Slotted Anode X-ray Tube

Abstract

The anode disc of an X-ray tube is provided with slots extending through anode material in the target area. Each slot has a width within the range of thermal expansion of the anode material so as to prevent thermal stresses in the anode body. In rotating anodes the slots extend from the circumference of the anode disc or the anode tyre well beyond the focal path. Radiation through the slots of the anode can be prevented by arranging the slots at an angle with respect to the direction of incidence of the electron.

Description

The invention relates to an X-ray tube having an anode on which a target spot is formed by an electron beam emitted by a cathode, the anode being provided with slots so as to prevent damage by thermal stresses.

An anode of an X-ray tube can be damaged in that in a tube in operation material of the anode on which the electron beam impinges is locally heated, the thermal conductivity of the anode material generally being too small for a quick and uniform distribution of the generated heat over the entire anode body. As a result, substantial local temperature differences occur in a radiated anode, particularly when the tube is switched on, which results in substantial thermal stresses. In general, hot parts of the anode which tend to expand will exert a tensile force on adjacent colder parts which cannot give in, which results in a compression force in the hot parts of the anode. It is known that these stresses can exceed the strength of the anode material, particularly in the case of severe loading of the anode when the X-ray tube is switched on. This can cause plastic deformation in the hot part and fracture in the colder and hence comparatively brittle part.

It is known that at a given dimension of the target spot of the electron beam the thermal stresses in the anode can be substantially reduced by putting the anode into motion so that the target spot describes a so-termed focal path over the anode surface. The radiation and the heat generated as a result thereof are then uniformly distributed over the material situated in the focal path, so that the temperature becomes less high and higher instantaneous loading is permissible while using the same anode material. Notably rotating anodes are widely used in X-ray tubes, particularly in X-ray tubes for medical applications where particularly high loading is desired. At these high loads, however, damage or fractures occur also in the rotating anodes as a result of the thermal stresses occurring therein.

Various measures are known so as to limit this damaging of anodes in X-ray tubes. It is known from German Pat. No. 667,039 that the strength of the anode can be increased by provision of supporting edges. It is also known that the mechanical strength or the elasticity of the anode material can be increased by using special alloys and combinations of alloys. All known methods are restricted to the reduction of the consequences of the thermal stresses occurring in the focal path and, consequently, they present only a limited improvement.

It is known from German Offenlegungsschrift No. 1,937,351 that the mechanical stresses which are transferred by the heated part of the anode, i.e., the focal path, to the colder part can be dealt with by means of a resilient construction of the colder part of the anode. To this end, a rotating anode is provided with slots in the anode material which is situated outside the focal path, i.e., in the anode material which is not directly heated. These slots extend, for example, radially through the anode material, as described in German Pat. No. 687,378, or spirally as indicated in German Offenlegungsschrift No. 1,937,351. As a result of the resilient construction the stresses generated in the directly heated anode material are better distributed over the brittle material. However, a stress concentration will then occur on the ends of the slots which are situated nearest to the centre so that fracturing is still liable to occur. Moreover, the discharge of heat from the focal ring is impeded in that the slots contain a tangentially directed component in this case. The stresses in the focal path itself are not significantly reduced either according to this method, so that plastic deformation can again occur at this area. The latter drawback can at least be partly eliminated by clamping, as described in the said Offenlegungsschrift, the material of the focal path as a loose ring onto the resilient construction. However, in that case the discharge of heat from the focal path is seriously hampered so that the material in the focal path becomes additionally warm.

The invention has for its object to provide an X-ray tube which does not have the above-mentioned drawbacks, and to this end an X-ray tube of the kind set forth is characterized according to the invention in that the slots extend through the anode material to be radiated by the electron beam.

Because according to the invention the material which is heated by the electron beam is provided with slots, the occurrence of thermal stresses is substantially prevented at this area so that stresses are transferred to the brittle material to a far lesser extent. When the invention is used for a rotary anode, the focal ring can expand and shrink, without internal or external thermal stresses and without any significant weakening of the anode disc, while maintaining proper thermal contact with the surrounding anode material. So as to prevent electrons of the electron beam from passing through the complete anode, or from impinging on a sub-layer of another material in the case of a composite anode, the slots can be provided at an angle with respect to the direction of incidence of the electron beam.

Some preferred embodiments according to the invention will be described hereinafter with reference to the drawing. In the drawing:

FIG. 1 is a diagrammatic view of an X-ray tube comprising a rotating anode which is provided with slots according to the invention,

FIG. 2 is a diagrammatic view of a rotating anode comprising slots according to the invention,

FIG. 3 is a diagrammatic view of a rotating anode in which the slots enclose an angle with the direction of incidence of the electron beam, and

FIG. 4 is a diagrammatic view of a type anode provided with slots according to the invention.

An X-ray tube 1 comprises a wall 2 with an X-ray exit window 3. The wall 2 furthermore accommodates a passage 4 for supply conductors 5 for a cathode 6. A filament 7 is divided into two portions 9 and 10 by means of a centre tapping 8. During operation the filament emits an electron beam 11 which is accelerated in the direction of an anode 12 and which is incident on this anode at a target spot 13. Via a shaft 14, the anode 12 is brought to rotation by a drive system 15 at a revolution speed of, for example, 9,000 r.p.m. Via a passage 16 in the wall 2, supply conductors for the rotating anode device are passed through the tube wall. As a result of the rotation of the anode 12, the target spot describes a circular target spot path and a beam of X-rays 17 is generated which departs through the window 3. By a suitable choice of the filament portion 9 or 10, the target spot path can be shifted in the radial direction over the anode in this double-focus tube, in which case, for example, a different focussing of the beam 17 is obtained or, because the anode disc consists of mutually different materials on the target spot surface, an X-ray beam of different wavelength is obtained. According to the invention, the anode 12 has a construction as shown in FIG. 2. An anode disc of this kind has a diameter of, for example, 90 mm and consists of an anode disc 20 which is made of, for example, tungsten. A rotation shaft 22 is centrally arranged in this anode disc by means of a rigid connection 21. An annular portion 23 which is denoted by broken lines and whose free surface 24 will act as the target spot surface, will be referred to hereinafter as focal ring. The focal ring can thus form an integral part of the complete anode disc or, in the case of composite anodes, an integral part of a cover layer of the anode, but can also be embedded as a separate ring in the anode disc. Instead of tungsten the anode disc, and in particular a cover layer thereof, can be made of, for example ruthenium, rhodium, palladium, molybdenum or of alloys such as tungsten with rhenium or tungsten with irdium. In the composite anode discs which are usually constructed in the form of a double layer, the two layers usually being sintered to each other, the layer which is not to be radiated is preferably made of molybdenum. In the case of a double focus ring, the two adjacent rings can be made of a different material. The anode disc is provided with slots 25 according to the invention. These slots extend substantially radially through the focal ring 23, preferably from the free radial end 26 of the anode disc well beyond the focal ring. In an anode disc having a diameter of 90 mm, the distance from the free limitation to the outer limitation of the focal ring amounts to, for example, 5 mm, the width of the focal ring is, for example, 15 mm and the slots extend, for example, another 10 mm beyond the focal ring. The ends 27 of the slots are then situated 15 mm from the centre of the disc. It will generally be advantageous to extend the slots as far as the free end 26 because thermal stresses are then optimally avoided. The provision of the slots is also easier. The width of the slots is, for example, approximately 50-200 microns and is determined by the maximum expansion in the focal ring, to be calculated, for example numerically, and the number of slots. To ensure proper division of the focal ring and a limitation of the slot width, the number of slots will amount to at least approximately eight, even though a smaller number of slots may be sufficient for some applications. The slots can be provided, for example, by spark chipping or electron beam machining. An advantage thereof is that the question whether or not slots will be provided need to be taken into account for the manufacture of anode discs. The slots can also be provided already during the formation of the anode discs.

So as to prevent electrons of the electron beam 11 from passing completely through the anode, the slots can be provided, as shown in FIG. 3, in planes which enclose an angle with the direction of incidence of the electron beam. The value of the angle is determined by the slot width and the thickness of the anode disc or the thickness of the focal ring material if the carrier material is to be protected against electron bombardment in the case of composite anodes. It is not always necessary that the slots extend through the entire anode part which is situated below the target spot path, viewed from the electron beam. For example, this is not necessary if the material does not become hot at this area. However, in general it will be more advantageous to allow the slots to extend through the material below the focal path.

FIG. 4 shows a preferred embodiment in the form of a tyre anode. A focal ring 30 forms a tyre as it were about a rotation-symmetrical carrier body 31. A rotation shaft 32 extends through the carrier body which is rigidly connected thereto by means of clamps 33 and 34. Both shaft portions 35 and 36 of the rotation shaft can then be mounted in bearings so that stable suspension is ensured. Slots 25 again divide the focal ring into, for example, 12 segments and can extend into the carrier body over some distance. The slots can again extend along planes which enclose an angle with the direction of incidence of the electron beam.

The advantages of the solution according to the invention are particularly significant in this tyre anode. In this case the slots are comparatively short with respect to the rotating anodes described above; they are in accordance with thickness of the disc and are shallow, for example, only a few millimetres, with the result that only comparatively little material has to be removed so that the provision of the slots is cheaper. Known solutions have the drawback that, if the focal ring is formed by a ring which is clamped about the carrier, the focal ring will become loose from the carrier when heated by the electron beam. The focal ring is then liable to fall from the disc, and in any case proper thermal contact between disc and focal ring is broken.

When the slots are provided, the angle at the limitation of the anode body can be somewhat rounded off. The internal limitations 27 can also be rounded off, it being permissible for the rounding off radius to be larger than half the slot width, which results in the shape shown in FIG. 4.

The invention enables the use of anode discs having a larger diameter. With known solutions it was found that larger discs always break down quickly, even though the focal ring used therein becomes less warm as a result of the larger target area and the greater discharge of heat. Considering the described invention it will be obvious that the thermal expansion of the focal ring which is larger in an absolute sense is responsible for such breakdowns. By division of the focal ring, this restriction is eliminated so that on the one hand higher loads can be permitted or, on the other hand, the choice of the anode material is less restricted at the same load. Both liberties can be utilized in particular in X-ray tubes for medical applications.

Claims

What is claimed is:

1. A rotating anode X-ray tube comprising a compact anode body having a target area against which an electron beam emitted by a cathode is directed, a plurality of substantially radial slots extending through the anode body from the circumference across said target area to a distance from center of rotation, the width of each slot corresponding substantially to the maximum thermal expansion of the anode portion at said target area.

2. An X-ray tube as claimed in claim 1, wherein said anode is formed by a body including a tyre portion at said target area.

3. An X-ray tube as claimed in claim 1, characterized in that the slots are situated in planes inclined to the direction of incidence of the electron beam.

4. A rotation symmetrical anode for an X-ray tube as claimed in claim 1, characterized in that it comprises a disc-like anode body having a diameter exceeding 100 mm and the width of the slots being in the range from 50 to 200 microns.