US3801847A

US3801847A - X-ray tube

Info

Publication number: US3801847A
Application number: US00295490A
Authority: US
Inventors: K Dietz
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1971-11-04
Filing date: 1972-10-06
Publication date: 1974-04-02
Anticipated expiration: 1991-04-02
Also published as: FR2158386B1; GB1414501A; AT331363B; FR2158386A1; DE2154888A1; JPS4855687A; ATA683472A

Abstract

An X-ray tube wherein electrons accelerated by an electrical voltage strike an anode which carries at least upon the surface receiving the electrons a thin layer of a substance having a high density which lies upon a body of lesser density. The invention is particularly characterized in that the layer is thinner than the depths of penetration of electrons in the material of the layer produced by acceleration voltages used for creating the rays.

Description

United States Patent [191 [111 3,801,847

Dietz Apr. 2, 1974 X-RAY TUBE Primary Examiner-Herman Karl Saalbach [75] Inventor: Kurt Dietz, Erlangen, Germany jff' i ip g f;

orney, gen ,or zrm 1c at s eier [73] Assignee: Siemens Aktiengesellschaft, Munich,

Germ 57 ABSTRACT [22] filed: 1972 An X-ray tube wherein electrons accelerated by an [21] Appl. No.: 295,490 electrical voltage strike an anode which carries at least upon the surface receiving the electrons a thin layer of a substance having a high density which lies upon a [30] Forelgn Apphcanon Pnonty Data body of lesser density. The invention is particularly NOV.4, 1971 Germany 2154888 characterized in that the y is thinner than he depths of penetration of electrons in the material of (ell. 3134686133532 the layer produced by acceleration voltages used n atin the r S 58 Field of Search 313/60, 330 m g ay 3 Claims, 3 Drawing Figures D a" 13 v" l5 X-RAY TUBE This invention relates to an X-ray tube wherein electrons accelerated by electrical voltage strike an anode carrying at least upon the surface receiving the electrons a thin layer of a substance having a high density which lies upon a body of lesser density.

Known X-ray tubes of this type have, for example, a carrying body of metal, such as molybdenum and its al loys, or of graphite. As material of higher density, which is the brake material, are used as a rule heavy metals, such as tungsten and its alloys, particularly those of tungsten and rhenium, tungsten and osmium or tungsten and iridium or tantalum. The coatings have a thickness located at least in the range of millimeters; however, they are not less than p. so that all electrons coming from the cathode and received in the focal point, are braked. All of them, or as many as possible, should be utilized for producing brake rays, namely, X- rays. As is known, in the course of this process about 99 percent of absorbed energy is changed into heat in the heavy metal layer which must be then transmitted to the carrier by heat conducting. This transmittal is necessary to avoid overheating in the focal point and a destruction of the ray receiving surface. The absorption of all electrons in the stricken surface produces there the maximum possible transformation of these electrons and thus also the greatest increase in heating.

An object of the present invention is to improve existing X-rays of the described type.

Other objects will become apparent in the course of the following specification.

In the accomplishment of the objectives of the present invention it was found possible to diminish the amount of heat transmitted to the carrying body by making the layer thinner than the depths of penetration of electrons in the material of the layer produced by acceleration voltage used for creating the rays. The layer has a thickness amounting to a few [1. when tungsten is used ranging between 0.5 to 5;:.. This layer absorbs only a part of the energy of striking electrons, so that the other portion of the electron energy reaches directly the supporting body and only there is absorbed. A substantial amount of produced heat is freed only at the supporting body; it is not necessary to transmit it to this body by heat conducting from the heavy metal layer. 0n the other hand experiments and considerations which have produced the present invention have shown that electrons which penetrate deeper into a heavy metal layer in case of braking produce only such rays which have a wave length about 1.5 times longer than the limit wave length. For the large part these rays cannot leave the outer surface of the anode.

This longer wave part of the ray is absorbed in the object and results there in an unnecessary ray load. Therefore in known tubes it is necessary to remove it by filters causing unavoidably a weakening of the useful rays. At the same time there is the advantage that the heat produced in the carrier does not have to be trans- I mitted any more from the brake layer to the carrier.

When the thickness of the heavy metal layer is set for an absorption of only 25 percent of the received elecextent. The more advantageous transmission of energy in the depth of the anode, by electronic conducting and heat conducting, produces a colder anode outer surface and makes possible higher loads, which leads to a more intensive ray emission.

The anode of the present invention can be constructed from known materials by the use of methods which are also known. It is advantageous to adapt the thickness of the layer of the material of higher density to the energy, i.e. the transmission capacity of electrons at acceleration voltages used in X-ray tubes. There results for tungsten in the range of 50 to 80 kV a layer thickness of 2p. and in the range of 80 to 150 kV a layer thickness of 5a. This arrangement produces thus a thickness which corresponds to 25 percent of the range of electrons in tungsten at the prevailing highest acceleration of electrons. The substance used as the material of higher density can also consist of a tungsten alloy with rhenium, osmium, iridium, etc., or another metal or compound, such as, for example, a carbide of tungsten, tantalum or hafnium. Different layer thicknesses result depending upon the density of these substances, if a corresponding part of the penetration depth is to be taken as a measure. The thickness must be increased for smaller densities and diminished for higher densities.

The invention will appear more clearly from the following detailed description when taken in connection tronic energy, the diminution of the useful rays behind the object amounts to only about 5 percent. This shows clearly that although the absorption of the electronic energy in the brake layer is smaller by a comparatively large percentage percent) than in known anodes, the amount of outgoing rays is smaller only to a small with the accompanying drawing showing by way of example only, preferred embodiments of the inventive idea.

In the drawing:

FIG. 1 is a diagrammatic side view of an X-ray tube with rotary anodes.

FIG. 2 is an enlarged section through a part of a rotary anode.

FIG. 3 is a similar section showing a rotary anode with two focal point paths of different thicknesses.

FIG. 1 shows a vacuum tight bulb l of a tube. At one end of the cylindrical bulb 1 there is a cathode device 2 and at its opposite end an anode device 3, which, as is known, includes a rotor 4 rotatably mounted upon a supporting pin 5. At its opposite end the rotor has an axle 5 upon which the rotary anode 6 is mounted. The cathode 2 consists of a casing 7 having an extension 8 in which a glow cathode 8 is located. The glow cathode can be actuated by connecting lines 9, l0 and 11. At these lines is also located the acceleration voltage for the electron current emerging from the cathode. The counter pole of this acceleration voltage is located at the support 5.

In the illustrated embodiment a heat voltage for the left half of the cathode is supplied between the lines 9 and 10. An electronic current 12 shown by broken lines emerges therefrom and strikes the outer focal point path 13 of the rotary anode 6. In this case the focal point path 14.does not receive the electron flow. It would have been struck by electrons if a heating voltage would have been applied between the lines 10 and 11.

FIG. 2 shows a section of a part of the anode 6 with a layer 15 upon which lie focal point paths l3 and 14. The layer 15 which is 3 p. thick and consists of tungsten, is applied upon the graphite body 6' which has a thickness of 10 mm. and a diameter of mm. The striking electrons of the ray bundle 12 are indicated by

arrows

16 and 17, whereby the arrows l6 symbolize those electrons which are transformed into rays in the layer 15, while the longer arrows l7 symbolize those electrons which pass through the layer. Electrons symbolized by the longer arrows give up energy only in the material of the body 6, so that their heating action does not affect the material of the layer 15, on the outer surface of which the X-rays are taken in well known manner.

In the embodiment of the present invention shown in FIG. 3 the two focal point paths 18, 19 lie upon the

layers

21 and 22 carried by the graphite body 20 having a thickness of 6 mm. and a diameter of 100 mm. The layer 21 has a thickness of 2p, and the layer 22 has a thickness of p. Both layers consist of tungsten. The layer 21 is for electrons of 50 to 80 kV and the layer 22 is for electrons of 80 to 150 kV. Thus the thickness of the layers is adapted to the diaphanous capacities of the electrons.

I claim:

1. An X-ray tube, comprising an anode having a body and at least one layer of material of higher density than that of said body carried by said body, and means having an electrical accelerating voltage for producing electrons and projecting them upon said layer with a depth which is greater than the thickness of said layer, the thickness of said layer amounting to at most 25 percent of the range of electrons at maximum accelerating voltage.

2. An X-ray tube in accordance with claim 1, wherein the anode has a plurality of focal points and a plurality of layers carried by said body and having different thicknesses, the material of said layers being tungsten, one of said layers having a thickness of 2p, for a focal point of 50 to kV and another one of said layers having a thickness of 5p. for a focal point of 80 to kV.

3. An X-ray tube in accordance with claim 1, wherein the material of said body is graphite.

Claims

2. An X-ray tube in accordance with claim 1, wherein the anode has a plurality of focal points and a plurality of layers carried by said body and having different thicknesses, the material of said layers being tungsten, one of said layers having a thickness of 2 Mu for a focal point of 50 to 80 kV and another one of said layers having a thickness of 5 Mu for a focal point of 80 to 150 kV.