X-ray tube anode arrangement

Abstract

A method of manufacturing an X-ray tube component, includes diffusion bonding or brazing an anode of rhodium, molybdenum or tungsten to a heat spreader of molybdenum, tungsten, or a composite of molybdenum and/or tungsten. Suitable joint materials for diffusion bonding include gold; suitable joint materials for brazing include an alloy of silver and copper, an alloy of silver, copper and palladium, an alloy of gold and copper or an alloy of gold, copper and nickel. The resulting tube component delivers reliable behaviors and the joint can withstand high temperatures, high temperature gradients, fast temperature changes, extremely high radiation and extremely high electric field, while maintaining good high vacuum properties.

Claims

1. A method of manufacturing an X-ray tube component, comprising: providing an anode of rhodium, molybdenum or tungsten; providing a heat spreader of a composite of molybdenum and/or tungsten having a matching thermal expansion coefficient to the anode; mounting the anode on the heat spreader with a layer of joint material therebetween, the joint material being gold or an alloy of gold; bonding the anode to the heat spreader with the joint material; wherein the step of bonding the anode to the heat spreader involves diffusion bonding the anode to the heat spreader.

2. The method according to claim 1, wherein the joint material is gold.

3. The method according to claim 1, wherein the joint material is a thin layer of thickness 5 to 200 m.

4. The method according to claim 1, wherein the step of bonding the anode to the heat spreader involves brazing the anode to the heat spreader.

5. The method according to claim 4, wherein the joint material is an alloy of gold and copper or an alloy of gold, copper and nickel.

6. The method according to claim 1, wherein the heat spreader is a composite of molybdenum and copper or a composite of tungsten and copper.

7. An X-ray tube component, comprising: an anode of rhodium, molybdenum or tungsten; a heat spreader of a composite of molybdenum and/or tungsten having a matching thermal expansion coefficient to the anode; and a bonding layer of gold or an alloy of gold diffusion bonding the anode to the heat spreader.

8. The X-ray tube component according to claim 7, wherein the bonding layer has a thickness 5 to 200 m.

9. The X-ray tube component according to claim 7, wherein the bonding layer is gold.

10. The X-ray tube component according to claim 7, wherein the bonding layer is an alloy of gold and copper or an alloy of gold, copper and nickel.

11. The X-ray tube component according to claim 7, wherein the heat spreader is a composite of molybdenum and copper or a composite of tungsten and copper.

12. An X-ray tube comprising an X-ray tube component according to claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An example of the invention will now be described with reference to the accompanying diagrams, in which:

(2) FIG. 1 is a cross section through an X-ray tube anode arrangement according to an embodiment of the invention;

(3) FIG. 2 shows a finite element analysis of an X-ray tube anode arrangement according to a comparative example;

(4) FIG. 3 shows a finite element analysis of an X-ray tube anode arrangement according to an embodiment; and

(5) FIG. 4 is a photomicrograph of a join in an X-ray tube anode arrangement according to an embodiment of the invention.

DETAILED DESCRIPTION

(6) Referring to FIG. 1, an X-ray tube anode arrangement includes anode 2 is made of rhodium mounted on a heat spreader 4. On the rear of the heat spreader there is provided a cooling arrangement 6,8.

(7) The heat spreader 4 is made of an alloy of molybdenum and copper, having an alloy composition chosen so that the thermal expansion coefficient matches the thermal expansion coefficient of rhodium.

(8) The inventors have tested a number of different approaches to fixing the anode to the heat spreader. Good results have been obtained using a diffusion bond of gold. Thus, a bonding layer of joint material 10, in this case of gold, is provided between the anode 2 and the heat spreader 4 to firmly fix the anode to the heat spreader.

(9) The layer of joint material may have a thickness of 5 to 200 m, in embodiments 10 to 100 m for example 50 m.

(10) A layer of corrosion resistant material 12 is provided on the rear of the heat spreader to avoid corrosion of the heat spreader 4. The corrosion resistant material 12 may be, for example, gold.

(11) The cooling arrangement 6,8 is formed by a pair of concentric tubes 6,8, an outer tube 6 around an inner tube 8. The end of the tubes are closed with the corrosion resistant material 12 on heat spreader 4. A coolant, for example deionised water, is used to transport heat in the cooling arrangement.

(12) In use, water is pumped along the inner tube 8 in the direction indicated by arrows, then flows across the corrosion resistant material 12 on heat spreader 4 where it takes heat from the heat spreader 4 and then is removed along a flow path between the inner 8 and outer 6 tubes. A circuit for the coolant is completed by a pump, filter, heat exchanger and stock barrel, which cools and recirculates the water.

(13) To manufacture the X-ray tube anode arrangement, the anode 2 and heat spreader 4 are brought together with the bonding layer in the form of a sheet of joint material 10 between them. The anode 2 is then diffusion bonded to the heat spreader 4 by heating under pressure, but not to a temperature where the gold melts. This creates a diffusion bond.

(14) In a specific embodiment, the diffusion bonding was carried out at a temperature between 700 C. and 950 C., for example 800 C., for between 15 minutes and 200 minutes, for example 120 minutes (two hours) in a forming gas atmosphere. The pressure used may be 10 bar to 500 bar, for example 80 bar; higher pressures may also be used. There is a trade off between temperature and time and higher temperatures may be used, for example, for shorter periods of time.

(15) Finite element analysis has been carried out to calculate the plastic deformation of the resulting anode arrangement in use compared with a comparative example of the same anode attached to a silver heat spreader. See the results presented in FIGS. 2 and 3. The drawings show deformationthe darker the region the more deformation there.

(16) Referring to FIG. 3, the embodiment, little plastic deformation is observed. This anode arrangement only has plastic deformation in the surface of the anode material, not in the heat spreader. This does not have a significant impact on the life of the X-ray tube.

(17) In contrast, referring to FIG. 2, the anode arrangement according to a comparative example has plastic deformation not merely in the anode but also in the heat spreader. This can materially affect the lifetime of the X-ray tube in use.

(18) The joint in the anode arrangement in particular could withstand: high temperatures of 850 C.; extreme temperature gradients of 100 C./mm, fast temperature changes of 100 C./s; extremely high radiation of 10 Sv/s; and extremely high electric fields of 15 kV/mm.

(19) Further, a section through a joint as illustrated in the photomicrogrpah of FIG. 4 shows a bond with a rhodium anode as the top layer on a 50 m layer of gold on a composite of molybdenum and copper. The result shown is an excellent bond, free of voids and cracks and with a complete contact between the different materials.

(20) Prototype X-ray tubes were made with the new anode construction and these were able to withstand an increase in the number of power switches before tube failure of a factor of 2 compared with the comparative example. The invention accordingly delivers surprisingly good results in terms of improved tube life and performance.

(21) Thus, the inventors have discovered how to reliably bond anodes of rhodium, molybdenum or tungsten to produce reliable joints in the extreme operating conditions of an X-ray tube.

(22) In alternative embodiments, different anode materials are used, in particular the anode may be of molybdenum or tungsten.

(23) As an alternative to diffusion bonding, brazing may also be used. In such a case, a metal layer of copper silver alloy or palladium copper silver alloy may be used. Such alloys are commercially available as Cusil or Pacusil respectively.

(24) Before brazing, it may be advantageous to coat either the anode, the heat spreader or both with a thin layer of nickel or gold plate before brazing.