TARGET STRUCTURE AND RADIATION GENERATOR

Technical Field

[0001] The present invention relates to a target structure that includes a target formed on a substrate and a radiation generator that includes the target structure.

Background Art

[0002] A radiation generator generates radiation by emitting electrons from an electron source and colliding the electrons with a target. When the electrons emitted from the electron source enter the target, most of their incident energy is converted into heat. Thus, the target generates heat. If the heat generated in the target fails to be sufficiently dissipated, the heat damages the target, which results in failure to obtain a stable amount of radiation.

[0003] PTL 1 discloses an X-ray anode that includes a target thin film disposed on a surface of a diamond

substrate and that rapidly dissipates heat generated in the target thin film from the diamond substrate.

Citation List

Patent Literature

[0004] PTL 1 PCT Japanese Translation Patent Publication No. 2003-505845 Summary of Invention

Technical Problem

[0005] The use of a diamond substrate enhances a heat dissipation effect. Thus, a stable amount of radiation can be obtained in cases of radiation output in a relatively short time.

[0006] However, targets in medical applications often require radiation output with a stable amount of radiation over a prolonged period. A long period of electron

irradiation of a target causes poor adhesion between a diamond substrate and a target thin film, which results in degradation of heat dissipation characteristics. Even in cases where an interlayer is interposed between the diamond substrate and the target thin film so as to improve the initial adhesion therebetween, a long period of electron irradiation causes poor adhesion between the diamond substrate and the interlayer, which results in degradation of heat dissipation characteristics.

[0007] The present invention provides a target structure that allows heat generated in a target to be rapidly dissipated due to a target being disposed on a specific surface of a diamond substrate, and a radiation generator that includes the target structure.

Solution to Problem

[0008] A target structure according to the present invention includes a target layer that is formed on a diamond substrate and emits radiation in response to irradiation with electrons. In at least a portion of a surface of the diamond substrate on which the target layer is formed, a dominant crystal plane is a (100) plane.

Advantageous Effects of Invention

[0009] The target structure according to the present invention has a structure that facilitates a bond between the diamond substrate and the target layer, thereby

improving adhesion therebetween. This allows heat generated in the target layer to be rapidly dissipated to the diamond substrate. Thus, the use of the target structure according to the present invention can produce a stable amount of radiation over a prolonged period.

Brief Description of Drawings

[0010] Fig. 1 is a cross-sectional view illustrating a configuration of an example of a radiation generator that includes a target structure according to the present invention;

Figs. 2A to 2D are cross-sectional views illustrating four exemplary target structures according to the present invention;

Fig. 3 is a cross-sectional view illustrating a

configuration of an example of a radiation source that includes a target structure according to the present invention; and

Fig. 4 is a drawing illustrating a configuration of a radiography system that includes a radiation generator according to the present invention.

Description of Embodiments

First Embodiment

[0011] Referring to Fig. 1, a radiation generator 13 that includes a target structure according to the present

invention will be described.

[0012] The radiation generator 13 includes a radiation source 1 and a driving circuit 14, which are disposed inside an envelope 11 having an emission window 10. The remaining space of the envelope 11 is filled with an insulating liquid 17 such as an insulating oil. The envelope 11 has a

grounding terminal 16.

[0013] The envelope 11 can be composed of a high-strength material, such as iron, stainless steel, or brass. The emission window 10 allows radiation 15 emitted from the radiation source 1 to exit to the outside of the radiation generator 13 therethrough. The emission window 10 can be composed of a plastic that contains no heavy elements, such as an acrylic resin or a polymethyl methacrylate resin.

[0014] The radiation source 1 is constituted by a vacuum vessel 6 that includes a transmission window 9, an electron source 3 disposed inside the vacuum vessel 6, and a target structure 8 held by a shield 7.

[0015] The electron source 3 includes an electron emission unit 2 that emits electrons. The electron source 3 can have any electron emission mechanism as long as the amount of electrons emitted by the electron source is controllable from the outside of the vacuum vessel 6. Examples of the electron source include a hot-cathode electron source and a cold-cathode electron source. The electron source 3 is electrically connected to the driving circuit 14 disposed outside the vacuum vessel 6 via a terminal 4 that penetrates through the vacuum vessel 6. This allows control of the amount of electron emission and the ON/OFF state of electron emission. Electrons emitted by the electron emission unit 2 are accelerated by applying an accelerating voltage to form an electron beam 5 having an energy of about 10 to 200 keV. Then, the electron beam 5 enters a target layer, which is disposed in the target structure 8 so as to face the

electron emission unit 2. The accelerating voltage is applied between the electron source 3 and the shield 7.

[0016] The target structure 8 includes a diamond substrate (hereafter, may be referred to as simply "substrate") and the target layer formed on the substrate. The target layer faces the electron emission unit 2. The diamond substrate is prepared so that the (100) plane is the dominant crystal plane in at least a portion of the surface of the substrate on which the target layer is formed. The term "(100) plane" refers to a plane represented by a Miller index of (100) . The target structure 8 is clamped with the shield 7 that includes a rear shield 7a and a front shield 7b, thereby being maintained in position.

[0017] The rear shield 7a includes an electron beam passing hole that guides the electron beam 5 to an electron beam irradiation region (radiation generation region) of the target layer. The rear shield 7a also serves as a shield against radiation emitted backward from among radiation emitted from the electron beam irradiation region in all directions. The front shield 7b has an aperture through which desired radiation emitted forward exits from among radiation emitted from the electron beam irradiation region in all directions. The front shield 7b also serves as a shield against unwanted radiation. The shield 7 can be composed of any material having electric conductivity and thermal conductivity. Specifically, the shield 7 can be composed of a material that intercepts radiation generated at 30 to 150 kV. Examples of such a material include tungsten, tantalum, molybdenum, zirconium, niobium, and alloys thereof.

[0018] The shield 7 and the target structure 8 can be joined with each other by brazing (not shown) . A brazing material is suitably selected depending on the material of the shield 7, the allowable temperature limit of the shield 7, and the like. For example, in cases where the

temperature of a target structure 8 is greatly increased, a Cr-V-based alloy, a Ti-Ta-Mo-based alloy, a Ti-V-Cr-Al-based alloy, a Ti-Cr-based alloy, a Ti-Zr-Be-based alloy, a Zr-Nb- Be-based alloy, or the like can be used as a brazing metal for a high-melting-point metal. In addition, a brazing material containing a Au-Cu alloy as a main component, a nickel solder, a brass solder, a silver solder, or a

palladium solder can be used.

[0019] The vacuum vessel 6 can be composed of glass, a ceramic, or the like. The vacuum vessel 6 has an internal space 12 that has been evacuated (depressurized) . The vacuum vessel 6 includes the transmission window 9 that allows the radiation 15 generated in the target layer of the target structure 8 to pass therethrough and exit to the outside through the emission window 10. Accordingly, the transmission window 9 can be composed of a material that can maintain an adequate degree of vacuum in the vacuum vessel 6 and that minimizes attenuation of the radiation 15 that passes therethrough. Examples of such a material include beryllium, carbon, diamond, and glass. Desirably, the material contains no heavy elements. The internal space 12 can be maintained at a degree of vacuum such that the mean free path of electrons is maintained, in other words, electrons can fly between the electron source 3 and the target layer that emits radiation. The degree of vacuum can be 1 x 10 ^~4 Pa or less. The degree of vacuum can be suitably selected in consideration of the electron source used, the operational temperature, and the like. In the case of a cold-cathode electron source or the like, the degree of vacuum can be 1 x 1CT ⁶ Pa or less.

[0020] Now referring to Figs. 2A to 2D, the target

structure 8 according to the present invention will be described.

[0021] Fig. 2A illustrates a target structure 8 including a single-crystal diamond substrate 8a and a target layer 8b formed on a (100) plane of the single-crystal diamond

substrate 8a. The (100) plane of diamond has a surface energy of σ = 9.2 J/m ² , which is greater than σ = 5.3 J/m ² of a (111) plane and σ = 6.5 J/m ² of a (110) plane. Therefore, the (100) plane bonds with the target layer 8b more easily than other crystal planes. The present invention employs these characteristics to enhance adhesion between the

substrate 8a and the target layer 8b. The (100) plane of the diamond substrate 8a need not necessarily be exposed over the entirety of the surface of the diamond substrate 8a. However, the dominant crystal plane needs to be the (100) plane in at least a portion of the surface of the substrate on which target layer 8b is formed. Preferably, the ratio of surface occupied by the (100) plane is 50% or more of a portion of the surface of the substrate on which the target layer 8b is formed. The same applies to the below-mentioned diamond substrates shown in Figs. 2B to 2D.

[0022] The (100) plane may be obtained during manufacture of the single-crystal diamond by promoting the growth of the

(100) plane, or alternatively, after the manufacture of the single-crystal diamond by cutting it so that the (100) plane is exposed at the surface of the single-crystal diamond.

[0023] The target layer 8b can be generally composed of a metal material having an atomic number of 26 or more. Metal materials having a higher thermal conductivity and a higher melting point are more suitable. Specifically, metal materials such as tungsten, molybdenum, chromium, copper, cobalt, iron, rhodium, palladium, rhenium and alloy

materials thereof can be suitably used. The thickness of the target layer 8b is 1 to 15 microns. However, the optimal thickness of the target layer 8b varies because the penetration depth of the electron beam into the target layer 8b, that is, the radiation generation region also varies depending on the accelerating voltage.

[0024] The target layer 8b is formed on the diamond substrate 8a by sputtering or vapor deposition.

Alternatively, the target layer 8b having a predetermined thickness can be formed by rolling or polishing and then diffusion-bonded to the diamond substrate 8a at a high temperature and pressure.

[0025] Fig. 2B illustrates a target structure 8 including a single-crystal diamond substrate 8a having asperities on the (100) plane of the substrate and a target layer 8b formed on the single-crystal diamond substrate 8a. Although not only the (100) plane but also other crystal planes, such as the (111) and (110) planes, of the single-crystal diamond substrate 8a are exposed since the asperities include edges and tapers in a microscopic view, it is considered that the (100) plane of the single-crystal diamond substrate 8a is mainly exposed. Particularly, in cases where a single- crystal diamond substrate 8a has asperities of about 0.01 to 0.2 microns, the (100) plane is dominant and the size of the asperities is appropriate, which produces a synergistic effect of chemical bonding and physical adhesion. The term "size of asperities" refers to a height difference between the peaks and the valleys in the surface of the substrate.

[0026] The asperities on the surface of the substrate can be formed by physically polishing the surface with a metal bonded abrasive, a scaife, or the like. In this case, the size of asperities varies depending on the type of metal bonded abrasive or the type of diamond abrasive grain used for scaife polishing. Generally, polishing with a metal bonded abrasive produces asperities having a size of about 0.1 to 2.0 microns. Scaife polishing produces asperities having a size of about 0.05 to 0.2 microns.

[0027] Alternatively, other than polishing, the asperities may be formed by heating a diamond substrate at about 750°C to 850°C to cause the surface of the substrate to be

slightly graphitized or amorphized, and then removing the graphitized or amorphized product from the surface with hydrofluoric acid or the like. The heating may also be performed using a laser beam. According to this method, the asperities can be arranged regularly at intervals of a few microns to several tens of microns. The asperities, which are formed by heating a diamond substrate by the above heating method and then removing the product with

hydrofluoric acid or the like, generally have a size of about 0.01 to 0.1 microns, which varies depending on the heating temperature.

[0028] In cases where the asperities of the surface of the substrate are small (for example, 0.01 to 0.2 microns) as shown in Fig. 2B, the surface of the target layer 8b formed on the substrate becomes substantially uniform.

[0029] In cases where the asperities of the surface of the substrate are large (for example, 1 micron or more) as shown in Fig. 2C, asperities occur on the surface of the target layer 8b so as to be aligned with the asperities of the surface of the substrate. However, a diameter of a region irradiated with an electron beam, that is, a focal diameter of the electron beam is generally about 0.1 to 1.5 mm, which is sufficiently large compared with the asperities of about a few microns. Thus, it is considered to be substantially uniform.

[0030] As described above, preferably, the surface of the substrate has asperities having a size of 0.01 microns or more and 2.0 microns or less.

[0031] Fig. 2D illustrates a target structure 8 including an interlayer 8c that is composed of titanium, niobium, or the like and formed on a surface of a substrate, and a target layer 8b formed on the interlayer 8c. Formation of the interlayer 8c on the (100) plane further enhances

adhesion. It is needless to say that, in this case, the diamond substrate shown in Fig. 2D may have asperities on the surface of the substrate as in Figs. 2B and 2C.

[0032] The target structure according to the present invention can be applied to both a reflection-type target and a transmission-type target. In a reflection-type target, the sizes of a target and a diamond substrate can be

increased in order to enhance the heat dissipation capacity. On the other hand, a transmission-type target is required to have a thin target and a thin diamond substrate, and thus it is difficult to increase the sizes of a target and a diamond substrate. Thus, the target structure according to the present invention is more suitably applied to a

transmission-type target.

Second Embodiment

[0033] Referring to Fig. 3, a radiation generator

including the target structure according to another

embodiment of the present invention will be described. This embodiment is similar to the first embodiment, except that the radiation source 1 has a different configuration.

[0034] In this embodiment, a target structure 8 also serves as a vacuum seal. A vacuum chamber 18 defines an enclosed space maintained at a vacuum by means of a flange 19 and a shield 7 which holds the target structure 8. The target structure 8 also serves as the transmission window 9 of the first embodiment. Therefore, a member corresponding to the transmission window 9 in the first embodiment can be omitted, which contributes to a reduction in attenuation of radiation emitted to the outside.

Third Embodiment

[0035] Referring to Fig. 4, a radiography system that includes the radiation generator according to the first or second embodiment will be described.

[0036] A radiation generator 13 is disposed so as to face a panel sensor 22 (radiation detector) , and a radiographic object 21 is interposed therebetween. While the radiation generator 13 emits radiation 15, a controller 23 controls the radiation generator 13 and the panel sensor 22 in a cooperative manner to allow the radiation 15 that has passed through the radiographic object 21 to be detected by the panel sensor 22. Image data output from the panel sensor 22 are analyzed by a personal computer 24 and displayed as an image. The radiographic object 21 may be a human body, an animal, an electronic circuit, or the like.

Examples

Formation of Asperities

[0037] The (100) plane of the diamond substrate was grown by a super-high pressure synthesis method, and the resulting diamond substrate was used as a diamond substrate for

deposition. The diamond substrate had a disk shape

(cylindrical shape) with a diameter of 5 mm and a thickness of 1 mm. Asperities were formed on a surface of the diamond substrate by one of the following methods of Examples 1 to 5. Example 1

[0038] The (100) plane of the diamond substrate for

deposition was polished with a metal bonded polishing plate that used diamond abrasive powder with a grain size of #200. Example 2

[0039] Diamond abrasive paste with a grain size of #1000 was diluted with olive oil and dropped onto a scaife. The (100) plane of the diamond substrate for deposition was polished using the scaife rotating at 1500 rpm. Example 3

[0040] The (100) plane of the diamond substrate for deposition was polished by the same polishing method under the same polishing conditions as in Example 2.

Example 4

[0041] The diamond substrate for deposition was maintained at 800°C for 10 minutes so that the surface of the substrate was graphitized. Then, the graphitized portion was removed by chemical etching with hydrofluoric acid.

Example 5

[0042] The (100) plane of the diamond substrate for deposition was irradiated with a laser beam at an output power of 100 W for an irradiation time of 0.2 msec so that the surface of the substrate was graphitized. Then, the graphitized portion was removed by chemical etching with hydrofluoric acid.

Deposition of Target Layer

[0043] Each of the diamond substrates obtained in Examples 1, 2, 4, and 5 was previously subjected to UV-ozone ashing so that organic matters on the (100) plane of the substrate was removed. Then, a tungsten layer having a thickness of 7 microns was formed as a target layer on each substrate by sputtering using Ar as a carrier gas. Thus, target

structures 8 were formed.

[0044] The diamond substrate obtained in Example 3 was also cleaned by the same method as described above so that organic matters on the surface of the substrate was removed. Then, a titanium layer having a thickness of 0.1 microns was formed as an interlayer on the substrate by sputtering using Ar as a carrier gas, and a tungsten layer having a thickness of 7 microns was formed as a target layer on the titanium layer. Thus, a target structure 8 was formed.

Preparation of Radiation Source

[0045] The target structure 8 manufactured by the above method was integrally attached to a shield 7 composed of tungsten. Next, as shown in Fig. 1, the target structure 8 was disposed so as to face an electron source 3 that was an impregnated thermionic-emission gun and then sealed in a vacuum to form a radiation source 1. In the same manner as above, five radiation sources were prepared.

Comparative Example 1

[0046] In Comparative Example 1, the (111) plane of the diamond substrate was grown by a super-high pressure

synthesis method, and the resulting diamond substrate was used as a diamond substrate for deposition. Diamond

abrasive paste with a grain size of #1000 was diluted with olive oil and dropped onto a scaife. The (100) plane of the diamond substrate for deposition was polished using the scaife rotating at 1500 rpm. The diamond substrate was cleaned so that organic matters on the surface of the substrate was removed. Then, a tungsten layer having a thickness of 7 microns was formed as a target layer on the substrate. Thus, a target structure 8 was formed. The method for removing the organic matters and the conditions and method for deposition of the target layer were the same as in Examples. Then, a radiation source including the target structure 8 was prepared by the same method as the preparation of the radiation sources of Examples.

Comparative Example 2

[0047] In Comparative Example 2, a polycrystalline diamond substrate produced by CVD was used as a diamond substrate for deposition. Then, a tungsten layer having a thickness of 7 microns was formed as a target layer on the substrate. Thus, a target structure 8 was formed. The conditions and method for deposition of the target layer were the same as in Comparative Example 1. Then, a radiation source

including the target structure 8 was prepared by the same method as the preparation of the radiation source of

Comparative Example 1.

Measurement of Orientation and Asperities of (100) Plane

[0048] The crystal planes of the diamond substrate were measured by the X-ray diffraction method. The ratio of

(100) plane is a ratio of diffracted intensity corresponding to the (100) plane relative to a total of 100 for all crystal planes. The size of asperities (surface roughness) of the crystal plane on the surface of the substrate was measured with a surface roughness tester.

Generation of Radiation and Method for Measuring

Radiation

[0049] The amount of radiation emitted from each of the radiation sources in Examples and Comparative Examples was measured with a semiconductor-type dosimeter. The operating conditions of the radiation sources were as follows:

acceleration voltage of 100 kV, current of 2 mA, irradiation time of 10 msec, and rest time of 90 msec.

Evaluation Result

[0050] Table 1 shows the surface roughness and the ratio of (100) plane of each of target structures in Examples and Comparative Examples. The surface roughness in Example is a maximum value of 1.2 microns in Example 1 and a minimum value of 0.01 microns in Example 5. Values in the other

Examples are in the middle range therebetween. The ratio of (100) plane is a minimum value of 50 in Example 1 and 90 or 95 in the other Examples. The surface roughnesses in

Comparative Examples 1 and 2 are 0.1 microns and 0.2 microns, respectively. The ratio of (100) plane in Comparative

Examples 1 and 2 are 5 and 20, respectively.

[0051] Table 2 shows a change in amount of radiation emitted from each of the radiation sources in Examples and Comparative Examples. During an operating time of 100 hours, the radiation sources in Examples 1 to 5 and Comparative Examples 1 and 2 show no decrease in the amount of radiation with time of pulse-generation. However, after 300 hours of operation, difference in amount of radiation gradually began to increase. After 700 hours, the amounts of radiation in Comparative Examples 1 and 2 are decreased to 50% of the respective initial values, whereas the amounts of radiation in Examples 1 to 5 are maintained at 75% to 80% of the respective initial values. Thus, it was confirmed that the present invention is effective.

[0052 ]

[Table 1]

[0053] [Table 2]

Other Embodiments

[0054] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0055] This application claims the benefit of Japanese Patent Application No. 2012-022976, filed February 06, 2012, which is hereby incorporated by reference herein in its entirety.

Reference Signs List

[0056] 8 target structure

8a diamond substrate

8b target layer