US2345722A

US2345722A - X-ray tube

Info

Publication number: US2345722A
Application number: US441166A
Authority: US
Inventors: Zed J Atlee
Original assignee: General Electric X Ray Corp
Current assignee: General Electric X Ray Corp
Priority date: 1942-04-30
Filing date: 1942-04-30
Publication date: 1944-04-04
Anticipated expiration: 1961-04-04

Description

Patented Apr. 4, 1944 UNITED STATES PATENT Y OFFICE v X-RAY TUBE c zea J. Atlee, Elmhurst, 111., assignor V,to General Electric X-Ray Corporation, Chicago, lll., a corporation of New York 1 Application April so, 1942, serial No. 441,166

2 claims. (cl. 25o- 148) The present invention relates in general to electronic devices and has more particular reference to X-ray generators, the invention more especially pertaining to anode construction for use in X-ray generating tubes.

An important object of the present invention is to provide an improved rotating anode structure especially well adapted to withstand the effects of electronic bombardment thereof in the generation of X-rays; a further object being to provide an anode structure includingan electron target having the ability to resist target deterioration under the nliuence of the excessive heat developed at the target when subjected to electronic bombardment. y

Another important object is to provide a rotating anode adapted to withstand the relatively high operating temperatures developed therein when subjected to electronic impact in the generation of X-rays, the anode structure of the present invention being particularly well suited for operation at high temperatures without cracking, peeling or flaking of the target.

Another important object is to provide an anode comprising a base carrying an electron target member mounted on the base in position for loombardment, including means for supporting the target member on the base for the transfer of heat developed in the target to the base while dissipating forces due fto thermal expansion in a fashion preventing mechanical deterioration of the target and its mounting; a further object being to utilize a target member comprising a disc or plate bonded to the base at the central portions of the disc to provide a target rim extending outwardly of the supported portions of the disc in position toV receive electron bombardment, whereby thermal expansion and contraction will take place mainly in the peripheral, mechanicallyunsupported portions of the target member, and be transferred thence inwardly to the relatively cooler, centralized, supported portions: of the plate, and transferred thence to the supporting basewithout danger of rupturing the bond between the plate and thebase, and without danger of the development of checks, cracks and akes at the working surface of the target.

\ A still further object of the invention is to form an anode target 'member having portions thermally isolatedA from the target-supporting base wherebyV the expansion and contraction `of said portions through the-,excessive range of temperature generated in saidl portions may be transferred fdrst to relatively cooler portions of the base to thereby minimize the deleterious effects of unequal rates of expansion of the target and the base materials, which tends to disrupt the mechanical bond between the target member and the base member on which it is supported.

Another important object is to provide an anode comprising a target plate bonded on a heat conducting Vbase for heat conduction from plate to base, the'plate being mounted in a manner presenting a portion of the plate in position thermally isolated from direct contact with the base so that, by: confining electronic bombardment of said target plate to a target area within said isolated plate portions, the same may be heated substantially uniformly vthroughout the entire thickness of said portions to thereby avoid surface cracking andflaking due to unequal heatingr of the said plate portions inwardly of the target surface. oa Thevforegoingp and numerous other important objects, advantages and inherent functions of the invention will become apparent as the same is more fullyunderstood from the following description, which, taken in connection With the accompanying drawing, discloses a preferred embodiment of the invention andthe manner of practicing the same.

f Referring to thedrawing:

The single figure comprising the drawing is a sectional View taken through an X-ray generator lcaving an anode embodying the present invenlon.

To illustrate .my invention, I have shown, in the drawing, an X-ray generator comprising an hermetic envelope I3 of tubular form, within the opposite ends of which is mounted electrode means comprising an electron emitting cathode i5 and an anodeull, said electrodes being sealed within the envelope in opposing relationship.

, The-cathode i5 comprises an electron emitting filament ladisposed in vposition to project elecytrons as a stream 2l-toward the facing end of plate before being dissipated into the Asupport 5l the anode which is provided with target means 23 disposed inthe path of the electron stream 2|, and it should be understoodthat impingement of electrons `projected `by and from the cathode upon the target 23 results in the production of `X-rays at the target and the projection of .such X-rays as a beam 25 outwardly of the envelope through a window 2l formed therein 0ppositethe target. yThe anode issupported within the envelope o n a spindle 29,1said spindle extending outwardly of the envelope `through a seal in the end thereof.` I l 5 In the .illustrated embodiment,` I have shown The anode base 3| is made of copper, or otherV cupreous material adapted to conduct heat read ily. The target member 23 preferably comprises tungsten, or other suitable material, the 'meme ber 23, in the illustrated embodiment, being formed as a plate and being secured upon the thermal expansion of copper is roughly three times the thermal expansion of tungsten. Unfore tunately,v there is no metal having high conductivity for heat dissipation having a low coefficient of thermal expansion comparable With the thermal expansion coefficient of tungsten and other usual target materials. Because of high heat conductivity, copper is the obvious choice for usein the backing or base. If use of other metals be attempted, the lack of heat storage and conducting capacity of the material used may ree vquire the target to function at destructively high temperatures, it being an important function of thebase to cool the target member rapidly to vpreventJ destructive heating thereof.

t Y In anodes built in accordance with existing closed end of the base 3| in position facing the Y cathode l5, the plate 23 being adhered or bonded tothe base in any preferred fashion, as by prefo-rming the plate 23 and casting the material of thebase 3| upon the pre-formed'plate in a suitable mold, the base material becoming .alloyed with the surface lmaterial of the plate during the casting operation so that the target plate is integrated with and becomes .a part of the base,

Whileat the same time 4maintaining the separate identity of the target material and thebase material. f Y

Impingement of-electrons on the target results in the generation of substantial heat at the tar'- get, and the anode, as a'result of electron bombardment, may reach a temperaturev of the order of several hundred degrees centigrade when thegenerator is inoperaton. This heat is-dissipated by conduction-through the anode and radiation therefrom outwardly Vof the envelope. 1 It will be seen, however, that all of the-hea generated in the-target member necessarily is dissipated thence through the relatively larger mass of metal comprising the base 3|. The materialfof *the base 43| is adapted to conduct heat away from the target rapidly, but because of the greater'mass of material in the base 31 and its higher'hea't `conducting ability, the base at all times during operationof thedevice will be at a temperature appreci-ably-lower than target temperature; The 'temperature in the target is a maximum at the surface lof that portion of the target lwhich is disposed in the path of the elec'-` tron stream and subjectedV to electronic bombardment. Ina rotating target 'the' target area is annular, each portion of the target area be'- ing` successively presented-for bombardment in the electron stream as the anode is rotated. The temperature -of theta-rget surface is a maximum, ofcourse, `at the `instant when it is subjected to electronic bombardment, the temperature vdroppingslightly due to rapid heat conduction therefrom into the Vbase during 4the periodwhile the target area isrotating out of the path of the elece tronstreami. When the generator is inopera-A tionv each portion of the target thus is succestively raised to a maximum temperatureand then cooled substantially. This results in alternate ex*- pansiona-nd contraction of the target material. riihe rate of thermal expansion of the material of the.y plate-23ris usually substantially lower than the rate" of thermal expansion ofthe material of the base, which condition, in conjunction with thelower operating temperature of the base 3|, tends to interfere with and damage ythe bond/between ine target member and the base;

The base ordinarily comprises copper and the target member usually comprises tungsten. The

principles of design, the back of the target plate is bonded throughout its entire area as tightly as possible on the base. The service life of X-ray generators containing such anodes is definitely limited by anode failure through. weakening or destruction of the target bond on its support base. Not only is there a tendency for the target member to work loose fromthe base, but also there is a tendency, heretofore unexplained, for the face of the target member to crack, peel and flake in the portions thereof which are directly exposed to electron bombardment. I have discovered that this surface cracking and flaking is due, at least in part, to the fact that the back of the plate oppositethe target area is mechanically and thermally bonded to the base. The temperature of the portions of the target plate bonded to the base at any instant While the generator is in' operation are at a substantially lower temperature than the material of the plate at the working surface thereof. This temperature differential through the plate is due to the fact that the base abstracts heat relatively rapidly from the base attached portions of the target member. It is thought, therefore, that surface cracking and peeling are'due to internal stresses set up within the `targetrnember itself as a result of the constantly fluctuating temperatures which prevail in the portions of the member under` electronic impact, which temperature is lower at the base connected portions 'than at the target surface portions of theplate. vWhere the back of the plate vertically beneath the target area is bonded to the base, it is mechanically held on the base and is at all times much cooler than the' target surface portions of the plate. The expansion and contraction of the target plate portions at and adjacent the base is thus much less than the expansion and contraction of the targetl surface portions, which consequently are highly susceptible to cracking and flaking.

In order to overcome the foregoing difficulties and to provide a tar-get carrying anode in which the bond between the target member and the base will be maintained substantially throughout the service life of the generator, and to inhibit Cracking and peeling at the electron Vreceiving surface Vof the target, I provide an anode in which there is neither mechanical nor heat conducting connection directly between' the base and the portions of the target member which are bombarded by the electron stream 2|. By

forming the anode in thisfashion, the'target.

member, when under bombardment, will become heated to substantially the same temperature throughout Ythe thicknessY ofthe plate, and also will be free to expand and contract without the restriction of a mechanical bond tending to pre.. vent expansion of the back of the plate portions` The target member thus is allowed to have substantially the same expansion and contraction throughout its thickness, thereby eliminating the tendency of the target surface to crack, flake or peel.

By thermally and mechanically isolating the target portions of the target member from direct thermal connection with the base, all of the heat generated in the target member at the zone of electronic impact will necessarily be caused to now toward relatively cooler portions of the member 23 for transfer thence to the supporting base from said relatively cooler plate portions which are bonded on the base. 'Ihese relatively cooler portions of the target member, since they are not subjected to periodic bombardment, will tend to have a substantially uniform temperature when the generator is in operation, which temperature will be but slightly higher than that of the attached base. Disruptive forces tending to destroy or impair the bond between such relatively cooler portions and the base are thus substantially eliminated, so that the bond may be maintained in unimpaired condition throughout the service life of the generator.

Thermal isolation of the electron receiving portions of the target member 23 may be accomplished, as shown in the illustrated embodiment, by slotting, grooving or otherwise cutting away the material oi the base immediately behind the portions of the target member 23, which, when the generator is in operation, are subject to electronic bombardment.

As shown in the drawin-g, the target carrying end of the anode base 3| is formed with an annular recess or groove 35 behind the marginal portions of the target plate 23. The groove 35 defines a central neck 31 which is bonded to and thermally connected with the central portions 39 of the target plate, the marginal portions 4I of which extend outwardly of the central neck to form an annular electron receiving target area, said outwardly extending plate portions being thermally isolated from direct contact with the base 3| by the cutout groove 35.

-It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing its attendant advantages, the form disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

l. An X-ray tube comprising a sealed envelope, an electron emitting cathode and rotary anode in said envelope, said anode comprising a support of heat conducting material having a target mounting neck, and a target plate having a central portion bonded on and thermally connected to said neck, said plate having an annular marginal portion projecting outwardly of said neck and forming an annular target surface isolated from direct thermal connection with said support, said neck having a tapered portion and said target plate having an integral peripheral portion in spaced relation with the tapered portion of said neck, whereby to preserve substantially uniform temperature conditions in said annular portion and thus prevent rupture from internal stresses set up in said portions.

2. An X-ray tube comprising a sealed envelope, an electron emitting cathode and a rotary anode in said envelope, said anode comprising a support of heat conducting material having a groove dening a neck at one end, and a target plate having a central portion secured on said neck and integrally bonded thereto, said plate having integral peripheral portions extending outwardly of said neck in position overlying said groove and isolated from direct thermal connection with said support, said peripheral portions forming an annular target zone in position to receive impact of electrons emitted by said cathode, and said neck being tapered in the direction of the central portion of said target plate, whereby heat developing in said peripheral portions as a result of electron impingement on said target zone may pass to said support through the central portions of said plate.

ZED J. ATLEE.