US2094395A - X-ray tube - Google Patents

Description

Patented Sept. 28, 1937 X-RAY TUBE Albert Bouwers, Eindhoven, Netherlands, as-

signor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application May 31, 1934, Serial No. 728,427 In Germany July 3, 1933 11 Claims.

The present invention relates to X-ray tubes and more particularly to improvements in the prevention of harmful stray radiation by such tubes.

As is well known, X-ray tubes, besides producing the useful X-ray beam directed at a selected angle to theoutside of the tube, also produce harmful X-rays, which-unless .prevented-emerge from the tube at various angles, either as direct or as difiused (scattered) stray rays.

The dangers involved in such stray X-ray radiation has been recognized since the early days of X-ray work, and various means have been developed, usually in the form of shields placed outside of the tubes, or screens provided within the tube, to intercept and absorb such stray rays.

For X-rays of a given hardness (wave-length) the ray-absorbing capacity of a shield, or screen depends on its thickness and on its material. This ray-absorbing capacity increases with the thickness of the layer intercepting the-rays and with the atomic weight of the material used. 7 l Lead, which has the highest atomic weight of the metals, commonly used, is generally used .either in;th e formof sheets or coatings, although inmany caseslead-glass or lead-containing compositions, are used. As a rule lead is unsuited for screens placed inside of the evacuated space of the tube, among other reasons because of the 'difliculty. of freeing lead from occluded gases. due

to the low melting point of this metal.

Tungsten, another high-atomic metal, which however, is more; expensive than lead; is usually employed forvinside screens. Copper, which has acomparatively-low atomicweight, is not a good screening material; neverthless in X-ray tubes operating. at comparatively low voltages the copper, anode body of thetube, on account of its comparatively great thickness, acts as a screen which absorbs such stray rays as-are directed 'through'thelanode towards the anode-end ofthe tube-,5 '1

. As is well known, the penetrative power of X-rays increases with their hardness, andfithe higherthe operating voltage of the tube, the -ha=rder (shorter.wave-lengths) rays are generlatt rl ;v

The absorbing capacity of lead is, with respect tothe hardestraysgenerated at 40 kv., only 1.8 .:times greater than that of. copper, so that for tubes of. this kind a copper, body having, in the direction of .the' rays, athickness of 18 mm; is as regards.raysabsorption-.equivalentto 10 mm. of .lead endless heavy. ForX-raytubes operating at higher voltages as are used for therapeutic work or for the examination of materials in metal industry very heavy screens would be required in the case of copper. For example to bring about a given absorption of the hardest rays generated at 125 kv., 10 mm. of lead are equivalent to about mm. of copper and the difference is still much greater when voltages of 200 kv. and higher are used.

For X-ray tubes operating at comparatively low voltages, for instance at 35,000 to 85,000 volts, as generally used in radiographic work, the problem of shielding against stray rays has been satisfactorily solved. As the largest amount of stray radiation is generated in the region of the focal spot of the anode and tends to leave the tube in directions which are more or less perpendicular to the axis of the tube, the X ray tube is surrounded at its X-ray generating portion, by a lead or lead-glass shield. Or in the X-ray tubes,

known under the trade-mark Metalix, this portion of the tube is formed of a metal section,

which absorbs a considerable portion of the 1 X-rays. In either case ray-windows or openings are provided for the passage of the useful X-ray beam.

Those stray rays which are directed towards the two ends of the tube can also be intercepted and absorbed in a comparatively simple manner in the case of tubes operating at comparatively -low,voltages. For instance, the rays which are directed towards the cathode-end of the tube can be absorbed by a ray-absorbing disc, for instance .of tungsten, placed in the tube in the rear of the cathode. Those rays which are directed towards the anode-end of the tube are to .a great extent absorbed by the copper body of the anode and by other metal portions in the rear of the anode intercepting same. In addition a lead or other heavy metal disc may be provided in the rear of the anode outside of the evacuated space of the tube. Furthermore, the copper anode body also absorbs those stray rays which are directed toward the anode end of the tube, but form a more or less acute angle with the axis of the tube.

Intubes for lower voltages the rays can also be absorbed by surrounding the insulating portiorrof the tube which supports the anode (and also-that supporting the cathode) with a coating of lead or with an insulating sleeve comprising lead.

However, in the case of X-ray tubes operating at several hundred thousand volts, for instance, 200,000 .to. 600,000 volts and. more, new problems arise in the protection against stray rays.

This.

applies primarily to the protection against those stray X-rays which are directed towards the anode-end of the tube in directions forming more or less acute angles with the axis of the tube. While the other stray rays generated in such very high voltage tubes, can be absorbed by screens and shields arranged in similar manner as in the case of lower voltage tubes, in regard to the just specified stray rays, the screening and shielding means used in lower voltage tubes copper anode body which intercepts the just specified rays, proves insufficient to absorb them.

Nor is it practical to surround the anode end of the tube with a lead coating or with an insulating sleeve containing lead, as this would require so thick and thus so heavy lead layers as to result in impractical tube constructions.

The main object of my invention is therefore to provide for X-ray tubes operating at very high voltages, means to absorb those stray rays which are directed through the anode body to the outside of the tube at more or less acute angles with the axis of the tube.

According to the invention I arrange within the body of the anode, which is of copper or other metal of good heat-conductivity, a screen formed of a material of high X-ray absorbing property, for instance, of lead, tungsten or the like, in such a manner that the heat dissipation of the target by heat conduction, through the anode body is not materially lessened.

In the drawing forming part of this specification I have illustrated by way of example one embodiment of my invention.

The drawing shows a cross-sectional side elevation of an X-ray tube having an anode structure made in accordance with my invention.

The drawing gives a more or less diagrammatic illustration'of an X-ray tube, with the omission of such features which are not needed for the understanding of the invention. (Such omission also applies to such features which usually characterize X-ray tubes for such extremely high voltages.) Z

The X-ray tube illustrated is of the type known under the trade-mark Metalix and comprises 'two cylindrical members! and 2'of glass, havmaterial, through a ray'window 6, provided in themetal portion 3. 7

The metal portion 3 is preferably provided with a lead coating or lead jacket IE to increase its ray-absorbing capacity.

In the rear of the cathode is provided a tungsten disc I! of sufficient thickness to prevent stray radiation towards'the rear ofthe cathode.

The copper anode body is provided with an annular cavity l8 coaxial with the axis of the anode and open towards the rear of the anode (outside of the evacuated space), in which cavity is inserted a cylindrical member 1 of a material maining portion of the annular cavity of high X-ray absorbing capacity, for instance of lead, tungsten or other heavy metal. The cavity l8 extends close to the frontal surface of the anode.

Due to the fact that with increasing distanceas measured from the frontal surface of the anode-the X-rays are increasingly absorbed, the member 1 instead of having uniform thickness, may be,'as shown in the drawing, tapered towards the rear end of the anode. Such tapering has the advantage of reducing the weight of the member I and thus of the anode. The re- IB is preferably filled out with a lighter metal of good heat-conductivity, as copper or brass. Preferably this metal forms an annular member 8 tapered towards'the front of the anode to form a complementary member to the member I, securing same in the cavity [3.

It is apparent that the provision of a coaxially disposed lead or other X-ray absorbing metal cylinder I in the anode body, does not materially reduce its heat conduction; whereas, if to obtain the same results, a ray-absorbing metal disc were to be placed perpendicularly through the anode,- it would have to extend throughout the whole cross-section of the anode body which would very materially reduce the heat conduction through the anode towards the cooling means.

While I have referred to the material of the anode body as being copper, it should be well understood that other good heat-conducting materials, for instance, aluminium, may be used therefor.

Similarly, the member 1, instead of being composed of lead, may be formed of tungsten or of other material of high X-ray absorbing capacity.

Furthermore, while I have illustrated my invention in connection with an X-ray tube of the so-called Metalix type, it can evidently be applied also to other types of tubes; also while the invention is specially useful in connection with X-ray tubes operating at extremely high voltages, it can also be applied with advantage to X-ray tubes operating at lower voltages.

Nor do I wish to be limited to the exact construction showing of my invention, as such construction may be modified without departing from the spirit of the invention.

I wish, therefore, the appended claims to be, construed as broadly as permissible in view of the prior art.

What I claim is:

1. An X-ray tube comprising an evacuated en- -velope,'a cathode and an anode structure there- -in, said anode structure comprising an anode body of a material having good heat conductivity and low X-rayabsorptioncapacity and having a target surface opposing the cathode, said body having an. annular cavity in the rear of said,

the evacuated space of the tube, and an annular member of a material having high X-ray absorption capacity disposed in said annular cavity.

.target surfaceand hermetically separated from .2. In. an X-ray tube, ananode structure com- 7 5 prising. a cylindrical anode bodyof copper, said -body having a frontal target surface and a cylin- .X-ray absorbing capacity and unsuitable for use in the evacuated space of the tube.

. 3. In an 'X-ray tube,.an anode structure com- .prising an anode .body of .a material having good heat conductivity and low X-ray absorption capacity, said body having a cylindrical cavity, and an annular member of a material having high X-ray absorption capacity disposed in said cavity, and a second annular member complementary to the first annular member disposed in said cavity, said second member being of a material having high heat conductivity.

4. In an X-ray tube, an anode structure comprising an anode body of a material having good heat conductivity and low X-ray absorbing capacity, said body having a cylindrical cavity, and an annular member of a material having high X-ray absorbing capacity disposed in said cavity, said member being tapered towards the rear end of the anode body, and a second annular member disposed in said cavity complementary to the first member, said second member being of a material of good heat conductivity.

5. An X-ray tube adapted to be operated at voltages of several hundred thousand volts and comprising an evacuated envelope, an anode structure and a cathode structure in said can-- velope, said cathode and anode structure opposing each other to' form a discharge path, the anode structure comprising an anode body of a material having high heat conductivity and low X-ray absorbing capacity, said body being provided with an annular cavity hermetically separated from the evacuated space and an annular member of a material having a high X-ray absorbing capacity disposed in said annular cavity.

6. An X-ray tube adapted to operate at voltages of several hundred thousand volts and comprising an evacuated envelope, an anode structure and a cathode structure in said envelope, the anode structure comprising an anode body of copper, said body being provided with a cylindrical cavity, and an annular member of a metal having high X-ray absorbing capacity disposed in said cavity of said anode body, said member being retained 'by a second annular member of a material having good heat conductivity.

7. An X-ray tube adapted to be operated at voltages of several hundred thousand volts and comprising a highly evacuated envelope, an anode structure and a cathode structure in said envelope, the anode structure comprising an anode body of a material having high heat conductivity and low X-ray absorbing capacity, and being provided with an annular cavity, an annular member of a material having high X-ray absorbing capacity disposed in said cavity to absorb stray X-rays directed towards the rear of the anode in directions forming acute angles with the axis of the tube, said member being tapered towards the rear of the anode body, and a second annular member disposed in said cavity complementary to said first annular member, said second member being of a material of good heat conductivity.

8. An X-ray tube comprising an evacuated envelope, a cathode and an anode structure mounted therein, and opposing each other toform a discharge path, said anode structure comprising an anode body of a material having a good heat conductivity and low X-ray absorption capacity, said body having a frontal face and an elongated annular cavity disposed to the rear of the frontal face and hermetically separated from the evacuated space, and an elongated annular member of high X-ray absorptive capacity disposed in said cavity to absorb the X-rays passing to the rear of the active surface. 1

9. In an Xray tube, an anode structure comprising a copper body having a frontal target surface and an annular cavity at the rear of said surface and. hermetically separated from the evacuated space of the tube, and an annular lead member disposed within said cavity to prevent X-rays from emerging through the anode end of the tube.

10. In an X-ray tube, an anode structure comprising a copper body having a frontal target surface and an annular cavity at the rear of said surface and hermetically separated from the evacuated space of the tube, and an annular tungsten body disposed in said cavity to prevent X-rays from emerging through the anode end of the tube.

11. An X-ray tube comprising an anode structure, a cathode structure and a vitreous evacuat-ed envelope having a sleeve portion, said anode structure comprising an anode body of a material of low X-ray-absorbing capacity and having a ring-shaped edge hermetically sealed to said sleeve portion, said body being provided with an annular cavity opening inside said ringshaped edge, and an annular metal member of high X-ray-absorbing capacity disposed in said cavity.

ALBERT BOUWERS.

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