Patent Grant
9589760
This application claims priority from on Japanese Patent Application No. 2013-215585 filed Oct. 16, 2013, the contents of all of which are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to an X-ray generator to be used for an X-ray inspection device for industrial or medical use, or various types of X-ray spectrometers or measuring devices using the diffraction or refraction of X-rays, and in particular to an X-ray generator in a system where X-rays are generated by making electrons collide with a target in a vacuumed atmosphere within an X-ray tube.
2. Description of Related Art
In X-ray generators, excluding special ones, a target and an electron source are placed within an X-ray tube that has been vacuumed, and electrons generated by the electron source are accelerated and made to collide with a target as an electron beam so that X-rays are generated. The generated X-rays are taken outside through an irradiation window that air tightly seals the inside of the X-ray tube from the outside.
Due to the difference between X-ray tubes in the means for holding a target, irradiating an electron beam, or taking out X-rays, the structure of the X-ray tube in the vicinity of the irradiation window is categorized as a transmission type or a reflection type as shown in the schematic diagrams of
In
Meanwhile, in
In both of the above-described transmission-type and reflection-type X-ray tubes, light metals such as Be or Al are used as the material of the irradiation windows 103 and 203.
Incidentally, the energy of the X-rays generated on the target is approximately 1% of the energy of the electron beam that strikes the target, and thus, the remaining 99% is converted to thermal energy. As a result, the temperature of the target 102 in the transmission-type X-ray tube shown in
Meanwhile, in the reflection-type X-ray tube shown in
When the temperature of the irradiation window of the X-ray tube becomes high, it may cause various problems such as a gas released into the vacuumed atmosphere within the X-ray tube, a load on a brazed portion in a vacuum due to thermal stress, or a thermal effect in the case where the object to be inspected approaches the irradiation window from the air side.
Therefore, various means for suppressing the increase in the temperature of the irradiation window have been provided according to the prior art. For example, the irradiation window or its periphery are water-cooled or air-cooled, or such a structure is adopted that the target in a transmission-type X-ray tube makes close contact with a diamond, which is a material with excellent thermal conductivity, so that the heat is led to a heat radiator (see Patent Document 1). In a reflection-type X-ray tube, such a structure is adopted that a shield member is provided within the X-ray tube so as to prevent the electron beam reflected from the target from colliding with the irradiation window (see Patent Document 2).
In the case where a material with poor thermal conductivity is adopted as the material of the irradiation window, which is not usual, the point irradiated with the electron beam easily reaches the melting point in a vacuum.
In order to prevent the temperature of the irradiation window from rising in the X-ray tube, it is most effective to reduce the intensity of the electron beam that strikes the irradiation window, which is the basic cause of heat generation. In a transmission-type X-ray tube, however, a reduction in the intensity of the electron beam means a reduction in the amount of generated X-rays, which affects the performance of the device.
In the case where the irradiation window is forcefully cooled by water or air, the space and cost for it are required. The technology disclosed in Patent Document 2, where a member for shielding electrons is provided in a reflection-type X-ray tube, also requires space and cost for the member to be placed within the X-ray tube.
Furthermore, an increase in the temperature can be reduced by modifying the irradiation window itself. For example, the irradiation window can be made thicker so that the thermal capacity is increased and the transfer of heat to the periphery is made easier, and thus, an increase in the temperature can be expected to be reduced. In the case of X-ray tubes used for non-destructive inspection, however, such a problem arises that the maximum enlargement ratio (magnification ratio of a radiograph) becomes smaller as the distance between the X-ray generating point (X-ray focal point) and the object to be inspected is increased in order to project an enlarged image of the object to be inspected. Such a problem also arises that the amount of X-rays absorbed by the irradiation window increases, and thus, the amount of X-rays that can be used effectively is reduced.
In the technology disclosed in Patent Document 1 where the heat is led to a heat radiator by making the target make close contact with a material having excellent thermal conductivity, the same problem as above where the magnification ratio is affected arises because the heat radiator protrudes from the target to the side in which X-rays are irradiated, and the irradiation window is provided in its end portion. When a material with excellent thermal conductivity is used for the irradiation window, an increase in the temperature in a local portion of the irradiation window can be suppressed. However, heat conducts uniformly throughout the irradiation window, and therefore, it is possible for the thermal effects on the object to be inspected to be greater when the object to be inspected approaches from the air side in order to increase the magnification ratio of the radiograph.
The present invention is provided in view of the above-described situations, and an object thereof is to provide an X-ray generator having a compact configuration where heat generated in the irradiation window can be prevented from conducting to a desired portion in accordance with the purpose of use, the method of use or the structure of the X-ray tube.
In order to achieve the above-described object, the X-ray generator according to the present invention is an X-ray generator for releasing X-rays generated by irradiating a target placed in a vacuumed atmosphere within an X-ray tube with an electron beam from an electron source to the outside of the X-ray tube through an irradiation window that air tightly seals an opening provided in the above-described X-ray tube, and is characterized in that the above-described irradiation window has thermal anisotropy where the thermal conductivity is different between the direction in which the irradiation window spreads and the direction of the thickness of the irradiation window.
In the present invention, either the structure where the thermal conductivity in the direction in which the irradiation window spreads is smaller than the thermal conductivity in the direction of the thickness of the irradiation window or the structure where the thermal conductivity in the direction in which the irradiation window spreads is greater than the thermal conductivity in the direction of the thickness of the irradiation window can be selected for the above-described irradiation window.
Concretely, an irradiation window made of a thermally anisotropic material, more concretely, an irradiation window made of a thermally anisotropic graphite can be used as the irradiation window in the present invention.
Furthermore, a multilayer material where materials having different thermal conductivities are alternately layered on top of each other can be used as another material for the irradiation window in the present invention.
Though the present invention can be applied to X-ray generators using either X-ray tube, transmission-type or reflection-type, the working effects are especially large when the invention is applied to a transmission-type X-ray tube where a target material is layered on top and integrated with the surface of the irradiation window on the inside of the X-ray tube.
An object of the present invention is achieved by providing thermal anisotropy to the irradiation window so that the main direction in which the heat conducts from the irradiation window is set. An object having thermal anisotropy is an object having different thermal conductivities depending on the direction of the object. In the case of an object in plate form, for example, the thermal conductivity is different between the direction of the thickness of the object and the direction in which the object spreads.
That is to say, less heat is conducted in a desired direction when an irradiation window having thermal anisotropy is provided so that the direction in which the thermal conductivity is smaller is matched with the desired direction. For example, the thermal conductivity in the direction in which the irradiation window spreads can be made smaller than the thermal conductivity in the direction of the thickness of the irradiation window so that the heat generated by the collision of electrons with the irradiation window mainly conducts in the direction of the thickness of the irradiation window, and thus is released to the air in the direction towards the air side. As a result, the heat can be prevented from conducting to the inside of the X-ray tube, and thus, the load on the brazed portions in the vacuum due to thermal stress and the load on the O ring for air tightly sealing the irradiation window can be reduced.
Conversely, the heat from the irradiation window mainly conducts in the direction in which the window spreads when the thermal conductivity in the direction in which the irradiation window spreads is greater than the thermal conductivity in the direction of the thickness of the irradiation window. In this case, heat can be prevented from conducting from the irradiation window to the air side, and thus, thermal effects can be reduced when the object to be inspected approaches the irradiation window.
The above-described irradiation window having thermal anisotropy can be implemented by using a thermally anisotropic material such as graphite. In addition, an irradiation window where the thermal conductivity in the direction in which the irradiation window spreads is greater than that in the direction of the thickness of the irradiation window can also be implemented by layering materials having different thermal conductivities on top of each other. That is to say, an irradiation window where good thermal conductors and poor thermal conductors are layered on top of each other allows heat to conduct throughout each layer of the good conductors while making it difficult for heat to conduct through the adjacent layers of poor conductors. As a result, the thermal conductivity in the direction of the thickness of the irradiation window is relatively smaller so that heat mainly conducts in the direction in which the irradiation window spreads, and thus, thermal anisotropy is achieved on the whole.
According to the present invention, the irradiation window of the X-ray tube has thermal anisotropy, and therefore, the direction in which the heat from the irradiation window mainly conducts can be regulated to a specific direction in accordance with the purpose or the structure of the X-ray tube. In the case where the brazed portions in the X-ray tube are desired to be prevented from being affected by thermal stress, for example, an irradiation window where the thermal conductivity in the direction in which the irradiation window spreads is smaller than the thermal conductivity in the direction of the thickness of the irradiation window can be used so as to lead heat mainly to the air side. In the case where an object to be inspected is desired to be less affected by heat in the X-ray tube for the purpose of gaining a radiograph of an enlarged image by making the object to be inspected approach the X-ray focal point, an irradiation window where the thermal conductivity in the direction in which the irradiation window spreads is greater than the thermal conductivity in the direction of the thickness of the irradiation window can be used to lead the heat mainly to the X-ray tube side. As described above, the X-ray tube can be fabricated based on the selection of the type of irradiation window in accordance with the portion to which it is desired to prevent heat from conducting.
In addition, it is not particularly necessary to add a member for suppressing heat conduction according to the present invention, and thus, the space for this is not necessary, and therefore, the structure can be made compact. Naturally, a member for conducting heat may be used together in order to further increase the cooling efficiency.
In the following, embodiments of the present invention are described in reference to the drawings.
The example in
This example is characterized in that the irradiation window 3 is formed of a thermally anisotropic material, for example, thermally anisotropic graphite. As shown in
As described above, the energy of the X-rays generated on the target 2 is approximately 1% of the energy of the electron beam B that has struck the target 2, and the remaining 99% is converted to thermal energy. In this type of X-ray tube, the target 2 is usually a thin film of several μm, and the heat generated on the target 2 is transferred to the irradiation window 3. The irradiation window 3 makes contact with the target holder 1, and therefore, the majority of heat is usually transferred to the target holder 1. In this embodiment, however, the heat that has been transferred to the irradiation window 3 mainly conducts in the direction of the thickness of the irradiation window 3 so as to be released to the air side. Accordingly, it becomes difficult for the heat that has been generated when the electron beam B strikes the target 2 and that has been transferred to the irradiation window 3 to be transferred to the X-ray tube (target holder 1) in this embodiment, which therefore is useful for X-ray tubes where it is necessary to take into consideration the thermal effects on the brazed portions and the O ring portions of the X-ray tube.
In the above-described embodiment, as shown in
In the example in
As described in the example in
Though in the example in
That is to say, as shown in
The irradiation window 23 in
Here, examples of the materials 23a with good thermal conductivity used for the irradiation window 23 in
The degree of thermal anisotropy of the irradiation window according to the present invention is described below. A light metal is used for the conventional irradiation window according to the prior art in order to make X-rays transmit well, and the thermal conductivity of the irradiation window is approximately 100 to 300 W/(m·K). According to the present invention, it is desirable for thermal anisotropy to mean that the ratio of the greater thermal conductivity to the smaller thermal conductivity is at least 2 and possibly 10 or greater. In a preferable example, the thermal conductivity is 1000 W/(m·K) or greater in the direction in which the thermal conductivity is greater, and the thermal conductivity is 10 W/(m·K) or less in the direction in which the thermal conductivity is smaller.
Next, the effectiveness of the structure according to the embodiment in
The irradiation window 13 has thermal anisotropy in the direction shown in
In the simulations, as shown in
As is clear from the results of the simulations, the temperature on the surface of the irradiation window on the air side could be lowered by 23.6° C. by using the thermally anisotropic material so that heat on the irradiation window mainly conducts in the direction in which the irradiation window spreads.
The irradiation window 23 had a three-layer structure where a layer of a material 23b with poor thermal conductivity was sandwiched between two layers of materials 23a with good thermal conductivity, where the thickness of each layer was 0.1 mm, the total thickness was 0.3 mm, the thermal conductivity of the materials 23a with good thermal conductivity was 100 W/(m·K), and the thermal conductivity of the material 23b with poor thermal conductivity was 5 W/(m·K). Another simulation was carried out as a comparative example in a case where the irradiation window 23 was a single layer (thickness: 0.3 mm) made of a material with a thermal conductivity of 100 W/(m·K) as a whole.
Table 2 shows the results of the calculations in temperature increments (° C.) at the point irradiated with the electron beam B at a point in time when a state of thermal equilibrium was achieved under the supposition that the same amount of heat was generated in the same area as in the model in
It can be seen from the results of the simulations that the temperature of the surface of the irradiation window 23 on the air side could be lowered by 8.4° C. in the case where the irradiation window 23 had a multilayer structure with thermal anisotropy.
Though examples where the present invention is applied to a transmission-type X-ray tube are illustrated in the above, the present invention can be applied to the irradiation window of reflection-type X-ray tubes as in
1 Target holder
2 Target
3, 13, 23 Irradiation window
4 Metal layer
23
a Material with good thermal conductivity
23
b Material with poor thermal conductivity
B Electron beam
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-215585 | Oct 2013 | JP | national |
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| Communication dated May 30, 2016, from the State Intellectual Property Office of the P.R.C., in counterpart Chinese application No. 20140509911.7. |
| Communication dated Dec. 2, 2016, from the State Intellectual Property Office of People's Republic of China in counterpart Application No. 201410509911.7. |
| Number | Date | Country | |
|---|---|---|---|
| 20150103979 A1 | Apr 2015 | US |
