US3160779A - Single crystal X-ray tube target - Google Patents

Description

Dec.

8, 1964 M. J. ZUNICK 3,160,779

SINGLE CRYSTAL X-RAY TUBE TARGET Filed April 30, 1962 F IG. 1

INVENTOR MICHAEL J. ZUNICK ATTORNEY llnitedistates Patent .S INGLE CRYSTAL X-RAY TARGET lVlichael J. Zunick, Hubertu s, WisL, assignor to General Electric Company, a corporation of New York Filed Apr. 30, 1962, Ser. No. 191,688 1 Claim. (U. 313330) invention relates to X-ray tubes, and particular.- ly, to the use of targets constituting a single crystal of metal in such tubes. The invention is broadly applicable to X-ray tubes used for diagnostic and therapeutic purposes as well as for tubes employed in X-ray emission or diffraction studies. i

An X-ray tube used for any of the above purposes includes an envelope that supports in its evacuated interior a cathode structure spaced from an anode. has a hot filament that emits electrons which are accelerated to the anode in a focused beam that produces X- rays at the point or line of impact on a target insert in the anode. Initially, the target surface is polished so that the emitted X-rays on any line of sight appear to emanate from a sharp lineor point, depending upon how the electron beam is focused. When the target insert is in this conditiona uniformly intense X-ray beam is emitted in any specified direction. Thus, if the tube is used for taking radiographs, sharp images, without penumbra, are produced because all rays appear to emanate from a single point. If the tube is of the line focus type commonly used to make X-ray diifr'action studies of crystalline materials, better resolution of the rays diffracted by the sample results. Users of X-ray tubes for both radiographic and analytical purposeshave demanded increased electron beam current ratings in order to obtain more intense -X-ray output and at the same time they have demanded smaller and sharper focal spots. An eifect of increasing the beam current and at the same time concentrating it is to overheat or even melt the target surface. The resultant roughening constitutes permanent damage to the target for it is nolonger possible to obtain a sharp focal region thereafter. Hence, commercial X-ray tubes are designed to compromise desired rating with a reasonable life.

, In radiographic X-ray tubes tungsten is the material most commonly used for targets. Tubes used for emission and diffraction work may employ targets of tungsten, molybdenum, vanadium, tantalum, chromium, nickel, copper, cobalt or iron. Although these metals are considered pure, they are characterized by a polycrystalline structure having minute gr'ain'boundaries that are separated b'y' oxid es andlother' impurities. The grains' are sometimes completely enveloped in impuritiesi This makes the target niaterialfmore susceptible to the" type of the mal deteriorationfmentioned above. I 7 Me llu'rgical' studies, particularly vphotomierographs, taken onfa'section ofthe target material, cut through the focal spot in prior art tubes reveals that a number of permanent changes may take place when a tube. is used. The crystallites often recrystallize into larger sizes and these ma'yalso breakdown into smaller sizes under thermal stressand theresultin'g migration and reformation causes subsurface voids and pronounced surface irregu; ar t to devs sn. The smal d s r te drys'tallites o grainsareeven more vulnerable to dislocation by the impinging electron" beam. Inextrerne c'assQpieCes of' the target material in the region of the focal spot 3. known to have brokeno'tf and deposited in undesirable places withinlthe X-ray tube envelope.

Surface irregularities in the regionlof the focal spot are especially harmful in tubes used for' Xrajy diffraction studies. Inthese' tube's'th'e X-raybea'mis ken on at an angle of about one to'five degreeswithrespect to the'plane The cathode of the target. .It. is evident that any disruptions, depressions or protuberances on the surface of the target will drastically affect X-rayoutput at these low angles because some parts vof the beam will be shaded or cutoff. Theseirregularities alsodisperse the X-rays at undesired angles so that, in addition to the intensity being reduced, resolution is seriously impaired. t ,A fundamental object of the present invention is..the' provision of a new X-ray tube target that facilitates producing a sharp focal regionand higher X-ray output, in: tensities from an X-ray tube without diminishing its usefullife. Other specific objects are to overcome theabove noted problems of target deterioration and poor thermal conductivity through the use of a target material a single crystal of pure substance having preferred thermal and other physical properties. v i V I Achievement of the foregoing andfothermore specific objects will appear from time to time throughout the course of the specification which follows. Single crystals have produced some surprisingresults when they are used for X-ray tube target inserts. Be:

cause they contain onlytrace amounts of impurities, they are ideal for X-ray diffraction tubes where the spectral purity of the characteristic radiation emitted from the target is of great consequence. Because of their mono cyrstal; linestructure they have no grain boundaries. This yields multiple advantages such as a thermal conductivity 50% greater than that of the polycrystallinemetals, and an electrical resistivity of about 80% ofthe usual polycrystalline sinteredand cold worked target materials. These latter proper ties are partly the result of-fthe single crys tals having a density which is immeasurably different than the 100% theoretical density of the pure metals. The new single crystal targets are non porous and have only minor gas occlusions so that outgassing is more certain and requiresless time. 1 l U v u e The absence of grain boundaries in thesingle crystal targets has virtually eliminatedtarget deterioration in the form of roughening, cracking, delamina'tion, recrystalliza tion, X-ray scattering and disintegration or fragmentation that was heretofore manifested when a previouslyoverloaded X -ray target was subjected to further heavy elec tron bombardment. A prior art diffraction-tube that was operated at 5.0 kilovolts-peak, 2Q milliamperes continuous duty and had a nominal focal spot size of 0j8 X 15 0 milli: meters could reasonably heexpeeted to have a useful life of about 600 hours. 7 A tube using the new single crystal target, ,which was in all other respects, similar to the prior art tube, was operated with the same focal spot size at twice the current, or 40 milliamperes, for almost twice as many hours as couldbe expected from the previous tubes, without any perceptible deterioration of the target 1 surface. s The sharpness of the beam and the purity of X- ray output remained.

Photomicrographs of the new tube targets showed no surface erosion of-any consequence nor was there any subsurface metallurgical alteration. Even though. there were a few minor cracks due to thermal stresses along the cleavage planes, they were across" the f'o'cal sp'ot' ma s didn'ot interfere significantly with the conductionof heat to the sidesor from the focal spot to the back of'thetargetf. A more detailed description of the invention, its use and advantages Willn'ow be set forth in connection with the following drawing in Which: 'FIG..1 shows an exemplary Xray tube, partly in sec-. tion, in which the new single crystal target material is employed; it i p 7 I n H FIG. 2 a plan view of the.X-ray tube anode and target assembly taken on the section lines 2-2 in FIG. 1; FIG. 3 is' a single crystal target insert'such as rnay be usedinthe anode shown in FIG. 2. 1

Refer now to FIG. 1 where the invention is illustrated I interiorly exposed surface X-rays are generated. It will I be observed that target insert 16, anode 15, extension 14- and flange 12 are all electrically connected and at the same potential. Since the tube is mounted on X-ray apparatus on flange 12 these parts, and the apparatus, are

usually at ground potential.

X-rays emitted from a line or spot source on the surface of target insert ldradiate in all directions but a useful beam passes exteriorly of the tube through an X-ray transmission window symbolically shown as item 17.' Windows 17 are usually beryllium a few mils thick so as to minimize X-ray attenuation. In diffraction tube up to four windows are commonly employed.

Envelope 11 is provided with a central re-entrant'pon.

tion 18 that terminates in a press 19 which is joined to a cathode assembly 20 through a conventional glass 'to' metal seal in the region of 21. 'On the end of cathode 2t), adjacent'target insert 16, there is a concavity 22 constituting a focusing cup of a character well known in the X-ray tube art. Insulatingly supportedwithin focusing cup 22 is an elongated helical filament 23 that is substantially coextensive with the long dimension of target insert 16. Conductors 24 supply current for heating filament 23 to a temperature where it emits --electrons that may be accelerated to target insert 16. Since the target isat ground potential, filament 23 is made very negative with respect to it. In X-ray tubes used for emission and :diffraction analysis, the potential difference between anode and cathode is usually 75 kilovolts orunder but' in diagnostic and therapeutic tubes this voltage isusuallyappreciably higher. 7 j

Tubes of this type commonly have their anode '15 water cooled but this feature has been omitted. because it is well known in the art. An exemplary water cooled X-ray tube may be seen in US. Patent No. 2,846,753, issued in this plane.

used. This is embedded in a copper base to enhance heat transfer.

Where single crystals of metals such as tungsten, molybdenum, vanadium, niobium and tantalum are used for target inserts it is desirable. to have the (110) atomic lattice plane parallel with the target surface because these and some other metals that one may wantto use have a body centered cubic crystal structure that is most dense Where other metals are used, other planes of maximum density may be preferred and specified by those knowledgeable in themetallurgical art. Single crystals whose lattice planes are identified with respect to the crystal-faces, and in some cases, crystals whose lattice planes are specified by the customer may be obtained from the Linde Company division of Union Carbide Corporation, Chicago, Illinois, and other suppliers.

The single crystal target insert should be as smooth as possible on itsX-ray emitting surface. By conventional shop methods crystals with satisfactory surface smoothness were obtained by surface grinding to a finish which 'was not irregular in excess of 125 microinches. The surface was then electrolytically etched by conventional techn-iques to assure that there was no cold worked metal present in the surface. This can be readily checked before assembly in an X-ray tube anode with an X-ray diffractorneter such as one schematized at the cited page and other locations in Klug and Alexanders book that waspreviously mentioned. In'the diifractometer the intensity and sharpness of the X-rays diffracted by the single'crystal sample serves as an indication of spectral purity and smoothness. Aphotomicrograph of a target surface treated in the aforegoing manner will reveal a homogeneous surface, without the presence of grains, grain boundaries or impurities. Thus, there is practically no tendency for the single crystal to recrystallize or un "dergo grain boundary separation or torough en when subjected to a concentrated, high energy electron beam.

' In certain cases, especially in X-ray tubes that are used for diffraction or emission analysis, it is desired to produce a spectrum of radiation with peak intensities at predetermined wavelengths. In such cases an alloyed target "may be desirablebut they are not known to have been used in the past because, when made by prior art sintermanner mentioned earlier.

to M. J. Zunick et al. August 12, 1958, and is assigned of sample-25 makes a low angle on with respect tofthe plane surface of target 16. Thus, even though thejwidth of the focal spot were equal to the width of target insert 16, the beam of X-rays 26 impinging on sample 25 would have a thin, elongated cross section or a line focus'as it is ordinarily termed. The sharpness of this focus affects the sharpness or resolution of the reflected or refracted beam 2 7' which may be directed to an "X ray detector through collimating slits, neither of which is shown. 'A more complete X-ray optical system in which tube 10 may be employed may be seen in Klug and Alexander,

X-Ray Diffraction Procedures, John Wiley & Sons, New

York, 1954, page 241. I V

The new single crystal metal target insert 16; rnay be vacuum cast into the anode 15 by conventional techniques that are described in more detail in the above cited ing and forming techniques, they had 'low thermal conductivity and were more disposed to deteriorate in the This problem may be overcome by using single crystal alloys. For example, dependent uponthe X-radiation spectrum desired, one may eleotto use crystals of hafnium boride, titanium nitride or tantalum carbide. Other combinations may become available in the future.

- The new single crystal X-ray targets have been described primarily in connection with an X-ray tube'that has a stationary anode but it will be appreciated by those versed in the art that single crystals may also be used in targets crystals in rotating'anode tubes will appear to those versed in thearta nr summary, there has been described .a new X-ray target material in the form of single crystals of composit1ons appropriate to the X-ray spectrum desired. The

Zunick et a1. Patent No. 2,846,573. For present purposes it is sufiicient ifone appreciates that target 16 is em bedded in good heat exchan e relationship withanode 15.

The single crystal target insert l'rnay have dimensions essentially the same as polycrystalline targets used heredimensions for economic reasons. one tube design, a x n X A inches tungsten insert is new targets result in X-ray tubes with higher ratings and,

longerlife; They are especially useful for X-ray analysis technics because of their capability for preserving a smooth target surface a'nd spectral purity under rated and reasonable overload conditions. I, s

Because a preferred use of the invention has been de scribed, many other applications will now appear to those. skilledin the art- It should, therefore, be understood that;

, such description' is illustrative rather than limiting and.

that the scopelof the invention should be determined. only by construing the claim which follows.

I I claim: transmitting a beam of X-rays from the one surface to the outside of the envelope,

(j) the said single crystal target element having a body centered cubic atomic lattice structure, and

(g) the (110) lattice plane of said crystal being insubstantial parallelism with the said one surface.

An X-ray tube comprising:

(a) a gas-tight envelope,

(b) a source of a focused electron beam sealed in the envelope, 5

(c) an anode structure,

(d) a target element that consists of a single crystal of metal which is fixed in the anode structure and which References Cited in the file of this patent UNITED STATES PATENTS has one surface that is impinged by the electron beam 10 2 482 053 Zunick Sept 13 1949 Pwduce 2,521,663 Zunick Sept, 5,1950 (2) an X-ray p rmeable wmdow 1n the envelope for 2 3 5 Robinson APR 21, 1959

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