Field of the Invention

This invention relates generally to X-ray tubes and more particularly to a target for the electron beam in an X-ray tube.

Background to the Invention

In the field of semiconductor devices operating at high current densities, it is known to employ as a heat sink a block of diamond, typically 0.75mm by 0.75mm square and 0.5mm thick, provided with a metallic coating, for example of titanium/platinum/gold or gold/tin eutectic, typically at least lOOμm thick. This diamond heat sink is located between the active region of the semiconductor device, for example a diode laser or a microwave power device, and a metal carrier, in order to provide rapid heat transfer by conduction from the device to the carrier, which may have fins for forced air cooling or be water cooled. Diamond is especially suitable for use in this field because of its high thermal conductivity, which is approximately five times the thermal conductivity of copper.

The Invention

According to the invention there is provided an X-ray tube wherein the target for the electron beam is constituted by a layer of a target metal coated on to a thermally conductive substrate, the layer being not more than 50μm thick, so as to offer a minimum thermal impedance.

The target metal is preferably copper or molybdenum, and the substrate is preferably a block of diamond.

Preferably the target metal layer is applied to a block of natural or artificial diamond by sputtering and/or electro-plating, and has a thickness equal to or very little larger than the range of the impinging electrons, that is between 5μm and 20μm.

The target of an X-ray tube is commonly cooled by a jet of cooling fluid which strikes its rear face. The choice of target material is dictated by the characteristic X-ray wavelength which the X-ray tube is intended to produce. One of the most commonly employed target materials is copper. It is known to be advantageous to employ the minimum thickness of copper for the target which will withstand the pressure of the cooling water, but without a backing substrate the scope for reducing the thickness is limited by the strength of the target materials.

The invention has the advantage of enabling use of a very thin metal target because of the high mechanical strength of the diamond backing, but without loss in heat conduction because the diamond itself has a very high thermal conductivity greater than that of copper or any other metal. For optimum heat transfer the diamond is about 200 m thick and 4mm in diameter.

Preferably, the cooling jet of water or compressed air impinges on the rear face of the diamond. This face may in turn be coated with a further layer of metal (preferably copper) which layer can be covered in one or more formations such as hemispherical dimples so as to increase the surface area of the cooled face and thus permit a larger heat flow into the cooling fluid. The diamond may be soldered to the wall of the vacuum vessel so that it acts as a seal between the vacuum in the X-ray tube and the water or other liquid or the compressed air.

Conveniently, the diamond acts as a heat distributor, rapidly transferring heat from the very thin target layer of the coolant. In general, the maximum X-ray output in an X-ray tube is related to the maximum power which can be dissipated at the target, which in turn is determined by the rate of the conduction away from the electron-focus on the front face of the target to the rear face of the target. The high thermal conductivity of the diamond target

preferably employed in the present invention can enable an improvement in power dissipation up to a factor of three or more, with correspondingly increased X-ray output in the X-ray tube, which is especially significant for crystallographic applications.

The nature of the target in the form of a thin layer deposited on the diamond substrate facilitates one embodiment of the invention in which the target material on the diamond substrate is removed in certain areas such as at the corners of the uniform central target area. The electron beam can then be scanned across the target area in a raster by means of deflecting coils or plates; by monitoring either the X-ray output or the target current during the scan it is possible to locate the corner areas where the X-ray output and the target current will be different and thus to determine the exact centre point of the target. Subsequently the electron beam is steered to this centre point, thus aligning the X-ray tube with a much greater precision than is possible without a fiducial pattern on the target.

Description of the Embodiment

An X-ray tube in accordance with the invention is now described by way of example with reference to the accompanying drawing, the single figure of which shows the target end of the X-ray tube in diagrammatic cross-section.

In the drawing, the vacuum wall of the X-ray tube is referenced 10. In use, an electron beam 12 within the evacuated interior 14 of the tube is focused on to a target. The focus 16 of the electron beam constitutes a source of X-rays 18. The means which are provided for altering the position of the focus on the target are not shown.

In the X-ray tube in accordance with the invention, the target is constituted by a very thin layer of the target material 22, in this case copper, supported by a substrate in the form of a diamond block 20.

The rear face of the diamond block 20 is jet-cooled. The drawing shows a coolant inlet 24 from which coolant exits just to the rear of the diamond block thence to flow away from the diamond block 20 to an outlet 26. However, the nature of the cooling system is not

important to the invention, except in that it is particularly convenient for the diamond block 20 to be hermetically bonded in position at 28, thereby to form a seal between the vacuum in the interior 14 of the X-ray tube, and the coolant.

The diamond block 20 enables a target layer 22 to be employed which has a thickness in the range 5 to 20μm. This layer 22 is conveniently applied by sputtering and electro-plating.

The high mechanical strength of the diamond block 20 enables this extremely thin copper target to be satisfactorily employed, while at the same time there is no loss in heat dissipation from the target, becuse of the very high thermal conductivity of diamond. Thus, the diamond will very rapidly conduct heat away from the electron focus or hot spot, and from the terget generally, to the cooling system. It is considered that heat dissipation can be improved by a factor of three or more, thus making possible a corresponding increase in maximum X-ray output.