US2836747A - X-ray tube cathode - Google Patents

Description

May 27, 1'958 z. J. ATLEE 2,836,747

x-RAY TUBE CATHODE Filed March 2a, 1955 u MVN HHHHHH 0, Y /9 20 /8 'f INVENTOR. ZED J'. ATLEE @M1/MM ATTORNE K2 United States Patent X-RAY TUBE CATHoDE Zed 3. Atlee, Chicago, Ill., assignor to Dunlee Corpi, Chicago, Ill., a corporation of Iitinois Application March 28, 1955, Serial No. 497,151 1 Claim. (Cl. 313-57) The present invention relates to X-ray tubes and more particularly to" new and'improved cathode structure for an X-ray tube.

Self-rectified X-ray tubes conventionally comprise a cathode and an anode mounted in facing, spaced relation within a sealed, evacuated glass envelope. The cathode is provided with a filament for generating a beam of electrous which is focused upon a wolfram target embedded in the anode, the filament being mounted in a metal body usually designated as the cathode head portion. The bombardment of the target by the electrons from the cathode lament causes X-rays to be emitted from the target,

but the continual bombardment of the target with electrons also causes it to become heated as a consequence of which it emits electrons. The electrons emitted from the target are usually designated as inverse electrons and are f may melt and vaporize, the vaporized metal depositing or condensing upon the colder parts of the tube such as the envelope which is undesirable since such deposits impair the efficiency of the tube and may even render a tube inoperable. Inasmuch as the intensity of the inverse elec- ,j

tron beam is directly proportional to the operating ratings of the tube, it has been necessary to operate a tube at ratings sufficiently low that the intensity of the inverse electron beam vlas below the point where it would cause excessive heating and vaporizing of the material of the cathode head portion. Heretofore, cathode head portions have been formed of nickel or iron or alloy thereof and which melted and vaporized under impingement by the inverse electron beam even though a tube was operated at a rating considerably below that at which it otherwise could have been operated. Attempts have been made to protect such head portions by mounting an insert of a high melting point material such as molybdenum or woifram therein at the inverse focal spot or by providing a layer of such material on the surface of the head portion. But such procedures are expensive and even with the protective material, the possibility of injuring the cathode remained the limiting factor in the tube operation. It is, therefore, a principal object of the invention to provide a new and improved X-ray tube structure which reduces or negates the problem of inverse electron emission from the anode target as a limiting factor in the operable rating of the tube.

More particularly, it is an object of the invention to provide a new and improved cathode structure for selfectied X-ray tubes which is not prone to injury upon ice bombardment by the inverse electron emission from the anode.

Still another object is to provide an inexpensive cathode structure not subject to vaporization or melting under bombardment by the inverse electron beam from the anode.

Other objects and advantages of the invention will become more app-arent hereinafter.

In accordance with the present invention, the tube cathode is constructed with a head portion-of copper which it has been discovered will not be injured even though the tube is operated at a rating greatly exceeding that which would be permissible with a cathode constructed in accordance with the prior art. in fact, with a copper cathode head the limiting factor in the tube ratingis the ability Vof the anode structure to withstand injury since the cathode head will not be injured even when the tube is operated at ratings which cause injury to the anode target.

For a more detailed description of the invention, reference is made to the accompanying drawings and the detailed description thereof wherein:

Fig. l is a longitudinal, sectional view taken through an X-ray tube constructed in accordance with the present invention;

Fig. 2 is an enlarged, longitudinal, sectional view through the anode and cathode of the X-ray tube;

Fig. 3 is a view looking in the direction of the arrows 3-3 of Fig. 2 and showing the face of the anode; and

Fig. 4 is a View looking in the direction of the arrows r-l-d of Fig. 2 and showing the face of the cathode.

The illustrated X-ray tube comprises an anode iii and a cathode l1 mounted in coaxial, facing relationship within a sealed glass envelope 12.

The anode 1t! comprises a copper body 14 secured in any suitable manner to the envelope i2 and formed with a target face which is inclined with respect to the longitudinal axis of the tube and in which face is embedded a planar faced target element l5 of Wolfram or similar material capable of generating X-rays upon bombardment with electrons. The face of the anode and target element is normally inclined with respect to the axis of the tube, as shown, so that a substantially uniform beam of X-rays indicated at lo will be emitted laterally through the walls of the envelope 12.

The cathode 11 comprises a mounting sleeve 1S, one end of which is fitted telescopically over the end of a re-entrant, cylindrical glass stem portion 19 of the envelope i?. and suitably secured thereto such as by means of a plurality of turns of Wire 2Q. Mounted in the end of the sleeve 18 facing the anode lli is a massive head block or head portion 22 in which is formed an electron focusing cup 23 wherein is disposed an electron emitting filament 24. The filament 24 is adapted to be energized through leads 2S extending outwardly through the stem 19 of the envelope. The head 22 may be secured to the sleeve 18 by any suitable means such as screws 26.

As is well known, heating of the filament 24 causes the same to emit electrons and the application of potential between the cathode 11 and anode itl causes the electrons to travel from the filament to the anode. The cathode cup 23 is shaped so as to focus the electrons upon the anode target in a concentrated electron beam indicated at 30, striking the target 15 in restricted area or focal spot 31. The impingement of the electron beam 3) upon the target 15 will heat the target in the area of the focal spot 31 whereupon electrons will be emitted from the heated portion of the target. As has been indicated hereinbefore, such electrons tend to leave the target in a direction at right angles to the plane of the target surface in an inverse electron beam such as indicated at 32 in Fig. 2.

lt will be understood that in the operation of a seif- X-ray Y head V22 is formed as a massive body of copper.

rectified tube an alternating current potential is applied between the anode and cathode 11. Duringthe half f n the cathode, the inverse electrons will flow from the target towards'the cathode.

The intensity of the X-rays generated atY the target of an X-ray tube is proportional to the intensity of the loading of the focal spot 31 of target 15, the loading ordinarily being expressed in terms of watts/sq.vc,m. of focal spot. The loading or tube rating may be increased by increasing the potential lapplied between the cathode and anode, by increasing the electron generation Vat thetilament, or Vby concentrating the electrons ina smaller focal 'spot on the anode target. As the loading or rating on the target 15 is increased, the temperature of the focal spot 31 will also increase, beamj32.

Ordinarily the spacing between the anode 10 and cath-Y ode 11 will be such that the inverse electron beam 32 will strike thecathode head .22. In the tube as illustrate-the inverse electron beam' 32 impingesupon the cathode head 22 in an inverse focal spot Yadjacent they bottom ofthe focusing cup 23, the spot being indicated Vin Fig; 4 at 34. It is'preferable, in fact, thatV the inverse beam strikethe cathode head so that the Ybeam will not impinge upon the envelope 12 since electrons impinging upon the envelope 12 may cause the envelope material to deteriorate or collect upon the wall,.subjecting the envelope to electrical strain under vwhich it may fail. However', it will be apparent that similarly to the heating of the focal spot 31 on thetarget, the inverse Velectron beam 32 will 'cause the cathode head to heat at the inverse focal spot 34, vand as the intensity of the inverse electron beam 32 increases, the heating at the inverse focal spot 34 will also increase. A limiting factor in tubes constructed in accordance with the prior art'has been't'nat the rating of the tube must be kept below the point where the cathode head melts and vapori/:es by reason of the heating at the focal spot 34. The vaporized metal condenses upon the colder portions of the tube, and particularly upon the envelope, interfering with the operation of thetube. In many cases a tube may be .rendered completely inoperable because of the kcondensed deposits.

In accordance with the present invention, the cathode Though giving rise to a .more intense inverse Velectron cathode, to withstand injury. That is, in

copper has much lower melting and boiling points than 'Y any of the materials used for cathodehead construction heretofore, boiling or Vvaporization of the copper at the g Y. n

`focal point thereon of ltherinverse electronrow fromtthe anode does not occur even when the tube is operated under ratings considerably higherthanpermissi'bleheretol fore since the heat Vis dispesed from -such point through the body of the head 22. As a consequence of the use of copper for constlucting Vthe increase in rating of a tube is permissible as compared with the rating at which `a tuberas manufactured heretofore could be operated under substantially similar conditions. ln fact, as mentioned hereinbefore, with a copper cathode head the permissible tube loading is dependent upon the ability of the anode structure, rather than the loading a tube the possibility of injuryto the copper cathode head portion can be disregarded and the maximum loading determined instead by the capacity of the anode torsurvive the loading since the Wolfram target of the anode ywill become overheated before the cathode head is injured. inasmuch as anodes have been greatly underloaded heretofore, the substitution of 'a copper cathode Vhead portion makes'possible an increase in permissible loading by as much as percent or more. While the greater heat conductivity of copper obviously attributes to its success, other Vfactors not at present fully understood may also contribute to the outstanding improvement inthe rating of an X-ray tube resulting from its use. p Having illustrated and described a preferred embodimentofrthe invention, it should be apparent to those skilled in the art that the invention permits of modifications in arrangement and detail. I claim Vas my invention all such modifications as come within the true spirit, and scope of the appended claim.

I claim: Y Y V, t An X-ray tube for self-rectified operation comprising an envelope having an anode unit and cathode. unit mounted therein vin facing relation, .said cathode comprising a body of copper on the end thereof adjacent vsaid anode having an electron focusing cup formed therein, 'and an electron generating lament mounted in said cup, said body being massive whereby heat generated at the focal point thereon of the inverseV electron How from said anode unit is dispersed through said body to prevent over heating and excessive vaporization of said bodyl at said focal point.

References Cited in the le of this Apatent Y.

UNITED STATES PATENTS 1,152,578 Wappler Sept. 7, 1915 1,275,542 Farrelly et al. Aug. 13, 1918 1,718,849 Bouwers June 25, 1929 2,671,867 Atlee Mar. 9, 1954 Y 2,719,240 Walker Sept. 27, 1955 head 22, a very significant

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