US1355126A - X-ray tube - Google Patents

Description

w. 0. coollnss. X-RAY TUBE.

Fig. 1,.

. Inventor Wijl Ham D. Coolidge,

His fittorney.

screen undesired WILLIAM D. COOLIDGE, OF

ELECTRIC COMPANY,

SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL A'COBPORATION OF NEW YORK.

X-ItAY TUBE.

To all wkom'ii may concern.

Be it known that I, WILLIAM D. Coomnen, a.citizen of the United States, residingat Schenectady, in the county of Schenectady, State of New York, have invented certain new and useful Improvements in X-Ray Tubes, of which the following is a specification.

The .present invention comprises a new X-ray device and its object is to cut off or X-rays coming-from the arts of'the target or anode other than the coal spot. With this object in view I have provided 'a screen or shield consisting of material such, for example, as a heavy metal which is substantially impervious to X-rays and which is no atively charged with respect to the anode uring the operation of the device.

The novel features and benefits of my in vention will be explained in connection with the accompanying drawings in which Figure 1 shows in outline an X-ray tube provided with a shield which is connected to the cathode; Fig. 2 is a detail sectional view of the cathode and focusing member; Fig. .3 is a detail perspective view of a shield supported at the anode end of the tube and electrically insulated from both electrodes; Fig. 4 illustrates a shield used in conjunction with an anode hood, and Fig. 5 is a detail view of a specialform of screen.

It has been suggested heretofore to place a screen composed of material transparent to X-rays about the anode of an X-ray tube which is dependent for its operation on the ionization of residual gas within the tube. The expressed object of this construction was to avoid the action of secondary rays. In a tube operating by the ionization of gas, there are present positive ions as well as negative electrons: An anode screen transparent to X-rays in this type of tube, even though insulated from the anode, will be continuously bombarded by negative electrons as any negative charge which tends to accumulate on the screen will be neutralized by the positive ions. The screen itself, therefore, is a source of X-rays.

However, in a tube operating independently of gas ionization, the positive ions are negligible and a screen insulated electrically from the anode accumulates a negative charge which prevents further bombard- Specification of Letters Patent.

Patented Oct. 12, 1920.

- Application and December 1c, 1916. Serial No. 137,355.

ment by electrons, and the shield hence does not become itself a source of X-rays.

Referring to Fig. 1, the X-ray device here shown comprises a tube in which are sealed a cathode 2 and an anode 3. The cathode, asshown in greater detail in Fig. 2, comprises a coiled filament of refractory metal, for example, tungsten, connected to conductors 4, 5, sealed into a stem 6. Surrounding the cathode proper is a metal tube 7, concave or dished in part, which focuses the discharge from the cathode. This focusing tube 7 is supported by a conductor 8 attached to a split tube 9 sprung over the stem 6, and is electrically connected to the cathode.

The anode or target 3' consists of tungsten or other suitable material, and is carried by a stem 10 supported by a tube 11 consisting of iron and held in place by friction within the tubular extension of the envelop. A sealed-in conductor 12 makes electrical connection to the anode The cathode is heated by a battery 13 in circuit with which is a switch 14.

The described device is evacuated to a pressure so low that positive gas ionization is substantially absent during its operation, as described in my Patent No. 1,203,495, issued October 31, 1916.

When a sufficiently high potential is impressed on the electrodes, as indicated by the connection of the cathode and the anode to terminals of the secondary of a transformer 15, by the conductors 16. 17, the cathode being heated to incandescence, a discharge of electrons is conducted through the evacuated space and is focused upon the face of the anode to strike within a limited area known as the focal spot. By the impact of the cathode rays or electrons upon the anode surface X-rays are generated. Secondary cathode rays are also generated at the focal spot and travel outwardl 1 from this point and are bent back upon the anode itself by the electrostatic repulsion of the cathode and the negatively charged tube walls.

These secondarv cathode rays give rise to the emission of X-rays over the entire surface of the target, but, of course, with lesser than at the focal spot. As a result intensitythere is some ictures of subjects giving intense contrasts.

n order to prevent these undesired secondloss of definition in X-ray'.

V ary X-rays from reaching the object which for example, tungsten or molybdenum. The

screen is located within the tube between the anode surface emitting secondary X-rays and the object tobe photographed or examined. The screen not only is opaque to undesired X-rays but when given the form illustrated, which does not extend entirely about the anode, it also acts by electrostatic repulsion to deflect cathode rays away from the side of the target facing the object to be examined.

As shown in Fig. i the screen may be me lchanically connected and in electrical contact with the cathode, or the screen may be supported as shown in Fig. 3 by being attached to circular strips 20 which are sprung I against the Walls of the anode extension of the envelop. If desired for any particular purpose the extension of the screen about the anode may be increased.

In either of the described constructions the screen, or deflector, 18 is negatively charged during operation. In the construc tion shown in Fig. 1 the screen is in direct electrical contact with the cathode and therefore operates at cathode potential, and in the construction shown in Fig. 3 the screen is charged to cathode potential by the electrons emitted from the cathode or the focal spot on the anode. As the space Within the tube is substantially free from a positive ions the screen remains negatively charged and, therefore, is not subject to continued bombardment by cathode rays and consequently emits no X-rays. A useful beam or cone of X-rays emitted from the focal spot passes through the window 19 upon .the object to be photographed or examined, while the stray rays emitted by the surface of the anode outside the focal spot are cut off. thereby giving sharp delineation.

In the construction shown in Fig. 4the ta.get or anode 3, is provided with a hood 21 consisting, for example .of molybdenum or tungsten. As described in my Patent No. 12111192. of January 2, 1917, onset the main functions of the hood is to suppress the escape of electrons emitted from the focal spot to the outer surface of the anode The presence of the ncgatii 'ely charged member 18. still further reduces the escape of electrons through the window 22 by electrostatic repulsion. Instead of escaping from the window and then impinging on the outer surface of the anode the electrons are and strike the outer surface of the target produce no appreciable effect as they are cut off by the screen 18 which is opaque to X-rays.

What I claim as new and desire to secure by Letters Patent of the United States, is

1 An X-ray tube comprising the combination of a refractory cathode, means for heating the same to incandescence, a cooperating anode, an inclosing sealed envelop, the space Within said envelop and the metal parts therein being evacuated to a pressure so low that conduction therein occurs substantially independent of positive ionization, and a shield substantially impervious to X- rays located to intercept X-rays from arts of the anode other than the face 0 the anode on which the cathode rays are focused, said shield being electrically connected to said cathode.

2. In an X-ray tube operating by substantially pure electron conduction independently of positive gas ionization, the combination ofan inclosmg envelop, a cathode, an anode, a hood for said anode having windows respectively for the entrance of cathode rays and the escape of X-rays and a member acting as deflector and screen charged negatively with respect to the anode located adjacent the window through which X-rays escape. said member being provided with a window to permit the passage of desired X-rays. i

3. In an Xray tube operating by substantially pure electron conduction without appreciable positive gas ionization and having a hood provided with a window for the escape of X-rays upon the charge receiving face of the anode, a screen adjacent the anode charged negatively with respect to the anode during operation. of the device and having a window registering with the window in the hood through which. Xa-ays escape, said screen being connected to the cathode and a grid of wires for the window of said screen.

In witness whereof I have hereunto set my hand this 14th day of December, 1916.

IVILLIAM D. COOLIDGE.

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