Method for forming an anode for a cold cathode X-ray source

Abstract

A method (P) for forming an anode (A) for a cold cathode X-ray source, comprising the steps of: forming a first material adapted to generate X-rays from the absorption of an electron beam, the first material being heated to a predetermined temperature T during use of the anode in the X-ray source; u The first thermal expansion coefficient C e,1 (T u forming an element, referred to as target (C), from a first material having a predetermined temperature T u The second coefficient of thermal expansion C e,2 (T u ) forming an element called a target support (SC) from a second material having a predetermined temperature T u and joining the target and the target support by hard brazing at a soldering temperature higher than the melting point of the solder material to form a solder film (FB) interposed between the target and the target support.

Description

[Technical field]

[0001] The present invention relates to the field of X-ray sources. [Background technology]

[0002] X-rays are nowadays used in imaging, especially in the medical field, in industry for non-destructive testing, and in security to detect dangerous objects or substances.

[0003] The most commonly used X-ray source is the X-ray tube. An X-ray tube typically contains a vacuum chamber. An envelope is formed of a metal structure and an electrical insulator. Two electrodes are disposed within the envelope. In a monoelectrode tube, a cathode electrode, held at a negative potential, contains the electron emitter. A second anode electrode, held at a positive potential relative to the first electrode, is associated with the target. The potential difference between the two electrodes accelerates electrons, which produce a continuous spectrum of Bremsstrahlung radiation when they strike the target. The metal electrodes are typically large in size and have a sufficient radius of curvature to minimize the electric field at their surface.

[0004] Depending on the power of the X-ray tube, it may have either a fixed anode or a rotating anode adapted to dissipate the thermal power. Fixed anode tubes have a power of a few kilowatts and are used especially in industrial, security and low power medical applications. Rotating anode tubes can exceed 100 kilowatts and are mainly used in the medical field for imaging requiring high doses of X-rays of short duration. As an example, the diameter of industrial tubes is of the order of 150 mm for 450 kV, 100 mm for 220 kV and 80 mm for 160 kV. The voltages indicated correspond to the potential difference applied between the two electrodes. For medical rotating anode tubes, the diameter varies from 150 to 300 mm depending on the power dissipated at the anode.

[0005] The anode forming the target must dissipate a large amount of heat power, which can be achieved by circulating a heat transfer fluid or by providing a large rotating anode, and the need for heat dissipation therefore also increases the size of the x-ray tube.

[0006] For stationary anodes, the technology currently standard is a subassembly formed of a thin insert of pure tungsten or tungsten-rhenium alloy, around which a copper body is significantly overmolded. Figure 1 shows a photograph of an example of a prior art anode AA for an X-ray source. This anode AA includes a tungsten insert C forming the target against which the electron beam strikes to produce the X-ray radiation. Overmolding of this target C forms a copper body BC adapted to remove the thermal power caused by the damping of the electron beam. The removal of the thermal power is now achieved by good thermal contact between the insert and the copper rod, due to the excellent thermal conductivity of the latter. Summary of the Invention [Problem to be solved by the invention]

[0007] The anode of FIG. 1 is satisfactory. However, the use of a copper overmolding to dissipate the power stored during the duty cycle of the X-ray source makes miniaturization of the X-ray source extremely difficult. In particular, to miniaturize the X-ray source, it is necessary to ensure a high level of electrical insulation between the cathode and the anode by using an electrical insulator. This mechanical element may for example form part of the vacuum chamber. These ceramics are therefore advantageous for forming miniaturized X-ray sources. However, copper has a higher thermal expansion coefficient compared to ceramics, making assembly with ceramics difficult.

[0008] Finally, overmolding copper onto a tungsten target is difficult to implement: the overmolding process requires melting the copper containing target at high temperatures, cooling it while ensuring perfect contact between the tungsten and copper, and then machining the surface and properly cleaning it to allow it to operate in a vacuum.

[0009] The present invention aims to overcome certain problems of the prior art. To this end, one subject of the invention is a method for forming an anode for a cold cathode X-ray source, comprising the step of hard-brazing a target and a target support (which provides for the removal of thermal power). The assembly of the target and the support is thus significantly easier, and the anode including the target support can be formed using materials that could not be used for overmolding the target in the prior art. Depending on the choice of material for the target support, the anode can be easily assembled with a ceramic insulating element in the X-ray source, which means that the X-ray source can be further miniaturized. [Means for solving the problem]

[0010] To this end, one subject of the invention is a method for forming an anode for a cold cathode X-ray source, comprising the steps of: A. a first material adapted to generate X-rays from the absorption of an electron beam, the anode being configured to have a predetermined temperature T during use in said X-ray source; u The first thermal expansion coefficient C e,1 (T u forming an element, referred to as a target, from a first material having a first thickness; B. Predetermined temperature T u The second thermal expansion coefficient C e,2 (T u forming an element, referred to as a target support, from a second material having a thickness of 100 nm; C. using a solder material, at a soldering temperature higher than the melting point of the solder material, to join the target and the target support by hard brazing to form a solder film interposed between the target and the target support, u is lower than the melting point of the solder material.

[0011] According to one embodiment of the present invention, the first and second materials have |C e,1 (T u )-C e,2 (T u )|≦XX holds true.

[0012] According to an embodiment of the present invention, the first material is based on tungsten and the second material is based on molybdenum, copper or an alloy containing copper, tungsten and nickel. Preferably, the solder material is based on gold, silver, copper, nickel or palladium. More preferably, the second material is an alloy of copper, nickel and tungsten and the solder material is an alloy of silver, copper and palladium or an alloy of silver, copper and gold.

[0013] According to an embodiment of the present invention, the bonding step is performed in a furnace at 700° C. to 1100° C. Preferably, the bonding step is performed under vacuum. More preferably, the solder material is an alloy of silver, copper and tin, the second material is molybdenum and the first material is tungsten.

[0014] According to an embodiment of the present invention, the bonding step is carried out in a hydrogen atmosphere.Preferably, the second material is molybdenum, the first material is tungsten and the solder material is an alloy of silver, copper and palladium.

[0015] According to one embodiment of the invention, the method includes an intermediate step B' between steps B and C of metallizing the target support to form a metal layer less than 150 μm thick around the periphery of the target support, the metallization preferably being based on copper.

[0016] According to one embodiment of the present invention, the target and the target support each have a shape adapted to allow the target to be fitted into the target support.

[0017] Another subject of the invention is an anode for a cold cathode X-ray source, said anode being obtainable by a method according to any one of the preceding claims, A. Goals and b. a target support; c. A solder film is interposed between the target and the target support to enable joining of the target and the target support.

[0018] Another subject of the invention is an X-ray source, d. A vacuum chamber; e. a cathode adapted to emit an electron beam within the vacuum chamber; and an anode according to claim 11 arranged to cause the electron beam to strike a target to produce X-ray radiation.

[0019] According to one embodiment of the x-ray source of the present invention, the cathode is adapted to emit a pulsed electron beam, and the target and target support each have a volume large enough that the predetermined temperature is less than 800° C. without any active thermal cooling elements within the x-ray source.

[0020] Other characteristics, details and advantages of the invention will appear on reading the following description and with reference to the accompanying drawings, given by way of example, in which: [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram showing an example of an anode AA of a prior art X-ray source. [Figure 2A] 1 illustrates a method of forming an anode for a cold cathode X-ray source according to the present invention. [Figure 2B] FIG. 2 is a schematic diagram of an anode of a cold cathode X-ray source according to the present invention. [Figure 3A] 1 illustrates a method of forming an anode for a cold cathode X-ray source according to one embodiment of the present invention. [Figure 3B] FIG. 2 is a schematic diagram of an anode of a cold cathode X-ray source according to one embodiment of the present invention. [Figure 4] 1 is a schematic diagram of a cold cathode X-ray source according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] In the drawings, elements are not drawn to scale unless otherwise indicated.

[0023] FIG. 2A shows a method P for forming an anode for a cold cathode X-ray source according to the invention. To avoid any doubt, the term "cold cathode X-ray source" means an X-ray source comprising a cathode which emits an electron beam and passes through the effect of a magnetic field. Cathode of this kind is described, for example, in WO 2006 / 063982 A1. Cold cathodes do not have the disadvantages of hot or thermionic cathodes (expansion or evaporation of the conductive element), can be switched very quickly between the light-emitting and off states of the electrode, and in particular are much more compact. The method of formation of FIG. 2A allows the formation of an anode A intended for use in a cold cathode X-ray source, which is shown in a schematic cross-sectional view in FIG. 2B.

[0024] The method of the present invention includes a first step A of forming a target C, an element known to those skilled in the art, made of a first material adapted to generate X-rays from the absorption of an electron beam. The first material is heated to a predetermined temperature T during use of the anode in an X-ray source. u So, C e,1 (T u ) The target may be made of a high atomic number material such as tungsten (pure or tungsten alloy) for optimal x-ray production yield, or a low atomic number material for x-ray sources used in the context of x-ray diffraction.

[0025] The method also includes: u The second coefficient of thermal expansion C e,2 (T u The method includes a step B of forming an element called the target support SC from a second material having a thickness of 100 nm. The role of the target support is to dissipate the thermal energy generated by the target when braking the electron beam. This second material preferably also has good thermal conductivity.

[0026] It should be understood that the order of steps A and B in method P is interchangeable.

[0027] Finally, the method of the invention includes a final step C of joining the target C to the target support SC by hard brazing with a solder material at a soldering temperature higher than the melting point of the solder material. Also, in order to prevent the two elements of the anode from becoming unsoldered during operation of the X-ray source, a predetermined temperature T u is lower than the melting point of the solder material. "Hard brazing" here means that the melting point of the solder material is above 600°C. Step C therefore makes it possible to obtain a solder film FB to be inserted between the target C and the target support SC.

[0028] As shown in Figure 2B, the anode obtained by the method of the invention therefore comprises a target C, a target support SC and a solder film FB inserted between the target and the target support to enable them to be joined to one another. The thickness of the film FB is preferably in the range of 5 to 140 μm after step C to ensure an accurate joining of the target and the target support.

[0029] The method of the invention allows the assembly of the target and the target support using a second material that could not possibly be used for overmolding the target as was done in the prior art with copper. Thus, according to one embodiment, the second material is based on molybdenum or an alloy containing copper, tungsten and nickel. These second materials are not suitable for overmolding the target C, but are advantageous because they provide a good thermal behavior of the anode when the temperature of the tungsten target increases. Furthermore, these materials can be soldered to the ceramic using hard soldering. The method of the invention therefore allows the formation of an X-ray source that includes ceramic elements to form both the cathode support and the anode support to electrically insulate the anode and the cathode and is soldered to the target support. Such an assembly is difficult to achieve with the copper overmolding BC of the anode of FIG. 1, because copper cannot be properly soldered to ceramic due to the high thermal expansion of copper compared to most ceramics.

[0030] In summary, the method of the present invention allows for the formation of an X-ray source that is much smaller than prior art X-ray sources while at the same time including a ceramic insulator soldered to the anode A of the present invention.

[0031] More generally, according to a preferred embodiment, in order to avoid thermal problems during operation of an X-ray source including an anode of the present invention, the first and second materials have a temperature of |C e,1 (T u )-C e,2 (T u )|≦4×10 -6 K -1 This feature ensures that the target and target support will not become unsoldered during operation of an x-ray source including the anode of the present invention.

[0032] In a preferred embodiment, designated MP, the joining step C is carried out in a furnace at temperatures ranging from 700° C. to 1100° C., the cycle may last for a minimum of 3 hours, with at least 3 minutes at the high temperature level. This minimum duration is necessary to ensure proper joining and a uniform solder film between the target and the target support. According to one embodiment, the furnace in which the soldering is carried out contains a thermal cooling element that is activated after the heating of the first phase allows the solder material to melt. This thermal cooling element allows the total time of the soldering cycle to be reduced.

[0033] In a first variant of embodiment MP, the bonding step C is carried out in a vacuum, i.e. at least 10 -3 The process is carried out in a furnace under high vacuum of 10 ...

[0034] Alternatively, according to a second variant of embodiment MP, the joining step C is carried out in a hydrogen atmosphere, which allows a more uniform temperature in the furnace and therefore a better quality solder joint. However, this second variant requires a subsequent step of degassing the anode before assembling it with the other elements of the X-ray source. This degassing step is necessary to remove gas trapped in the solder, on the surfaces of the subassembly, thereby avoiding desorption phenomena occurring during the operation of the X-ray source.

[0035] Preferably, the solder material is based on gold, silver, palladium, copper or nickel. More preferably, if the soldering is performed under vacuum, the solder material is an alloy of silver, copper and tin when the second material is molybdenum and the first material is tungsten. Alternatively, according to another embodiment, if the soldering is not performed under vacuum, the solder material is preferably an alloy of silver, copper and palladium. The alloy of two of the above solder materials can compensate for the low wettability of molybdenum.

[0036] Alternatively, if the second material is an alloy of copper, nickel and tungsten and the first material is tungsten, the solder material is preferably an alloy of silver, copper and palladium, or an alloy of silver, copper and gold, so that the solder completely wets and diffuses into the copper deposit of the second material, allowing for an overall superior soldering.

[0037] 2B, the target C optionally has an inclined plane FI, which is obtained by reaching a compromise regarding different physical parameters of the X-ray source, namely the required thermal incidence angle and focus of the electron beam.

[0038] 2B, the target and target support each have a shape adapted to allow the target to be snapped into the target support. This feature facilitates assembly of the target support and the target during step C of the method of the present invention.

[0039] A method for forming an anode according to one particular embodiment of the present invention is shown in Figure 3A, and a schematic cross-sectional view of a negative electrode formed by the method of Figure 3A is shown in Figure 3B.

[0040] In comparison with the method of Fig. 2A, the method of Fig. 4 includes an intermediate step B' between steps B and C of metallizing the target support to form a metal layer CM around the target support with a thickness of less than 0.15 mm. This step B' further improves the wettability of the target support and limits the diffusion phenomenon of the solder material into the target support, thus maintaining the hermeticity of the brazed joint even under ultra-high vacuum. This step B' is particularly advantageous when the second material is an alloy of copper, nickel and tungsten.

[0041] Figure 4 shows: g. a chamber EV adapted to be placed under vacuum; h. a cathode Cat adapted to emit an electron beam FE within the vacuum chamber; Schematically shown is an X-ray source 2 comprising an anode A according to the invention arranged to strike an electron beam on a target to produce X-ray radiation, denoted i.FX.

[0042] Preferably, the second material is molybdenum, copper or an alloy containing copper, tungsten and nickel, and the chamber EV is partly or entirely made of ceramic to electrically insulate the anode and the cathode. Thus, according to an advantageous embodiment, the chamber EV is assembled with the anode A by brazing the target support and the ceramic parts P1, P2 of the chamber EV together. This final soldering step is particularly facilitated as a result of the choice of the second material, which is itself made possible by the inventive forming method. Moreover, these second materials exhibit a mechanical behavior under increased temperature that is much more suitable for soldering to ceramic than the copper used in the prior art. This makes it possible to obtain a joint between the chamber EV and the anode that is particularly gas-tight and compatible with operation in a chamber under ultra-high vacuum, without prior metallization of the ceramic.

[0043] Preferably, the cathode is adapted to emit a pulsed electron beam through the effect of a magnetic field, and the target and target support are each heated to a predetermined temperature T u The target has a volume large enough that the target and target support have a temperature T u The anode's thermal dissipation area and volume are sufficient to maintain the anode's temperature T below 800° C. This maintains the integrity of the anode and ensures its correct operation throughout its life cycle. u No active thermal cooling elements are required within the x-ray source to maintain

[0044] An exhaustive description of all elements that may be included in an X-ray source is beyond the scope of the present invention. However, according to certain embodiments of the present invention, the X-ray source of the present invention includes various elements found in X-ray sources and known to those skilled in the art. For example, the X-ray source 2 includes an electrode (not shown in FIG. 4) located near the cathode and capable of focusing the electron beam FE on a target C. This type of electrode is known as a beam focusing electrode.

Claims

1. A method (P) for forming an anode (A) for a cold cathode X-ray source, comprising the steps of: A. a first material adapted to generate X-rays from the absorption of an electron beam, the anode being heated to a predetermined temperature T during use in the X-ray source; ｕ The first thermal expansion coefficient C ｅ，１ (T ｕ forming an element, referred to as target (C), from a first material having a B. The predetermined temperature T ｕ and the second thermal expansion coefficient C ｅ，２ (T ｕ forming an element, called a target support (SC), from a second material having a thickness of 100 nm; C. Using a solder material, at a soldering temperature higher than the melting point of the solder material, to join the target and the target support by hard brazing to form a solder film (FB) interposed between the target and the target support, and ｕ is lower than the melting point of the solder material.

2. In the first and second materials, |C ｅ，１ (T ｕ ) -C ｅ，２ (T ｕ ) |≦4.10 －６ K －１ The method of claim 1 , wherein

3. 3. The method of claim 1, wherein the first material is primarily tungsten and the second material is primarily molybdenum, copper or an alloy containing copper, tungsten and nickel.

4. The method of claim 3 , wherein the solder material is based on gold, silver, copper, nickel, or palladium.

5. 5. The method of claim 4, wherein the second material is an alloy of copper, nickel and tungsten, and the solder material is an alloy of silver, copper and palladium, or an alloy of silver, copper and gold.

6. The method of any one of claims 1 to 5, wherein the bonding step is carried out in a furnace at a temperature in the range of 700°C to 1100°C.

7. The method of claim 6 , wherein the bonding step is performed under vacuum.

8. 8. The method of claim 7, wherein the solder material is an alloy of silver, copper and tin, the second material is molybdenum, and the first material is tungsten.

9. The method of claim 5 , wherein the bonding step is performed in a hydrogen atmosphere.

10. 10. The method of claim 9, wherein the second material is molybdenum, the first material is tungsten, and the solder material is an alloy of silver, copper and palladium.

11. The method according to any one of claims 1 to 10, comprising an intermediate step B' between steps B and C of metallizing the target support to form a metal layer (CM) around the target support with a thickness of less than 150 μm.

12. The method of claim 11 , wherein the metallization is copper-based.

13. A method according to any preceding claim, wherein the target and the target support each have a shape adapted to enable the target to be fitted into the target support.

14. An anode for a cold cathode X-ray source, said anode being obtainable by the method according to any one of claims 1 to 13, The goal; The target support portion; an anode including a solder film interposed between the target and the target support to enable joining of the target and the target support;

15. a vacuum chamber (EV), a cathode (Cat) adapted to emit an electron beam (FE) within said vacuum chamber; an anode according to claim 14, arranged so that said electron beam impinges on said target to produce X-ray radiation (FX).

16. 16. The x-ray source of claim 15, wherein the cathode is adapted to emit a pulsed electron beam, and the target and the target support each have a volume large enough that the predetermined temperature is less than 800° C. without any active thermal cooling elements within the x-ray source.

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