A neutron generation target and a method for manufacturing the same.

Abstract

To provide a target for generating neutrons which can suppress generation of blistering.SOLUTION: The target for generating neutrons include a beryllium layer formed of a plurality of beryllium crystal grains with a columnar structure which has grown in the thickness direction, and the beryllium layer has gaps among the beryllium crystal grains.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a neutron generation target and a method for manufacturing the same. [Background technology]

[0002] Neutrons are used in various applications, including boron neutron capture therapy (BNCT), a cancer treatment, neutron diffraction for structural analysis and identification of materials, and neutron imaging, a type of non-destructive testing. Besides conventional nuclear reactors, accelerators are known to generate neutrons by irradiating a metal target with accelerated protons. While there are large accelerators, several kilometers in size, used in facilities like J-PARC and SPring-8, smaller accelerators (for example, those with a minimum installation area of ​​50 m²) are more convenient in terms of ease of installation. 2 (Less than) is advantageous.

[0003] In small accelerators, the acceleration energy of protons is lower compared to large accelerators, so a target is needed that can generate neutrons even at low energies. In this regard, beryllium is known as a material for neutron generation targets used in small accelerators and the like. However, beryllium targets have a problem in that bubbles (swelling) called blistering occur during use, and these bubbles eventually rupture as they grow larger. In other words, the diffusion rate of hydrogen is extremely slow in beryllium, so the irradiated protons remain in the beryllium and turn into hydrogen gas, and this generated hydrogen gas appears as blistering.

[0004] Several techniques have been proposed to suppress blistering. For example, Non-Patent Document 1 (T. Rinckel et al., "Target Performance at the Low Energy Neutron Source" Physics Procedia 26, pp.168-177 (2012)) proposes increasing the proton acceleration energy or reducing the target thickness to allow protons to pass through the target without accumulating, and stopping the protons in the cooling water behind the target. Also, Patent Document 1 (Japanese Patent No. 6713653) discloses a neutron generation target comprising a neutron generation target material containing beryllium and a blistering-resistant intermediate material made of a metal capable of accumulating hydrogen, wherein the thickness of the neutron generation material is less than or equal to the range of the incident protons. According to such a neutron generation target, the irradiated protons stop at the blistering-resistant intermediate material, thereby suppressing the hydrogenation of protons inside the neutron generation target material and suppressing the occurrence of blistering. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6713653 [Non-patent literature]

[0006] [Non-Patent Document 1] T. Rinckel et al., "Target Performance at the Low Energy Neutron Source", Physics Procedia 26, pp.168-177(2012) [Overview of the project]

[0007] However, increasing the proton acceleration energy necessitates larger accelerators and radiation shields, which presents a problem. Furthermore, thinning the target can lead to a decrease in the amount of neutrons generated and a reduction in the structural mechanical strength. Therefore, a neutron generation target capable of suppressing blistering, regardless of proton acceleration energy or target thickness, is desirable.

[0008] The present inventors have now found that by creating gaps between beryllium crystal grains in a beryllium layer composed of multiple beryllium crystal grains, it is possible to provide a neutron generation target that can suppress the occurrence of blistering.

[0009] Therefore, an object of the present invention is to provide a neutron generation target capable of suppressing the occurrence of blistering.

[0010] The present invention provides the following embodiments. [Aspect 1] A neutron generating target comprising a beryllium layer composed of a plurality of beryllium crystal grains having a columnar structure grown in the thickness direction, wherein the beryllium layer has gaps between the plurality of beryllium crystal grains. [Aspect 2] The neutron generating target according to embodiment 1, wherein the plurality of beryllium crystal grains as a whole form a frost columnar structure. [Aspect 3] A neutron generating target according to embodiment 1 or 2, wherein the intercenter distance between adjacent beryllium crystal grains is 2 to 50 μm. [Aspect 4] A neutron generating target according to any one of embodiments 1 to 3, wherein the beryllium crystal grains have an aspect ratio of 1 to 3000. [Aspect 5] A neutron generating target according to any one of embodiments 1 to 4, wherein the beryllium layer is permeable to helium gas. [Aspect 6] A neutron generating target according to any one of embodiments 1 to 5, wherein the beryllium layer is a physically deposited film. [Aspect 7] The neutron generation target according to any one of Aspects 1 to 6, wherein the beryllium layer has a thickness of 0.02 to 6 mm. [Aspect 8] The neutron generation target according to any one of Aspects 1 to 7, further comprising a substrate on one surface of the beryllium layer. [Aspect 9] The neutron generation target according to Aspect 8, wherein the substrate is made of copper. [Aspect 10] A method for manufacturing a neutron generation target, comprising growing a plurality of beryllium crystallites in the thickness direction of a substrate by physical vapor deposition on one surface of the substrate, thereby obtaining a beryllium layer having gaps between the plurality of beryllium crystallites. [Aspect 11] The method for manufacturing a neutron generation target according to Aspect 10, wherein the physical vapor deposition method is at least one selected from the group consisting of resistance heating vapor deposition, electron beam vapor deposition, and sputtering.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic cross-sectional view showing one aspect of the neutron generation target of the present invention. [Figure 2] It is a graph plotting the leak rate against the elapsed time in the helium leak test using the beryllium target of Example 1. [Figure 3] It is a graph plotting the leak rate against the elapsed time in the helium leak test using the beryllium target of Example 2. [Figure 4] It is a SEM image (magnification 1000 times) of the beryllium target of Example 1 observed from directly above. [Figure 5] It is a SEM image (magnification 3000 times) of the beryllium target of Example 1 observed from directly above. [Figure 6] It is a SEM image (magnification 500 times) of the polished cross-section of the beryllium target of Example 1 observed. [Figure 7]This is an SEM image (magnification 2000x) of the polished cross-section of the beryllium target in Example 1. [Figure 8] This is an optical microscope image (magnification 200x) of the cross-section of the beryllium target in Example 2 after etching. [Figure 9] This is an optical microscope image (magnification 500x) of the cross-section of the beryllium target in Example 2 after etching. [Modes for carrying out the invention]

[0012] Neutron generation target As conceptually shown in Figure 1, the neutron generation target 10 of the present invention includes a beryllium layer 12. Optionally, the neutron generation target 10 may further include a substrate 14 on one side of the beryllium layer 12. The beryllium layer 12 is composed of a plurality of beryllium crystal grains 12a. Each beryllium crystal grain 12a has a columnar structure grown in the thickness direction of the beryllium layer 12. The beryllium layer 12 has gaps between the plurality of beryllium crystal grains 12a. In this way, by providing gaps between the beryllium crystal grains 12a in the beryllium layer 12 composed of a plurality of beryllium crystal grains 12a, a neutron generation target 10 capable of suppressing blistering can be provided.

[0013] Conventional beryllium targets are generally composed of beryllium block material (sintered body), and therefore, there are no gaps between the beryllium crystal grains. As a result, as described above, it is thought that accelerated protons remain in the beryllium layer and turn into hydrogen gas, causing blistering. In contrast, in the neutron generation target 10 of the present invention, even if accelerated protons turn into hydrogen gas in the beryllium layer 12, this hydrogen gas is released to the outside through the gaps between the beryllium crystal grains 12a. Therefore, it is possible to suppress the occurrence of blistering regardless of the proton acceleration energy or the thickness of the beryllium layer 12.

[0014] Preferably, each of the multiple beryllium crystal grains 12a constituting the beryllium layer 12 is composed of elemental beryllium metal. However, the beryllium layer 12 may also contain beryllium compounds, beryllium alloys, etc., in addition to elemental beryllium metal, as long as the desired amount of neutrons can be generated. Furthermore, the beryllium layer 12 may contain unavoidable impurities resulting from the raw material components or the film formation process.

[0015] Each beryllium crystal grain 12a has a columnar structure that grows in the thickness direction of the beryllium layer 12, as shown in Figure 1. In a preferred embodiment of the present invention, a plurality of beryllium crystal grains 12a form a frost-pillar structure as a whole. With such a structure, the beryllium layer 12 can be said to have three-dimensional gaps between the crystal grains, which allows for more efficient release of hydrogen gas generated inside the beryllium layer 12, and as a result, the occurrence of blistering is more effectively suppressed. In the schematic diagram shown in Figure 1, each beryllium crystal grain 12a is depicted as extending uniformly from the bottom surface to the top surface of the beryllium layer 12, but the beryllium crystal grains 12a are not limited to this configuration, and may have grain boundaries between the top and bottom surfaces of the beryllium layer 12.

[0016] The intercenter distance between adjacent beryllium crystal grains 12a is preferably 2 to 50 μm, and more preferably 2 to 25 μm. Within this range, hydrogen gas is released more efficiently, and as a result, the occurrence of blistering is suppressed more effectively. The intercenter distance between crystal grains can be measured by cross-sectional observation of the beryllium layer 12 with a scanning electron microscope.

[0017] The beryllium crystal grains 12a preferably have an aspect ratio of 1 or more, more preferably 2 or more, and even more preferably 5 or more. When within such a range, the hydrogen gas generated within the beryllium layer 12 is more likely to move in the thickness direction of the beryllium layer 12, and as a result, the occurrence of blistering is more effectively suppressed. The upper limit value of the aspect ratio of the beryllium crystal grains 12a is not particularly limited, but typically is 200 or less. In the present invention, the aspect ratio of the beryllium crystal grains 12a means the maximum length L w in the width direction of the beryllium layer 12 of the beryllium crystal grains 12a, relative to the maximum length L T in the thickness direction of the beryllium layer 12 of the beryllium crystal grains 12a (= L T / L w ). The maximum length L w in the width direction of the beryllium layer 12 of the beryllium crystal grains 12a is not particularly limited, but typically is 2 to 50 μm. Also, the maximum length L T in the thickness direction of the beryllium layer 12 of the beryllium crystal grains 12a is not particularly limited, but typically is 2 μm or more. The above maximum length L W , maximum length L T and aspect ratio can be measured by observing the cross-section of the beryllium layer 12 with a scanning electron microscope.

[0018] The beryllium layer 12 preferably has helium gas permeability. Here, the kinetic molecular diameter of helium is 0.26 nm, which is slightly smaller than the hydrogen molecular diameter of 0.289 nm. Therefore, it can be said that when the beryllium layer 12 has helium gas permeability, hydrogen gas is even more likely to be discharged from the beryllium layer 12. In the present invention, "having helium gas permeability" means that when a helium leak test is performed in accordance with the vacuum spraying method (spray method) defined in Appendix 1 of JIS Z2331:2006, a leak of helium gas is detected. For example, in the helium leak test, the leak rate (Pa·m 3It is preferable that the maximum value of ( / s) is 10 times (preferably 100 times or more) greater than the leak rate detected by the leak detector before helium gas is blown in.

[0019] The beryllium layer 12 preferably has a thickness of 0.02 to 6 mm. Within this range, it is easier to generate a desired amount of neutrons and sufficient strength as a target can be ensured. The surface area of ​​the beryllium layer 12 when viewed from above (i.e., the surface area of ​​the surface irradiated by protons) can be appropriately determined according to the specifications of the neutron generator used (e.g., a small accelerator) and is not particularly limited, but is typically 30 to 400 cm². 2 That is the case.

[0020] The beryllium layer 12 may be fabricated by any method, but is preferably a physically deposited film. A preferred method for manufacturing the neutron generation target 10 including the beryllium layer 12 will be described later.

[0021] As described above with reference to Figure 1, the neutron generation target 10 may further include a substrate 14 on one side of the beryllium layer 12. The substrate 14 can function as a cooling element for the beryllium layer 12 irradiated with accelerated protons.

[0022] The substrate 14 is preferably made of copper, which allows for more efficient cooling of the beryllium layer 12. The thickness of the substrate 14 is not particularly limited, but is typically 0.1 mm or more.

[0023] Method for manufacturing a neutron generation target A preferred embodiment of the present invention provides a method for manufacturing a neutron generation target 10. This method involves growing a plurality of beryllium crystal grains 12a on one surface of a substrate 14 in the thickness direction of the substrate 14 by physical vapor deposition. In this way, a beryllium layer 12 having gaps between the plurality of beryllium crystal grains 12a can be obtained.

[0024] Preferred examples of physical vapor deposition methods include resistance heating deposition, electron beam deposition, and sputtering, with resistance heating deposition or electron beam deposition being more economically advantageous. Film deposition by resistance heating deposition is performed under a vacuum of 2 × 10⁻⁶. -2 It is preferable to perform deposition by heating the beryllium raw material to 1287°C (the melting point of beryllium) or higher under conditions of Pa or less and a substrate temperature of 350-550°C, until the beryllium layer reaches the desired thickness. For film deposition by electron beam deposition, the conditions should be the same as for the resistance heating deposition method described above, except that the voltage and current are controlled so that the beryllium melts. For film deposition by sputtering, after creating a high vacuum in the chamber, Ar gas should be filled to about 1 Pa, and a voltage should be applied between the substrate and the beryllium until the desired beryllium layer thickness is achieved.

[0025] The beryllium used as a raw material in the physical vapor deposition method is preferably obtained by processing a beryllium ingot manufactured by vacuum casting into small pieces. The purity of the beryllium ingot is preferably 98% by weight or higher, and more preferably 99% by weight or higher. [Examples]

[0026] The present invention will be further described in detail by the following examples.

[0027] Example 1 A beryllium target was fabricated as a neutron generation target in the present invention by the following procedure.

[0028] (1) Prepare the beryllium ingot A beryllium ingot (99% by weight purity) obtained by vacuum casting was prepared. This beryllium ingot was manufactured as follows: First, granular metallic beryllium was placed in a magnesium oxide crucible in a vacuum melting furnace, and the furnace was maintained at approximately 1300°C until the beryllium melted. After the beryllium had completely melted, the furnace temperature and vacuum level were increased to remove impurities. After the refining in the vacuum melting furnace was completed, the molten material was poured into a graphite crucible in a vacuum and solidified to produce a beryllium ingot.

[0029] (2) Preparation of targets by physical vapor deposition The prepared beryllium ingot was cut into small pieces, and these pieces were placed in a crucible inside a vacuum chamber. A 0.12 mm thick beryllium layer was formed on one surface of a commercially available pure copper plate (0.15 mm thick) prepared as a deposition substrate using resistance heating deposition under the following apparatus and conditions. - Equipment: Resistance heating deposition system (manufactured by Sanyu Electronics Co., Ltd., model number: SVC-700TMSG / 7PS80) - Beryllium heating temperature: 1300℃ - Deposition time: 60 minutes - Vacuum degree: 2×10 -2 Pa or less - Substrate temperature: 450℃

[0030] For the various evaluations described later, the pure copper plate with the beryllium layer formed on it was etched off with nitric acid to obtain a beryllium target with a diameter of 50 mm and a thickness of 0.12 mm.

[0031] Example 2 (comparison) A beryllium target was fabricated using a conventional powder sintering method. The specific procedure is as follows:

[0032] (1) Prepare the beryllium ingot A beryllium ingot similar to that used in Example 1 was prepared.

[0033] (2) Fabrication of targets by powder sintering After mechanically crushing the prepared beryllium ingot, it was sieved to obtain beryllium powder with a particle size of 300 mesh. This beryllium powder was sintered using a vacuum hot press method to form a molded body. Specifically, the beryllium powder was filled into a mold in a vacuum furnace, and a molded body was obtained by applying a pressure of 8 MPa in the vertical direction while vacuum heating at a temperature range of 1025 to 1125°C. This molded body was cut to a thickness of 10 mm, and then subjected to repeated hot rolling and softening annealing. During this process, both hot rolling and softening annealing were performed at a temperature range of 750 to 950°C. In this way, a beryllium target with a diameter of 50 mm and a thickness of 0.025 mm was obtained.

[0034] Various evaluations The beryllium targets obtained in Examples 1 and 2 were evaluated in the following ways:

[0035] <Helium leak test> Helium leak tests were performed on beryllium targets in accordance with the vacuum spraying method (spray method) of JIS Z2331:2006. Specifically, the beryllium target was fixed to a sample holder inside the test specimen (workpiece) connected to a leak detector (ULVAC, Inc., HELIOT901W1). After evacuating the inside of the test specimen, helium gas was sprayed from the outside of the test specimen using a spraying probe, and the helium gas leaking into the test specimen was detected by the leak detector. Helium gas spraying was performed twice for each example.

[0036] The helium leak test results showed that in the beryllium target of Example 1, gas leakage occurred with each helium gas injection (maximum leak rate: approximately 5 × 10⁻¹⁶). -5 Pa·m 3 A leak rate of approximately 2 × 10⁻⁶ ( / s) was detected. The leak rate before helium gas injection was approximately 2 × 10⁻⁶. -7 Pa·m 3The rate was / s. Therefore, it was confirmed that the beryllium target in Example 1 has gaps large enough to allow helium permeability. Here, the dynamic molecular diameter of helium is 0.26 nm, which is slightly smaller than the molecular diameter of hydrogen, which is 0.289 nm. For this reason, when accelerated protons are converted into hydrogen gas within the beryllium target in Example 1, the hydrogen gas will be released to the outside of the beryllium target in the same way as the helium gas, and as a result, it is thought that the occurrence of blistering will be suppressed. For reference, Figure 2 shows a graph plotting the leak rate against elapsed time in a helium leak test using the beryllium target in Example 1.

[0037] On the other hand, in the beryllium target of Example 2, even when helium gas is blown onto it, the leak rate is within the detection limit (1.0 × 10⁻⁶). -14 Pa·m 3 The leak rate was less than 0.2 / s, confirming that it does not have helium permeability. Therefore, when accelerated protons are converted into hydrogen gas within the beryllium target of Example 2, this hydrogen gas is unlikely to be released outside the beryllium target, which is thought to cause blistering. For reference, Figure 3 shows a graph plotting the leak rate against elapsed time in a helium leak test using the beryllium target of Example 2.

[0038] <Tissue observation of beryllium> The beryllium target of Example 1 was observed from directly above using a scanning electron microscope (SEM, JEOL Ltd., part number: JSM-6490LA) under conditions of an acceleration voltage of 15kV and magnifications of 1000x and 3000x. The obtained observation images are shown in Figure 4 (magnification 1000x) and Figure 5 (magnification 3000x). Next, the beryllium target of Example 1 was embedded in resin and its cross-section was polished. Then, the polished cross-section of the beryllium target was observed using the above SEM under conditions of an acceleration voltage of 15kV and magnifications of 500x and 2000x. The obtained observation images are shown in Figure 6 (magnification 500x) and Figure 7 (magnification 2000x). As shown in Figures 4 to 7, it was confirmed that the beryllium target of Example 1 (beryllium layer 12) is composed of a plurality of beryllium crystal grains 12a having a columnar structure, and that there are gaps between the plurality of beryllium crystal grains 12a.

[0039] On the other hand, in the case of the beryllium target in Example 2, it was difficult to confirm the metallic structure using SEM observation similar to that in Example 1, so cross-sectional observation was performed after etching. Specifically, the beryllium target in Example 2 was embedded in resin and its cross-section was polished. By contacting this polished cross-section with an etching solution and performing etching, the metallic structure was made visible. Then, the cross-section after etching was observed using an optical microscope at magnifications of 200x and 500x. The obtained observation images are shown in Figure 8 (magnification 200x) and Figure 9 (magnification 500x). As shown in Figures 8 and 9, it was confirmed that the beryllium target in Example 2 has densely packed, flattened crystal grains with no gaps. [Explanation of Symbols]

[0040] 10. Neutron generation target 12 Beryllium layer 12a Beryllium crystal grains 14 circuit boards

Claims

1. A neutron generating target comprising a beryllium layer composed of a plurality of beryllium crystal grains having a columnar structure grown in the thickness direction, wherein the beryllium layer has gaps between the plurality of beryllium crystal grains.

2. The neutron generating target according to claim 1, wherein the plurality of beryllium crystal grains as a whole form a frost columnar structure.

3. The neutron generating target according to claim 1 or 2, wherein the intercenter distance between adjacent beryllium crystal grains is 2 to 50 μm.

4. The neutron generating target according to claim 1 or 2, wherein the beryllium crystal grains have an aspect ratio of 1 to 3000.

5. The neutron generating target according to claim 1 or 2, wherein the beryllium layer is permeable to helium gas.

6. The neutron generating target according to claim 1 or 2, wherein the beryllium layer is a physically deposited film.

7. The neutron generating target according to claim 1 or 2, wherein the beryllium layer has a thickness of 0.02 to 6 mm.

8. The neutron generating target according to claim 1 or 2, further comprising a substrate on one side of the beryllium layer.

9. The neutron generating target according to claim 8, wherein the substrate is made of copper.

10. A method for manufacturing a neutron generation target, comprising growing a plurality of beryllium crystal grains on one surface of a substrate in the thickness direction of the substrate by physical vapor deposition, thereby obtaining a beryllium layer having gaps between the plurality of beryllium crystal grains.

11. The method for manufacturing a neutron generating target according to claim 10, wherein the physical deposition method is at least one selected from the group consisting of resistance heating deposition, electron beam deposition, and sputtering.

Images