Neutron shielding member and method for manufacturing the same

Abstract

The present invention provides a neutron shielding member having multiple shielding layers containing ethylene-based resins, capable of efficiently shielding neutrons, and a method for manufacturing the same. [Solution] The neutron shielding member of the present invention comprises at least two layers, from the source side: a first shielding layer (A-1) containing an ethylene resin and a neutron absorbent, and a second shielding layer (A-2) containing an ethylene resin and a neutron absorbent, wherein the mass-average particle diameter Φ1 (μm) of the neutron absorbent contained in the first shielding layer (A-1) and the mass-average particle diameter Φ2 (μm) of the neutron absorbent contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 1). Φ1

Description

Technical Field

[0001] The present invention relates to a neutron shielding member and a method for manufacturing the same, and more particularly, to a neutron shielding member having a plurality of shielding layers containing an ethylene-based resin and capable of efficiently shielding neutrons, and a method for manufacturing the same.

Background Art

[0002] Radiation is roughly classified into electromagnetic radiation and particle radiation. Here, the main electromagnetic radiations are gamma rays and X-rays, and the main particle radiations are alpha rays (α-rays = helium nuclei) generated from radioactive isotopes, beta rays (β-rays = electrons), highly energetic electrons, protons, neutrons, and heavy particles (heavy ions) generated in the cosmic environment, accelerator facilities, etc. Among these, electrically neutral neutrons are not blocked by matter due to electromagnetic interaction. Therefore, in order to protect the human body, electronic devices, etc. from now on, a shielding member especially adapted to its properties is required.

[0003] With the recent technological progress, the need for radiation shielding members has been increasing more than ever. In the field of electronic technology, as a result of the high integration of semiconductors, the so-called "soft errors" of communication devices caused by neutrons derived from cosmic rays are increasing. A soft error is a malfunction of an electronic device caused by the inversion of bit data due to the intrusion of the above neutrons into semiconductor elements. Since the cosmic ray dose fluctuates significantly at unexpected times due to solar flares, etc., if vehicle automatic driving using GPS technology becomes widespread in the future, there is concern that once a soft error occurs, it will cause great human and material damage. In the field of medical technology, for example, in facilities that perform particle beam capture therapy, which has attracted attention in recent years, a member for protecting medical staff from radiation particle exposure is required. In the field of space technology, a radiation shielding member for protecting electronic devices in artificial satellites (large to small) and for suppressing cosmic ray exposure during long-term stays in rockets and space stations as much as possible is desired. In the field of energy technology, the need for shielding members is increasing in nuclear reactor facilities with active expansion plans and in the field of IT technology, such as in data center facilities.

[0004] An efficient method for shielding highly penetrating neutrons involves two stages: first, reducing the kinetic energy of neutrons by repeatedly colliding them with the atomic nuclei that make up each component of the shielding material; and second, absorbing the neutrons (thermal neutrons) whose energy has decreased to equilibrium with the thermal motion of surrounding molecules through a capture reaction by the atomic nuclei. The former stage is called "deceleration," and the latter stage is called "absorption."

[0005] In the deceleration process, neutrons can most efficiently reduce their energy by colliding with atomic nuclei of the same mass. Therefore, hydrogen atoms with a mass number of 1 are preferred. For this reason, the number of hydrogen atoms per unit area is used as an indicator of the neutron deceleration effect. Materials with a high number of hydrogen atoms per unit volume include resins such as ethylene-based materials and metal hydrides.

[0006] The neutron absorption cross-section is used as an indicator of the absorption process; the larger this value, the more efficiently the neutron capture reaction is carried out. This value has been determined through previous nuclear research, and elements such as boron, cadmium, and gadolinium are known to have large absorption cross-sections for thermal neutrons.

[0007] Since different elements are highly effective for deceleration and absorption, both moderators containing these elements and neutron absorbers are required for shielding.

[0008] If the neutrons deviate from this pattern, for example, when high-energy neutrons collide with neutron-absorbing materials, secondary gamma rays and particle beams may be generated, potentially increasing the risk of radiation exposure.

[0009] For example, Patent Document 1 discloses a neutron shielding member in which boron carbide particles, bismuth oxide particles, and / or gadolinium oxide particles are dispersed within a resin. It also states that the preferred particle size for the gap distance between neutron-absorbing material particles is 2 μm to 20 μm (mass-average diameter).

[0010] Next, Patent Document 2 discloses a three-layer neutron shielding structure using polyethylene. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2017-26563 [Patent Document 2] Japanese Patent Publication No. 2015-10826 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, with a single-layer neutron shielding member like the one described in Patent Document 1, there is a problem in that it is difficult to adjust the composition of the moderator and neutron absorber necessary for shielding high-energy neutrons.

[0013] The neutron shielding structure described in Patent Document 2 consists of three layers, but it does not describe the range of neutron-absorbing material content in each layer, its particle size, or its dispersion state, and therefore does not consider how to achieve efficient shielding.

[0014] Therefore, the object of the present invention is to provide a neutron shielding member capable of efficiently shielding neutrons and a method for manufacturing the same. [Means for solving the problem]

[0015] As a result of diligent research, the inventors of the present invention have found that the above objective can be achieved by having multiple shielding layers made of ethylene resin and controlling the mass-average particle size of the neutron-absorbing material contained in each shielding layer, and have completed the present invention.

[0016] In other words, the present invention has the following configuration (1) to (5). (1) A neutron shielding member comprising at least two layers, namely a first shielding layer (A-1) containing an ethylene-based resin and a neutron absorber, and a second shielding layer (A-2) containing an ethylene-based resin and a neutron absorber. The mass average particle diameter Φ1 (μm) of the neutron absorber contained in the first shielding layer (A-1) and the mass average particle diameter Φ2 (μm) of the neutron absorber contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 1). Φ1 < Φ2 (Equation 1)

[0017] (2) The neutron shielding member according to (1) above, wherein Φ1 is 0.5 μm or more and 30 μm or less, and Φ2 is more than 30 μm and 200 μm or less.

[0018] (3) The neutron shielding member according to (1) or (2) above, wherein the content C1 (mass %) of the neutron absorber contained in the first shielding layer (A-1) and the content C2 (mass %) of the neutron absorber contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 2). C1 < C2 (Equation 2)

[0019] (4) The neutron shielding member according to any one of (1) to (3) above, wherein the neutron absorber is at least one selected from any one of the atoms of boron B, hafnium Hf, cadmium Cd, gadolinium Gd, europium Eu, tantalum Ta, and compounds containing any one of these atoms.

[0020] (5) A method for manufacturing a neutron shielding member according to any one of (1) to (4) above, the method including a step of forming the first shielding layer (A-1) and the second shielding layer (A-2) by using an extrusion lamination method or a coextrusion method.

Advantages of the Invention

[0021] According to the present invention, by arranging a plurality of shielding layers of an ethylene-based resin, a neutron shielding member capable of efficiently shielding neutrons and a method for manufacturing the same can be provided. Controlling the mass average particle diameter makes it easier to improve various properties such as mechanical properties and flexibility by optimizing the structure, and in addition, an improvement in productivity is also expected. Further, by increasing the proportion of the ethylene-based resin in the layer closer to the radiation source and increasing the proportion of the neutron absorber in the layer farther from the radiation source, efficient neutron shielding performance can be obtained.

[0022] When shielding high-energy neutrons, it can also be adjusted by increasing the thickness of the layer closer to the radiation source. As a result, the neutron shielding member of the present invention can be expected to be applied in a wide range of fields such as the medical technology field, the electronic technology, the energy technology, and the space technology fields where shielding of high-energy neutrons is required.

Mode for Carrying Out the Invention

[0023] Hereinafter, the present invention will be described in detail based on one embodiment.

[0024] A neutron shielding member according to one embodiment of the present invention (hereinafter also referred to as the present neutron shielding material) includes at least two layers of a first shielding layer (A-1) and a second shielding layer (A-2), and the first shielding layer (A-1) and the second shielding layer (A-2) contain a neutron absorber and an ethylene-based resin. It is only necessary to have two or more layers of the first shielding layer (A-1) and the second shielding layer (A-2), and three or more layers can be provided, and other layers (for example, an adhesive layer or a layer containing other than the ethylene-based resin) can be arranged.

[0025] (Neutron Absorber) In the present neutron shielding member, the neutron absorber contained in the first shielding layer (A-1) and the second shielding layer (A-2) is not particularly limited, but any one of atoms such as boron B, hafnium Hf, cadmium Cd, gadolinium Gd, europium Eu, tantalum Ta, its compounds (oxides, nitrides, carbides, oxynitrides, carbon oxides, carbonitrides, carbon oxynitrides, etc.), or a mixture, etc. can be used. The cross-sectional area of the neutron absorber is preferably on the order of tens of thousands of barns to 100 barns. In this neutron shielding member, boron B, gadolinium Gd, and their compounds (such as boron oxide, boron carbide, and gadolinium oxide) are preferably used from the viewpoint of shielding effect, handling, and cost-effectiveness. Furthermore, the neutron-absorbing materials contained in each layer may be the same or different.

[0026] In this neutron shielding member, the mass-average particle diameter Φ1 (μm) of the neutron absorber contained in the first shielding layer (A-1) and the mass-average particle diameter Φ2 (μm) of the neutron absorber contained in the second shielding layer (A-2) satisfy the following relationship (Equation 1). Φ1<Φ2 (Equation 1)

[0027] In this specification, the mass-average particle diameter is the mass average of the primary particle diameter. The mass-average particle size of the neutron absorber is not particularly limited as long as it satisfies Equation 1, but it is preferable that the total particle size of the neutron absorber contained in the first shielding layer (A-1) and the second shielding layer (A-2) be between 0.5 μm and 200 μm. Within this range, aggregation and non-uniform dispersion are less likely to occur when mixed with ethylene-based resin, making it easier to ensure fluidity and moldability.

[0028] The mass-average particle diameter Φ1 of the neutron absorber contained in the first shielding layer (A-1) is preferably 0.5 μm to 30 μm, more preferably 0.7 μm to 20 μm, and even more preferably 1 μm to 10 μm. Within this range, sufficient distance is ensured between particles, and the resin present in the gaps efficiently slows down neutrons.

[0029] The mass-average particle diameter Φ2 of the neutron absorber contained in the second shielding layer (A-2) is preferably greater than 30 μm and less than or equal to 200 μm, more preferably between 40 μm and 170 μm, and even more preferably between 50 μm and 150 μm. Within this range, it is possible to efficiently absorb slowed neutrons.

[0030] The mass-average particle size difference (Φ2 - Φ1) between the mass-average particle size Φ1 and the mass-average particle size Φ2 is not particularly limited as long as it satisfies Equation 1, but is preferably 10 μm or more, more preferably 30 μm or more, and even more preferably 50 μm or more. Within this range, the first shielding layer (A-1) can efficiently exhibit the function of mainly slowing down neutrons, and the second shielding layer (A-2) can efficiently exhibit the function of mainly absorbing neutrons. Note that the upper limit of the mass-average particle size difference is 180 μm or less, preferably 120 μm or less, and more preferably 100 μm or less.

[0031] In addition, if the mass-average particle size of the neutron absorber is within the above range, two or more types with different mass-average particle sizes can be combined and used. When two or more types are combined and used, the mass-average particle size is calculated using the weighted average. For example, when three types of neutron absorbers with different mass-average particle sizes of 100 μm, 10 μm, and 1 μm are blended in the same mass, the mass-average particle size is 37 μm.

[0032] In the present invention, the mass-average particle size of the neutron absorber can be calculated assuming a uniform density, using, for example, the volume-based distribution obtained by a laser diffraction particle size distribution measuring device.

[0033] The content of the neutron absorber in each of the above shielding layers may be appropriately adjusted according to the desired shielding performance. However, it is preferable to increase the ratio of the ethylene-based resin in the first shielding layer (A-1) to slow down neutrons and the ratio of the neutron absorber in the second shielding layer (A-2) to absorb neutrons. That is, it is preferable that the content C1 (mass%) of the neutron absorber contained in the first shielding layer (A-1) and the content C2 (mass%) of the neutron absorber contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 2). C1 < C2 (Equation 2)

[0034] Specifically, the neutron absorber content C1 of the first shielding layer (A-1) is preferably 1% by mass or more and 40% by mass or less, more preferably 5% by mass or more and 30% by mass or less, and even more preferably 10% by mass or more and 20% by mass or less. Within this range, high-energy neutrons can be efficiently slowed down, and mechanical properties can also be easily ensured.

[0035] The neutron absorber content C2 in the second shielding layer (A-2) is preferably more than 20% by mass and 90% by mass or less, more preferably more than 25% by mass and 80% by mass or less, and even more preferably more than 30% by mass and 60% by mass or less. Within this range, neutrons with reduced energy can be absorbed efficiently, and mechanical properties can also be easily ensured.

[0036] (Ethylene-based resin) In this neutron shielding member, the ethylene-based resin contained in the first shielding layer (A-1) and the second shielding layer (A-2) is not particularly limited, but examples include low-density polyethylene, linear low-density polyethylene, linear ultra-low-density polyethylene, medium-density polyethylene, and high-density polyethylene. In addition, copolymers or polypolymers of ethylene as the main component, such as copolymers or polypolymers of ethylene with one or more comonomers selected from propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, heptene-1, octene-1, etc., which have 3 to 10 carbon atoms; vinyl esters such as vinyl acetate and vinyl propionate; unsaturated carboxylic acid esters such as methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate and their ionomers; and unsaturated compounds such as conjugated dienes and unconjugated dienes. Mixed compositions thereof, graft-modified products (such as maleic anhydride modification and amine modification), and ethylene-vinyl alcohol copolymers are also possible. The ethylene unit content in the ethylene-based resin is usually more than 50% by mass, and preferably 60% by mass or more.

[0037] Among these ethylene-based resins, at least one ethylene-based polymer selected from the group consisting of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer is preferred from the viewpoint of economy and dispersibility with neutron absorbers described later. Among these, high-density polyethylene and linear low-density polyethylene are even more preferred from the viewpoint of hydrogen atom content and mechanical properties.

[0038] In this neutron shielding member, it is preferable to use high-density polyethylene or linear low-density polyethylene for the shielding layer (A-1), and it is preferable to use at least one ethylene-based polymer selected from the group consisting of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer for the shielding layer (A-2).

[0039] The polymerization method for the above-mentioned ethylene-based resin is not particularly limited, but known polymerization methods such as gas-phase polymerization, solution polymerization, and slurry polymerization can be employed. Furthermore, the catalyst used can be a multi-site catalyst such as the Ziegler-Natta catalyst, or a single-site catalyst such as the metallocene catalyst, as appropriate. In addition, various monomers can be used as raw materials for polymerization, including those derived from petroleum, biomass, or recycled materials.

[0040] In this neutron shielding member, the content of ethylene resin is not particularly limited, but in the first shielding layer (A-1), it is preferably 60% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 95% by mass or less, and even more preferably 80% by mass or more and 90% by mass or less. Within this range, it is preferable because it mainly acts to slow down neutrons with ethylene resin while also having a neutron absorption effect.

[0041] Various additives may be added to each shielding layer of this neutron shielding member as needed. Examples of such additives include silane coupling agents, antioxidants, ultraviolet absorbers, weather stabilizers, light diffusing agents, nucleating agents, colorants (e.g., pigments, dyes), flame retardants, flame retardant enhancers, and discoloration inhibitors. These can be used individually, in combination of two or more types, or the same additive may be added to each shielding layer, or different additives may be added to each layer. When additives are added, the amount is preferably 30% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of each shielding layer. Depending on the type of additive, it may become activated or react with other components in environments exposed to radiation. Therefore, it is preferable to limit the amount to the minimum necessary required by the environment of use.

[0042] (Other layers) This neutron shielding member may also include other layers as described above. For example, it may include an adhesive layer (mainly composed of adhesive polymers or adhesives) that adheres each shielding layer to one of the following: a metal layer, a CFRP layer, a ceramic layer, or a concrete layer; or a layer containing something other than ethylene-based resin, specifically a layer mainly composed of engineering plastics, super engineering plastics, or thermosetting resins. Examples of the metal layer include layers containing aluminum, iron, stainless steel, lead, and tungsten, and examples of the ceramic layer include layers containing oxides, nitrides, carbides, oxynitrides, carbonites, carbonitrides, and carbonatenitrides. Furthermore, examples of adhesive polymers for the adhesive layer include polymers copolymerized by grafting polar functional groups such as maleic anhydride, carboxylic acids, oxazolines, aziridines, and amines. Examples of adhesives for the adhesive layer include acrylic, urethane, epoxy, ethylene-vinyl acetate, and silicone adhesives.

[0043] In this invention, aluminum or iron can be suitably used as the metal layer from the viewpoint of lightness and ease of secondary processing, lead or tungsten from the viewpoint of radiation shielding, and stainless steel from the viewpoint of environmental resistance.

[0044] (Layer composition) In this neutron shielding member, when there are three or more layers, various layer configurations can be appropriately selected, but specifically, the following layer configurations can be mentioned: First shielding layer (A-1) / Second shielding layer (A-2) / First shielding layer (A-1), First shielding layer (A-1) / Second shielding layer (A-2) / Metal layer, CFRP layer / First shielding layer (A-1) / Second shielding layer (A-2), Ceramic layer / First shielding layer (A-1) / Second shielding layer (A-2), First shielding layer (A-1) / Other layers (such as adhesive layers and layers other than ethylene-based resin layers) / Second shielding layer (A-2), First shielding layer (A-1) / Second shielding layer (A-2) / Other layers (adhesive) Examples of configurations include a three-layer structure (such as a layer other than an ethylene-based resin layer), a metal layer / first shielding layer (A-1) / second shielding layer (A-2) / first shielding layer (A-1), a metal layer / first shielding layer (A-1) / second shielding layer (A-2) / concrete layer, a CFRP layer / first shielding layer (A-1) / second shielding layer (A-2) / concrete layer, and a metal layer / first shielding layer (A-1) / second shielding layer (A-2) / first shielding layer (A-1) / concrete layer.

[0045] In this neutron shielding member, the configuration can be appropriately selected depending on the application and location, but from the viewpoint of neutron shielding effect, mechanical properties and secondary processability, a two-layer configuration of a first shielding layer (A-1) / second shielding layer (A-2), a three-layer configuration of a metal layer / first shielding layer (A-1) / second shielding layer (A-2) or a CFRP layer / first shielding layer (A-1) / second shielding layer (A-2), and a four-layer configuration of a metal layer / first shielding layer (A-1) / second shielding layer (A-2) / concrete layer can be suitably used. When lightweight is important in addition to neutron shielding effect, a two-layer configuration is suitable, when secondary processability is important, a configuration of three or more layers including a metal layer is suitable, and when the shielding effect of radiation including neutrons is important, a configuration including a concrete layer can be suitably used.

[0046] (Thickness) The overall thickness of this neutron shielding member can be adjusted as appropriate depending on the energy of the target neutrons, the required shielding performance, durability, and handling. While not particularly limited, considering the overall balance, a length of 1.0 mm to 1000 mm is preferred, 2 mm to 500 mm is more preferred, and 5 mm to 300 mm is even more preferred. Regarding the proportion each layer occupies, generally, the higher the neutron energy, the larger the first shielding layer (A-1) required compared to the second shielding layer (A-2). This is because, as the energy increases, more steps are needed to slow down the neutrons to an energy level that the absorber can absorb. Here, the thickness of the first shielding layer (A-1) is preferably 10% or more thicker than the thickness of the second shielding layer (A-2), and more preferably 20% or more thicker.

[0047] (Linear attenuation coefficient of this neutron shielding member) The linear attenuation coefficient of this neutron shielding material is a useful value for comparing the performance of neutron shielding materials, but it changes depending on the energy of the neutrons being shielded. Generally, materials containing a large amount of ethylene-based resin, which slows down neutrons, exhibit a high linear attenuation coefficient when the neutron energy is high.

[0048] (Manufacturing method for this neutron shielding member) Next, a method for manufacturing the neutron shielding member will be described, but it is not limited to this. Specifically, examples include a co-extrusion method in which a neutron absorber is melt-kneaded into an ethylene-based resin in an extruder and laminated using a T-die; an extrusion lamination method in which one or more layers of either the first shielding layer (A-1) or the second shielding layer (A-2) are pre-formed into films and wound on a roll or T-die and supplied, and then the layers to be laminated are melt-extruded and heat-laminated; and a method in which each layer is laminated by heating and pressurizing using a press or roll. In the present invention, the extrusion lamination method and the co-extrusion method can be suitably used.

[0049] The molding temperature for manufacturing this neutron shielding member is adjusted as appropriate depending on the flow characteristics and film-forming properties of the ethylene resin used, but is generally 300°C or lower, preferably 230-270°C. While single-screw or multi-screw extruders can be suitably used for this neutron shielding member, it is preferable that the extruders for each layer have a venting function. The venting function is preferable because it can be used for drying the ethylene resin used in the first shielding layer (A-1) or the second shielding layer (A-2) and for removing trace amounts of volatile components, resulting in a laminate with fewer defects such as bubbles. It is also preferable because it contributes to reducing outgassing of the neutron shielding member.

[0050] The ethylene-based resin used in this neutron shielding member may be used after pre-mixing each component using a mixer such as a tumbler, V-type blender, Banbury mixer, or extruder. Alternatively, each component may be directly supplied after being measured to the feed port of an extruder, or the components may be supplied separately to each feed port of an extruder having two or more feed ports. Furthermore, known methods can be used for mixing the various additives. For example, (a) a masterbatch may be prepared separately by mixing various additives at a high concentration (typically around 3 to 70% by mass) in a suitable base resin, and then mixing this with the resin to be used after adjusting the concentration; or (b) various additives may be directly mixed with the resin to be used.

[0051] (Application) This neutron shielding material is suitably used in a wide range of fields, including medical technology, electronics technology, energy technology, and space technology. Specifically, it can be used in a wide variety of applications such as radiation therapy equipment, protective clothing, shielding vests, shielding trousers, walls, floors, ceilings, and roofs of living spaces, shielding plates for data center facilities, shielding materials for electronic circuit boards and package elements, radioactive waste containers, mobile devices, artificial satellites, and components for spacecraft and rockets. Furthermore, this neutron shielding material can also be applied to shield against radiation other than neutrons, depending on the purpose. [Examples]

[0052] The present invention will be described in more detail below based on one embodiment, but this will not limit the present invention in any way.

[0053] Using the components listed below, ethylene-based resin sheets containing the neutron absorbers shown in Experimental Examples 1 to 14 were prepared. The surface appearance, aggregation, and neutron shielding performance of each experimental example were evaluated using the following evaluation methods.

[0054] <Ethylene-based resin> • PE-1: High-density polyethylene (Nippon Polyethylene Co., Ltd., product name: Novatec HD HJ360, density: 0.951 g / cm³) 3 MFR (190℃, 2.16kg load): 5.5g / 10min)

[0055] <Neutron absorber> • Boron oxide: 98% purity, manufactured by Fujifilm Wako Pure Chemical Industries (mass-average particle size: 300 μm, density: 2.46 g / cm³) 3 ) The above boron oxide was pulverized in an aqueous solvent using a microbead mill (manufactured by Hiroshima Metal & Machinery Co., Ltd., product name: Ultra Apex Mill), and the particle size was adjusted by varying the pulverization time. After removing the microbeads, the mixture was vacuum-dried at 80°C for 8 hours to produce particles with mass-average particle sizes of 100 μm, 10 μm, and 1 μm (boron oxide A-C). The mass-average particle size (mass average of the primary particle size) was calculated by measuring the volume-referenced distribution of boron oxide in the aqueous boron oxide dispersion after microbead removal using a particle size analyzer (manufactured by Otsuka Electronics Co., Ltd., product name: FPAR-1000AS), assuming a uniform density of boron oxide particles. • Boron A oxide: Mass-average particle size 100 μm • Boron oxide B: Mass-average particle size 10 μm Boron oxide (C): Mass-average particle size 1 μm • Boron oxide D: Mass-average particle size 300 μm (unground product)

[0056] • Boron carbide A: Manufactured by Fujifilm Wako Pure Chemical Industries (Mass-average particle size: 50 μm, Density: 2.51 g / cm³) 3 ) • Gadolinium oxide A: 98% purity, manufactured by Fujifilm Wako Pure Chemical Industries (mass-average particle size: 100 μm, density: 7.07 g / cm³) 3 ) Boron carbide A and gadolinium oxide A were also calculated using the same wet measurement method in an aqueous solvent as described above for boron oxides A-D.

[0057] [exterior] Regarding the surface appearance, visible light was shone on the surface of each experimental sample, and surface distortion originating from the neutron-absorbing material was visually confirmed and judged according to the following criteria. (〇) The surface is smooth and shows almost no irregularities. (△) Some unevenness is visible on the surface. (×) The entire surface is uneven.

[0058] [Agglutination] Surface aggregation was observed within a 5mm x 5mm area. The particle size of the neutron-absorbing material was confirmed using a microscope (manufactured by KEYENCE Corporation, product name VHX-5000) and judged according to the following criteria. (〇) Almost no secondary particles formed by the aggregation of individual particles are observed. (△) Some secondary particles are visible. (×) Secondary particles are visible throughout.

[0059] [Neutron shielding performance] • Neutron beam: Neutrons (p-Be system) generated when protons from a cyclotron (manufactured by Sumitomo Heavy Industries, product name CYPRIS HM-18C) are irradiated onto beryllium were slowed down to thermal neutron levels by a moderator (polyethylene) installed inside the device. Irradiation time: 3 hours. • Measurement method: The gold foil method was used. Gold foil with a diameter of 10 mm and a thickness of 200 μm, covered with cadmium (Cd), was attached to both sides of the test material. After irradiation with thermal neutrons for a predetermined time, the gold foil was collected. The shielding performance was evaluated from the degree of activation (197Au + n → 198Au) of the gold foil on both sides. Measurement time: 30 minutes.

[0060] Specifically, shielding tests were conducted using test materials consisting of 50mm (length) x 50mm (width) x 1mm (thickness) experimental examples and comparative examples of 50mm (length) x 50mm (width) x 2mm (thickness), which were left standing at 25°C for two weeks. Using the difference in the degree of activation of the gold foil attached to the front and back of the test material, and the total thickness of the neutron shielding material excluding the thickness of the gold foil, the neutron attenuation per unit distance, i.e., the linear attenuation coefficient (cm²), is calculated. -1 The linear attenuation coefficient (cm) obtained in Experimental Example 1 was then calculated for each experimental example. -1 The linear attenuation coefficient ratio for each experimental example was calculated and recorded using ) as the baseline (1.00) (rounded to the third decimal place).

[0061] (Experimental Example 1) The aforementioned PE-1 and boron oxide A were dry-blended to a ratio of 90% by mass / 10% by mass, and a film was formed using a twin-screw compounding extruder (manufactured by Technovel Co., Ltd., product name KZW) at a set temperature of 230°C to produce an ethylene-based resin sheet with a thickness of 1 mm. The evaluation results are shown in Table 1.

[0062] (Experimental Example 2) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that the mixing ratio was changed to 70% by mass / 30% by mass. The evaluation results are shown in Table 1.

[0063] (Experimental Example 3) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that the mixing ratio was changed to 50% by mass / 50% by mass. The evaluation results are shown in Table 1.

[0064] (Experimental Example 4) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that boron oxide A was replaced with boron oxide B. The evaluation results are shown in Table 1.

[0065] (Experimental Example 5) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 4, except that the mixing ratio was changed to 70% by mass / 30% by mass. The evaluation results are shown in Table 1.

[0066] (Experimental Example 6) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 4, except that the mixing ratio was changed to 50% by mass / 50% by mass. The evaluation results are shown in Table 1.

[0067] (Experimental Example 7) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that boron oxide A was replaced with boron oxide C. The evaluation results are shown in Table 1.

[0068] (Experimental Example 8) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 7, except that the mixing ratio was changed to 70% by mass / 30% by mass. The evaluation results are shown in Table 1.

[0069] (Experimental Example 9) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 7, except that the mixing ratio was changed to 50% by mass / 50% by mass. The evaluation results are shown in Table 1.

[0070] (Experimental Example 10) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that boron oxide A was replaced with boron oxide D. The evaluation results are shown in Table 1.

[0071] (Experimental Example 11) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that boron oxide A was replaced with boron carbide A. The evaluation results are shown in Table 1.

[0072] (Experimental Example 12) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that boron oxide A was replaced with gadolinium oxide A. The evaluation results are shown in Table 1.

[0073] [Table 1]

[0074] Table 1 shows that surface irregularities become visible at concentrations of 30% or more by mass for boron oxide A (mass-average particle size 100 μm) (Experimental Example 2), 50% or more by mass for boron oxide B (mass-average particle size 10 μm) (Experimental Example 6), and 10% or more by mass for boron oxide D (mass-average particle size 300 μm) (Experimental Example 10). This confirms that the larger the mass-average particle size, the more likely the surface appearance is to deteriorate. On the other hand, while boron oxide C (mass-average particle size 1 μm) exhibits excellent surface appearance, secondary particles due to aggregation become visible at concentrations of 30% by mass or more (Experimental Examples 8, 9), and boron oxide B (mass-average particle size 10 μm) becomes visible at concentrations of 50% by mass or more (Experimental Example 6). This confirms that aggregation is more likely to occur as the mass-average particle size decreases. Regarding neutron shielding performance, it is found that it increases with higher neutron-absorbing material content, but the balance with surface appearance and aggregation is also important.

[0075] (Experimental Example 13) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that the dry blend of PE-1 and boron oxide A was changed from 90% by mass / 10% by mass to 40% by mass / 60% by mass. The evaluation results are shown in Table 2.

[0076] (Experimental Example 14) An ethylene-based resin sheet was prepared in the same manner as in Experimental Example 1, except that PE-1 and boron oxide A were replaced with PE-1 and boron oxides A, B, and C, and the respective concentrations were changed to 40% by mass / 20% by mass / 20% by mass / 20% by mass. The mass-average particle size in this case was calculated to be 37 μm. The evaluation results are shown in Table 2.

[0077] [Table 2]

[0078] Table 2 shows that, when the neutron-absorbing material content is the same, using a combination of boron oxides A, B, and C with smaller mass-average particle sizes results in the production of ethylene-based resin sheets with improved neutron shielding performance, surface appearance, and reduced aggregation, compared to using boron oxide A with a larger mass-average particle size alone.

[0079] (Example 1) Using a two-layer co-extrusion apparatus, a multilayer ethylene resin sheet was fabricated at a set temperature of 210-230°C, with the first shielding layer (A-1) and the second shielding layer (A-2) having a composition of Experimental Example 7 / composition of Experimental Example 2 = 1 mm / 1 mm, to serve as a neutron shielding member. The neutron shielding performance of the obtained neutron shielding member was measured with the first shielding layer (composition side of Experimental Example 7) facing the radiation source. The evaluation results are shown in Table 3.

[0080] (Comparative Example 1) An ethylene-based resin sheet was prepared in the same manner as in Example 1, except that the formulations of the first shielding layer (A-1) and the second shielding layer (A-2) in Example 1 were changed from those of Experimental Example 7 and Experimental Example 2 to those of Experimental Example 1 and Experimental Example 8, respectively, to form a neutron shielding member. The neutron shielding performance of the obtained neutron shielding member was measured with the first shielding layer (the side with the formulation of Experimental Example 1) on the source side. The evaluation results are shown in Table 3.

[0081] [Table 3]

[0082] In each example and comparative example, the linear attenuation coefficient (cm) obtained in Comparative Example 1 is... -1 The linear attenuation coefficient ratio was calculated and listed using ) as 1.00 (reference) (rounded to the third decimal place). Table 3 confirms that a configuration in which the mass-average particle size of the contained neutron absorber is increased from the first shielding layer (A-1) closest to the radiation source to the second shielding layer (A-2) is preferable in terms of neutron shielding performance. [Industrial applicability]

[0083] As described above, the neutron shielding member of the present invention can be suitably used in a wide range of fields, including medical technology, electronic technology, energy technology, and space technology. Specifically, it can be used in a wide range of applications such as radiation therapy equipment, protective clothing, shielding vests, shielding trousers, walls, floors, ceilings, and roofs of living spaces, shielding plates for data center facilities, shielding members for electronic circuit boards and package elements, radioactive waste containers, mobile devices, artificial satellites, and components for spacecraft and rockets. Furthermore, the neutron shielding member of the present invention can also be applied to shield against radiation other than neutrons, depending on the purpose.

Claims

1. A neutron shielding member comprising at least two layers, from the radiation source side, a first shielding layer (A-1) containing an ethylene resin and a neutron absorbent, and a second shielding layer (A-2) containing an ethylene resin and a neutron absorbent, wherein the mass-average particle diameter Φ1 (μm) of the neutron absorbent contained in the first shielding layer (A-1) and the mass-average particle diameter Φ2 (μm) of the neutron absorbent contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 1). Φ1<Φ2 (Formula 1)

2. The neutron shielding member according to claim 1, wherein the Φ1 is 0.5 μm or more and 30 μm or less, and the Φ2 is greater than 30 μm and 200 μm or less.

3. The neutron shielding member according to claim 1, wherein the content C1 (mass%) of the neutron absorbing material contained in the first shielding layer (A-1) and the content C2 (mass%) of the neutron absorbing material contained in the second shielding layer (A-2) satisfy the following relational expression (Equation 2). C1<C2 (Formula 2)

4. The neutron shielding member according to claim 1, wherein the neutron absorbing material is at least one selected from any of the atoms of boron B, hafnium Hf, cadmium Cd, gadolinium Gd, europium Eu, tantalum Ta, or a compound containing any of these atoms.

5. A method for manufacturing a neutron shielding member according to any one of claims 1 to 4, comprising the step of forming the first shielding layer (A-1) and the second shielding layer (A-2) using an extrusion lamination method or a co-extrusion method.

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