X-ray tube with improved temperature control

Abstract: In order to enhance the dissipation of heat, a metal structure (58) is provided between an anode target layer (12) and a support for the anode target layer (10) in an X-ray tube. In the case of a target transmission tube, notably the dissipation of heat to the window wall is enhanced, whereas in the case of an anode target layer provided on a suitably thermally conductive anode body, the dissipation of heat to said body is enhanced.

Description: The invention relates to an X-ray tube, comprising a cathode for generating an electron beam and an anode having a comparatively thin anode target layer, for generating X-rays in response to the impingement of the electron beam forming an electron target spot on the inner side of the anode target layer, the anode comprising a metal layer which is situated on the anode target layer near an electron target spot on the anode target layer and which is thermally conductively connected to the tube wall for dissipating heat from the anode target layer. An X-ray tube of this kind is known from the United States Patent No. US 3 992 633. In connection with the present invention an anode having a comparatively thin anode target layer should be understood as an anode target layer having a thickness which is much smaller than the thickness of an anode target layer having a sufficient thickness for dissipating the heat without causing a temperature which is too high to avoid deterioration of the cathode layer. In general an X-ray tube comprises a radiation exit window which is made of, for example beryllium and an inner side of which is provided with a thin layer of metal which acts as the anode target layer. In the anode target layer notably the X-rays are generated, which X-rays emanate directly via the exit window in this case. A thin anode target layer of this kind may also be provided on an anode support of a suitably thermally conductive material mounted in an X-ray tube. In the case of such thin anode target layers, the degree of dissipation of the heat generated by the incident electron beam has a strong effect on the service life of the tube. This problem is significant in target transmission tubes because of the comparatively poor thermal conductivity of the thin anode target layer itself as well as of the comparatively thin beryllium exit window. In the case of anode target layers provided on a metal anode support the problem of locally excessive temperatures may arise because the transition between the anode target layer and the anode support constitutes a heat barrier. To mitigate these drawbacks in known X-ray tubes the anode target plates comprise means for enhancing the dissipation of heat. Because the anode target layer itself is provided with means for enhancing the dissipation of heat, the temperature of this layer as a whole, and notably at the area of the electron target spot, will become less high, so that the layer will be less readily damaged and the service life of the tube is prolonged. In the said patent document No. US 3 992 633 the means for enhancing the dissipation of heat constitute a metal layer which is provided against the anode target plate and which is thermally conductively connected to a wall portion of the X-ray tube. It is an object of the invention to further enhance the heat dissipation of the anode target plate; to achieve this, the X-ray tube of the kind set forth in accordance with the invention is characterized in that the electron target spot has a substantially annular shape, the thermally conductive metal layer being situated on the inner side of the anode target layer and within the anode target spot ring. The metal layer is provided notably within a substantially annular electron target spot, so that the spot exhibits suitable dissipation of heat to both radial sides and a central part of the window as well as an irradiated part of the window will become considerably less hot. It should be remarked that GB 1 249 341 discloses an X-ray tube comprising a cathode for generating an electron beam, and an anode having a comparatively thin anode target layer, for generating X-rays in response to the impingement of the electron beam forming an electron target spot on the anode target layer, the anode comprising a metal layer which is situated on the anode target layer near the electron target spot on the anode target layer and which is thermally conductively connected to the tube wall. However, this document does not disclose or indicate or suggest the characterising features of the present invention. EP 0 432 568 discloses an X-ray tube comprising a cathode for generating an electron beam and an anode target layer for generating X-rays in response to the impingement of the electron beam forming an electron target spot on the anode target layer. However, the anode does not comprise a metal layer which is situated on the anode target layer near the electron target spot on the anode target layer and which is thermally conductively connected to the tube wall. Moreover, this document does not disclose or indicate or suggest the further features of the present invention. EP 0 275 592 discloses an X-ray tube comprising a cathode for generating an electron beam and an anode target layer for generating X-rays in response to the impingement of the electron beam forming an electron target spot on the inner side of the anode target layer. However, the anode does not comprise a metal layer which is situated on the inner side of the anode target layer near the electron target spot on the anode target layer and which is thermally conductively connected to the tube wall. Furthermore, the electron target spot has a substantially annular shape. The window layer consists of for example beryllium. Moreover, this document does not disclose or indicate or suggest the further features of the present invention. In a preferred embodiment, an anti-diffusion layer is provided between the anode target layer and an adjoining layer in order to reduce detrimental interactions between the two layers of material. Using such a layer, a reduction of the thermal conduction between the two layers can be prevented, for example due to the appearance of intermetallic compounds. Such an anti-diffusion layer can also reduce other adverse interactions between the layers; for example, the loss of vacuum-tightness of the window can thus also be prevented. An anti-diffusion layer of this kind is provided notably between a window plate of beryllium and an anode target plate which is provided thereon and which consists of, for example rhodium scandium or another known anode target plate material. Some embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Fig. 1 shows an X-ray tube comprising a target transmission anode and an annular electron target spot, Fig. 2 shows an embodiment of an anode target layer and a metal structure for enhancing the dissipation of heat in such a tube, and Figs. 3 and 4 show an anode window with a locally deposited metal layer acting as a heat dissipation means. An X-ray tube as shown in Fig. 1 comprises an envelope 1 with a conical ceramic base 2, a cathode 4 with an emissive element in the form of a filament 6, a cylindrical wall 8 and an exit window 10. An anode 12 is provided in the form of an anode target layer on an inner side of the exit window. The anode consists of, for example chromium, rhodium, scandium or another anode material. The thickness of the layer is adapted to the desired radiation, to the radiation absorption properties of the material, notably to the electron absorption thereof, and to the desired high voltage for the tube, and amounts to, for example a few µm. In the envelope there is provided a cooling duct 14 with an inlet 16, an outlet 18 and a flow duct 20 which encloses the exit window. A high-voltage connector can be inserted into the base 2. A high-voltage connector of this kind is connected to a high-voltage cable, to supply leads for the filament and to supply leads for any further electrodes to be arranged in an anode-cathode space 22. Around the envelope there is provided a mounting sleeve 24 with a mounting flange 26 and an additional radiation shield 28 which also bounds the flow duct 20. Around the tube there is also arranged a thin-walled mounting sleeve 30 in which the cooling ducts are accommodated and which also has a temperature-equalizing effect. Fig. 2 shows the window-anode construction at an increased scale. The window 10 is provided, for example by local diffusion at the area of a mounting edge 33, in a window support 31 in the envelope. When it is ensured that the window support 31 adjoins the flow duct 20 and is in suitable thermal contact with the envelope 24 and the shield 28, suitable dissipation of heat from the edge of the window is ensured. A comparatively thick construction of the elements 24 and 28 benefits the dissipation of heat as well as the absorption of scattered radiation. On an inner side of the window 10 there is provided the anode 12 in the form of a vapour-deposited thin anode target layer. Besides vapour-deposition, sputtering or electroplating are also suitable techniques for the deposition of the anode layer. The anode customarily operates substantially at ground potential, so that no problems will be encountered as regards the electrical insulation of the comparatively thin beryllium window 10. In the present embodiment, the electron-emissive element 6 is arranged in the cathode-anode space at a comparatively small distance from the anode. The emitter is shaped as a loop-shaped filament 40 with input and output leads 42. The filament is preferably freely suspended. Around the emitter there is arranged a sleeve-shaped electrode 46 and an electrode sleeve 48 is arranged within the filament 40. In addition to the diameter of the filament loop, a transverse dimension of a ring focus 56 to be formed can thus be varied by varying either potentials of the electrode sleeves or by varying the height position of at least one of the sleeves 46 or 48. The ring focus can be focused on the anode layer to a greater or lesser extent by optimizing the positioning and potentials carried by the sleeves. Between the beryllium window 10 and the anode target layer 12 there is provided a gauze structure 58. Such a metal gauze of silver or gold has a pitch and a wire thickness such that the X-ray focus, being the object of a subsequent radiation optical system, is not adversely affected thereby. Such a gauze structure may also be provided on an outer side of the window and may constitute, for example a honeycomb structure of silicon carbide of another suitably thermally conductive and comparatively strong material. Figs. 2 and 3 show a preferred embodiment of an exit window of an exit window target transmission tube comprising a metal heat dissipation construction 62 in the form of a metal disc 62, arranged within an annular electron target spot 56, and a radial dissipation conductor 64 constituting a connection between the disc 62 and a tube wall portion 33. In this tube the focus ring has a fixed diameter, so that the metal layer 62 can be provided so as to be adjacent thereto.

Claims: An X-ray tube, comprising a cathode (4) for generating an electron beam, and an anode having a comparatively thin anode target layer (12), for generating X-rays in response to the impingement of the electron beam forming an electron target spot (56) on the inner side of the anode target layer, the anode comprising a metal layer (62) which is situated on the anode target layer near the electron target spot on the anode target layer and which is thermally conductively connected to the tube wall for dissipating heat from the anode target layer (12) characterized in that the electron target spot (56) has a substantially annular shape, the thermally conductive metal layer (62) being situated on the inner side of the anode target layer (12) and within the anode target spot ring (56). An X-ray tube as claimed in Claim 1, in which an anti-diffusion layer is provided between the anode target layer and an adjoining layer in order to reduce detrimental interactions between the two layers of material. An X-ray tube as claimed in Claim 2, in which the anode target layer forms part of a window plate of an X-ray exit window of the X-ray tube, plate further comprising an exit window layer and the anti-diffusion layer, the anti-diffusion layer being provided between the anode target layer and the window layer, the anti-diffusion layer acting as a support for the anode target layer, of the X-ray exit window layer.

Inventors: GIJSBERS JAN CORNELIS, JENNESKENS THEODORUS J J M, SMIT HUBERT HERMAN A, GIJSBERS, JAN CORNELIS, JENNESKENS, T.J.J.M., SMIT, HUBERT HERMAN A., Gijsbers, Jan Cornelis, c/o INT. OCTROOIBUREAU BV, Jenneskens, T.J.J.M., c/o INT. OCTROOIBUREAU BV, Smit, Hubert Herman A., c/o INT. OCTROOIBUREAU BV
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H01J35
IPC codes: H01J35, H05G1