Abstract
An object of the invention is to provide an X-ray generator having a simple 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 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 through an irradiation window of the X-ray tube, the 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, and therefore, the thermal conductivity in the direction in which the heat from the irradiation window is desired not to conduct is made relatively smaller.
Claims
1. 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 said X-ray tube, characterized in that said irradiation window is formed of a thermally anisotropic material and 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.
2. The X-ray generator according to claim 1, characterized in that the thermal conductivity of said irradiation window 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.
3. The X-ray generator according to claim 1, characterized in that the thermal conductivity of said irradiation window 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.
4. The X-ray generator according to claim 1, characterized in that said thermally anisotropic material is graphite.
5. The X-ray generator according to claim 1, characterized in that a material of the target is layered on and integrated with a surface of said irradiation window on the vacuumed atmosphere side.
6. The X-ray generator according to claim 1, wherein a ratio of the thermal conductivity in a direction in which said irradiation window spreads to the thermal conductivity in a direction of the thickness of said irradiation window is two or greater.
7. The X-ray generator according to claim 1, wherein a ratio of the thermal conductivity in a direction in which said irradiation window spreads to the thermal conductivity in a direction of the thickness of said irradiation window is 10 or greater.
Description
DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a schematic cross-sectional diagram showing the structure in the vicinity of the irradiation window of the X-ray tube where heat conduction to the target holder is suppressed according to an embodiment of the present invention;
(2) FIG. 2 is a diagram for illustrating a way for heat to conduct through the irradiation window in FIG. 1;
(3) FIG. 3 is a schematic cross-sectional diagram showing the structure in the vicinity of the irradiation window of the X-ray tube where heat conduction to the target holder is suppressed according to another embodiment of the present invention;
(4) FIG. 4 is a schematic cross-sectional diagram showing a modification of FIG. 3;
(5) FIG. 5 is a schematic cross-sectional diagram showing the structure in the vicinity of the irradiation window of the X-ray tube where heat conduction to the air side is suppressed according to still another embodiment of the present invention;
(6) FIG. 6 is a diagram for illustrating away for heat to conduct through the irradiation window in FIG. 5;
(7) FIG. 7 is a diagram for illustrating the structure of the irradiation window made of a multilayer material having the same functions as the irradiation window made of a thermally anisotropic material in FIG. 5 and a way for heat to conduct through the irradiation window;
(8) FIG. 8 is a diagram for illustrating a temperature simulation model for each portion when X-rays are generated in the embodiment in FIG. 5;
(9) FIG. 9 is a diagram for illustrating a temperature simulation model for each portion when X-rays are generated in the case where the irradiation window in FIG. 7 is used;
(10) FIG. 10 is a schematic cross-sectional diagram showing an example of the structure in the vicinity of the irradiation window of a transmission-type X-ray tube according to the prior art; and
(11) FIG. 11 is a schematic cross-sectional diagram showing an example of the structure in the vicinity of the irradiation window of a reflection-type X-ray tube according to the prior art.
DETAILED DESCRIPTION OF EMBODIMENTS
(12) In the following, embodiments of the present invention are described in reference to the drawings.
(13) FIG. 1 is a schematic cross-sectional diagram showing the irradiation window of the X-ray tube and its vicinity according to an embodiment of the present invention, which is an example of the structure for preventing heat from the irradiation window from conducting to the X-ray tube (target holder) side.
(14) The example in FIG. 1 basically has the same structure as that in FIG. 10, and a target holder 1 provided in an end portion of an X-ray tube that has been vacuumed air tightly holds an irradiation window 3 on the inside of which a target 2 is layered so as to be integrated. In addition, an electron source made of a filament or the like (not shown) is placed within the X-ray tube. When an electron beam B generated by converging and accelerating electrons from this electron source strikes the target 2 integrated with the irradiation window 3, X-rays are generated at an X-ray generating point 2a, which is the spot at which the electron B strikes the target 2. The generated X-rays are released out of the X-ray tube mainly in the direction DT that is the same direction in which the electron beam B progresses.
(15) 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 FIG. 2, the way for heat to conduct is characterized in that the thermal conductivity in the direction in which the irradiation window 3 spreads (direction perpendicular to the electron beam B) is smaller than the thermal conductivity in the direction of the thickness of the irradiation window 3 (the same direction as the electron beam B). That is to say, the majority of the heat that has been transferred from the target 2 to the irradiation window 3 conducts in the direction of the thickness of the irradiation window 3.
(16) 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 m, 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.
(17) In the above-described embodiment, as shown in FIG. 3, a metal layer 4, which allows heat to diffuse uniformly, may be provided on the surface of the irradiation window 3 on the air side for the purpose of preventing the temperature of the irradiation window 3 made of a thermally anisotropic material such as graphite from locally increasing. This metal layer 4 can be provided so as to protrude towards the air side as shown in FIG. 4, and in this case, heat is not transferred to the target holder 1 through the metal layer 4, and thus, heat can be efficiently released to the air side. The metal layer 4 can be made of Be, Al or the like.
(18) FIG. 5 is a schematic cross-sectional diagram showing the irradiation window of the X-ray tube and its vicinity according to another embodiment of the present invention, which is an example of the structure for preventing heat from the irradiation window from being transferred to the air side. The structure illustrated in FIG. 5 is basically the same as in FIG. 1, and therefore, the same symbols are attached to the same members as in FIG. 1 and the descriptions thereof are omitted.
(19) In the example in FIG. 5, the irradiation window 13 is different from that in the example in FIG. 1. That is to say, though a thermally anisotropic material such as graphite as in FIG. 1 is used for the irradiation window 13 in the example in FIG. 5, the way for heat to conduct is characterized as shown in FIG. 6 in that the thermal conductivity in the direction in which the irradiation window 13 spreads (direction perpendicular to the electron beam B) is greater than the thermal conductivity in the direction of the thickness of the irradiation window 13 (the same direction as the electron beam B). Namely, the majority of the heat that has been transferred to the irradiation window 13 from the target 2 conducts in the direction in which the irradiation window 13 spreads.
(20) As described in the example in FIG. 1, the heat on the target 2 that has been generated when the electron beam B strikes the target 2 is transferred to the irradiation window 13. In the example in FIG. 5, the heat in the irradiation window 13 mainly conducts in the direction in which the irradiation window 13 spreads, and thus, the amount of heat that is transferred from the irradiation window 13 to the air side can be kept low. Accordingly, it is 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 13 to be transferred to the air side where an object to be inspected is placed in the example in FIG. 5. This is useful for X-ray generators where the thermal effects on an object to be inspected need to be taken into consideration when an image of the object needs to be taken with a high magnification ratio, that is to say, an X-ray image of the object needs to be taken when the object is as close as possible to the X-ray generating point (X-ray focal point) 2a.
(21) Though in the example in FIG. 5 an irradiation window 13 where the thermal conductivity in the direction in which the window spreads is made greater than the thermal conductivity in the direction of the thickness of the window by using a thermally anisotropic material such as graphite, the same functions as those of the irradiation window 13 using an thermally anisotropic material can be provided in the case where an irradiation window having materials with different thermal conductivities that are layered on top of each other is used.
(22) That is to say, as shown in FIG. 7, an irradiation window 23 made of a multilayer material where materials 23a with good thermal conductivity and materials 23b having poor thermal conductivity are alternately layered on top of each other may be replaced with the irradiation window 13 in FIG. 5 so that the same effects as in the example in FIG. 5 can be provided.
(23) The irradiation window 23 in FIG. 7 has such a structure that a layer of a material 23a with good thermal conductivity is provided adjacent to the target 2 and a material 23b with poor thermal conductivity is provided next to the material 23a, which is then followed by repeatedly layering materials 23a and 23b. In this irradiation window 23, heat generated on the target 2 is transferred to the layers of the materials 23a with good thermal conductivity in such a manner that heat conducts uniformly within these layers. However, it is difficult for heat to be transferred to the adjacent layers of the materials 23b with poor thermal conductivity. That is to say, on the whole, heat easily conducts in the direction in which the layers spread while it is difficult for heat to conduct in the direction in which the layers are provided on top of each other in the multilayer body. In other words, the thermal conductivity in the (lateral) direction in which the irradiation window 23 spreads is greater than the thermal conductivity in the (longitudinal) direction of the thickness of the multilayer body, and thus, the same functions as of the irradiation window 13 made of a thermally anisotropic material in FIG. 5 are provided.
(24) Here, examples of the materials 23a with good thermal conductivity used for the irradiation window 23 in FIG. 7 are light metals such as Be and Al, while SiO2 can be cited as an example of the materials 23b with poor thermal conductivity.
(25) 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.Math.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.Math.K) or greater in the direction in which the thermal conductivity is greater, and the thermal conductivity is 10 W/(m.Math.K) or less in the direction in which the thermal conductivity is smaller.
(26) Next, the effectiveness of the structure according to the embodiment in FIG. 5 and a simulation that was carried out in order to verify the effectiveness of the structure where the irradiation window in FIG. 5 was replaced with the multilayer body in FIG. 7 are described.
(27) FIG. 8 is a diagram showing a model used for the simulation concerning the structure in the embodiment in FIG. 5. Since the actual structure is symmetric relative to the electron beam B, FIG. 8 is a cross-sectional diagram showing a half (right side) in the lateral direction. A simulation using this model was carried out on the basis of the finite element method as a verification test.
(28) The irradiation window 13 has thermal anisotropy in the direction shown in FIG. 6, and the thermal conductivities used for the simulation were 1700 W/(m.Math.K) in the direction in which the irradiation window 13 spread and 7 W/(m.Math.K) in the direction of the thickness of the irradiation window 13. Another simulation on the irradiation window 13 (dimensions were the same as in FIG. 8) made of a thermally isotropic material with a thermal conductivity of 1700 W/(m.Math.K) was carried out as a comparative example.
(29) In the simulations, as shown in FIG. 8, heat was generated in an area having a radius of 5 m at a point irradiated with an electron beam B until the amount of generated heat became 5 W, and thus, the temperature in each portion was calculated at the point in time when a state of thermal equilibrium was achieved. Table 1 shows the results of the calculations in temperature increments ( C.) on the vacuum side and on the air side along the axis of the electron beam B.
(30) TABLE-US-00001 TABLE 1 Vacuum side Air side Isotropic material 192 C. 33.6 C. Anisotropic material 1955 C. 10.0 C.
(31) 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.
(32) FIG. 9 is a diagram showing a model used in a simulation where an irradiation window 23 having a multilayer structure as in FIG. 7 was adopted in place of the above-described thermally anisotropic irradiation window 13. This simulation model had the same structure as in FIG. 8 except for the irradiation window 23 and also had the same way of carrying out the simulation.
(33) 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.Math.K), and the thermal conductivity of the material 23b with poor thermal conductivity was 5 W/(m.Math.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.Math.K) as a whole.
(34) 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 FIG. 8.
(35) TABLE-US-00002 TABLE 2 Vacuum side Air side Single layer 2213 C. 50.5 C. Three layers 2282 C. 42.1 C.
(36) 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.
(37) 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 FIG. 11, and in this case as well, the same effects as those for the transmission-type X-ray tubes can be gained.
EXPLANATION OF SYMBOLS
(38) 1 Target holder
(39) 2 Target
(40) 3, 13, 23 Irradiation window
(41) 4 Metal layer
(42) 23a Material with good thermal conductivity
(43) 23b Material with poor thermal conductivity
(44) B Electron beam
