US1665637A - X-ray tube - Google Patents

Description

J. W. MARDEN X-RAY TUBE Filgd June 18, 1927 L9 (UEANIVM) INVENTOR J. w. HARD N ATTORNEY Patented Apr. 10, 1928.

UNITED STATES PATENT OFFICE.

JOHN WESLEY MARDEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

x-BAY TUBE.

Application filed June 18, 1927. Serial No. 199,698.

The present invention relates to thermionic devices and more particularly to X-ray tubes operating by an electron discharge gen erated substantially independently of gas ionization.

In an X-ray tube employing a tungsten filamentary cathode as the sourceof electrons and containing a target consisting of thorium, efi'ective cooling means must be em ployed otherwise the target becomes heated and some of the thorium is sputtered onto the cathode. When such action occurs, the thermionic current is materially increased which at times results in the target becoming overheated and also causes the tube to become erratic. This action occurs as a result of employing cathode and target materials which have the property of emitting electrons of corresponding value at widely different temperatures.

It is well known in radio. tubes that the electron emissivity of tungsten is considerably increased by the provision of a thorium film on the surface thereof. Furthermore, that in such a filament a copious electron emission takes place at temperatures far be low that at which the same electron emission occurs from tungsten alone. It is thus readily seen in the case of an X-raytube containing a thorium target and a filament of tungsten as the cathode, that the de osit of thorium on the filament resulting roin sputtered particles of thorium from the target will tremendously increase the electron emission of the filament. When such action occurs, asexplained above, the tube becomes erratic.

The present invention overcomes the foregoing difficulties by providing within the X-ray tube, target material and cathode material having the property of emitting electrons of substantially the same value at very close to the same temperature so that the increased electron emissivity referred to does not take place. More specifically, the invention contemplates the employment of a target of uranium and a filament of molybde num. The temperature at which uranium emits electrons is very near the temperature at which molybdenum gives its electron emission of corresponding value.

It is also to be appreciated that uranium as target. material is more desirable than such metals as tungsten or even thorium, since its atomic weight is' higher and also because it is a better ray emitter.

In the accompanying drawings, Fig. 1 is a longitudinalview of an X-ray tube of the hot cathode type, and

Fig. 2' is an enlarged sectional view of the ilectrodes of such a tube as is illustrated in The tube as shown in Fig. 1 comprises a glass bulb 3 having oppositely extending arms 4 and 5- into which the electrodes 6 and 7 are sealed. The cathode, as shown in Fig. 2, comprises a spiral filament 8 which in accordance with my invention, is made of molybdenum. The cathode is connected to lead wires 9 and 11 which are sealed into a glass stem 12 and joined to contacts 13 and 14:. The cathode filament 8 may be heated to electron emitting temperature by any suitable source of low voltage current, as for example, a battery 15, in circuit with a switch 16.

Surrounding the cathode 8 is the usual focusing cup 17 the function of which is well understood in the art. The focusing device is connected to one of the cathode lead wires 9 or 11.

The anode or target comprises a mass of copper 18 or other suitable heat conducting material, to which is intimately united a uranium button or insert 19, which forms the focal spot of the target. It is this portion of the target upon which the electrons emanating from the cathode 8 impinge. Obviously, the impacts of the electrons thereon cause the focal spot to become heated. in addition, the impact of the electrons causes sputtering of the uranium. These sputtered particles, if deposited upon the filament 8 of molybdenum, do not cause any increased emission of electrons from the filament since the electron emissivity of the filament for any given value is approximately the same as the electron emissivity of uranium. It is thus apparent that by reason of the employment of a focal spot of uranium and a cathode of molybdenum a novel result is obtained as the electron emissivity of the filament is maintained substantially constant regardless of the deposit thereon of any particles of uranium. In addition, the advantages obtained by the use of uranium as the target material are preserved.

The usual high volta e source 21 is applied to-the electrodes 0 the device.

Modifications of the invention may occur to those skilled in the art but such modifications are contemplated by me as come within the scope of the invention as defined by the appended claims.

What is claimed is:

1. A thermionic device comprising an anode and a cathode adapted to be heated to electron-emitting temperature, said anode being formed of a material having the property of emitting electrons of corresponding value at the same temperature as that to which said cathode may be heated and also possessing better ray emitting properties than tungsten.

of molybdenum, and an enclosing envelope.

for said anode and cathode.

4. An X-ray tube COll'lPllSlDg an envelope, a thermionic cathode of molybdenum sealed therein and a target consistlng of uranium. In testimony whe eof, I have hereunto snsbcribed my name tlllii 17th day of June,

JOHN WESLEY MARDEN.

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