the sintered body necessary for the neutron shielding material.

Patent No. 5112105 Patent No. 5813258 Retable No. 2018-181395 (Patent application No. 2018-517656) Special Table 2018-514325 (Patent Application No. 2017-557373) Japanese Patent Application Laid-Open No. 51-94098

H. Tanaka et al., Applied Radiation and Isotopes 69 (2011) 1642-1645 H. Tanaka et al., Applied Radiation and Isotopes 69 (2011) 1646-1648 Takeshi Hoshino, J.Plasma and Fusion Res.Vol.89,No.1 (2013) 3-10 Fuji Dempa Koki Co., Ltd. website (http://sps.fde.co.jp/jp/whats1.html) manual: "Spark Plasma Sintering (Discharge Plasma Sintering) What's SPS" Masao Tokita, "Present state and future image of ceramic sintering by spark plasma sintering (SPS) method", Ceramics, February 2014, Vol. 49, No. 2, pp. 91-96 H. Kumada, et al., 2020. Evaluation of the characteristics of the neutron beam of a linac-based neutron source for boron neutron capture therapy. Radiat. Isot. , 165, 109246

Means to solve the problem and its effect

The present invention has been made in view of the above problems, for example, to remove radiation species such as neutrons that are unnecessary or harmful to treatment in a therapeutic beam used in a BNCT device, and / or to prevent this therapeutic beam from leaking outside the treatment range, and/or to prevent equipment failure or activation due to the leaked beam outside the treatment range, and/or similarly Radiation that has a precise structure, high mechanical strength, and a size and shape suitable for radiation shielding materials to be installed in order to prevent radiation exposure of medical personnel due to beams and activation. An object of the present invention is to provide a sintered body for a shielding material, a radiation shielding material, and a method for producing the same.

Furthermore, in order to prevent the occurrence of side effects near the patient's outer skin due to treatment, a new alternative to cadmium (Cd) used to reduce or absorb thermal neutrons in the "irradiation beam" before irradiation The purpose is to provide a "thermal neutron shielding material (thermal neutron filter)".

As described above, the "leakage radiation" includes "leakage radiation that leaks out of the system from the deceleration system (perimeter of the deceleration system and the periphery of the exit port)" and "leakage radiation that leaks out of the gap between the exit port (collimator) and the patient. radiation”.

In order to reduce this "leakage radiation", it is first necessary to develop a "shielding material" with excellent shielding performance.
Neutrons adjusted for therapeutic use mainly consist of epithermal neutrons, thermal neutrons and slow neutrons (i.e., cold neutrons), and also include fast neutrons that have not been completely moderated by the moderator.

A property required for the "shielding material" is to have effective shielding performance against neutrons of such a wide range of energy levels.
As described above, the source of the shielding performance is the absorption cross section for neutrons at a specific energy level possessed by specific elements such as hydrogen atoms, nitrogen atoms, ⁶ Li, ¹⁰ B, ¹¹² Cd, and ¹⁵⁷ Gd, or radioactive isotope elements. depends on the size of
Therefore, it is difficult to shield all neutrons in a wide range of energies with a single shielding material.

Therefore, this time, we focused on the development of a so-called "thermal neutron shielding material" that can effectively and reliably shield mainly low-energy level neutrons, that is, thermal neutrons and slow neutrons lower than thermal neutrons.

As described above, naturally occurring Li contains ^7.5 atom% of ^6Li and 92.5atom% of 7Li isotopes, of which ^6Li contains low-energy neutrons such as thermal neutrons. It has a large absorption cross section for level neutrons.
⁶ In order to obtain an “excellent shielding material” by actually using Li, it is necessary to have a high density and high homogeneity (high homogeneity means that the variation in density is small, there is no residual strain, and there are no defects such as cracks). ), it is necessary to develop a method that ensures machining strength and that can stably and inexpensively manufacture large-sized products.

The raw materials are required to be neither dangerous nor deleterious substances, to pose no major problems in handling, to be stable during heating and agglomeration processes, and to have no secular change after agglomeration.

Furthermore, the lump should be highly homogeneous and ensure mechanical working strength.
The reason why "high homogeneity" is necessary is that the higher the density, the higher the shielding performance against radiation such as neutrons, and the less uneven the density, the smaller the uneven shielding performance.

In addition, since shielding materials of various sizes and shapes are required, it is essential to machine the sintered body. If residual stress remains in the sintered body, cracks or fissures are likely to occur during machining.

the main issues mentioned above,
(b) Requirements for ensuring neutron shielding performance,
(b) Requirements for ensuring the mechanical strength required for sintered bodies for neutron shielding materials,
(C) Requirements for ensuring the size and shape required for sintered bodies for neutron shielding materials,
With regard to the three points above, we considered solutions to the problems as follows.

Regarding the main issues (a) and (c), the shielding performance required as a shielding material for radiation, especially for neutrons, depends on the density and size of the shielding material (specifically, is attributed to the relationship between "area" and "thickness") and shielding performance.

Specifically, a shielding material is placed around each moderator in the moderator system to prevent leakage from the moderator to the outside, and to prevent adverse effects of leaked radiation on the measurement device and control device of the BNCT device. It is assumed that a plate-shaped one having a certain amount of area and thickness is required.
Therefore, in the plate-shaped shielding material, in addition to the shielding performance, the shape maintenance performance and the restrictive conditions as the device are considered important.

As for the thickness of the sintered body, the thin limit is several millimeters or more, specifically, in order to secure the shielding performance and to secure the shape maintenance performance so that cracks and cracks do not occur during handling. It was recognized that 2 mm or more was necessary.

The thickness limit is a structural constraint of the BNCT device, for example, for prevention of leakage radiation in the gap between the therapeutic beam irradiation port and the patient's affected area, the energy loss of the therapeutic beam exceeds 100 mm from the irradiation port. It was recognized that it would be large, and it was determined that 100 mm or less was desirable.

However, in the case of the shielding material for preventing leakage of radiation leaking from the outer periphery of the moderator system, the thickness is required to obtain the required shielding performance, and there is no upper limit for the thickness of the shielding material. Only the lower limit of "2 mm or more" was set as the limiting condition for the thickness.

In order to achieve the above object _, a sintered body for radiation shielding material _{( 1} ) according to the present invention contains LiF in the range of 99 wt. , and/or containing one or more fluorides selected from YF ₃ in the range of 1 wt. It is characterized by having physical properties.

First, in order to ensure neutron shielding performance, a Li fluoride, that is, lithium fluoride (LiF) was selected as the main raw material.
In general, it has been considered difficult to sinter LiF alone (synonymous with "single").
Conventionally, in most cases, LiF uses other raw materials as the main raw material. (meaning)” or as a “supplementary ingredient”.

When a raw material of LiF alone is sintered, it is violently vaporized during the sintering process to generate fluorine gas (generate a sublimation phenomenon), and the sintered body foams remarkably to form large cavities inside.
As a result, the sintered body has a low density, and it is impossible to increase the size of the sintered body, so that the intended characteristics cannot be obtained. Thus, there is a limit to securing the shielding performance relying only on the concentration of ⁶ Li.

Therefore, the inventors
(1) Securing the Li necessary for obtaining excellent shielding performance as a LiF raw material based on naturally occurring Li,
(2) LiF is a representative raw material that is difficult to sinter. Therefore, in order to stably and uniformly sinter large objects, other fluorides such as MgF ₂ should be used instead of LiF alone. , CaF ₂ , AlF ₃ , KF, NaF and/or YF ₃ are selected and mixed with LiF to form a multicomponent fluoride sintered body.

Here, the reason why other fluorides were selected as substances to be mixed with LiF is that
If it is a fluoride similar to LiF, it is easy to form a solid solution in the sintering process, and a eutectic point is generated to lower the sintering temperature, resulting in the decomposition and vaporization of fluoride such as LiF (that is, This is because it is considered that the sublimation phenomenon is suppressed, foaming is prevented, and a dense sintered body can be easily obtained.

A LiF- _MgF2 - _CaF2 ternary system in which _MgF2 and _CaF2 are mixed with LiF will be described as an example.
FIG. ₁ shows the equilibrium phase diagram of the LiF--MgF.sub.2-- _CaF.sub.2 ternary system.
In this case, the mixing ratio of LiF 59.0 mol% (that is, 35.7 wt.%), MgF ₂ 13.6 mol% (40.5 wt.%), CaF ₂ 6.4 mol% (23.8 wt.%) ( "B point") is the eutectic point.
The melting point temperature at point B is the lowest value among the isotherms in the drawing showing melting point temperatures at various mixing ratios.

This LiF- _MgF2 - _CaF2 ternary system is a so-called "eutectic ternary equilibrium diagram", and at temperatures below the eutectic point, all component ratios form a solid phase, or a solid phase and a solid solution. are in a state of coexistence.
At a temperature higher than that, all liquid phases or two phases of a liquid phase and a solid solution coexist.

Considering the sintering of LiF alone, the rule of thumb is that "the approximate appropriate range of sintering temperature in solid-phase sintering is about 80% of the melting point of LiF (mp = 847°C), that is, about 670-680°C." It is speculated that
On the other hand, the temperature of the eutectic point in the above equilibrium diagram is 672° C., which is almost the same temperature as the estimated proper sintering temperature of 670 to 680° C. in the above LiF single sintering.

The proper sintering temperature (in solid phase sintering) in this ternary system is much lower than the eutectic point temperature (according to the rule of thumb, the temperature is 10% lower than the eutectic point temperature or more). guessed. This temperature is considerably lower than the sintering temperature for LiF alone, and is expected to have a high possibility of suppressing the decomposition and vaporization of LiF.

Here, in consideration of obtaining high shielding performance, in order to search for the mixing ratio of the above three components in which the mixing ratio of LiF is high, the melting point temperature is low, and the sintering reaction easily proceeds, the following is performed. performed such an operation.

LiF, which is the main raw material, 100 mol %, the lower end vertex (point A) and the eutectic point (point B) are connected by a straight line, and the straight line is extended to connect the vertex of _MgF2 and the vertex of _CaF2 (ridge line) ) is the point C.
A sintering test was conducted while changing the compounding ratio on the straight line connecting A-C, the quality properties of the test samples were examined, and raw material compounding conditions, sintering conditions, etc. suitable for this application were determined.

As a result of the test, it was found that if the mixing ratio of LiF is about 5 wt. % to 70 wt.
However, even in the binary system sintering method, when the LiF mixing ratio exceeds 70 wt. .

The reason why the lower limit of the mixing ratio of LiF is set to 5 wt.

The present inventors selected two or more types from LiF and other fluorides (e.g., MgF ₂ , CaF ₂ , AlF ₃ , KF, NaF and/or YF ₃ ), and sintered ternary or higher multicomponent systems. When sintered by the method,
It was found that significant foaming can be suppressed even when the mixing ratio of LiF exceeds 70 wt.%.

However, even in a ternary or higher multicomponent sintering method, if the LiF mixing ratio exceeds 99 wt. It is recognized that the sintered body will foam, and metal parts and members in the sintering furnace will react with the fluorine gas and contaminate the inside of the sintering furnace and the sintered body. was there.
Therefore, it was determined that 99 wt.

In the ternary or higher multicomponent sintering method, the eutectic reaction is promoted compared to the binary sintering method of LiF alone and LiF and other fluorides, and the sintering temperature is low. It was found that significant foaming can be suppressed if the mixing ratio of LiF is 99 wt.% or less.
Based on these test results, it was determined that the mixing ratio of LiF is proper in the range of 5 wt.% to 99 wt.%.

In addition, in order to exhibit excellent shielding performance, it is necessary to reduce as much as possible air bubbles and voids in the sintered body that hinder the shielding of radiation.
Bubbles and voids are zero in a sintered body in a true density state (that is, 100% relative density), and the relative density decreases as they increase.

The inventors of the present invention have found by investigation that between the relative density of about 100% and about 95%, there is not much difference in shielding performance compared to the relative density of 100%, and the difference in shielding performance is observed from about 94%. It started to appear slightly, and a decrease in shielding performance was observed from about 92%, and it was found that a significant decrease was observed at 90% or less.
Therefore, it was determined that the relative density should be at least 92% or more, preferably 94% or more.

The relative density of the multicomponent sintered body is obtained by first multiplying the true density of each compound by its compounding ratio, and adding the resulting value (that is, the weighted average value) as the "true density" of the multicomponent sintered body. It was calculated by dividing the "bulk density" calculated by dividing the weight of the sintered body by its bulk volume by its "true density".

For example, the blending ratio of the ternary sintered body on the straight line based on the above-mentioned ternary equilibrium diagram (Fig. 1) is LiF: 98.8 wt.%, _MgF2 : 0.8 wt.%, _CaF2 : 0 ^{In the} _case of _.4 ^wt . The value obtained by multiplying and adding is "2.646 g/cm ³ ".

According to the sintered body for radiation shielding material (1), a sintered body for radiation shielding material containing Li having a large absorption cross section for neutrons at a high concentration and being excellent for radiation, especially low energy neutrons, is provided. can do.
Moreover, it is possible to stably and inexpensively provide an excellent shielding material with a high density, high homogeneity, sufficient mechanical strength, and a large shape.
Therefore, it is possible to provide a sintered body for radiation shielding materials that can be applied to products of various sizes and shapes.

Regarding the concentration of the isotope ⁶ Li, even when a ⁶ Li-enriched form is used, sintering by the method of the present application provides a sintered body with sufficient physical properties such as density and mechanical strength. We were able to confirm that the sintered body would have a much higher neutron shielding performance than the sintered body using the Li raw material produced from natural sources. (Abundance ratio of ⁶ Li in naturally occurring Li is 7.5 atom %).

In addition, the sintered body for radiation shielding material (2) according to the present invention is the multicomponent fluoride containing LiF as a main phase described in the sintered body for radiation shielding material (1), and further contains B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B ₄ O ₇ is added externally as the boron isotope ¹⁰ B in a proportion of 0.1 to 5 wt. , relative density of 92% or more, bending strength of 40 MPa or more, and Vickers hardness of 80 or more.

Furthermore, in the present application, Li, which has a large absorption cross section for neutrons, contains B (isotope ¹⁰ B), which has a larger absorption cross section than Li, at a high concentration, and has a high density, high homogeneity, and sufficient machining. To stably provide an excellent shielding material with strength and a large shape at a low cost.

Since there is a limit to improving the neutron shielding performance only with the multi-component fluoride sintered body mainly composed of the LiF raw material based on the above-mentioned naturally occurring Li, a raw material containing ¹⁰ B as another substance exhibiting shielding performance. was added to form a sintered body.
The ¹⁰ B abundance ratio in naturally occurring B (this is referred to as “natural abundance ratio” in the technical field) is 19.9 atom%, and the remaining 80.1 atom% is occupied by the stable phase ¹¹ B. there is

The boron compound was added to the multi-component fluoride raw material at 0.1 to 5 wt.% as ¹⁰ B as an external addition. On the other hand, if the addition exceeds 5 wt.%, the density of the sintered body will fall below a predetermined value (92% or more). This is because it has been confirmed by tests that the density is further lowered and it becomes difficult to maintain the shape of the sintered body.
Based on the test results, the appropriate range of addition of the boron compound was determined to be 0.1 to 5 wt.% of ¹⁰ B.

According to the radiation shielding material sintered body (2), since it contains a high concentration of B (isotope ¹⁰ B), which has a larger absorption cross section for neutrons than Li, it further has excellent radiation shielding performance against neutrons. A sintered body for a shielding material can be provided.

Further, the sintered body for radiation shielding material (3) according to the present invention is the multicomponent fluoride having LiF as a main phase described in the sintered body for radiation shielding material (1), and further includes Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ is added as gadolinium isotope ¹⁵⁷ Gd at a rate of 0.1-2 wt. It is characterized by physical properties such as a strength of 40 MPa or more and a Vickers hardness of 80 or more.

In the present application, it further contains Li, which has a large absorption cross section for neutrons, and Gd ( ¹⁵⁷ Gd), which has a larger absorption cross section than Li, at a high concentration, and has a high density, high homogeneity, and sufficient machining strength. To stably and inexpensively provide an excellent shielding material with a large shape.

The reason why the gadolinium compound is added to the multicomponent fluoride raw material as ¹⁵⁷ Gd in an amount of 0.1 to 2 wt. On the other hand, if the addition exceeds 2 wt.%, the density of the sintered body will fall below a predetermined value (92% or more). This is because it has been confirmed by tests that the density of the sintered body is further lowered, making it difficult to maintain the shape of the sintered body.
Based on the test results, the appropriate range of addition of the gadolinium compound was determined to be 0.1 to 2 wt.% addition of ¹⁵⁷ Gd.

According to the radiation shielding material sintered body (3), since it contains a high concentration of Gd ( ¹⁵⁷ Gd), which has a larger absorption cross section for neutrons than Li, it further has excellent radiation shielding performance against neutrons. It is possible to provide a sintered body for materials.

Further, the sintered body for radiation shielding material (4) according to the present invention is the multi-component fluoride containing LiF as a main phase described in the sintered body for radiation shielding material (1), and further includes B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B ₄ O ₇ is added externally as the boron isotope ¹⁰ B in a proportion of 0.1 to 5 wt. Furthermore, a gadolinium compound selected from among Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ is added externally as gadolinium isotope ¹⁵⁷ Gd in a proportion of 0.1 to 2 wt. It is characterized by having physical properties such as a density of 92% or more, a bending strength of 40 MPa or more, and a Vickers hardness of 80 or more.

In the present application, it further contains Li, which has a large absorption cross section for neutrons, B ( ¹⁰ B isotope), and Gd ( ¹⁵⁷ Gd), which has a larger absorption cross section than Li, at a high concentration and at a high density. To stably and inexpensively provide an excellent shielding material with high homogeneity, sufficient mechanical strength, and a large shape.

In order for the sintered body to obtain the necessary shielding performance, it is desirable to have a high density and a high concentration of the isotopes ⁶ Li, ⁶ Li plus ¹⁰ B and ¹⁵⁷ Gd.

According to the radiation shielding material sintered body (4), since it contains B (isotope ¹⁰ B) and Gd (isotope ¹⁵⁷ Gd) at high concentrations, which have a larger neutron absorption cross-section than Li, A sintered body for a radiation shielding material having excellent neutron shielding performance can be provided.

Here, for the purpose of improving radiation shielding properties, a multicomponent sintered body obtained by adding a boron compound containing ¹⁰ B and/or a gadolinium compound containing ¹⁵⁷ Gd to a multicomponent fluoride containing LiF. I will describe how to calculate the relative density of .
The relative density of the sintered body is calculated by dividing the "mass" obtained by measuring the weight of the sintered body by the "bulk volume" calculated from the external dimensions, as shown in the following formula (1). A method of calculating by dividing the "bulk density" by the "true density" calculated by the method described below.

Relative density (%) of sintered body = [bulk density (g/cm3)] x 100 / [true density (g/cm3)] (1)

However, the value of [true density] was calculated by adding the density value of each composition of the multicomponent sintered body by the mixing ratio of each composition.
Further, when a boron compound containing ¹⁰ B and/or a gadolinium compound containing ¹⁵⁷ Gd is added, the natural abundance ratio of ¹⁰ B and ¹⁵⁷ Gd (the natural abundance ratio of ¹⁰ B is 19.9 atom%, ¹⁵⁷ Gd The natural abundance ratio is 15.65 atom%), or the enrichment ratio when ¹⁰ B is enriched is restored to the original element (i.e., B, or, and Gd), and the addition ratio of each compound is added and calculated. method.
Incidentally, as will be described later, for each [Example] and [Comparative Example], the values of [True Density] and [Relative Density] calculated by the above method are displayed.

Further, the radiation shielding sintered body (5) according to the present invention is characterized in that, in any one of the radiation shielding sintered bodies (1) to (4), the radiation is a neutron beam.
According to the sintered body for radiation shielding material (5), it is possible to provide a sintered body for radiation shielding material having extremely excellent neutron shielding performance.

Further, the radiation shielding material (1) according to the present invention is characterized by being formed by machining any one of the radiation shielding material sintered bodies (1) to (5). .
All of the above sintered bodies for radiation shielding materials ensure high mechanical working strength, and according to the above radiation shielding material (1), a radiation shielding material having a desired shape can be easily obtained.

Further, in the radiation shielding material (2) according to the present invention, the thickness of the shielding material formed by machining the sintered body in the radiation field in the radiation shielding material (1) is 100 mm or less. , the value obtained by dividing the thermal neutron flux (N1) emitted from the shielding material by the thermal neutron flux (N0) incident on the shielding material, that is, the thermal neutron attenuation rate (N1/N0) is 1/100 or less. It is characterized by having neutron shielding performance.

As a structural limitation of the BNCT apparatus, for example, in order to prevent leakage radiation in the gap between the therapeutic beam irradiation port and the patient's affected area, it is recognized that the energy loss of the therapeutic beam increases when the distance exceeds 100 mm from the irradiation port. It was decided that the following would be desirable:

According to the radiation shielding material (2), it is possible to provide a radiation shielding material that is thin enough to prevent the energy loss of the therapeutic beam from increasing and yet has excellent thermal neutron shielding performance.

In addition, the method (1) for producing a sintered body for a radiation shielding material according to the present invention includes:
high-purity LiF raw material,
and one or more fluoride raw materials selected from MgF ₂ , CaF ₂ , AlF ₃ , KF, NaF, and/or YF ₃ all of which are of high purity,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of forming a sintered body by heating in the same normal pressure and atmosphere as in the previous step at a temperature range of 570 to 800 ° C. (secondary sintering step);
It is characterized by having

In general, it has been considered difficult to sinter LiF alone.
When a raw material of LiF alone is sintered, it is violently vaporized during the sintering process to generate fluorine gas (generate a sublimation phenomenon), and the sintered body foams remarkably to form large cavities inside.
As a result, the sintered body has a low density, and it is impossible to increase the size of the sintered body, so that the intended characteristics cannot be obtained.

The fundamental reason why LiF, which is the main element for obtaining shielding properties, is difficult to sinter is that Li and F are easily decomposed by heating during the sintering process of LiF, and the vaporization of F proceeds, resulting in the generation of many bubbles. It is in.
Close observation of this phenomenon reveals that the temperature at which Li and F begin to decompose is relatively low. It was speculated that the degree of densification of the sintered body is determined by whether the sintering temperature exceeds or falls below the temperature at which decomposition begins.

The first step in the idea of the present invention is the eutectic reaction between LiF and other fluorides such as CaF ₂ and MgF ₂ in a binary system sintering method (by making the fluorides a binary system). , the eutectic point is generated, and the sintering temperature can be lowered in the binary system sintering than in the sintering temperature with a single raw material). rice field.

Therefore, instead of sintering a raw material of LiF alone, the present inventors devised MgF ₂ and CaF ₂ , which are the same fluoride-based raw materials that are expected to be mixed with LiF raw material and easily sintered. , AlF ₃ , KF, NaF and/or YF ₃ .

Regarding the main issue (b) above, it is about securing the machining strength of the block (sintered body) necessary to obtain an excellent shielding material. As shown in FIG. 3, various factors affecting the securing of the mechanical working strength of the sintered body are mainly governed by density, residual stress and mineral structure.
In a good state of density and mineral structure, it can be said that it is determined simply by the magnitude of "residual stress".

As described in the section of Patent Document 3 above, the “discharge plasma sintering (SPS) method” has an extremely large residual stress, and in the case of “pressure sintering methods such as hot press method and HIP method” However, like the SPS method, there was a concern that a large residual stress would be generated.

In addition, the mechanical working strength required for the sintered body used in the above-mentioned wide variety of applications is not only the "Vickers hardness" that indicates compression resistance in one direction, but also the "bending strength" that is an index of impact resistance. There is a need to.
In addition, a method for manufacturing this large lump (sintered body) inexpensively, stably, and of high quality as described above is the sintering method. It was considered desirable to use the pressure sintering method as a base and, if necessary, to pressure-sinter the sintered body obtained by the normal pressure sintering method.
For this reason, in the present invention, at least the primary sintering process and the secondary sintering process adopt the pressureless sintering method.
As a result of such research on sintering methods, the "residual stress" inside the sintered body has been completely eliminated, and there is no need to worry about securing mechanical strength, resulting in a very stable and good product.

According to the method (1) for producing a sintered body for a radiation shielding material, it is possible to provide a radiation shielding material, particularly an excellent shielding material for low-energy neutrons.
More specifically, it is possible to stably and inexpensively provide a high-density, large mass containing Li, which has a large absorption cross section for neutrons, at a high concentration.

Furthermore, it is possible to provide a sintered body for a radiation shielding material that has high homogeneity, that is, has little unevenness in density, has extremely little residual stress, and is excellent in ensuring mechanical strength, It is possible to provide a sintered body for radiation shielding materials that can be easily applied to products of various sizes and shapes.

In addition, the method (2) for producing a sintered body for a radiation shielding material according to the present invention includes:
high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B, each of which consists of a natural boron raw material and/or a boron raw material enriched with the isotope ¹⁰ B as its boron (B) source. A boron compound raw material selected from ₄ O ₇ ,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of forming a press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP forming step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of forming a sintered body by heating in the same normal pressure and atmosphere as in the previous step at a temperature range of 570 to 800 ° C. (secondary sintering step);
It is characterized by having

Raw materials containing ¹⁰ B include, for example, boron oxide (B ₂ O ₃ ), boric acid (B(OH) ₃ ), lithium triborate (LiB ₃ O ₅ ) or tetraboric acid (all of which are of high purity). Using lithium (Li ₂ B ₄ O ₇ ), apart from the multicomponent fluoride raw material, the boron compound containing ¹⁰ B is also pulverized to a predetermined particle size similar to the multicomponent fluoride raw material, and then , 0.1 to 5 wt.% of ¹⁰ B is added (that is, 0.1 to 5 wt.% of a boron compound as ¹⁰ B is added to the multi-component fluoride raw material by external coating), mixed for a predetermined time, and further pulverized. did. A predetermined amount of pure water was added to this mixed raw material powder, and the mixture was kneaded for a predetermined time to obtain a starting raw material.

Here, the reason why the multi-component fluoride raw material and the boron compound were individually finely pulverized, then mixed together, and then finely pulverized is that in a preliminary fine pulverization test of the raw materials, it was found that each raw material was finely pulverized. This is because it has been confirmed that the method is different.

When boron (B) is added to the multi-component fluoride raw material, the use of the ¹⁰ B-enriched type is technically satisfactory and can be used in the sintering method of the present application.
In BNCT, the ¹⁰ B-enriched form is used as the boron drug pre-administered to the patient immediately before treatment. This use of BNCT has been exempted from security regulations for some time, and there is no problem with its use.

In the examples of the present application, the ¹⁰ B-enriched type was used in only one example, and in the other boron-added examples, natural boron raw materials were used in consideration of exporting the products overseas.

According to the method (2) for producing a sintered body for a radiation shielding material, since B (isotope ¹⁰ B), which has a larger absorption cross section for neutrons than Li, is contained at a high concentration, furthermore, the shielding performance against neutrons is improved. An excellent sintered body for radiation shielding material can be provided.

In addition, the method (3) for producing a sintered body for a radiation shielding material according to the present invention includes:
high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and a gadolinium compound raw material selected from Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ , all of which are of high purity, wherein the gadolinium (Gd) source is a natural gadolinium raw material,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
It is characterized by having

In the present application, a method of adding a raw material containing ¹⁵⁷ Gd as a substance exhibiting shielding performance and sintering was devised. The "natural abundance" of ¹⁵⁷ Gd in naturally occurring Gd is 15.65 atom %.

As a raw material containing ¹⁵⁷ Gd, for example, gadolinium oxide (Gd ₂ O ₃ ), gadolinium hydroxide (Gd(OH) ₃ ) or gadolinium fluoride (GdF ₃ ) (all of which are of high purity) is used, and a multicomponent system Apart from the fluoride raw material, this gadolinium compound containing ¹⁵⁷ Gd is also pulverized to a predetermined particle size similar to the multicomponent fluoride raw material, and then 0.1 to 2 wt.% of ¹⁵⁷ Gd is added (that is, 0.1 to 2 wt.% of a gadolinium compound as ¹⁵⁷ Gd was added to the multicomponent fluoride raw material, mixed for a predetermined time, and then pulverized.
A predetermined amount of pure water was added to this mixed raw material powder, and the mixture was kneaded for a predetermined time to obtain a starting raw material.

Here, the reason why the multi-component fluoride raw material and the gadolinium compound were individually finely pulverized, then mixed together, and further finely pulverized is that in a preliminary fine pulverization test of the raw materials, it was found that each raw material was pulverized. This is because it has been confirmed that the method is different.

According to the method (3) for producing a sintered body for a radiation shielding material, since it contains a high concentration of Gd ( ¹⁵⁷ Gd), which has a larger absorption cross section for neutrons than Li, it has excellent neutron shielding performance. It is possible to provide a sintered body for a radiation shielding material.

In addition, the method (4) for producing a sintered body for a radiation shielding material according to the present invention includes:
high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B, each of which consists of a natural boron raw material and/or a boron raw material enriched with the isotope ¹⁰ B as its boron (B) source. A boron compound raw material selected from ₄ O ₇ and a gadolinium (Gd) source consisting of a natural gadolinium raw material, all of which are selected from among high-purity Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ Gadolinium compound raw material,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
It is characterized by having

According to the method (4) for producing a sintered body for a radiation shielding material, B (isotope ¹⁰ B) and Gd (isotope ¹⁵⁷ Gd), which have a larger absorption cross-section for neutrons than Li, are contained at high concentrations. Furthermore, it is possible to provide a sintered body for radiation shielding material having excellent neutron shielding performance.

Further, the method (5) for producing a sintered body for a radiation shielding material according to the present invention is the method (2) for producing a sintered body for a radiation shielding material or the method (4) for producing a sintered body for a radiation shielding material. ,
It is characterized by adding the boron compound raw material as boron isotope ¹⁰ B in an amount of 0.1 to 5 wt. there is

According to the method (5) for producing a sintered body for a radiation shielding material, it is possible to stably and inexpensively provide a sintered body for a radiation shielding material having excellent neutron shielding performance.

Further, the method (6) for producing a sintered body for a radiation shielding material according to the present invention is the method (3) for producing a sintered body for a radiation shielding material or the method (4) for producing a sintered body for a radiation shielding material. ,
0.1 to 2 wt.% of the gadolinium compound raw material is added as the gadolinium isotope ¹⁵⁷ Gd in the natural gadolinium raw material to the multicomponent fluoride raw material composed of LiF and fluorides other than LiF. It is characterized by

According to the method (6) for producing a sintered body for a radiation shielding material, it is possible to stably and inexpensively provide a sintered body for a radiation shielding material having excellent neutron shielding performance.

Further, in the method (7) for producing a sintered body for a radiation shielding material according to the present invention, after the secondary sintering step described in the method (1) for producing a sintered body for a radiation shielding material, Alternatively, it is characterized by comprising a process (hot press process) of heat-pressing at a temperature range of 450 to 700° C. and a uniaxial molding pressure of 0.05 MPa or more in an inert gas atmosphere at normal pressure.

When the pressure method is used from the first sintering, the degree of sintering progresses greatly depending on the part of the sintered body (for example, the progress of sintering progresses fastest in the outer peripheral part of the sintered body, while the progress in the inner part slowest), and if a constant pressurizing force is applied to each part in such a state, it causes a large residual stress.
On the other hand, a homogeneous sintered body that has been once sintered by the pressureless sintering method, even if pressure sintering is subsequently performed, the sintered body will remain the same as in the case of performing pressure sintering from the first sintering described above. No large residual stress is generated inside, and only a small amount of stress is generated.
This is a great advantage when pressure sintering is performed after normal pressure sintering.

According to the method (7) for producing a sintered body for a radiation shielding material, the sintered body is further improved in homogeneity, has less density unevenness, has extremely low residual stress, and has an excellent mechanical strength. It is possible to provide a sintered body for a radiation shielding material, which can be applied to products of various sizes and shapes.

Further, a method (8) for producing a sintered body for a radiation shielding material according to the present invention is any one of the methods (1) to (7) for producing a sintered body for a radiation shielding material, wherein the radiation is a neutron beam. It is characterized by a

According to the method (8) for producing a sintered body for a radiation shielding material, it is possible to provide a radiation shielding material, particularly an excellent shielding material for low-energy neutrons.

Further, the method (1) for producing a radiation shielding material according to the present invention is the production of the sintered body for a radiation shielding material according to any one of the above methods (1) to (8) for producing a sintered body for a radiation shielding material. The radiation shielding material is formed by further subjecting the sintered body for radiation shielding material manufactured by the method to machining.

According to the method (1) for manufacturing a radiation shielding material, it is possible to easily provide radiation shielding materials having various sizes and desired shapes.

FIG. 2 is a diagram showing an example of a LiF—MgF ₂ —CaF ₂ ternary system equilibrium diagram and raw material mixing ratios in the same ternary system raw material. FIG. 2 is a factor analysis diagram that analyzes the relationship between the manufacturing process of the shielding sintered body and the product performance. FIG. 2 is a relational diagram showing the relationship between physical properties of a sintered body and physicochemical factors that affect machining. 1 is a flow chart showing a manufacturing process when adding various fluoride raw materials and further boron-based raw materials or gadolinium-based raw materials. 2 is a graph showing the results of a small-scale sintering test using Li ₂ B ₄ O ₇ as a boron compound to be added to the LiF—MgF ₂ —CaF ₂ ternary system. 2 is a graph showing the results of a compact sintering test using Gd ₂ O ₃ as a gadolinium compound added to a LiF—MgF ₂ —CaF ₂ ternary system. FIG. 10 is a graph showing the results of a simulation performed on shielding performance of shielding materials obtained by machining various sintered bodies obtained in a small-sized sintering test. FIG. 1 is a table showing raw materials and sintering conditions in Examples and Comparative Examples, and evaluation results of physical properties of sintered bodies, machining strength, neutron shielding performance, and the like.

BEST MODE FOR CARRYING OUT THE INVENTION A sintered body for a radiation shielding material, a radiation shielding material, and a method for producing the same according to the present invention will be described below with reference to the drawings.
In order to manufacture a sintered body for a radiation shielding material according to the embodiment, a multicomponent fluoride containing LiF is used as a raw material, and/or a boron compound containing ¹⁰ B is added to the multicomponent fluoride. and/or a compound obtained by adding a gadolinium compound containing ¹⁵⁷ Gd to the multicomponent fluoride.

In order to manufacture a sintered body for a radiation shielding material according to the embodiment, first, a high-purity (98.5 wt.% or higher purity) LiF powder as a raw material and a high-purity (99.9 wt.% or higher purity) LiF powder and one or more fluoride powders selected from MgF ₂ , CaF ₂ , AlF ₃ , KF, NaF, and/or YF ₃ are weighed out.

The raw material was pulverized by inserting alumina balls (φ5 mm: 1800 g, φ10 mm: 1700 g, φ20 mm: 3000 g, φ30 mm: 2800 g) into an alumina rotating mill (inner diameter: 280 mm, length: 400 mm) as a ball mill. 3,000 g of raw material for pulverization is put in and pulverized by rotating for a predetermined period of time.
Other pulverization methods, such as "bead mill pulverization method", "dynamic pulverization method", etc., use "medium agitation fine pulverization method" in which a medium such as alumina is agitated and finely pulverized together with the raw material to be pulverized. You may

FIG. 4 shows a process flow for adding a boron compound to this multicomponent fluoride.
The boron compound is selected from high _- purity (99.5 wt.% or higher) _B2O3 , _{B (} OH) _{3 , LiB3O5} or _Li2B4O7 . Also, the boron source is naturally occurring boron and/or enriched with the ¹⁰ B isotope of naturally occurring boron.
After the multicomponent fluoride raw material and the boron compound raw material to be added are separately pulverized (primary pulverization) for 2 weeks by the following pulverization method, predetermined amounts of each are weighed and mixed overnight using a V-type mixer. carry out the work;
Further, the mixed raw materials are pulverized (secondary pulverization) for one week by the following pulverization method.
The median diameter of the average particle size after pulverizing for 2 weeks was 8 μm or less for all raw materials, regardless of the type of raw material.
After the mixture was ground for one week, the average particle diameter was 6 μm or less in terms of median diameter.

FIG. 4 shows the process flow for adding a gadolinium compound to the multi-component fluoride, as in the case of adding the boron compound.
The gadolinium compound is selected from among _{high-purity (99.9 wt.% or higher) Gd2O3, Gd(OH)3 or GdF3 .} Also, the gadolinium source is natural gadolinium.
As in the case of adding a boron compound, the multicomponent fluoride raw material and the gadolinium compound raw material to be added are separately pulverized (primary pulverization) for 2 weeks by the following pulverization method, and then weighed and taken in predetermined amounts. Mixing work was carried out all day and night using a V-type mixer.
Further, the mixed raw material was pulverized (secondary pulverization) for one week by the following pulverization method.

3 wt.% of pure water was added to these finely pulverized raw materials, and the mixture was kneaded for 12 hours using a kneader to obtain starting raw materials (raw material blending step).
Here, the reason why pure water is added to the mixed raw material after the secondary pulverization is to maintain the shape of the molded body in the subsequent uniaxial press molding process, CIP molding process, and between both molding processes.
Generally, a sintering aid is often used to maintain the shape, but if the sintering aid remains after sintering, it becomes an impurity, which may have a large effect on the neutron shielding performance. Therefore, pure water was used here.
Preliminary tests have revealed that the range of the amount of pure water to be added for shape retention is 1 wt.% or more and 5 wt.% or less. Furthermore, it has been found that a range of 2 wt.% or more and 4 wt.% or less is a more desirable range.

The starting material is filled in a wooden mold and molded using a hydraulic uniaxial press molding machine at a molding pressure of 5 MPa or more, preferably 20 MPa or more (uniaxial press molding step).
Here, the reason why the uniaxial press molding is performed is to enable the maintenance of the shape of the compact between the next CIP process and within the next CIP process.
The shape of the mold used in this uniaxial press molding process and the shape of the press plate that applies press pressure from above are square, round, ring, etc. in plan view, so that the shape in plan view of the molded product can be varied. It can be of any shape.

Furthermore, for example, by making it a ring shape in plan view and a horizontal concave shape in cross section, it is suitable for closing the gap between the affected part of the patient and the beam exit of the BNCT apparatus. A sintered body having a shape that is thin on the side and thick on the outer diameter side can also be used.

The press-molded product is placed in a plastic bag and sealed, the air inside is sucked to remove the air inside the plastic bag, and the plastic bag is sealed again. After the sample loading section is closed and sealed, clean water is filled between the sample loading section and the press molded product in the plastic bag.
Thereafter, a molding pressure of 5 MPa or more, preferably 20 MPa or more is applied to the filled clean water (that is, the sample is pressed through the vinyl) to mold. This makes it possible to maintain the shape of the press-formed product between and within the following steps (CIP forming step).

The CIP molded product is pre-sintered by heating in the air atmosphere at a temperature range of 350 to 470° C. (pre-sintering step).
In this temporary sintering step, vaporization and vaporization of the water contained in the raw material and added pure water and solid-phase reaction between the raw material particles are promoted.
The above temperature range is set because if the heating is less than 350°C, the vaporization and evaporation of the moisture is too slow, and if the heating is over 470°C, the solid-phase reaction becomes too fast and the vaporization and evaporation of the moisture is delayed. This is because it is insufficient.

Next, the preliminary sintered body is sintered by heating in a temperature range of 480 to 560° C. in an air atmosphere at normal pressure or in an inert gas atmosphere at normal pressure (primary sintering step).
Heating at 480° C. or higher accelerates the solid-phase reaction, and heating at 560° C. or lower suppresses the sublimation reaction of the raw material.
Subsequently (that is, the meaning of “continuously heating without cooling once”), the sintered body is formed by heating at a temperature range of 570 to 800 ° C. in the same atmospheric pressure and the same atmosphere as in the previous step (second next sintering step).
By setting the temperature within this range, a solid solution is generated and an excessive sintering reaction is suppressed.
If necessary, after the secondary sintering step, the sintered body (that is, the sintered body obtained by normal pressure sintering) is heated at 450 to 700 ° C. in a vacuum atmosphere or an inert gas atmosphere. Pressure molding is performed at a molding pressure of 0.05 MPa or more while heating within a temperature range (hot press step).

The reason why the heating temperature range is set to 450 to 700° C. in this hot pressing process is that if the temperature is less than 450° C., the viscosity of the sintered body becomes too high and the viscous flow slows down, making smooth hot compacting impossible. On the other hand, if the temperature exceeds 700° C., the sintered body and the forming frame (mold) react violently and hot forming cannot be performed.
The reason why the uniaxial molding pressure is set to 0.05 MPa or more is to make the molding pressure suitable for the viscous flow characteristics of the sintered body in the above-described heated state.

Further, if necessary, the sintered body (a sintered body obtained by normal pressure sintering, or a sintered body obtained by subjecting the sintered body obtained by normal pressure sintering to a hot pressing step) is machined (machining step).
The above sintered body (a sintered body obtained by normal pressure sintering, or a sintered body obtained by subjecting the sintered body obtained by normal pressure sintering to a hot press step, or a sintered body thereof machined) is used as a shielding material for neutron beams.

The specific uses of the neutron beam shielding material for this sintered body alone are the first (prevention of neutron leakage from the outer periphery of the moderator system) and the third (control equipment around the radiation generator). (for malfunction and failure prevention).

[Small sintering test]
As a preliminary test prior to the following examples, a small sintered body with approximate dimensions of 75 mm in diameter and 60 mm in thickness was evaluated for neutron shielding performance by Monte Carlo transport analysis, which will be described later.
Here, the reason why the approximate dimensions of the sintered body in the small-sized sintering test were set to 75 mm in diameter and 60 mm in thickness is that if these dimensions are increased (generally referred to as "scale-up"), This is because it was found in the test in advance that the result of the sintering test was similar to the result of the small-sized sintering test.
From this evaluation result, the thermal neutron shielding performance of the sintered body in which a boron compound or a gadolinium compound is added to the LiF _{- MgF2 - CaF2} ternary system is It was found to be even more improved than the body and to have extremely excellent shielding performance.

When a boron compound is added to this LiF- _MgF2 - _{CaF2 ternary system, the above four boron compounds, namely boron oxide B2O3, boric acid B(OH)3 , lithium} triborate _{LiB3O 5} or lithium tetraborate Li ₂ B ₄ O ₇ was individually added in a predetermined amount, and the sintering temperature was changed in the temperature range of 400° C. to 800° C. to prepare sintered bodies. The maximum value of the relative density of these sintered bodies and the sintering temperature at which the maximum value was obtained were investigated.
As a result, the maximum relative density was in the order of Li ₂ B ₄ O ₇ ≈LiB ₃ O ₅ >B ₂ O ₃ ≈B(OH) ₃ .

It was also found that Li ₂ B ₄ O ₇ is the most suitable boron compound to be added to this LiF—MgF ₂ —CaF ₂ ternary system.
As shown in FIG. 5, it was found that the sintered body to which Li ₂ B ₄ O ₇ was added had a good relative density equivalent to that of the high-density ternary system.
Thus, the most suitable boron-based additives were found to be Li ₂ B ₄ O ₇ and LiB ₃ O ₅ , followed by B ₂ O ₃ and other boron compounds. did.
The difference in sinterability (specifically, relative density) of the sintered body between the two lithium borate-based boron compounds and the other two boron compounds is due to lithium borate The melting point of the boron compound in the system is about 900°C, which is higher than the sintering temperature, and vaporization and decomposition do not occur during the sintering process, while boron oxide B ₂ O ₃ and boric acid B(OH) ₃ do not. It is presumed that this is because the melting point of is low and vaporization and decomposition are likely to occur.

Originally, the amount of ternary Li element, which is the source of the shielding performance, is diluted by the addition of the boron compound according to the addition ratio, and the shielding performance derived from Li is reduced accordingly. However, the sintered body to which Li ₂ B ₄ O ₇ is added has the advantage in terms of shielding performance because the Li element derived from Li ₂ B ₄ O ₇ provides “replenishment of Li element”. presumed to have happened.

Similarly, when a gadolinium compound is added to the LiF- _MgF2 - _CaF2 ternary system, the above _{three gadolinium compounds, that is, Gd2O3, Gd(OH)3 , or} GdF3 _, are added separately. A small-sized sintering test was performed in the same manner as above after adding a constant amount.
As an example of the results, FIG. 6 shows the case of adding Gd ₂ O ₃ .
By adding a gadolinium compound, a sintered body with a high density equivalent to that of a ternary system was obtained, exhibiting good sinterability. Moreover, no clear difference was observed in the relative density values due to the addition of individual compounds of different types.
Based on these preliminary small-scale sintering test results regarding the LiF—MgF ₂ —CaF ₂ ternary system and the addition of a boron compound or gadolinium compound to the same ternary system, next, a large-scale apparatus It was decided to implement the example.

[Example]
EXAMPLES The present invention will be described in more detail below based on examples, but these examples are merely examples, and the present invention is not limited to these examples.

Various characteristic evaluation tests were conducted using samples taken from the sintered body. FIG. 8 shows the sintering conditions of the sintered body and the results of the characteristic evaluation test of the sintered body.
Here, the characteristic evaluation test method for the sintered body will be described.
Bending strength and Vickers hardness were used as evaluation indices for mechanical strength.
Preparatory samples for bending strength were prepared according to JIS C2141. The sample size was 4 mm×46 mm×t3 mm, and the upper and lower surfaces were optically polished. A three-point bending test was performed on this prepared sample in accordance with JIS R1601.
The Vickers hardness was measured using a product name "Micro Hardness Tester" manufactured by Shimadzu Corporation, pressing an indenter with a load of 100 g and a load time of 5 seconds, measuring the diagonal length of the indentation, and converting the hardness as follows.
Hardness = 0.18909 x P/(d) ²
where, P: load (N), d: diagonal length of indentation (mm)

The bending strength and Vickers hardness of the sintered body are not determined only by the density of the sintered body (that is, the relative density). Specifically, as shown in FIG. It is determined by factors such as "mineral texture", "residual stress", and "bubbles".
Therefore, if the component system is different, the numerical limits of bending strength and Vickers hardness will be different. Here, the former is composed of the same fluoride-based compound, the latter is a mixture system of fluoride and other compounds, and the former mixture of the same fluoride-based compounds has higher mechanical strength. A trend was observed.

Regarding the evaluation of shielding performance, the BNCT-related development team has traditionally used a particle/heavy ion transport calculation code (called “PHITS”)
We are designing and manufacturing a neutron generator and evaluating the shielding performance of the facility by the Monte Carlo transport analysis described in . The same analysis method was used to evaluate the neutron shielding performance of the sintered body.

FIG. 7 shows the results of a simulation on the shielding performance of various sintered bodies obtained in the small-sized sintering test described above.
Specifically, by inputting characteristic values such as the composition and components of the sintered body and relative density, the thickness of the sintered body after machining, that is, the thickness of the neutron shielding material, is changed, and thermal neutrons are mainly emitted. (Thermal neutron flux at the incident point of the sintered body is 1.0E+09: constant) is incident on the sintered body, and the thermal neutron flux at the exit point after transmission is compared ([Exit thermal neutron flux ] / [incident thermal neutron flux] = [thermal neutron decay rate]). The thinner the thickness of the sintered body (unit: mm) that reaches the thermal neutron attenuation rate (1/100) required as a shielding material, the higher the shielding performance of the sintered body.

Regarding the upper limit of the thickness of the sintered body, as described in the section [Means for Solving the Problems and Its Effect], ``The limit of the thickness is due to structural restrictions of the BNCT apparatus, for example, In order to prevent leakage of radiation in the gap between the therapeutic beam irradiation port and the patient's affected area, it was recognized that the energy loss of the therapeutic beam would increase if the distance exceeded 100 mm from the irradiation port, and it was determined that 100 mm or less was desirable.
However, in the case of the shielding material for preventing leakage of radiation leaking from the outer circumference of the moderator system, the thickness that can obtain the required shielding performance is required, and there is no upper limit for the thickness of the shielding material. Assuming that it will be used in the vicinity, the upper limit of the thickness is 100 mm.

Therefore, the required shielding performance (here, the thickness of the sintered body (unit: mm) at which the thermal neutron attenuation rate is 1/100) is limited to 100 mm or less.
As shown in the following examples and comparative examples, the shielding performance when the component system is a LiF-MgF ₂ -CaF ₂ ternary system and the LiF blending ratio is 5 wt. Since the thickness of the sintered body is exactly 100 mm, the LiF compounding ratio is required to be 5 wt.% or more.

The "comprehensive evaluation (◎: excellent, ○: good, △: difficult, ×: unacceptable)" of each sintered body shown in FIG. This is a comprehensive evaluation based on the results of
The component system in the example is LiF-MgF ₂ -CaF ₂ ternary system and the LiF blending ratio is 50 wt.% or more. Thermal neutron shielding performance is high (thermal neutron attenuation The thickness of the shielding material is thin when the ratio is 1/100), and it has extremely excellent shielding performance.

[Example 1]
High-purity LiF (using naturally occurring Li): 98.8 wt.%, MgF ₂ : 0.8 wt.%, CaF ₂ : 0.4 wt.% as raw materials [Mode for Carrying Out the Invention] 3 wt.% of pure water was added to the pulverized material and kneaded to obtain the starting material.

This starting material was filled in a wooden mold (dimensions: 420 mm x 420 mm x t 150 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 420 mm x 420 mm x t90 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water. ×406 mm ×t85 mm).

This CIP compact was placed in a presintering furnace and presintered in an air atmosphere at 400° C. for 6 hours to obtain a presintered compact with dimensions of approximately 389 mm×389 mm×t81 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 490 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 610 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering). After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 363 mm x 363 mm x t76 mm.

In the following [Examples] and [Comparative Examples], the atmospheric gas pressure conditions in the preliminary sintering process, the primary sintering process and the secondary sintering process were all set to "normal pressure".

The bulk density of the sintered body was calculated to be 2.514 g/cm ³ from the approximate dimensions and weight. The relative density was 95.0% because the true density calculated from the composition ratio was 2.646 g/cm ³ .
The “bulk density” referred to here means that the appearance of the sintered body is square in plan view, so the bulk volume is obtained from the measured two sides and thickness of the square, and the separately measured weight is the bulk volume. We adopted the method of dividing by . The same procedure was followed below.

Various characteristic evaluation tests were conducted using samples taken from this sintered body.
The results are shown in FIG.
As described above, the shielding performance required for the sintered body for shielding material (here, the thickness of the sintered body at which the thermal neutron attenuation rate is 1/100 (unit: mm)) is 100 mm or less. , the shielding performance of this embodiment was 15 mm.
The shielding performance of "a mixture of 50 wt.% LiF powder and 50 wt.% polyethylene (hereinafter abbreviated as "LiF + PE")", which is said to have the highest shielding performance among shielding materials currently in use, is 25mm. Taking this into consideration, it can be seen that the shielding performance of Example 1: 15 mm is "extremely excellent shielding performance". In addition, other physical properties such as relative density and mechanical working strength of the sintered body were also good.

The shielding performance of the above "LiF + PE": 25 mm simulation calculation preconditions are that the LiF powder is uniformly dispersed in polyethylene (resin) and that the particle size distribution of the same powder is constant. This value is based on the assumption that the material has an ideal texture and structure.
Usually, when powder of inorganic material such as LiF is kneaded into such polyethylene resin, a dispersant such as a surfactant is used in order to disperse the powder more uniformly. Contamination with impurities is strictly prohibited and cannot be used for wood applications.
Therefore, the LiF powder is not excellent in uniform dispersibility in the polyethylene resin, and is easily presumed to be unevenly distributed. If there is such an uneven distribution, it is expected that the overall shielding performance will decline because the shielding performance will vary locally.
Therefore, it can be said that the actual shielding performance of “LiF+PE” is highly likely to be a value larger than the value of 25 mm used here (slightly inferior shielding performance).

[Example 2]
High-purity LiF (using naturally occurring Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.% as raw materials [Mode for Carrying Out the Invention] 3 wt.% of pure water was added to the pulverized material and kneaded to obtain the starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 275 mm×275 mm×t89 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 620 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering). After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.

The bulk density of the sintered body was calculated as 2.584 g/cm ³ . The relative density was 96.0% because the true density calculated from the composition ratio was 2.692 g/cm ³ .

The shielding performance of the sintered body of Example 2 is 16 mm, which is within the standard (100 mm or less), and is better than the comparative material "LiF + PE", and has excellent shielding performance. In addition, the mechanical working strength was also good without any problem.

[Example 3]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 289 mm). ×289 mm×t94 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×274 mm×t88 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 620 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering). After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 259 mm x 259 mm x t85 mm.
The relative density of the sintered body was calculated as 96.9%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 4]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 21.0 wt.%, MgF ₂ : 49.8 wt.%, CaF ₂ : 29.2 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 250 mm×250 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 250 mm x 250 mm x t100 mm) was placed in a thick vinyl bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 242 mm). ×242 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered in an air atmosphere at 400° C. for 6 hours to obtain a presintered compact with dimensions of about 229 mm×229 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 620 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering). After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 217 mm x 217 mm x t82 mm.
The relative density of the sintered body was calculated as 98.8%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 5]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 9.0 wt.%, MgF ₂ : 57.3 wt.%, CaF ₂ : 33.7 wt. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 450 mm x 450 mm x t 130 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 450 mm x 450 mm x t60 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 431 mm). ×431 mm×t57.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 410° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 409 mm×409 mm×t54.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 510 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 630 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering). After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 388 mm x 388 mm x t49 mm.
The relative density of the sintered body was calculated as 97.2%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 6]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 5.3 wt.%, MgF ₂ : 59.7 wt.%, CaF ₂ : 35.0 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 420° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 520 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 630 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 96.6%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 7]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a circular wooden mold (dimensions: φ550 mm×t180 mm) in a plan view, and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press-molded body (dimensions: about φ550 mm×t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded body (dimension: about φ531 mm) by CIP molding. ×t94 mm).

This CIP molded body was placed in a pre-sintering furnace and pre-sintered at 400° C. for 6 hours in an air atmosphere to obtain a pre-sintered body with dimensions of about φ503 mm×t90 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 480 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 610 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were φ477 mm×t83 mm.
The relative density of this disk-shaped sintered body was calculated to be 95.6%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 8]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a ring-shaped wooden mold (dimensions: outer φ350 mm×inner φ100 mm×t200 mm) in plan view, and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press-formed body (dimensions: approx. 350 mm outer diameter, 100 mm inner diameter, and 120 mm t) was placed in a thick plastic bag, degassed and sealed, and then loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine is filled with clean water, and an isotropic pressure of 20 MPa is applied to the clean water to form a molded body by CIP molding (dimensions: approx. φ337 mm×inner φ96 mm×t111 mm).

This CIP molded body was placed in a pre-sintering furnace and pre-sintered at 400° C. for 6 hours in an air atmosphere to obtain a pre-sintered body with dimensions of approximately outer φ317 mm×inner φ93 mm×t106 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in an air atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 610 ° C. The temperature was raised at a constant rate over 6 hours and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were outer φ300 mm×inner φ88 mm×t100 mm.
The relative density of this ring-shaped sintered body was calculated to be 96.2%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 9]
As raw materials, similarly to Example 1, high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (mold dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 5.5 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine is filled with clean water, and an isotropic pressure of 5.5 MPa is applied to the clean water to form a molded body by CIP molding (dimensions: about 290 mm x 290 mm x t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 350° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 480 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 580 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t83 mm.

The relative density of the sintered body after secondary sintering was 93.0%, which was slightly lower than the prescribed relative density of 92%.
Therefore, this sintered body is heated in a vacuum atmosphere using a hot press machine, and hot-formed by applying a load of 0.05 MPa with a pressure press while maintaining at 540 ° C. for 0.25 Hr, and stopping the load. After that, the heating was stopped, and the sintered body was taken out after the specified workpiece take-out temperature became 100° C. or less by furnace cooling.
The relative density of the sintered body after hot pressing was as good as 98.9%. In addition, both mechanical strength and neutron shielding performance were good.

[Example 10]
As a raw material, as in Example 1 above, a multicomponent system consisting of high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% Fluoride and high-purity boric acid (B(OH) ₃ ) using an enriched boron raw material in which the isotope ¹⁰ B is enriched to 96% as a boron compound, respectively, are separately described above [In order to carry out the invention morphology] section for ² weeks by the method using a ball mill, and 0.5 wt. ), and then pulverized for one week after mixing, and 3 wt.

This starting material was filled in a wooden mold (mold dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 5.5 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine is filled with clean water, and an isotropic pressure of 5.5 MPa is applied to the clean water to form a molded body by CIP molding (dimensions: about 292 mm x 292 mm x t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 610 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t84 mm.

The relative density of the sintered body after secondary sintering was 92.4%, which was slightly lower than the prescribed relative density of 92%.
Therefore, it is heated in a vacuum atmosphere using a hot press machine, hot-formed by applying a load of 0.10 MPa with a pressure press while maintaining at 570 ° C. for 0.25 Hr, and stopping the heating after stopping the load. , and the sintered body was taken out after the specified workpiece take-out temperature reached 100°C or less by furnace cooling.
The relative density of the sintered body after hot pressing was as good as 99.4%. In addition, both mechanical strength and neutron shielding performance were good.

[Example 11]
As a raw material, as in Example 1 above, a multicomponent system consisting of high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% Fluoride and high-purity lithium tetraborate (Li ₂ B ₄ O ₇ ), which is a natural boron raw material as a boron compound, are individually mixed in the ball mill described in the above [Mode for Carrying out the Invention] section. and added 1.0 wt.% of the boron compound as isotope ¹⁰ B to the multicomponent fluoride. % and kneaded was used as the starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 640 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 10 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 95.6%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 12]
As a raw material, as in Example 1 above, a multicomponent system consisting of high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% Fluoride and high-purity gadolinium oxide (Gd ₂ O ₃ ), which is a natural gadolinium raw material as a gadolinium compound, are individually mixed in a ball mill as described in the above [Mode for Carrying Out the Invention] section. and added 0.52 wt.% of the same gadolinium compound as the isotope ¹⁵⁷ Gd to the multicomponent fluoride. , kneaded was used as a starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and then continued to 700 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 94.0%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 13]
As a raw material, as in Example 1 above, a multicomponent system consisting of high-purity LiF (using natural Li): 70.0 wt.%, MgF ₂ : 18.9 wt.%, CaF ₂ : 11.1 wt.% Fluoride and high-purity gadolinium oxide (Gd ₂ O ₃ ), which is a natural gadolinium raw material as a gadolinium compound, are individually mixed in a ball mill as described in the above [Mode for Carrying Out the Invention] section. 1.56 wt.% of the boron compound as the isotope ¹⁵⁷ Gd is added to the multi-component fluoride, and after mixing, the mixture is further ground for 1 week, and 3 wt.% of pure water is added to the ground material. , kneaded was used as a starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 520 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and then continued to 700 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 10 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 95.5%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 14]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 5.3 wt.%, MgF ₂ : 59.7 wt.%, CaF ₂ : 35.0 wt.%, and a boron compound As a natural boron raw material, high-purity lithium tetraborate (Li ₂ B ₄ O ₇ ), and as a gadolinium compound, high-purity gadolinium oxide (Gd ₂ O ₃ ), which is a natural gadolinium raw material, are individually described above. Grind for ² weeks by the method using a ball mill described in the section [Mode for carrying out the invention], add 1.5 wt. 0.52 wt.% of the same gadolinium compound as the isotope ¹⁵⁷ Gd was added, and after mixing, the mixture was further pulverized for 1 week.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 420° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 520 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and then continued to 700 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 10 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 94.3%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 15]
As raw materials, similarly to Example 1, high-purity LiF (using naturally occurring Li): 9.0 wt.%, MgF ₂ : 57.3 wt.%, CaF ₂ : 33.7 wt.%, and a boron compound and high-purity lithium tetraborate (Li ₂ B ₄ O ₇ ), which is a natural boron raw material, for 2 weeks by a method using a ball mill described in the section [Mode for Carrying Out the Invention]. pulverize, add 1.5 wt.% of the boron compound as isotope ¹⁰ B to the multicomponent fluoride, mix and pulverize for another week, add 3 wt.% of pure water to the pulverized raw material, and knead. The resulting product was used as a starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 420° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 520 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 630 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 10 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated as 95.2%. Also, the sintered body had good mechanical strength and neutron shielding performance.

[Example 16]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 9.0 wt.%, MgF ₂ : 57.3 wt.%, CaF ₂ : 33.7 wt.%, and a gadolinium compound and high-purity gadolinium oxide (Gd ₂ O ₃ ), which is a natural gadolinium raw material, are individually pulverized for 2 weeks by the method using the ball mill described in the above [Mode for Carrying out the Invention] section, 0.52 wt.% of a gadolinium compound as isotope ¹⁵⁷ Gd is added to the multicomponent fluoride, and after mixing, the material is ground for a further week, 3 wt.% of pure water is added to the ground material, and the material is kneaded to obtain the starting material. and

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 420° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 520 ° C. at a constant rate over 6 hours in an air atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 700 ° C. The temperature was raised at a constant rate over 6 hours and held at the same temperature for 10 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t81 mm.
The relative density of the sintered body was calculated as 96.0%. Also, the sintered body had good mechanical strength and neutron shielding performance.
In the above examples, examples using one or more fluorides selected from AlF ₃ , KF, NaF, and/or YF ₃ were not given. If so, it is within the range of fluorides that can be easily conceived, and is within the scope of the technical ideas constituting the present invention.

[Comparative Example 1]
As raw materials, as in Example 1 above, high-purity LiF (using naturally occurring Li): 3.0 wt.%, MgF ₂ : 61.1 wt.%, CaF ₂ : 35.9 wt. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 630 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t81 mm.
The relative density of the sintered body was calculated as 96.7%. Although the mechanical strength of the sintered body was good, the neutron shielding performance was remarkably poor.

[Comparative Example 2]
As raw materials, as in Example 1 above, high-purity LiF (using naturally occurring Li): 4.5 wt.%, MgF ₂ : 60.2 wt.%, CaF ₂ : 35.3 wt.%. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 20 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 20 MPa was applied to the clean water to obtain a molded product (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 500 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 625 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t81 mm.

The relative density of the sintered body was calculated as 97.0%. Although the mechanical strength of the sintered body was good, the neutron shielding performance was remarkably poor.

[Comparative Example 3]
As raw materials, similarly to Example 1, high-purity LiF (using naturally occurring Li): 91.5 wt.%, MgF ₂ : 5.4 wt.%, CaF ₂ : 3.1 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm) and compressed and molded using a uniaxial press with a uniaxial press pressure of 5 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 5 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 290 mm). ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 460 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 560 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated to be 91.2%, which was lower than the prescribed value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 4]
As a raw material, similarly to Example 1 above, high-purity LiF (using natural Li): 100 wt. Then, 3 wt.% of pure water was added to the pulverized raw material, and the mixture was kneaded to obtain the starting raw material.

This starting material was filled in a wooden mold (mold dimensions: 300 mm x 300 mm x t 180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 4 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 4 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 290 mm). ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 480 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 560 ° C. The temperature was raised at a constant rate over 4 hours and held at the same temperature for 4 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated to be 91.0%, which was lower than the prescribed value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 5]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (mold dimensions: 300 mm x 300 mm x t 180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 4 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 4 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 290 mm). ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 450 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 550 ° C. The temperature was raised at a constant rate over 4 hours and held at the same temperature for 4 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated to be 90.2%, which was lower than the specified value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 6]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (mold dimensions: 300 mm x 300 mm x t 180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 4 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 4 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 290 mm). ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 380° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t91 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 460 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 570 ° C. The temperature was raised at a constant rate over 4 hours and held at the same temperature for 4 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82.5 mm.
The relative density of the sintered body was calculated to be 90.7%, which was lower than the prescribed value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 7]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.% were used. The material was pulverized by the method using a ball mill described in the section [Mode for Carrying Out the Invention], and 3 wt.

This starting material was filled in a wooden mold (mold dimensions: 300 mm x 300 mm x t 180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 4 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded product and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 4 MPa was applied to the clean water to obtain a compact by CIP molding (dimension: about 290 mm). ×290 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t91 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 470 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 570 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated to be 91.0%, which was lower than the prescribed value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 8]
As raw materials, similarly to Example 1, high-purity LiF (using naturally occurring Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.%, and a boron compound and high-purity boric acid (B(OH) ₃ ) using an enriched boron raw material in which the isotope ¹⁰ B is enriched to 96% as shown in Example 10 above, and each separately described above [In order to carry out the invention form] for ² weeks by the method using a ball mill described in the section above, add 1.5 wt. Then, 3 wt.% of pure water was added to the pulverized raw material, and the mixture was kneaded to obtain the starting raw material.

This starting material was filled in a wooden mold (dimensions: 300 mm×300 mm×t180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 3 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-formed body and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 3 MPa was applied to the clean water. ×292 mm×t95 mm).

This CIP compact was placed in a presintering furnace and presintered at 400° C. for 6 hours in an air atmosphere to obtain a presintered compact with dimensions of about 274 mm×275 mm×t91 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 480 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 570 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t83 mm.
The relative density of the sintered body was calculated to be 89.5%, which was below the specified value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

[Comparative Example 9]
As raw materials, similarly to Example 1 above, high-purity LiF (using natural Li): 90.0 wt.%, MgF ₂ : 6.3 wt.%, CaF ₂ : 3.7 wt.%, and a gadolinium compound and high-purity gadolinium oxide (Gd ₂ O ₃ ), which is a natural gadolinium raw material, are individually pulverized for 2 weeks by the method using the ball mill described in the above [Mode for Carrying out the Invention] section, 0.52 wt.% of the same gadolinium compound as the isotope ¹⁵⁷ Gd is added to the multicomponent fluoride, and after mixing, the mixture is further pulverized for one week, and 3 wt.% of pure water is added to the pulverized raw material and kneaded. used as the starting material.

This starting material was filled in a wooden mold (dimensions: 300 mm x 300 mm x t 180 mm), and compressed and molded using a uniaxial press with a uniaxial press pressure of 10 MPa.
This press molded body (dimensions: about 300 mm x 300 mm x t100 mm) was placed in a thick plastic bag, degassed and sealed, and loaded into the molding section of a cold isostatic pressing (CIP) machine.
The gap between the plastic bag containing the press-molded body and the molding part of the CIP machine was filled with clean water, and an isotropic pressure of 10 MPa was applied to the clean water to obtain a molded body (dimension: about 290 mm) by CIP molding. ×290 mm×t94.5 mm).

This CIP compact was placed in a presintering furnace and presintered in an air atmosphere at 390° C. for 6 hours to obtain a presintered compact with dimensions of approximately 274 mm×275 mm×t90.5 mm.
This temporary sintered body is placed in a sintering furnace, heated from room temperature to 470 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, held at the same temperature for 6 hours (primary sintering), and continued to 630 ° C. The temperature was raised at a constant rate over 6 hours, and held at the same temperature for 6 hours (secondary sintering).
After that, the heating was stopped and the product was naturally cooled to 100° C. (cooling time was about one day and night), after which it was taken out from the sintering furnace. The sintered state was good, and the approximate dimensions were 260 mm x 260 mm x t82 mm.
The relative density of the sintered body was calculated to be 90.5%, which was lower than the specified value (92% or more). The neutron shielding performance of the sintered body was good, but the mechanical strength was significantly inferior.

Claims (16)

LiF in the range of 99 wt.% to 5 wt.% and one or more fluorides selected from MgF ₂ , CaF ₂ , AlF ₃ , KF, NaF, and/or YF ₃ in the range of 1 wt.% to 95 wt. A sintered body for a radiation shielding material, characterized by having physical properties such as a relative density of 92% or more, a bending strength of 50 MPa or more, and a Vickers hardness of 100 or more. _Boron selected from _B2O3 , _{B (} OH) ₃ , _LiB3O5 or _Li2B4O7 in addition to the multicomponent fluoride having LiF as a main phase according to claim ₁ The compound is added as the boron isotope ¹⁰ B in an amount of 0.1 to 5 wt. A sintered body for a radiation shielding material, characterized in that In addition to the multicomponent fluoride having LiF as a main phase according to claim 1, a gadolinium compound selected from Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ is added as the gadolinium isotope ¹⁵⁷ Gd A radiation shielding material characterized by being added at a rate of 0.1 to 2 wt.% as an external coating and having physical properties such as a relative density of 92% or more, a bending strength of 40 MPa or more, and a Vickers hardness of 80 or more. Sintered body for _Boron selected from _B2O3 , _{B (} OH) ₃ , _LiB3O5 or _Li2B4O7 in addition to the multicomponent fluoride having LiF as a main phase according to claim ₁ A compound is added as the _{boron isotope} ¹⁰ _B in an amount of 0.1 to ₅ wt. , Gadolinium isotope ¹⁵⁷ Gd is added at a rate of 0.1 to 2 wt. A sintered body for a radiation shielding material, characterized by: The sintered body for radiation shielding material according to any one of claims 1 to 4, wherein the radiation is a neutron beam. A radiation shielding material formed by machining the sintered body for a radiation shielding material according to any one of claims 1 to 5. In the radiation field, the thickness of the shielding material formed by machining the sintered body is 100 mm or less, and the thermal neutron flux (N1) emitted from the shielding material is transferred to the thermal neutron flux incident on the shielding material. 7. The radiation shielding material according to claim 6, characterized in that it has a thermal neutron shielding performance in which a value divided by (N0), that is, a thermal neutron attenuation rate (N1/N0) is 1/100 or less. high-purity LiF raw material,
and one or more fluoride raw materials selected from MgF ₂ , CaF ₂ , AlF ₃ , KF, NaF, and/or YF ₃ all of which are of high purity,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
A method for producing a sintered body for a radiation shielding material, comprising: high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B, each of which consists of a natural boron raw material and/or a boron raw material enriched with the isotope ¹⁰ B as its boron (B) source. A boron compound raw material selected from ₄ O ₇ ,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
A method for producing a sintered body for a radiation shielding material, comprising: high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and a gadolinium compound raw material selected from Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ , all of which are of high purity, wherein the gadolinium (Gd) source is a natural gadolinium raw material,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
A method for producing a sintered body for a radiation shielding material, comprising: high-purity LiF raw material,
and one or more fluoride raw materials selected from _MgF2 , _CaF2 , _AlF3 , KF, NaF, and/or _YF3 , all of which are of high purity;
and B ₂ O ₃ , B(OH) ₃ , LiB ₃ O ₅ or Li ₂ B, each of which consists of a natural boron raw material and/or a boron raw material enriched with the isotope ¹⁰ B as its boron (B) source. A boron compound raw material selected from ₄ O ₇ and a gadolinium (Gd) source consisting of a natural gadolinium raw material, all of which are selected from among high-purity Gd ₂ O ₃ , Gd(OH) ₃ or GdF ₃ Gadolinium compound raw material,
Each of them is individually finely pulverized (primary pulverization) so that each average particle size is 8 μm or less in terms of median diameter,
After that, these primary pulverized individual raw materials are mixed in a predetermined ratio,
Furthermore, fine pulverization (secondary pulverization) is performed so that the average particle size is 6 μm or less in terms of median diameter,
After that , a step of adding 3 wt.
A step of molding the kneaded compounded raw material at a press pressure of 5 MPa or more using a uniaxial press molding machine (uniaxial press molding step),
A step of molding the press-formed product by applying a water pressure of 5 MPa or more using a cold isostatic pressing (CIP) machine (CIP molding step);
A step of pre-sintering by heating the CIP molded product in a temperature range of 350 to 470 ° C. in a normal pressure atmosphere (pre-sintering step);
A step of heating and sintering the temporary sintered body in a normal pressure atmospheric atmosphere or a normal pressure inert gas atmosphere at a temperature range of 480 to 560 ° C. (primary sintering step);
Subsequently, a step of heating at a temperature range of 570 to 800 ° C. in the same normal pressure and atmosphere as in the previous step to form a sintered body (secondary sintering step);
A method for producing a sintered body for a radiation shielding material, comprising: The boron compound raw material is added as a boron isotope ¹⁰ B in an amount of 0.1 to 5 wt. The method for producing a sintered body for radiation shielding material according to claim 9 or 11. 0.1 to 2 wt.% of the gadolinium compound raw material is added as the gadolinium isotope ¹⁵⁷ Gd in the natural gadolinium raw material to the multicomponent fluoride raw material composed of LiF and fluorides other than LiF. 12. The method for producing a sintered body for radiation shielding material according to claim 10 or 11, characterized in that: After the secondary sintering step according to claim 8, a step of further heat-pressing at a temperature range of 450 to 700 ° C. and a uniaxial molding pressure of 0.05 MPa or more in a vacuum atmosphere or a normal pressure inert gas atmosphere. (a hot press step). 15. The method for producing a sintered body for radiation shielding material according to any one of claims 8 to 14, wherein the radiation is a neutron beam. A radiation shielding material is formed by further subjecting the sintered body for a radiation shielding material manufactured by the method for manufacturing a sintered body for a radiation shielding material according to any one of claims 8 to 15 to machining. A method for manufacturing a radiation shielding material, characterized by:

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