X-RAY TUBE AND ASSOCIATED MANUFACTURING PROCESS

Abstract

An X-ray tube includes a vacuum-sealed tube housing evacuated to a pressure of 10.sup.?7 mbar or lower, a cathode assembly inside the housing including an electron emitter adapted to emit electrons when heated at a temperature included in a defined working temperature range and at least one component containing carbon in an amount of at least 20% by weight, especially at least 30% by weight, even more especially at least 50% by weight, the at least one component being preferably designed for holding the emitter, and an anode assembly inside the housing including a target layer for receiving electrons emitted by the electron emitter, wherein the electron emitter preferably includes boride, preferably lanthanum hexaboride (LaB.sub.6), and wherein the cathode assembly is designed such that if the emitter temperature is included in the working temperature range.

Claims

1. An X-ray tube comprising: a vacuum-sealed tube housing evacuated to a pressure of 10.sup.?7 mbar or lower; a cathode assembly inside the housing comprising an electron emitter adapted to emit electrons when heated at a temperature comprised in a defined working temperature range and at least one component containing carbon in an amount of at least 20% by weight, the at least one component being preferably designed for holding the emitter, an anode assembly inside the housing comprising a target layer for receiving electrons emitted by the electron emitter, the X-ray tube being wherein the electron emitter comprises boride, and the cathode assembly is designed such that if the emitter temperature is comprised in the working temperature range, the partial vapor pressure of carbon inside the housing, remains less than 10.sup.?4 mbar.

2. The X-ray tube according to claim 1, wherein the at least one component is holding the emitter.

3. The X-ray tube according to claim 1, wherein the vacuum-sealed tube housing is sealed using materials suitable for bake out temperatures higher than 470K, advantageously higher than 520K, even more advantageously higher than 570K, preferably the vacuum-sealed tube housing is sealed using exclusively these materials.

4. The X-ray tube according to claim 3, wherein the vacuum-sealed tube housing comprises metal and ceramic.

5. The X-ray tube according to claim 3, wherein the vacuum-sealed tube housing has been sealed after being baked-out at a temperature higher than 470K.

6. The X-ray tube according to claim 5, wherein the partial pressure of oxygen, especially molecular oxygen, inside the housing is less than 10.sup.?8 mbar after the bake-out and pumping procedure.

7. The X-ray tube according to claim 5, comprising a crimped pump tube.

8. The X-ray tube according to claim 1, wherein the diameter of the electron emitter is larger than 100 ?m.

9. The X-ray tube according to claim 1, wherein the working temperature range is 1400 K to 2100 K.

10. The X-ray tube according to claim 1, wherein the at least one component is in the form of an elongated portion protruding towards the anode assembly, the elongated portion extending in a longitudinal direction between a first end at which it is fixed and a second free end, and the electron emitter is located at the second free end.

11. The X-ray tube according to claim 10, wherein the elongated portion is so configured that an electrical current flowing in the cathode assembly flows along the elongated portion in a forward current supporting portion, in the longitudinal direction, towards the second free end thereof and back towards its first end in a backward current supporting portion, the forward and backward current supporting portions being designed such to keep the partial vapor pressure of carbon inside the housing.

12. The X-ray tube according to claim 10, wherein the elongated portion comprises an electrical insulating layer or a gap delimiting two electrically conductive paths joined at their end.

13. The X-ray tube according to claim 10, wherein the emitter is embedded in the elongated portion and the electron emitting surface of the emitter comprises a flat emitting surface facing the target layer and wherein the flat emitting surface is coplanar with the second free end of the elongated portion.

14. The X-ray tube according to claim 1, comprising an electrical power supply adapted to deliver an electrical current to the cathode assembly for heating the emitter, the power supply preferably being configured to deliver electrical power so that during operation the emitter temperature is comprised in the working temperature range.

15. The X-ray tube according to claim 1, wherein the electron emitter is supported by an electrically conductive supporting base including the at least one component, wherein the electrically conductive supporting base is designed for resistively heating the emitter, and wherein the electron emitter and the electrically conductive supporting base are designed such that at a temperature of the emitter comprised in the working temperature range, the partial vapor pressure of the carbon contained in the at least one component remains less than 10.sup.?4 mbar.

16. The X-ray tube according to claim 15, wherein the electrically conductive supporting base is operationally connected to the electrical power supply to deliver electrical current to the conductive supporting base.

17. The X-ray tube according to claim 15, wherein the cathode assembly, especially the electron emitter and the electrically conductive supporting base, are designed such that with an electrical heating current comprised between 0.5 A and 3 A the emitter temperature is comprised in the working temperature range.

18. The X-ray tube according to claim 15, wherein the cathode assembly, especially the electron emitter and the electrically conductive supporting base, are configured so that the temperature of the at least one component is as close as possible from the temperature of the electron emitter, when the emitter temperature is comprised in the working temperature range.

19. The X-ray tube according to claim 18, wherein the cathode assembly, is designed such that the temperature difference between the at least one component and the electron emitter is less than 300 K, K, when the emitter temperature is comprised in the working temperature range.

20. The X-ray tube according to claim 15, wherein the cathode assembly, especially the electron is designed such that the temperature of the at least one component is less than 2500 K when the emitter temperature is comprised in the working temperature range.

21. A method for manufacturing an X-ray tube, comprising the steps of: a. providing for cathode assembly inside a housing, the cathode assembly comprising an electron emitter adapted to emit electrons when heated at a temperature comprised in a defined working temperature range and at least one component containing carbon in an amount of at least 20% by weight, the at least one component being designed for holding the emitter, the electron emitter comprising boride, b. providing for an anode assembly inside the housing comprising a target layer for receiving electrons emitted by the electron emitter, c. evacuating the housing, d. baking-out the X-ray tube at a temperature sufficient to reach a partial pressure of oxygen inside the housing less than 10.sup.?8 mbar, wherein the X-ray tube is baked-out at a temperature of at least 470K, e. sealing the housing.

22. The method of claim 21, wherein the housing is sealed by closing, a pumping tube through which the housing is evacuated.

23. Use of an X-ray tube according to claim 1 for creating X-rays wherein the electron emitter is heated to reach a temperature comprised in a defined working temperature range for emitting electrons towards the anode assembly and wherein the partial vapor pressure of carbon inside the housing is kept lower that 10.sup.?4 mbar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The invention will now be explained in more details with reference to particular and non-limitative embodiments of the invention. The figures depict, in a simplified, not-to-scale representation, schematic views of X-rays tubes or parts thereof, according to these particular embodiments of the invention:

[0053] FIG. 1 is an overall schematic view of an X-ray tube according to an embodiment of the present invention; and

[0054] FIG. 2 is a schematic view showing an electron emitter mounted on an elongated portion of the cathode assembly.

[0055] FIG. 1 schematically illustrates an X-ray tube 100 according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0056] In a conventional manner, the X-ray tube 100 comprises a vacuum-sealed tube 10 typically formed of a cylinder 12 (often a glass or a metal cylinder) of axis X, pumped out so as to define an evacuated internal chamber 14. The pressure inside the tube is typically around 10.sup.?7 or 10.sup.?8 mbar, and the tube is advantageously baked-out to reach a partial pressure of oxygen lower than 10.sup.?8 mbar. The tube 10 itself may be enclosed by a metal enclosure 20 provided with a window 22 through which X-ray radiation issuing from the tube 10 emerges into the outer space.

[0057] An anode assembly 30 forming the X-ray generating part and a cathode assembly 40 forming the electron generating part of the X-ray tube 100 are disposed inside the tube 10 opposite each other, as shown along axis X.

[0058] The anode assembly 30 is conventionally made of a metal piece electrically biased with respect to the cathode assembly 40 in order to accelerate the electrons to a kinetic energy of several thousands of electron volts. The anode assembly typically comprises a base 32 of high conductive material, for example copper, molybdenum or graphite, and an upper target layer 34 directly facing the cathode 40.

[0059] The cathode assembly 40 is a so-called hot type cathode. It comprises an electron thermionic emitter 50 comprises boride, advantageously lanthanum hexaboride (LaB.sub.6), and is adapted to emit electrons when heated to an ideal working temperature of around 1760 K.

[0060] The thermionic emitter 50 is supported by an electrically conductive supporting base 42 which in the particular context of the present invention includes at least one component including at least 20% carbon in weight.

[0061] In the non-limitative illustrated example, more specifically, the electrically conductive supporting base 42 comprises two legs 44A, 44B-made for example of a molybdenum-rhenium alloy or carbon-rigidly fixed in a base 18, for example made of ceramic material, and bent towards the center in an inverted V. Legs 44A, 44B both act as a clamp maintaining, at its distal end, the at least once component containing carbon, which in is this embodiment is in the form of an elongated portion 48 protruding towards the anode assembly.

[0062] An electrical power supply 60 supplies an electrical current to the cathode assembly 40 for heating the supporting base 42 and the elongated portion 48 by resistive heating and thus the emitter 50 by heat conductivity.

[0063] A separate electrical power supply 62 supplies a high voltage between the anode and cathode assemblies 30, 40 for accelerating electrons which are already out of the electron emitter material towards the target layer 34, as shown in FIG. 1.

[0064] The thermionic emitter 50 is heated mostly by heat conductivity through its contact surfaces with the elongated portion 48 and to a lesser extent by heat radiated from said elongated portion 48. Considering heat losses, the temperature of the elongated portion and/or the conductive base has to be higher than the temperature of the emitter 50. According to the invention, the cathode assembly is however designed such that when the emitter temperature is comprised in the working temperature range, the temperature of the components of the cathode assembly containing carbon is low enough to ensure that the partial vapor pressure of carbon inside the housing, especially the partial vapor pressure of the carbon contained in the elongated portion 48, remains less than 104 mbar, preferably less than 10.sup.?5 mbar, still more preferably less than 10.sup.?6 mbar.

[0065] FIG. 2 illustrates in more details a possible embodiment for the electrically conductive supporting base 42 comprising the two legs 44A and 44B and the elongated portion 48 on which the emitter 50 is mounted. In this embodiment, the elongated portion comprises an electrical insulating layer 49, or a gap delimiting two electrically conductive paths 1 and 2 joined at their end at a position adjacent to the electron emitter 50. As illustrated by the arrows in this figure, the electrical current I is directed, thanks to the insulating layer 49, towards the emitter 50, allowing for a resistive heating of the elongated portion 48 and thus to a temperature of the elongated portion as close as possible to the temperature of the emitter 50 when the latter is maintained at a temperature in the working temperature range.