US1211092A - X-ray tube. - Google Patents

Description

W. D. COOLIDGE.

X-RAY TUBE. APPLICATION FILED JUNE 5. 1 916.

1,21 1,092. Patented Jan. 2, 1917.

Inventor: William D. Coolidge,

His fitto rney.

WILLIAM D. COOLIDGE, OF SCHENEGTADY, NEW YORK, ASSIGNOR 'IO GENERAL."

ELECTRIC COMPANY, A CORPORATION OF NEW YORK. I

X-RAY TUBE.

Specification of Letters Patent.

Patented Jan. 2, 1917.

Application filed June 5, 1915. Serial No. 82,381.

\ IDGE, a citizen of the United States, residing at Schenectady, 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 an improved X-ray tube.

One of the main objects of my invention is to provide a tube which may be operated with an alternating current with a current input high enough to heat the working face of the anode to incandescence without becoming appreciably conductive to current waves having a negative polarity with re spect to the electrode normally operating as anode.

Other objects of my invention are to reduce X-ra'y production from the back of the target and to improve focusing of the X-rays by preventing distortion of the focal spot and these aspects of my invention are of importance when applied to operation with direct current as well as alternating current.

In the Physical Review for December, 1913, I have described an X-ray tube having a cathode adapted to be heated to incandescence to emit electrons and operating in a space so highly evacuated that positive ionization of residual gas is negligible. This tube acts as a rectifier of its operating current as long as the cooperating anode is not heated much above redness by the cathode discharge. Vhenoperated with alternating current it is conductive only to the current waves which are negative with respect to the incandescent cathode. In some cases the energy input is so great that the anode as a whole or in part is heated to bright incandescence, for example, the anode at the focal spot may become highly incandescent especially when using a sharp focus. The device then becomes conductive to current in both directions causing deleterious bombardment of the cathode and the bulb wall back of the cathode by cathode rays.

Another characteristic of an X-ray tube operating with a pure electron discharge is the bombardment of the back of the target by secondary cathode rays. Apparently the walls of the tube become negatively charged and cause the secondary rays to follow a path bent back upon the target itself, in- 4 stead of traveling outward in straight lines to the glass.

One embodiment of my invention herein-- after described comprises an X-ray tube opcrating with a substantially pure electron discharge and having an anode provided w1th a longitudinal extension of conductive material which in part incloses the path of the cathode rays entering the target. This target extension in most cases is constructed to suppress undesired primary and secondary cathode rays emitted from the face of the target.

In the accompanying drawings illustratmg my mvention, Figure 1 is a perspective and somewhat diagrammatic view of an X- ray tube equipped with means for preventing discharges from the anode to the cathode; Fig. 2 is an enlarged sectional view of a cathode and one form of anode embodying my invention; Figs. 3 and 4 show in perspective modifications which my invention may assume; and Fig. 5 is a detailed view of one form of cathode.

The tube shown in Fig. 1 comprises an envelop 1 consisting of glass or the like and a cathode with its accompanying focusing cup 2 and an anode 3. The cathode proper comprises a spiral filament 4 adapted to be heated as indicated in Fig. 5. As indicated in Fig. 2, this filament spiral preferably is given a slight curvature following the shape of the cup 2 but it may also be fiat if desired. The cathode is'supplied with heating current through conductors 5 sealedinto a glass stem 6 in the usual manner, external connections being made by a threaded cap 7. The focusing device 2 is supported by a wire 8 also embedded in the glass stem. The focusing device is electrically connected to the cathode. The focusing device may this end in view the bulb is first evacuated by approved lamp exhaust methods. The last portions of gas are removed by the Gaede molecular pump or any other means capable of removing vapors of oil and water as well as gases. When the evacuation has proceeded to such a high degree that acurrent may be sent through the device without destructive positive ionization, the cathode is heated to incandescence and a potential high enough to conduct current across the vacuous space is impressed upon the electrodes. The bombardment of the anode by the electron discharge liberates ionizable gas. As soon as appreciable fluorescence of the glass is observed, the discharge isdiscontinued and pumping continued until a high vacuum has been restored when the process is repeated. When the electrodes have been thus sufficiently freed from gas to operate at high voltages without evolving ionizable gas and the space within the tube has been exhausted to a vacuum so high that positive ionization is substantially absent, the tube is sealed off from the pump.

The anode is provided, in accordance with the present invention, with a conductor extending toward the cathode over the face of the anode and surrounding the path of the cathode rays. This conductor may assume the form of an inclosing hood or shell 12, as shown in Fig. 1, consisting preferably of metallic molybdenum or tungsten and containing an opening 13 for the admission of the cathode rays and an opening 14 for egress of X-rays.

The hood -'12 tions:

(1) By its uniform radial electrostatic field it prevents the cathode ray stream from being deflected toward the nearest edge of the inclined face of the anode away from the axis of the tube. v i

(2) By modifying the distribution of the potential at the anode, the hood suppresses inverse electron conduction from the heated focal spot when of negative potential. This result may be ascribed to two causes. The electric force within the hood tending to drawv the electrons out is very small.v In the secondplace the focusing action is so strong that the electrons tending to leave the heated-focal spot when of negative potential are crowded into a narrow beam by the negatively charged hood. The electrostatic and electromagnetic fields of the electrons in the cathode stream exert upon each other a repulsion known as the space charge effect, which limits the current. As the space charge effect at the anode with its narrow opening 13 is greater than the space charge effect at the cathode with the wide opening in the ring 2, the electron current tending to leave the anode even when highly heated is very much less at a given voltage has several distinct fun ccathode rays emitted from the focal spot,

the hood greatly reduces undesired X-ray radiation from the back of the target and thus promotes sharpness of definition in radiographs and fluoroscopic images.

(4) Incidentally the hood reduces blackening of the bulb and lessens the liability of melted globules flying fromthe focal spot to the walls of the bulb and cracking the glass. The hood also cuts off all but the narrow cone of X-rays which is emitted through the opening 14, and thus reduces the danger of X-ray burns to the operator and patient.

The modification of the hood illustrated at 15, Fig. 3, permits a greater dissipation of heat by radiation from the hot focal spot by reason of the space between the ribs 16 and, therefore, enables the tube to be used with a greater energy input. In this modification of the'anode the secondary cathode rays and the X-rays are not screened to the same extent as in the type shown in Fig. 1.

When the X-rays are used for sterilization or other industrial purposes where focusing is of no consequence this is an advantage. The space charge effect while not as great as in the structure shown in Fig. 2 is still prescut and enables a tube having this form of anode to be operated with an alternating current source.

In Fig. 4 is shown a modification in which the space charge effect is still less than in the modification shown in Fig. 3. Here the static field modifying means assumes the form of a ring 17 supported by a wire 18 attached by a split ring 19 to the anode 3.

The ring 17 by surrounding the beam of cathode rays effectively prevents deflectionof the focal spot away from the axis of thetube.

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

1. The combination of an envelop, a cathode adapted to be independently heated, an anode, means for focusing a discharge from said cathode upon a spot of restricted area on the surface of the anode, and conductive means electrically connected to said anode and surrounding at least in part the path of said discharge, said envelop being so highly evacuated that electrical conduction occurs by a. pure electron discharge.

2. An X-ray device comprising the combination of a sealed envelop, the space within which is evacuated to a pressure so low that an electron discharge may occur therein substantially Without evidences of positive ionization, an electron-emitting cathode, an anode, means for focusing a discharge from said cathode on the face of the anode, and conductive shielding means surrounding at least in part the face of said anode, While leaving an unobstructed path between said cathode and said anode for the passage of cathode rays.

3. A Roentgen-ray device comprising the combination of an envelop, an incandescing cathode, an anode, means for concentrating a discharge from said cathode upon the face of the anode, and a conductor connected to said anode and extending at least in part about the path of said discharge toward the cathode, said device being evacuated to a pressure so low that a discharge may occur therein substantially Without positive ionization.

4.- An electrical discharge device comprising the combination of a sealed envelop evacuated to a pressure so low that an electron discharge may occur therein substantially Without positive ionization, a cathode adapted to be independently heated, an anode, means for focusing a discharge from said cathode upon the anode, and a conductive member, surrounding in part the charge-receiving face of the anode but providing an opening for admitting an electron discharge from the cathode, whereby an electric field is established suppressing the emanation of an electron discharge from the face of the anode.

5. The combination of a source of alternating current, an X-ray tube operable with a substantially pure electron discharge Without positive ionization connected to said source, said tube having an incandescing cathode, an anode, means for focusing a discharge from said cathode upon a face of the anode, and a conductive member, surrounding the path of the beam of cathode rays near the point of impingement on the anode.

In Witness whereof, I have hereunto set my hand this 4th day of June, 1915.

WILLIAM D. COOLIDGE.

Images