JP2001033593A - Neutron carbon mirror and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a neutron mirror that can withstand stricter irradiation environment by polishing a carbon material containing graphite-retardant carbon being obtained by performing such treatment as burning to an organic resin material or the like to a specific surface roughness for manufacturing. SOLUTION: A neutron mirror with carbon as a material is manufactured, for example, by a furan resin. First, for example, a B stage film is created by a film-forming machine, the two films are laminated by a specific method for joining integrally, then are machined to an appropriate shape, and then are subjected to heating curing treatment. An obtained multilayered structure composition is, for example, subjected to carbonization treatment at approximately 1,400 deg.C in nitrogen atmosphere, and is further burnt at approximately 2,000 deg.C, thus obtaining a neutron carbon mirror material. By performing specific polishing machining treatment to the mirror material, a neutron carbon mirror whose surface roughness is for example approximately 0.64 nm is obtained. The obtained mirror has a high neutron reflection factor and a large total reflection critical angle as shown in a figure and has less neutron absorption.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the effective use of neutrons, which are being used for structural analysis of materials and medical treatment, and more particularly, to a neutron carbon mirror for reflecting neutrons and a method for producing the same. Things.

[0002]

2. Description of the Related Art Neutron scattering experiments using slow neutrons
Along with X-rays and synchrotron radiation, it is one of the most effective means for determining the atomic structure of a substance. X-rays and synchrotron radiation are effective for heavy elements, while neutrons are an effective method for light elements and magnetic materials. Taking advantage of this feature, researches on metal magnetic materials utilizing thermal neutrons having energy at about room temperature have been conventionally conducted. In recent years, it has also been used in fields such as structural analysis of high-temperature superconductors, polymers, and biological materials. Use of cold neutrons with lower energy, etc., suitable for examining large structures Is also being promoted.

As described above, neutron utilization has been promoted. However, neutron sources capable of performing neutron scattering experiments are very large facilities, and there are only three neutron sources in Japan, and they are counted even in the world. is there. For this reason, it is extremely important to use the generated neutrons efficiently.

Further, since neutrons are generated in a region having an extremely high irradiation dose, such as a reactor core or an accelerator target, a special technique is required for neutron extraction. Initially, a horizontal experimental hole was inserted around the reactor core, and neutrons flying in the space inside the experimental hole were simply used outside.However, a neutron conduit combined with a neutron mirror was used to remove slow neutrons far away. When it could be guided, large-scale beam holes became possible, and the utilization efficiency was dramatically improved.

[0005] In the case of thermal neutrons and cold neutrons that have been used in the past, it was necessary to install a neutron conduit so as to look at the tip of the horizontal experimental hole from a relatively long distance in order to prevent radiation damage to the neutron conduit. The critical angle of total reflection on the neutron mirror for thermal neutrons and cold neutrons was sufficiently small even in such a way to look from afar in this way, so there was no problem.However, in order to extract extremely cold neutrons with lower energy, neutrons were required. The tip of the conduit must be brought closer to the core. This required a neutron mirror that could withstand more severe irradiation environments.

The neutron mirrors that have been used so far include 1) a glass substrate deposited film, 2) a polished aluminum substrate deposited film, 3) a silicon substrate deposited film, 4) a polished stainless steel, and 4) a replica mirror. The features of each are described below.

[0007] 1) The glass substrate vapor-deposited film is formed by depositing or sputtering a metal suitable for neutron reflection such as nickel on the surface of float glass or polished borosilicate glass. In some cases, a multilayer film combining nickel and titanium is used. Since glass is used, the surface accuracy is good and the area can be easily increased. Many neutron conduits use this neutron mirror. However,
Since glass is vulnerable to irradiation, there is a drawback that its life is shortened when it is used at a position where the dose is high.

[0008] 2) A polished aluminum substrate vapor-deposited film is a thin film processed on a mechanically polished aluminum plate surface. Aluminum is less activated by neutrons,
Suitable for use under high dose. It is also good for large areas. However, because of machining, the planar accuracy is poor and the neutron reflectivity is often not very good even with a single layer film. Further, it is almost impossible to form a multilayer film for a neutron mirror.

3) The silicon substrate vapor-deposited film is a thin film processed on the surface of a silicon wafer. The surface accuracy of the silicon wafer is extremely good, and it is ideal as a substrate for a neutron mirror. Therefore, it is used for many experimental neutron mirrors, but since the size is determined by the rod diameter,
Since only relatively small ones can be manufactured, they cannot be used for neutron conduits, which can be several tens of meters long. In addition, the behavior of silicon in a high irradiation field has not been elucidated yet, and the lifetime cannot be evaluated.

[0010] 4) Polished stainless steel mirrors have not yet obtained sufficient surface accuracy, and thus have not obtained sufficient reflectivity for thermal neutrons and cold neutrons. Therefore, it is used only for ultracold neutrons with moderate conditions. Also, stainless steel contains cobalt as an impurity,
This cobalt has poor activation properties, and is practically impossible to use in an irradiation field when maintenance is considered.

[0011] 5) The replica mirror is manufactured by depositing a nickel or nickel-titanium multilayer film on a float glass, electrodepositing copper having a thickness sufficient to make it self-supporting, and peeling it off. Usually, the thickness of the electrodeposited copper is about 1 mm. As can be seen from the manufacturing method, the thin film surface has extremely high smoothness because the surface of the float glass is transferred. However, it has a drawback that it involves a step of stripping after electrodeposition, and that it is easy to bend because it is essentially desired to make the metal to be a substrate thin. When targeting very slow neutrons such as extremely cold neutrons, the drawback of relatively easy bending is less noticeable, but the loss caused by that point cannot be ignored,
It is not used for thermal or cold neutrons.

[0012]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a novel neutron mirror which solves the above-mentioned problems and a method for manufacturing the same.

[0013]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, when extracting neutrons, do not disturb the neutron field and do not lower the neutron flux. The present inventors have come to realize that the problem can be solved by using carbon, which is a substance and has been conventionally used as a neutron reflector in a reactor core, and completed the present invention.

The neutron carbon mirror of the present invention is obtained by firing an organic resin material having a shape-forming property and having three-dimensional cross-linking, or a natural organic material which undergoes solid phase carbonization. Specifically, one or a mixture of two or more kinds of organic polymer substances and their monomers / oligomers, tar pitches, carbonized pitches, thermoplastic resins, thermosetting resin initial polymers, and the like are fired. It is gained by things.

Here, the organic polymer substance is a substance other than a thermoplastic resin and a thermosetting resin described below.
Lignin, cellulose, gum tragacanth, gum arabic, natural gums and derivatives thereof, sugars, chitin, compounds having condensed polycyclic aromatics such as chitosan in the basic structure of the molecule,
And indanthrene-based vat dyes derived from naphthalene sulfonic acid formalin condensate, dinitronaphthalene, pyrene and the like, and intermediates thereof.

As the thermoplastic resin, polyvinyl chloride,
Polyvinylidene chloride, polyacrylonitrile, post-chlorinated polyvinyl chloride, polyvinyl acetate, and other ordinary thermoplastic resins, and polyphenylene oxide, polyimide, polyamide imide, polybenzimidazole, and other heat-resistant thermoplastic resins, At the time of carbonization, a carbon precursor treatment such as oxidative crosslinking is performed.

As the thermosetting resin, a phenol resin,
Furan resin, epoxy resin, xylene resin, copna resin, etc. are used, flow by heating, and cause intermolecular cross-linking to be three-dimensionally hardened, and high carbon residue yield without special carbon precursor treatment It shows.

The pitches include petroleum pitch, coal tar pitch, asphalt, and pitch-distilled pitches of hydrocarbon compounds such as these pitches and synthetic resins, which have been subjected to a non-graphitizing treatment such as an oxidation treatment for crosslinking. It is.

The resin composition and the amount used in the present invention are not particularly limited, but are preferably those which do not contain impurities required for the intended neutron carbon mirror, particularly, neutron absorbing action. In addition, the composition can be appropriately selected depending on the coefficient of thermal expansion, the final surface roughness, and the like, and can be used alone or as a mixture of two or more kinds. However, it is particularly preferable that the composition be a liquid composition.

Hereinafter, a method for producing a neutron carbon mirror according to the present invention will be described. First, a resin composition having the above-mentioned shapeability, which becomes glassy carbon after firing, is appropriately selected, and a molding machine used for performing ordinary plastic molding such as an extrusion molding machine, a vacuum molding machine, and an applicator is used. To make a B-stage film.

Next, an arbitrary number of these B-stage films are adhered and combined into a single body to obtain a multilayer structure composition. The adhesive used here can be a resin solution selected arbitrarily, but it seems that it is preferable to use the starting material itself of the B-stage film. The obtained multilayer structure composition is subjected to a curing treatment and a carbon precursor treatment in an air oven, and then at a temperature of 1000 ° C. to 3000 ° C. while controlling the temperature rising rate in a non-oxidizing atmosphere such as nitrogen or argon. By firing to the extent, carbonization is completed, and a neutron carbon mirror material having a multilayer structure is obtained. Further, by baking under vacuum to about 1400 to 3000 ° C., it is possible to remove raw materials and impurity elements mixed in the steps up to baking. For impurity elements that cannot be removed by vacuum firing,
If necessary, further purification can be performed by treating at 2000 to 3000 ° C. in the presence of chlorine gas.

It is not always necessary to have a multi-layer structure, but if it is made to have a single-layer structure, the thickness may be large, so that air bubbles involved before the curing treatment and desorption of condensed water generated at the time of baking are eliminated. Often does not work. In this case, pores appear on the mirror-finished surface, and it is difficult to obtain a good mirror surface. Further, in the case of a multi-layer structure, although the pores exist inside, the probability of existence in the surface layer can be extremely reduced, so that the yield at the time of mirror finishing can be improved. Furthermore, there is an advantage that the strength is substantially improved when the multilayer structure is used.

Next, a method of polishing the obtained carbon material will be described. As a neutron mirror, it can be used even if the surface roughness is not very good, but considering the scattering of neutrons, a super smooth mirror surface without scratches is ideal. is there. The polishing method may be an ordinary method, and is not particularly limited. Despite the surface hardness being Hv = 450 to 600, when silica is used as the abrasive, the polishing hardly proceeds, and alumina or carbonized Even if silicon is used, considerable time is required. Therefore, it is effective to use diamond.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

[0025]

EXAMPLES (Example 1) First, a B-stage film consisting of only a furan resin (Hitafuran VF-302 manufactured by Hitachi Chemical Co., Ltd.) was prepared using a film forming machine. The obtained two B-stage films were bonded and integrated with a solution obtained by mixing a curing agent in a furan resin, processed into an appropriate disc shape, and then subjected to a heat curing treatment. The obtained multilayer structure composition was carbonized to 1400 ° C. in a nitrogen gas atmosphere to obtain a carbon body. The carbon body thus obtained was further fired under vacuum to 2000 ° C. to obtain a neutron carbon mirror material. The obtained neutron carbon wafer material was ground on both sides with a # 600 GC abrasive using a double-sided lapping machine to obtain a neutron carbon mirror material having a surface roughness of about 1 μm (Ra). Subsequently, polishing was performed using a combination of a tin surface plate and a diamond slurry (2 microns) to obtain a neutron carbon mirror material having a surface roughness of about 5 nm (Ra). Further, a tin platen and a diamond slurry (0.
The neutron carbon mirror was polished with a combination of 5 μm and a surface roughness of 0.64 nm (Ra).

The neutron reflection performance was measured by the Nuclear Research Institute JRR-3.
M was measured by the θ-2θ method in the C3-1-2 experimental hole. The experimental arrangement is shown in FIG. In FIG. 1, 10 is a neutron conduit, 12, 16, 18, and 22 are slits, 14 is a multilayer monochromator, 20 is a sample, and 24 is a neutron detector.

The divergence of the incident direct beam at the detector position, which was measured without placing the sample 20 at the position shown in FIG.
Shown in The divergence of the reflected beam by the neutron carbon mirror is shown in FIG. 3, and the measurement result of the reflectance is shown in FIG.

Example 2 Using a film forming machine, a B-stage film made of a mixed resin of a furan resin (Hitafuran VF-302 manufactured by Hitachi Chemical Co., Ltd.) and a phenol resin (no added hardener, manufactured by Gunei Chemical Co., Ltd.) was prepared. Produced. The obtained two B-stage films are bonded and integrated with a solution obtained by mixing a furan resin / phenol resin mixed resin and a furan resin hardener, and then processed into an appropriate disc shape, followed by heat curing. gave. After that, a neutron carbon mirror was obtained in the same manner as in Example 1. The measurement results of the neutron reflection performance were almost the same as in Example 1.

(Example 3) A phenolic resin (without addition of a curing agent, manufactured by Gunei Chemical Co., Ltd.) / Isopropyl alcohol solution was applied using a film forming machine to produce a B-stage film composed of only a phenolic resin. The two B-stage films thus obtained were bonded and integrated with a solution obtained by mixing a curing agent (hexamine) with a phenol resin / IPA solution, processed into an appropriate disc shape, and then subjected to a heat curing treatment. After that, a neutron carbon mirror was obtained in the same manner as in Example 1. The measurement results of the neutron reflection performance were almost the same as in Example 1.

(Comparative Example 1) For comparison, a comparison is made with the reflection result of a silicon wafer having extremely good flatness and smoothness. FIG. 5 shows the divergence of the reflected beam performed under the same conditions as in Example 1, and FIG. 6 shows the measurement results of the reflectance.

The reflected beam in FIG. 5 was in good agreement with the direct beam in FIG. 2, and the half-width of the divergence angle was 4 × 10 ^-4 rad. On the other hand, the neutron carbon mirror of the present invention (FIG. 3) has a half-width of the divergence angle slightly increased to 5.4 × 10 ^-4 rad. Conceivable. However, this spread is sufficiently small, and it can be said that the influence on neutron reflection is small.

A comparison between FIG. 4 and FIG. 6 reveals that the total reflection critical potential at which the reflectivity starts to decrease is 54 neV for silicon and 130 neV for carbon, which is excellent as a neutron mirror.

Comparative Example 2 Table 1 shows a comparison between the neutron carbon mirror according to the present invention and other neutron mirrors.

[Table 1]

As described above, the neutron mirror of the present invention 1) has a good surface smoothness. Since it has extremely high smoothness, it has high neutron reflectivity. 2) Good surface flatness. Since it is hard to bend, it has a good mirror performance that can be used for thermal neutrons and cold neutrons. 3) Since carbon has a high potential for neutrons, the critical angle for total reflection is large. Therefore, the effective neutron reflection angle at the time of reflection can be increased, and the performance when applied to a neutron conduit or the like is good. 4) As carbon is used as a reflector in the core,
Low neutron absorption and extremely high irradiation performance against fast neutrons and gamma rays. For this reason, even when inserted near the core, there is no bad effect on the core neutron flux. 5) Relatively high strength, not deformed by normal handling. In addition, it is chemically stable, withstands high temperatures, has high resistance to the environment, and is easy to handle. 6) Large area mirrors up to about 30 cm in diameter can be manufactured. 7) The surface of nickel or the like can be processed into a thin film. This makes it possible to further improve the neutron reflection performance as compared with carbon alone. At present, there is no other material having all these advantages.

[0035]

The beam extraction portion of the reactor requires a large cavity as a horizontal experimental hole in the reflector region such as heavy water. Since this must be installed at the position where the thermal neutron flux is the highest, it inevitably causes a large disturbance in the neutron flux distribution and lowers the performance. And if it can be inserted to this position, the cavity size for taking out can be minimized. This will minimize disturbances on the core neutron flux distribution,
The extracted neutron flux can be substantially increased. Therefore, the use of the neutron mirror of the present invention greatly improves the neutron utilization efficiency.

[Brief description of the drawings]

FIG. 1 is a diagram showing an arrangement for measuring neutron reflection performance.

FIG. 2 is a graph showing the divergence of an incident direct beam.

FIG. 3 is a graph showing divergence of a reflected beam by a neutron carbon mirror in Example 1.

FIG. 4 is a graph showing the measurement results of the reflectance of the neutron carbon mirror in Example 1.

FIG. 5 is a graph showing divergence of a reflected beam by a silicon wafer of Comparative Example 1.

FIG. 6 is a graph showing the measurement results of the reflectance of the silicon wafer of Comparative Example 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Neutron conduit 12, 16, 18, 22 ... Slit 14 ... Multilayer monochromator 24 ... Neutron detector

Claims (6)

[Claims] 1. A neutron carbon mirror wherein 99% or more of the components are carbon. 2. The neutron carbon mirror according to claim 1, wherein the surface roughness Ra is 2 nm or less. 3. The neutron carbon mirror according to claim 1, wherein the carbon includes non-graphitizable carbon. 4. The method according to claim 1, further comprising the step of firing the raw material to become glassy carbon after firing in a non-oxidizing atmosphere at a temperature in the range of 1000 ° C. to 3000 ° C.
The method for producing a neutron carbon mirror according to any one of Items 1 to 3. 5. The method for producing a neutron carbon mirror according to claim 4, further comprising a step of firing in a vacuum atmosphere at a temperature of 1400 ° C. to 3000 ° C. 6. 2000 ° C. to 3000 ° C. in the presence of chlorine gas
6. The method for producing a neutron carbon mirror according to claim 4, further comprising a step of performing treatment in a temperature range of: