JP4352136B2 - Target material - Google Patents

Description

The present invention relates to a target material used in a target material composed of carbon.
The target material prepared according to the present invention can be used as a target material that generates a gamma ray by irradiating a charged particle beam such as an ion beam, and is used in basic science experimental fields such as physics, semiconductors, electronics, chemistry, and biology. In particular, it can be suitably used as a target material for generating gamma rays in the fields of astrophysical nuclear physics and elemental synthesis.

2. Description of the Related Art Conventionally, gamma ray source devices that use gamma rays (X-rays) have been used in medical radiography apparatuses, industrial nondestructive inspection apparatuses, X-ray microanalysis apparatuses, and the like. In the gamma ray source device, it is known to use gamma rays (X-rays) of bremsstrahlung generated when a metal target (target member) is irradiated with charged particles such as electrons and positrons. The wavelength (energy) output from the gamma rays differs depending on the target material used.
As such a gamma ray source device, a technique described in Patent Document 1 (Japanese Patent Laid-Open No. 2000-314800) is conventionally known.

  In Patent Document 1 (Japanese Patent Laid-Open No. 2000-314800), an appropriate material is selected from among atomic numbers 1 to 100 as a target material, which may be a metal or a synthetic resin, and is liquid or gas at room temperature and below room temperature It is described that a substance coagulated at a low temperature may be used.

In addition, targets for enriched isotope carbon 12 and carbon 13 are indispensable in scientific experiments to study the origin, evolution, space celestial nuclear physics, elemental synthesis, etc. using the charged particle beam of the accelerator. When these targets are used in ultraprecision experiments such as space astrophysics, the following requirements (1) to (3) must be satisfied.
(1) The target material is of higher purity.
(2) The target film is uniformly formed with the required thickness.
(3) Stable without evaporation, peeling or breakage due to high-intensity beam current and long-time irradiation.

JP 2000-314800 A ("0029") JP-A-6-158350

(Problems of conventional technology)
Conventionally, as a method for producing a target material in which a target material layer is coated on a substrate, a metal plating method, a vacuum deposition method, or a spray coating method is generally used. However, in the metal plating method, there are restrictions on materials that can be coated, and there is a problem that the equipment becomes large-scale because the waste liquid treatment of plating and high power are required.
The vacuum deposition method is a method in which the target material is heated and vapor-deposited. However, the deposited material has a high activity, and there is a problem that it contains impurities by reacting with a residual gas or the like. There is also a problem that adhesion is weak. Moreover, since it is a point evaporation source, there also exists a problem that uniformity is bad.
Further, the spray coating method has a problem that it is difficult to form a film uniformly and densely and the adhesion is weak.

In addition, valuable and expensive carbon isotopes are available in the form of methane gas or amorphous powder (flaked debris), and carbon foils are usually made by a pyrolysis method that thermally decomposes methane gas. However, the thickness of the carbon foil, which is created by the thermal decomposition method, there is 100μg ^/ cm ² ~2000μg ^/ cm 2 and a width, there is a problem that it is difficult to uniformly form. Further, the carbon foil produced by the pyrolysis method has a problem that it is damaged in a short time due to the heat generated by the foil (about 1000 K) with respect to the beam irradiation.
In order to alleviate the damage caused by heat, a high-temperature-conducting gold foil is used as a backing film, and the surface of the gold foil is vapor-deposited by heating with a thermal decomposition powder or amorphous powder using an electron or laser beam. It is also made by vapor deposition by the vapor deposition method or sputtering method.

However, in this method, as described above, since it is a vapor deposition method, the adhesion between the backing film and the carbon foil (carbon film) is poor. Therefore, the film thickness of the vapor deposited film is limited to 80 μg / cm ² at the maximum. In addition, there is a problem that the uniformity is poor and the yield (the ratio of the product to the starting material) is 10% or less and the efficiency is poor.
In order to increase the yield, an attempt was made to produce a disc-shaped target with a high-pressure press molding machine, but the target was not able to be produced because it did not solidify at all.

At present, the only method for producing a target having a carbon film formed thereon is the vapor deposition method, and the appearance of a new method is expected.
Accordingly, Patent Document 2 (Japanese Patent Laid-Open No. 6-158350) invented and filed by the inventor of the present application describes a technique relating to a method for coating a substrate, and between a pair of electrodes and an electrode. The substrate is placed in the space between the placed insulating members and the particles of the coating material are accommodated, and the particles of the coating material strike the surface of the substrate by reciprocating the particles by applying a voltage between the electrodes. The film is formed. Therefore, it is conceivable to create a target material for outputting gamma rays using this method.
Here, in the case of outputting gamma rays when performing physical property inspection or the like, depending on the material used as the target material, it may be difficult to output gamma rays or irradiation with strong charged particle beams may be required. . As the target material, when using the relatively easily available natural carbon, carbon, carbon 12 ⁽¹² C) of about 98.9% in the natural state, the carbon 13 ⁽¹³ C) is about 1.1% In addition, there is a problem that high-precision data cannot be collected if a very small amount of carbon 14 ( ¹⁴ C) is present and mixed.

FIG. 3 is an explanatory diagram when the film is formed by the conventional carbon 13 film forming method.
However, when carbon 13 is used to form a film by the method described in Patent Document 2, that is, as shown in FIG. 3, a flat conductive substrate 01 as an electrode and a hollow cylindrical insulating member 02 When the carbon 13 powder is accommodated in the space surrounded by and a voltage is applied to reciprocate the carbon 13 powder, the carbon 13 coating 03 is formed between the insulating member 02 and the conductive substrate 01. It has been confirmed by experiments of the present inventors that a large amount adheres to the boundary portion and the film 03 is not formed uniformly.

  In view of the above-described circumstances, the present invention has a first technical problem of uniformly forming highly enriched isotope carbon as a target material with high adhesion.

(Invention)
In order to solve the technical problem, the target material for generating gamma rays according to claim 1 is:
A target substrate is disposed in a space formed by at least a pair of electrodes and an insulating member disposed between the electrodes, and carbon particles as a surface layer forming powder are accommodated, and the surface layer is formed between the electrodes. When a target material having a carbon surface layer formed on the surface of the target substrate is applied by applying a voltage for reciprocating the powder for use, amorphous carbon particles are heated to a high temperature of 2500 K or higher in an inert gas atmosphere. It is characterized by being composed of carbon particles graphitized by heating.

  According to the first aspect of the invention, since the graphitized carbon particles are used, carbon as a target material can be uniformly formed on the surface of the target substrate with high adhesion.

Next, specific examples (examples) of the embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples.
In the following description using the drawings, illustrations other than members necessary for the description are omitted as appropriate for easy understanding.

FIG. 1 is an explanatory diagram of a gamma ray source apparatus according to Embodiment 1 of the present invention.
In FIG. 1, the gamma ray source device 1 according to the first embodiment includes a device housing 1 a that is evacuated by an exhaust device P. A positron beam source device 2 as an example of a charged particle beam source is disposed inside the device casing 1a. The positron beam 2a outputted from the positron beam source device 2 is irradiated to the target material 3 for generating gamma rays. The target material 3 has a target substrate 3a and a surface layer 3b formed on the surface of the target substrate 3a. The target material 3 is configured so as to be movable in the apparatus housing 1a by a moving device (not shown), and is irradiated to reduce damage caused by continuing irradiation with the positron beam 2a at the same position. Can be changed. In Example 1, the substrate 3a of the target material 3 is made of gold (Au) that hardly affects gamma rays, and the surface layer 3b is made of carbon 13 ( ^13C ) that easily generates gamma rays. Yes.
On the surface layer 3b of the target material 3 irradiated with the positron beam 2a, bremsstrahlung gamma rays 4 are emitted, and the generated gamma rays 4 are output to the outside through the window material 6 of the apparatus housing 1. The window material 6 can be made of a material having a high transmittance of gamma rays 4, and can be made of beryllium, aluminum, or the like, for example.

FIG. 2 is an overall explanatory view of the target material manufacturing apparatus according to the first embodiment of the present invention.
In FIG. 2, the target material manufacturing apparatus 11 for generating gamma rays according to the first embodiment of the present invention has a plate-like base 12. A first electrode member 13 is supported on the base 12, and a second electrode member 14 is disposed above the first electrode member 13 so as to face the first electrode member 13. In Example 1, the target substrate 3a of the target material 3 was used as the first electrode member 13, whereby the first electrode member 13 and the target substrate 3a were shared (shared). In Example 1, gold (Au) was used as the target substrate 3a (backing layer). Furthermore, in Example 1, although the distance between the said electrode members 13 and 14 is set to 10 mm (= 1 cm), the said distance can be changed arbitrarily according to a design, a specification, etc.
A power supply device E is connected to the pair of electrode members 13 and 14. A hollow cylindrical (ring-shaped) insulating member 18 is sandwiched between the pair of electrode members 13 and 14. A powder storage space 19 in which the powder F is stored is constituted by a space surrounded by the pair of electrode members 13 and 14 and the insulating member 18. In Example 1, the insulating member 18 is formed in a ring shape having an inner diameter of 25 cm. However, the diameter can be arbitrarily changed according to the design or the like. Further, although glass is used as the insulating member 18, the material is not particularly limited, and any insulating member (ceramic, resin, etc.) can be employed.

A support column 21 is supported at the end of the base 12, and an arm 22 extending to the second electrode member 14 side is screwed to the tip of the support column 21. A shaft member 23 is supported at the tip of the arm 22 so as to be movable in the vertical direction. A plate-like pressing member 24 that contacts the upper surface of the second electrode member 14 is supported at the lower end of the shaft member 23. A spring 26 is mounted on the shaft member 23 between the pressing member 24 and the arm 22 and always urges the pressing member 24 downward. Therefore, the second electrode member 14 is pressed toward the first electrode member 13 by the pressing member, and the powder containing space 19 is sealed so that the powder F does not leak outside.
In Example 1, the powder storage space 19 is not evacuated but is disposed at room temperature under atmospheric pressure. Then, a DC voltage of about 12 kV (that is, an electric field strength of 12 kV / cm) and an AC voltage are superimposed and applied between the electrode members 13 and 14 by the power supply device E. The voltage is not limited to 12 kV, and can be set to any voltage at which a carbon film can be formed, for example, a value between about 3 kV to 30 kV.

In Example 1, the powder containing space 19 contains surface layer forming particles for forming the surface layer 3b of the target material 3 as the powder F. In Example 1, the surface contains the surface. The layer-forming particles F are composed of graphitized carbon 13 ( ¹³ C). In addition, it is not limited to carbon 13, Carbon 12 ( ^13C ) can also be used.
The graphitized carbon 13 is prepared by the following method. First, the concentration of carbon 13 is increased to 99% or higher by a conventionally known isotope separation method such as centrifugal separation, laser diffusion, or electromagnetic isotope separation. Next, the separated concentrated isotope carbon 13 is heated from room temperature in a 1 atm atmosphere of nitrogen (N ₂ ) gas as an example of an inert gas and heated at 2500 K to 3500 K for 3 hours or more. To do. The powder of carbon 13 obtained by heating is pulverized in an agate mortar to form a fine powder of about several tens of μm, and this fine powder is used.

(Operation of Example 1)
In the target material manufacturing apparatus 11 for generating gamma rays of Example 1 having the above-described configuration, when carbon 13 heated at 3000 K is used, the surface layer 3b of carbon 13 is uniformly formed on the surface of the target substrate 3a. Confirmed by At this time, when carbon 13 in a state obtained by centrifugation and carbon 13 after heating are measured by an X-ray diffraction method, carbon 13 in a state where isotopes are separated is amorphous (amorphous) carbon. On the other hand, it was found that the carbon 13 after heating at 2500 K or more was converted to a graphite crystal.

  Therefore, in the target material manufacturing apparatus 11 for generating gamma rays of the first embodiment, the uniform surface layer 3b can be formed by graphitizing the carbon 13 and using the graphitized carbon 13. Further, the produced carbon film has high adhesion, and does not deteriorate the purity of a valuable and expensive carbon sample, and can be formed with a necessary thickness without loss (high yield). And it can form into a film thicker than before. Furthermore, since the target material manufacturing apparatus 11 can be executed at room temperature and does not involve heating of the target substrate 3a, the target substrate 3a is not deformed or altered, and can be taken out immediately after the carbon film is formed. . Therefore, the gamma ray source device 1 of Example 1 uses the target material 3 having the surface layer 3b of the carbon 13 that has a uniform thickness, extremely high adhesion and durability, and does not decrease in film thickness due to charged particle beam irradiation. Thus, stable gamma rays can be generated.

(Deposition experiment)
(Experimental example 1)
In Experimental Example 1, the amorphous powder was crystallized from the flaky amorphous carbon 13 by high-temperature treatment at 2500 K to 3500 K, and the graphitized powder was pulverized into a fine powder having an average particle size of 40 μm in an agate mortar. Using this fine powder, a gold backing film having a thickness of 0.2 mm was used as the electrode substrate 13, and a DC high voltage of 16 kV was applied by the target material manufacturing apparatus 11 of Example 1 to form a carbon 13 film. As a result, a target was produced in which a uniform carbon film having high adhesion and high yield was formed on the surface of the target substrate (first electrode substrate) 3a. The thickness of the carbon film when the target material manufacturing apparatus 11 was operated for 3 hours was 500 μm / cm ² , six times that of amorphous carbon. The uniformity of the carbon 13 film was within 2% at the maximum regardless of the location. As a result of the experiment, the maximum carbon film thickness was 5000 μg / cm ² .
(Experimental example 2)
In Experimental Example 2, a target was produced under the same conditions and the same apparatus except that carbon 12 was used instead of carbon 13.

(Comparative Example 1)
In Comparative Example 1, a concentrated isotope amorphous carbon powder without high-temperature treatment was deposited on the surface of a gold backing film having a thickness of 0.2 mm by an electron beam evaporation method to form a carbon film having a thickness of 80 μg / cm ^2. Produced.
(Comparative Example 2)
In Comparative Example 2, a concentrated isotope amorphous carbon powder without high-temperature treatment was produced in the same manner as in Experimental Example 1 using the target material production apparatus 11 of Example 1, and a target having a thickness of ˜100 μg / cm ² was produced. did.

The targets of Experimental Examples 1 and 2 and Comparative Examples 1 and 2 were set in an electrostatic accelerator, and each target was irradiated with an ion beam having an energy of 3.2 MeV, Ne ⁺ , 3 μA, and a beam spot diameter of 3.5 mm. In Comparative Example 1, the carbon layer peeled off from the gold backing film 45 minutes after the start of irradiation. In Comparative Example 2, small cracks occurred 2 hours after the start of irradiation, gradually spread over the whole, and finally peeled off.
In Experimental Examples 1 and 2, both were irradiated for 25 hours, but peeling, film thickness reduction, and deformation did not occur at all and were stable. In Experimental Examples 1 and 2, a plurality of carbon films were formed by increasing the thickness. However, the carbon films were not damaged and were stable regardless of the thickness.
In addition, the commercially available carbon foil (comparative example 3) produced by thermally decomposing methane gas was damaged within 20 minutes under the same irradiation conditions.

(Graphitization experiment)
Next, an experiment was conducted to verify the relationship between the temperature at which the high temperature treatment is performed and the graphitization.
(Experimental conditions)
The separated concentrated isotopic amorphous carbon flakes (500 mg) were placed in a crucible of a vacuum high-temperature sintering furnace and heated in a nitrogen gas atmosphere for 5 hours. Experiments were performed at a heating temperature of 2000K (Comparative Example 3), 2500K (Experimental Example 3), 3000K (Experimental Example 4), and 3500K (Experimental Example 5).
When the powder produced at each temperature was analyzed by X-ray spectrum (X-ray diffraction method), it did not appear in Comparative Example 3, but in Experimental Examples 3 to 5, spectra showing graphite crystals were observed. When this high-temperature-treated powder was pulverized in an agate mortar and formed into a film using the target material manufacturing apparatus 11 of Example 1, a uniform film could be formed.

(Example of change)
As mentioned above, although the Example of this invention was explained in full detail, this invention is not limited to the said Example, A various change is performed within the range of the summary of this invention described in the claim. It is possible. Modification examples (H01) to (H06) of the present invention are exemplified below.
(H01) In the above embodiment, the target material manufacturing apparatus 11 for generating gamma rays was operated at room temperature and atmospheric pressure. However, the present invention is not limited to this, and the powder storage space 19 is heated or cooled, evacuated, or high-pressure gas. It can be arbitrarily changed such as encapsulation. That is, the target material 3 can be produced in any of a vacuum pressure, an atmospheric pressure, and a high pressure state. At this time, since the discharge conditions vary depending on the temperature, pressure, powder size, etc., it is necessary to apply a voltage that does not discharge and reciprocates the particles with the power supply device.

(H02) In the above embodiment, gold (Au) is exemplified as the target substrate 3a. However, the target substrate 3a is not limited to this. For example, a metal material such as tantalum or copper can be used. In addition, ceramics such as alumina (aluminum oxide) can be used. In this case, since the ceramics and the like are insulative, the first electrode member 13 cannot be used in common with the first electrode member 13. It is necessary to dispose a ceramic target substrate on the surface side of the substrate. Needless to say, the target substrate 3a is preferably a substrate having a high performance of transmitting the generated gamma rays. Further, although the target substrate 3a and the electrode 13 are made common, it is also possible to form a surface layer 3b on the surface of the target substrate 3a by arranging the target substrate 3a inside the electrode 13 separately.

(H03) In the embodiment, the size of the device and the voltage to be applied are not limited to the values exemplified in the embodiment, and can be changed according to the design. At this time, since the distance between the electrodes and the voltage are related to the discharge conditions and the reciprocating motion of the powder, it is possible to set the voltage and the distance between the electrodes so as not to discharge and reciprocating. Further, the voltage to be applied is not limited to a voltage in which an AC voltage is superimposed on a DC voltage, and may be an AC voltage, only a DC voltage, or a pulsed voltage.
(H04) In the above examples, the powder pulverized with an agate mortar was exemplified, but the present invention is not limited to this, and the size is such that it can reciprocate by any method (although it varies depending on the specific gravity, etc., about 500 μm or less is preferable ). Further, since the diameter of the produced spherical particles depends on the size of the powder F, it is desirable to make the size of the powder F uniform in order to reduce the variation in the diameter of the produced spherical particles.

(H05) In the above embodiment, the positron beam source is exemplified as the charged particle beam source. However, the present invention is not limited to this, and a charged particle beam source capable of generating gamma rays from carbon 13 is used. A charged particle beam source such as an ion beam generator can be used.
(H06) In the above embodiment, the temperature, gas used, pressure and time for graphitizing amorphous carbon 12 and carbon 13 are not limited to the exemplified conditions, and the carbon 12 and carbon 13 are in a state of amorphous to graphite. It can be changed to any number that can be For example, the gas used can be a rare gas as an inert gas, and can also be in a vacuum state. Also, the pressure can be higher or lower than 1 atm, and the time can be changed to a time sufficient for graphitization.

FIG. 1 is an explanatory diagram of a gamma ray source apparatus according to Embodiment 1 of the present invention. FIG. 2 is an overall explanatory view of the target material manufacturing apparatus for generating gamma rays according to the first embodiment of the present invention. FIG. 3 is an explanatory diagram in the case where the film is formed by the conventional carbon 13 film forming method.

Explanation of symbols

1 ... gamma ray source device,
2 ... charged particle beam source,
2a ... charged particles,
3 ... Target material for generating gamma rays,
3a, 13 ... target substrate,
3b ... surface layer,
11 ... Target material manufacturing apparatus for generating gamma rays,
13, 14 ... electrodes,
18 ... Insulating member,
19 ... space,
E: Power supply device.

Claims (1)

  A target substrate is disposed in a space formed by at least a pair of electrodes and an insulating member disposed between the electrodes, and carbon particles as a surface layer forming powder are accommodated, and the surface layer is formed between the electrodes. When a target material having a carbon surface layer formed on the surface of the target substrate is applied by applying a voltage for reciprocating the powder for use, amorphous carbon particles are heated to a high temperature of 2500 K or higher in an inert gas atmosphere. A target material comprising carbon particles that have been graphitized by being heated.

Images