US2521663A - Electron target and means for making the same - Google Patents

Description

Sept. 5, 1950 M. J. ZUNICK ELECTRON TARGET AND MEANS FOR MAKING THE SAME Filed Nov. 4, 1947 Patented Se t. 5, 1950 ELECTRON TARGET AND MEANS FOR MAKING THE SAME Michael J. Zunick, Greenfield, Wis., assignor to General Electric X-Ray Corporation, Chicago, 111., a corporation of New York Application November 4, 1947, Serial No. 783,879

2 Claims.

My present invention relates in general to electronics, and has more particular reference to an' improved electron target structure and method of making the same; the invention having specific reference to an electron target structure well suited for use as an anode in an X-ray generator.

An X-ray generator comprises an electron emitting cathode and an anode at which X-rays may be generated in response to impingement of electrons, emitted by the cathode, upon a suit able electron target at the anode. Electron impingement on the anode target during operation of the generator tends to disintegrate the target material due to excessive heat liberated at the target as the result of impingement of high speed electrons thereon. Target deterioration may take the form of target deformation, splitting, flaking, and curling, with the result that the target becomes progressively less efiicient as such, more or less rapidly, eventually becoming entirely useless.

Materials commonly used for anode targets, such as tungsten, nickel, copper, cobalt, iron, chromium, molybdenum, and other metals, all tend to deteriorate in the manner mentioned when exposed to electronic bombardment; and an important object of the present invention is to minimize deterioration of the character mentioned, particularly in electron targets of tungsten, although the principles of the present invention may have beneficial application in connection with electron targets of other materials than tungsten.

Another important object is to minimize electron target deterioration by controlling the grain size of target material, more particularly by controlling grain structure to provide very coarse, elongated grains substantially uniformly distributed throughout the body of the target material.

Another important object is to provide material for electron targets, having very coarse, elongated grain structure distributed substan tially uniformly throughout the material, by repeatedly rolling a slab of target material to reduce the sectional dimensions thereof to desired size, and by recrystallizing the material while it is being Worked, whereby to obtain maximum grain growth in the material, as by heating the slab during an interval, or intervals, between successive passes of the slab through the working rollers.

Another important object is to provide electron target material, such as worked tungsten, having very coarse, elongated grain structure of the order of about LOGO-2,000 grains per square millimeter, by working the material, as by rolling it, for the reduction of its sectional size, and more or less completely recrystallizing the material during the working thereof, but prior to final working of the material, to obtain maximum grain growth therein, the desired internal grain structure comprising relatively large grains which are substantially elongated, in contrast with the compact or pebbly grain form of so-called equiaxed material, produced by heating the material Without working it, which has poor strength and which tends to crack and disintegrate under the high heat generated in the material when subjected to electronic impingement.

Another important object of the invention is to provide electron'target material characterized by relatively large recrystallized grain structure, substantially free of laminations, and worked sufficiently to produce an elongated grain structure, thereby preventing cracking and curling of the material when exposed to electron impact, without, however, carrying the working to the point of producing laminae in the material or breaking up or otherwise reducing the desired coarse and elongated grain structure.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawings, discloses a referred embodiment of the invention.

Referring to the drawings:

Fig. l is a longitudinal section taken through an X-ray generator having an anode providing an electron target;

Fig. 2 is a sectional view taken substantially along the line 2-2 in Fig. l to illustrate the electron target end of the anode;

Fig. 3 is a representation of a micro picture of tungsten having typical fine grain structure of the sort heretofore commonly employed for electron target purposes in X-ray generators;

Fig. 4; is a view similar to Fig. 2, showing the character of deterioration under extended electron bombardment of target material of the fine grain structure shown in Fig. 3;

Fig. 5 is a representation of a micro picture of tungsten having relatively fine grain structure partially recrystallized and worked to increase grain size and length;

Fig. 6 is a representation of a micro picture of tungsten of relatively coarse, elongated grain structure provided by working the material while substantially recrystallizing the same;

Fig. '7 is a representation of a micro picture of tungsten having very coarse, elongated grain structure provided by working the material and more or less completely recrystallizing the same; and

Fig. 8 is a sectional view comparable to Fig. 2, showing the minimum character of deterioration in an electron target comprising material of the sort illustrated in Fig. '7.

To illustrate the invention, the drawings show an X-ray generator ll comprising a sealed envelope [2 containing an anode l3 and a cathode l4 embodying an electron emitting filament Hi. The cathode and anode, as shown, may be mounted in co-axial, spaced and facing relationship within the envelope !2. Suitable conductors for energizing the electron emitting filament l5, and for applying suitable operating electrical potential, between the filament and the anode, from an electrical power source disposed outwardly of the envelope, for the operation of the generator, may, of course, be provided.

While the invention is not necessarily limited to any particular type or form of X-ray generator, the device, as shown in Fig. 1, comprises a generator of the sort in which the anode I3 is mounted for rotation within the envelope upon an axle or support stem IS, the anode being caused to rotate by the operation of electrical motor means including the stationary stator ll, mounted on and outwardly of the envelope at the anode containing end thereof.

At its cathode facing end, the anode may be provided with an electron target 18, which may comprise an annular ring of suitable target material, such as tungsten, mounted concentrically upon the anode, which may comprise abody or shell of copper forming a mounting base for the target ring, said base in turn being journalled for rotation on the axis stem l3. It should be understood, of course, that the anode is so positioned as to present the target ring i8 continuously in the path of an electron beam l9, emitted by the filament l as the anode rotates.

The present invention, of course, is not necessarily limited to targets for rotating anodes, but applies equally to stationary targets in which the target may comprise a target member disposed in fixed position in the path of the electron beam.

A generator operates to produce X-rays at the surface of the target as the result of the impingement, on the surface of the target, of electrons emitted at the filament l5, when electrically energized, said emitted electrons being impelled toward, and caused to impinge upon, the surface of the target, under the influence of electrical potential applied between the filament and the anode, the higher the anode-cathode potential, the greater the force of electron impingement and the greater the intensity of resulting X-rays.

X-rays generated as a result of electron impingement on the target surface pass outwardly of the envelope and may be utilized for various useful purposes. When the generator is in operation, a considerable amount of heat is developed at the target surface as a result of electron impingement thereon, such heat being dissipated through the body of the anode and thence outwardly of the envelope. The temperature at the target surface depends not only on the rate of heat dissipation from the anode, but also upon the intensity of electron bombardment, which,

in turn, is a function of anode-cathode operating potential and of the rate of electron emission from the filament; and the temperature developed at the target surface may be of the order of the melting temperature of the target material, particularly in high power generators in which high electron driving potential is applied between the anode and the cathode, it being usual, in the interests of operating efficiency, to operate the generator so that its anode, at the target surface, functions at a temperature just short of the softening temperature of the anode material. The excessively high temperature to which the target is subjected tends to cause target deterioration, which is evidenced by bulging, buckling, cracking, curling, and flaking of the target material, and the consequent deformation of the target surface from a desirable fiat condition. Such deformation, of course, impairs the eficiency of the generator, and fre-- quently results in rendering the generator completely useless for its intended purpose.v

Heretofore it has been thought that target deterioration, including bulging, buckling, cracking, curling, and flaking, is the result of grain growth within the body of the target material; and previous attempts to eliminate target deterioration have contemplated the utilization of target materials of ultra fine grain, and to the preservation of the fine grain character of the material during the service life of the generator, by operation at potentials such that the device will not be subjected to target deteriorating temperatures. As a consequence, target material has been worked in such fashion as to produce ultra fine grain texture therein, it being presently the practice to employ tungsten having fine grain structure of the order of more than 2,500 grains per square millimeter of surface, such fine grain structure being illustrated in Fig. 3 of the drawings.

Targets of fine grain material give satisfactory performance if not overloaded; but the present trend of development toward ever higher powered generators demands target loadings which rapidly deteriorate fine grain targets comprising material of the sort shot-m in Fig. 3; and when so overloaded, a target of such material is found rapidly to disintegrate by cracking, flaking, and curling, in the manner illustrated in Fig. 4, which shows a fine grain target H3 in deteriorated condition such that the target is useless for practical purposes, the same having its surface portions flaked and curled up from the body of the target to form. separated shreds or flakes 20.

In order to prevent target deterioration of the sort shown in Fig. i, the present invention contemplates the use of target material having relatively coarse granular structure of the order of substantially less than 2,500 grains per square millimeter, in which the constituent grains are elongated to an appreciable extent. This type of grain structure may be obtained by recrystallizing the target material during the working of a slab of the material, such working consisting of successive slab rolling operations, whereby to reduce the sectional area of the work piece to desired dimensions. The working of the material, of course, tends to reduce the size ofconstituent grains by breaking them down as a result of the rolling operations. Working the material also tends to develop laminae in the worked material, material thus laminated to any appreciable extent tending to split along the laminae when under electron bombardment. Recrystallization of the material may be accomplished by heating the Work piece at controlled temperatures in order to accomplish recrystallization and resultant grain growth, such recrystallization being accomplished by heating the work piece during the period between successive rollings; and recrystallization preferably is accomplished as fully as possible by heating the work piece at a temperature, and for a period of time, sufficient to produce grain growth to maximum size, although favorable results are obtained if recrystallization is allowed to proceed to a degree of partial recrystallization short of the maximum possible recrystallizing efiect.

Desirable results may be obtained by heating the work piece for either complete or partial recrystallization, several times during the working process, or recrystallization may be accomplished during the rest interval immediately prior to the final rolling step of the working operation.

By thus either partially or completely recrys tallizing the target material during the working, the resultant target material may be obtained substantially free of laminae with a grain structure of the order of 2,000 grains per square millimeter, accomplished by partially recrystallizing the target material, such partially recrystallized material having appreciably elongated grains being illustrated in Fig. 5 of the drawings.

A somewhat coarser grain structure, accom plished by more completely recrystallizing the target material and having a grain structure of the order of 1,500 grains per square millimeter, is illustrated in Fig. 6, while Fig. '7 pictures a grain structure of maximum coarseness of the order of 1,000 grains per square millimeter, accomplished by working the material during and after substantially complete recrystallization of the same. Material of the character depicted in Fig. 7, and also as shown in Figs. 5 and 6, are all characterized by the presence of relatively large and substantially elongated grain structure, as compared with the typical fine grain structure depicted in Fig. 3, which is obtained by working the material without recrystallizing the same, and which has heretofore been considered essential for satisfactory electron targets.

The present invention, in teaching the recrystallization of target material in conjunction with. the working of the same, also distinguishes from so-called equiaxed material, which is produced by recrystallization without working and which is of coarse grain pebbly structure in which the grains are rounded and not elongated.

Applicant, thus, has discovered that by recrystallizing the target material as completely as possible while working the same, a very coarse grained material of the sort shown in Fig. 7 is obtained which is exceedingly and unexpectedly resistant to deterioration under electron bombardment. Although there is a tendency for recrystallized target material to crack, as indicated at in Fig. 8, when bombardment by electrons under excessively high voltage, targets of such recrystallized material are able successfully to resist deterioration under bombardment of very much greater intensity than that which results in the rapid and complete deterioration of targets comprising fine grain unrecrystallized material, or targets of equiaxed material. Targets of material which has been worked while being recrystallized, accordingly, may be employed without deterioration under loads which have heretofore been considered destructively high; and even where overloaded and deleteriously afiected as by cracking, in the manner shown in Fig. 8, electron targets of recrystallized material do not materially lose utility. As a consequence, apparatus embodying an electron target of the sort here described does not become useless for further service, even if overloaded sufiiciently to crack the target element in the manner indicated in Fig. 8, although overloads of such character would completely destroy the utility of targets of the best quality and character heretofore provided.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous 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 any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. An electron target for an X-ray tube comprising mechanically worked tungsten having recrystallized grain structure of the order of 2000 grains per square millimeter or less, wherein the constituent grains are substantially elongated as a result of working the tungsten, whereby the same under heavy electron impact load is highly resistant to cracking, splitting, flaking, and curling at the electron receiving surface of the target.

2. An Y-ray generator embodying an anode having an electron target comprising tungsten having at least partially recrystallized grain structure in the range of 1000-2000 grains per square millimeter, in which the grains are substantially elongated.

MICHAEL J. ZUNICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,826,514 Gero et al. Oct. 6, 1931 2,332,044 Bell Oct. 19, 1943 2,430,800 Atlee Nov. 11, 1947 OTHER REFERENCES Tungsten, by C. J. Smithells, 2nd edition, 1945; Figs. 24-29 and page 55. l

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