X-RAY TARGET ASSEMBLY, X-RAY ANODE ASSEMBLY AND X-RAY TUBE APPARATUS

Abstract

An X-ray target assembly includes a cylindrical base and a cylindrical multilayered X-ray target that includes at least a heat transfer layer, an X-ray source layer and an adhesion layer provided between the heat transfer layer and the X-ray source layer , wherein the X-ray target is oriented such that the heat transfer layer is closest to the base, wherein the X-ray target is placed on top of a cylindrical carrying element, wherein the in-plane coefficient of thermal expansion of each of the heat transfer layer, the X-ray source layer, the adhesion layer and of the material of the carrying element is different, wherein the in-plane coefficient of thermal expansion of the heat transfer layer is the lowest and that of the material of the carrying element the highest.

Claims

1. X-ray target assembly comprising a cylindrical base, and a cylindrical multilayered X-ray target that comprises at least a heat transfer layer, an X-ray source layer and an adhesion layer provided between the heat transfer layer and the X-ray source layer, wherein the X-ray target is oriented such that the heat transfer layer is closest to the base, wherein the X-ray target is placed on top of a cylindrical carrying element, wherein the in-plane coefficient of thermal expansion of each of the heat transfer layer, the X-ray source layer, the adhesion layer and of the material of the carrying element is different, wherein the in-plane coefficient of thermal expansion of the heat transfer layer is the lowest and that of the material of the carrying element the highest, wherein the carrying element featuring a height DH and a diameter DD is attached to the base and positioned between the base and the heat transfer layer, wherein the diameter DD of the carrying element is smaller than the diameter BD of the base, wherein the ratio R of the height DH over the diameter DD of the carrying element is larger than or equal to 0.1 and smaller than or equal to 0.2, and wherein the diameter TD of the X-ray target is substantially equal to the diameter DD of the carrying element.

2. X-ray target assembly according to claim 1, wherein the base and the carrying element are coaxial.

3. X-ray target assembly according to claim 1, wherein the diameter BD of the base is at least 1.5 time larger than the diameter DD of the carrying element.

4. X-ray target assembly according to claim 1, wherein the base and the carrying element are made out of copper or silver or a combination thereof.

5. X-ray target assembly according to claim 1, wherein the heat transfer layer exhibits an in-plane thermal conductivity of at least 500 W/m.Math.K.

6. X-ray target assembly according to claim 5, wherein the heat transfer layer (4a) is made out of diamond.

7. X-ray target assembly according to claim 1, wherein the X-ray source layer is made out of tungsten, rhenium, molybdenum or an alloy thereof.

8. X-ray target assembly according to claim 1, wherein the X-ray target (14) comprises several heat transfer layers and X-ray source layers in alternation.

9. X-ray target assembly according to claim 8, wherein an adhesion layer is present between each heat transfer layer and X-ray source layer.

10. X-ray target assembly according to claim 1, wherein the one or more adhesion layer is made out rhenium, rhodium, molybdenum or chromium.

11. X-ray target assembly according to claim 1, wherein the X-ray source layer and/or the adhesion layer is deposited on the heat transfer layer by means of ion beam sputtering, chemically vapor deposition or thermally vapor deposition.

12. X-ray target assembly according to claim 1, wherein the base comprises cooling fins on its side opposite to the carrying element.

13. X-ray target assembly according to claim 1, wherein the base comprises a recess in which the carrying element is located.

14. X-ray target assembly according to claim 14, wherein the recess of the base possesses a depth smaller than half of the height DH of the carrying element.

15. X-ray anode assembly comprising an X-ray target assembly according to claim 1.

16. X-ray anode assembly according to claim 15, wherein it comprised a cylindrical body with a target socket configured to receive the X-ray target assembly.

17. X-ray anode assembly according to claim 16, wherein the socket is configured such that the X-ray source layer of the target assembly is tilted with respect to the longitudinal axis of the anode body.

18. X-ray tube apparatus comprising an X-ray target assembly according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The foregoing and other objects, features and advantages of the present invention are apparent from the following detailed description taken in combination with the accompanying drawings in which:

[0032] FIG. 1 is a schematic side view of an X-ray target assembly according to a first preferred embodiment of the present invention;

[0033] FIG. 2 is a schematic side view of an X-ray target assembly according to a second preferred embodiment of the present invention;

[0034] FIG. 3 is a perspective view of an X-ray target assembly according to a third preferred embodiment of the present;

[0035] FIG. 4 is a schematic side view of an X-ray target assembly according to the third preferred embodiment of the present; and

[0036] FIG. 5 is a sectional side view of an X-ray anode assembly according to a preferred embodiment of this aspect of the present invention.

DETAILED DESCRIPTION

[0037] FIG. 1 illustrates a schematic side view of an X-ray target assembly 1 according to a first preferred embodiment of the present invention. The target assembly 1, here a fixed target assembly, comprises a cylindrical base 2, a cylindrical carrying element 3 and a target 4. As illustrated, the target 4 is provided on top of the carrying element 3 and comprises a heat transfer layer 4a and an X-ray source layer 4b. The heat transfer layer 4a has the purpose of supporting the source layer 4b and of optimally transferring the heat created in the source layer 4b during X-ray production under the effect of an impinging high-energy electron beam.

[0038] As the electron beam is focused onto a tiny region of the source layer 4b, it is advantageous if the heat transfer layer 4b is able to spread the heat in directions substantially perpendicular to the longitudinal axis of the carrying element 3 and the base 2, i.e. “in-plane”. With heat spreading, the cooling efficiency of the base 2 and the carrying element 3 is higher than if the heat produced in the source layer 4b would be substantially transmitted to the carrying element 3 by the heat transfer layer 4a in a direction parallel to the longitudinal axis of the target assembly 1.

[0039] The heat transfer layer 4a shall not only have a high thermal conductivity in order to transfer the heat produced in the source layer 4b but shall also have a high melting point. In order to efficiently produce X-rays at the source layer 4b, its temperature shall be kept slightly below its melting point. As in many applications, the use for the source layer 4b of high melting point materials such as tungsten or tungsten alloys, rhenium or molybdenum, is required, the heat transfer layer 4b shall have a melting point higher than the expected temperature at the interface between the source layer 4b and the heat transfer layer 4a. Materials that combine the ability to spread the heat in directions other than parallel to the longitudinal axis of the target assembly 1, a high thermal conductivity and high melting point are for examples carbons allotropes and especially diamond or highly oriented pyrolytic graphite (HOPG). For that purpose, diamond can be in the form of chemically vapor deposited (CVD) diamond or crystal diamond. Wherein the latter has better thermal and mechanical properties.

[0040] In order to efficiently transport the heat produced in the target 4 away, the base 2 as well as the carrying element 3 are made out of materials with a thermal conductivity of at least 100 W/m.Math.K, advantageously of at least 200 W/m.Math.K, even more advantageously of at least 300 W/m.Math.K. Examples of possible materials are copper, silver or a combination thereof. Important to note is that the base 2 and the carrying element 3 can be made out of the same material or out of two different materials. Furthermore, the base 2 and the carrying element 3 can advantageously be produced from one piece of material. The base 2 and the carrying element 3 are advantageously cylindrical shape and coaxial. Nevertheless, the base 2 and the carrying element 3 could exhibit other sections, such as for example rectangular or squared sections, and they do not need to be coaxial. Furthermore, and as can be observed in FIG. 1, the base 2 comprises cooling means 2a, on its face opposite to the carrying element 3. The cooling means can have any kind of shape in order to increase the contact surface to a given fluid or gaseous cooling medium.

[0041] As mentioned above, the X-ray source layer 4b is provided on top of the heat transfer layer 4a as actual source for the X-rays. The source layer 4b is made out of the material suitable for the production of X-ray with the desired wavelength respectively energy. Advantageously, the source layer 4b of the X-ray target assembly 1 is made out of a high melting point material such as tungsten, a tungsten alloy, for instance tungsten carbide, rhenium or molybdenum. The source layer 4b is advantageously deposited onto the heat transfer layer 4a by means of ion beam sputtering, CVD or thermal evaporation. The source layer 4b shall be thick enough such that all imping electrons decay before reaching the heat transfer layer 4a. For electron energy in the range of 100 key a thickness of 2 to 10 microns, especially 2 to 5 microns, depending on the exact material of the source layer 4b, is sufficient to completely attenuate the electron beam. Providing for a thicker source layer does not allows for producing any additional X-rays but would diminish the cooling efficiency. This can be especially a problem when the source layer 4b is made out of a material with low thermal conductivity such as tungsten or a tungsten alloy.

[0042] In order to improve the adhesion properties of the X-ray source layer 4b on the heat transfer layer 4a, it can be advantageous to provide for an adhesion layer 4c between these two layers. The adhesion layer can, for instance, be a layer of 1 to 10 microns, especially 5 to 50 nm, of rhenium, rhodium, chromium or molybdenum. This is especially advantageous in cases where the heat transfer layer 4a is made out of diamond since layers deposited on diamond have reduced adhesion in case of several metals.

[0043] FIG. 2 illustrates a schematic side view of an X-ray target assembly 10 according to a second preferred embodiment of the present invention. As can be seen in this figure, the target 14 comprises a heat transfer layer 14a and on top of it several adhesion layers 14c and source layers 14a in alternation. The presence of intermediate adhesion layers ensure that the X-ray source layer will not have a single columnar crystalline structure. The multi columnar crystalline structure ensures a higher mechanical strength under thermal stress. Apart from the presence of additional layers in the target 14, the target assembly 10 is similar to target assembly 1. Especially, the target 14 is placed on top of carrying element 3 and base 2 and the target assembly comprises cooling means 2a on the face of the base 2 opposite to the carrying element

[0044] FIG. 3 illustrates a perspective view of an X-ray target assembly 20 according to a third preferred embodiment of the present invention. The target assembly 20, comprises a cylindrical base 2, a cylindrical carrying element 3 and a target 24. As can be seen in this figure, the base 2 comprises a base recess 2b in which the carrying element 3 is located. The presence of the base recess 2b implies that the upper edge of the base is higher than the lower edge of the carrying element. The base recess 2b has the advantage of allowing for a target assembly with a reduced overall height. A skilled person would however understand that the presence of the base recess 2b is not an essential feature of the present invention and that an X-ray target assembly according to the present invention can be foreseen without a base recess 2b. A sectional side view of the target assembly 20 according to the third preferred embodiment of the present invention is shown in FIG. 4. As illustrated, the base 2 of the target assembly 20 comprises, on its face opposite to the carrying element 3, cooling means in the form of cooling fins 2a. Of course, cooling means in the form of cooling fins could be used in all embodiments of the target assemblies according to the present invention. The target 24 of the target assembly 20 comprises, similar to target 4 and 14, a source layer 24b and a heat transfer layer 24a. An adhesion layer 24c can be provided between these two layers.

[0045] As can be seen in FIGS. 1 to 4, the targets 4, 14, 24 of the target assembly 1, 10, 20 are, contrary to X-target assemblies known from the prior art, not directly provided on or embedded into the base 2, but are placed on top of a carrying element 3. The inventors have observed that, by providing for a carrying element 3 of substantially the same diameter as the target between the base 2 and the target, the mechanical stress in the heat transfer layer and the source layer due to different thermal expansion coefficient can be reduced. Thanks to the carrying element 3 the material below the heat transfer layer can freely expand without producing irremediable mechanical stress in the target and partly cancel the stress caused by the heat expansion in the source layer relative to the heat transfer layer. The presence of carrying element 3 especially inhibits the usually observed mechanical cracking of the target at high power regime, i.e. when the X-ray source layer is almost at its melting point.

[0046] It has been observed that the value of the ratio R between the height DH of the carrying element 3 and its diameter DD has an important impact on the mechanical and thermal properties of the target assemblies according to the present invention. Advantageously, the height DH is larger by at least 10% than DD but smaller than 20% of DD, or mathematically expressed: 0.1.Math.DD<DH<0.2 DD. This implies, for example, that for a carrying element diameter DD of 5 mm, the carrying element height DH shall be in the range 0.5 mm to 1 mm. For DD =8 mm, DH shall be in the range 0.8 mm to 1.6 mm and for DD =10 mm, DH shall be in the range 1 mm to 2 mm. Of course, the diameter TD of the target 4, precisely of the heat transfer layer 4a and the X-ray source layer 4b, is substantially equal to the diameter DD of the carrying element 3. Furthermore, in order to obtain optimal cooling efficiency, the diameter BD of the base shall be at least 1.5 time larger than the diameter DD of the carrying element 3.

[0047] FIG. 5 displays a sectional side view of an anode assembly 30 according to a preferred embodiment of this aspect of the present invention. The anode assembly 30 comprises an anode body 31 with a target socket 32 configured to receive the target assembly 1. The anode body 31 comprises an electron opening 33 through which electrons are entering the anode assembly 30 and an X-ray opening 34 through which X-ray produced at the target assembly 1, 10, 20 are exiting the anode assembly 30. As illustrated in this figure, the target assembly is tilted with respect to the longitudinal axis Z of the anode body 31. With this, it is possible to provide for an angle of approximately 90° between the electron impinging direction A and the X-ray emitting direction B. The anode body 31 further comprises a cooling opening 35 configured for the coupling of the target assembly with active cooling means (not shown here). For this purpose, attachment recess 36 such as threaded holes are provided in the anode body 31.

[0048] Finally, it should be pointed out that the foregoing has outlined pertinent non-limiting embodiments. It will be clear to those skilled in the art that modifications to the disclosed non-limiting embodiments can be carried out without departing from the spirit and scope thereof. As such, the described non-limiting embodiments ought to be considered merely illustrative of some of the more prominent features and applications. Other beneficial results can be realized by applying the non-limiting embodiments in a different manner or modifying them in ways known to those familiar with the art.