US1808430A - X-ray tube - Google Patents

Description

JuneZ, 1931'. M.- MORRISON X'-RAY`TUBE Filed Ms? 9. 1922.

INVENTOR vMonVrf--ORD MORRIS() N ATTORNEY Patented June 2, 1931 .UNITED STATES PATENT ortical MONTFORD MORRISON, OF NEW YORK, N. Y., ASSIGNOR, BY MESNE ASSIGNMENTS, TO WESTINGHOUSE LAMP COMPANY, A CORPORATION F PENNSYLVANIA X-RAY TUBE Application filed May 9, 1922. Serial No. 559,569'.

My invention relates to certain improvements 1n apparatus for the generation of Roentgen or X-rays.

An object of my invention is the provision of a more eiicient"'X-ray device in which a larger percentage of the generated X-rays may be utilized than in tubes at present employed.

Another object of my invention is the provision of an X-ray tube which does not rev quire a special device for focusing the cathode rays. I

. Another object of the invention is the provision of an X-ray tube whlch does not require, for its incandescent cathode, material of such high specic electronemissivity as those 'now employed.

Anotherobj ect is the provision of an X'l-ray tube 'in which the generation of X-rays will Y be confined to the area intended for that purose.

A further object of this invention is the provision of an X-ray tube which may be op-` erated without substanti al positive ionization while containing gas at a pressure which would otherwlseleadto an excessive amountof gaseous ionization in tubes of the prior art.

A still further object is the provision of shielding means for stray X-rays which may be less expensive and cumbersome than the shielding means now employed.

Other objects of the invention may be perceived from a reading of the following specication. v

In the operation of X-ray tubes used fo certain purposes, it is desirable, for wellknown reasons, to avoid positive ionization of the residual gas within the tube. To avoid a large amount of positive ionization in X-ray tubes at high voltages, it has been proposed to evacuate the tube to thehighest possible vacuum. Such a degree of vacuum is partly necessary in tubes of the prior art on account of the locally intense electric fields caused by the presence of structures having sharp edges or surfaces of small radii of curvature. The samedeg-ree of vacuum is also partly necessary by reason of the structure of tubes of the prior art in which the electrodes were placed comparatively far apart.

intensity of the electric field or potential gradient, vupon which ionization may depend, is reduced to a value insuiiicient to cause a large amount of 1on1zat1on. In order to reduce the potential gradient to such degree, I may elim-v inate structures having sharp points or edges and surfaces of small radii of curvature or l may take advantage of the fact that close lspacing of the electrodes allows a higher gas pressure to be used than when the electrodes are far apart. Accordingly, I construct at least one of the electrodes of my X-ray tube and other structures therein in such manner that they shall be free from sharp oints and edges and have relatively large ra 'i of cunvature or I may place electrodes in close proximity to each other, but, preferably, I dispose the electrodes in close proximity to each other vand eliminate from all structures the sharp points and edges which may cause electric fields of such intensity as to result in ionization ofthe residual gas.

While vmy invention covers these features boardly, I have found thatl there is a special construction ofthe tube elements, hereinafter to be illustrated, which affords the abovementioned advantages and others inaddition thereto.l

Although, when the electrodes of an X-ray tube consist of parallel planes, the voltage gradient varies inversely as' vthe distance between the electrodes, it is found that for certain other forms of electrodes,y the voltage gradient becomes a minimum for a definite separation thereof. For example, itis well known thatthe maximum voltage gradient between concentric cylindrical electrodes is a minimum when the outer diameter of the inles - ization than when- ,should be of such .not wish to times the interior diameter, of the outer electrode. A

The diameters of the cylinders,.however,

size that the,spacin'g between them will be less than that at which a substantial or detrimental amount of ionization will occur with the gas pressure used and the voltage applied to the electrodes. As a specific example, it may b e stated thatthe interior diameter of the outer cylinder may be one centimeter and that, for a minimum potential gradient, the diameter of the inner electrode would then be approximately.3.7 mms., the cylinders being concentrically disposed, the separation would then be about 3.1 mms.

Concentric c linders are considered the preferred form gbr the electrodes of my X-'ray tube but they may assume other forms in which the voltage gradient between the electrodes is notl inversely proportional to the distance between themfor all 'values of space separation or, in other words, the form of the electrodes may be such that the maximum electrical stress or potential gradient between them is approximately a minimum for a certain separation. It will be appreciated that, according to this feature of my invention, a higher'gas pressure may exist in the'tube without obtainingdetrimental positive ionthe form and spacin of electrodes now in use are employed. he tubes now in use and referred to herein as tubes .having the highest possible vacuum, in order to avoid positive ionization, are illustrated and described in 1,203,495, issued Oct. 31, 1916, wherein the upper limit of pressure is fixed at 0.6 of a micron.

While, und v s irable pto place the electrodes at a distance resulting in A be confined in thiseinventionto such separation of electrodes, inasmuch as I have pr vided a construction, as hereinafter shown, w ich has novel advantages even when the electrodes are disposed'at a distance other tential gradient. Such than that which produces the mmimum povariations'of separation may be made, for example,- with refer- Y ence to the degree of vacuum in the tube,

in order toproperly balance the advantages and 'disadvantages of agiven separation of ionizationvis diminished.

Coolidge Patent N o. h

er certain conditions, it ,is de- `minimum electrical stresses, I do the electrodes with a given degree of vacuum.

In tubes of the prior art, a large percentage of theX-rays enerated have been wasted by reason of the act that the construction has been such that the rays have emanated from their source at an jective of the X-rays and from points other than the focal spot. Not only have the wasted rays detracted from the eiliciency of the -ray device, but they have called for expensive shielding means of comparatively large area to prevent them from causing personal injury. Y

My invention, however, provides means, includlng a novel form and arrangement of the tube elements, whereby a larger percentage of the X-rays are u tilizedE and whereby the means used to absorb stray X-rays is of com' paratively small area, the latter feature serving to simplify the cumbersome and ex ensive shielding means now employed., e adequacy of shielding means of small area is accomplished by absorbing stray X-rays close to their source, as will hereinafter be more clearly illustrated by reference to a drawing of a specific embodiment of this feature.

In the arrangement ofthe electrodes, according to my invention, I place a cathode, which may be incandescible if desired,-in such relation to an anode that all the high velocity electrons may impinge only/where it is desired'to have them do so, namely, on the focal spt of the target or X-ray-generating surface. Such arrangement of the electrodes has one embodiment lwhen a cathode is placed within a hollow cylindrical anode. of my invention also obviatesr v This feature the necessity for a focusing device now used in X-ray tubes. The form of construction in which the ca. thode is encircled by the anode is also of advantagein that alower .electron density is effected, whereby the liability of causing A further advantage to bederived from my invention resides in the fact that the cathode, which is preferably made incandescent, may be made df a'material which as a lower electron emiss'ivitythan'the materials now frequently employed. 'Ihis adyantage arises from the fact that a cathode of larger area may be utilized in one form of my new type of tube.

angle from the intended ob- The foregoing and other novel advantages i will be explained-in connection with the accompanying drawings in which,

Fig. 1 is a view in elevation, partly in section, of an X-ray 'tube embodying my invenf tion; and,

Fig. 2 is' a`fragmentary view, partly in sectionshowing a modification'of my device.

Figure l shows an X-ray tube, partly in section, comprising a spherical evacuated glass envelope 1 having arms 2 and 3 extendmg at right a'ngles to each other, into which are sealed, respectively, an anode 4 anda cathode 5, the former having a leading-in conductor 6 and the latter having leading-in conductors 7 and 8, these conductors being preferably free from sharp points and edges. The .leading-in conductors of both electrodes and the inactive parts of the electrodes are preferably covered with an insulating material. To the exterior ends of the leading-in conductors of the anode and the cathode are secured, respectively, terminals 9 and 10 hav ing large radii of curvature,such structure being disclosed and clai-med in my copending application Serial No. 540,141, filed March 1, 1922, X-Ray Tube, which eventuated into Patent No. 1,621,852, March 22, 1927. rllhe anode 4 is a hollow, metallic cylinder shown in longitudinal cross section and has its upper and lower ends 12 and 13, respectively, rounded off to a comparatively large radius of curvature to avoid excessively intense electric fields at those parts, whereby the tendency of the gas to ionize may be reduced. Supported within the envelope 1 by means of a rod 14 secured to the neck 3 is a mushroom shaped element 15, shown partly in section, having a lstem 16 around which the cathode 5 is wound in helical form.

The anode may consist entirely of a refractory metal of high atomic weight, or ymay be a composite metallic body, the inner cylindrical portion of which should consist of a metal of high atomic weight, such, for example, as tungsten, uranium or thorium, since the inner surface constitutes the target from which X-rays emanate, while the outer portion may be a metal having high heat conductivity.

The cathode 5 is shown as a helix placed within the anode 4 and wound around the cylindrical stem 16 of the shielding element and cathode support 15, the longitudinal axis of the stem 16 being coincident with the lon gitudinal axis of the cylindrical anode 4. As above set forth, the diameter of the cathode 5 may have a definite relation to the interior diameter of the anode 4 so that the voltage gradient between the anode and the cathode may be a minimum, the outer diameter of the cathode being times the interior diameter of the outer electrode.

The cathode 5 preferably consists of a refractory metal, such as tungsten, so that it may be heated to an electron-emitting temperature by an electric current passing through the leading-in conductors 7 and 8.

The element 15, the vertical axis of which is coincident with that of the anode 4, may be a mushroom-shaped member, the stem 16 of which preferably consists of a refractory insulating material, while the upper or body portion consists preferably of a metal for the purpose of absorbing X-rays which pass through the upper opening ofthe anode 4.

Fig. 2 shows the cathode 5 in a modified form, being there illustrated as a ring or toroid placed concentrically with the anode 4 so that the electrostatic pull on the cathode is reduced to a.. minimum. lIn this form of construction thastem 16 of the element 15, as shown in Fig. 1, is dispensed with, the cathode 5 being held in position by the leading-in conductors 7 and 8. The leading-in conductors and the filament preferably consist of stout wire so that they may resist the strong electrostatic pull exerted on them and are preferably free from sharp points and edges.

During the operation of the, tube, a high potential being applied to the anode and the cathode while the latter is incandescent, a stream of electrons proceeds at high `velocity from the cathode 5 and impinges on the inner surface of the anode 4 from which X-rays,

emanate. The portion of 'X-rays which are directedthrough the opening in the upper part of the anode are absorbed by the shield 15 which may be comparatively close to the origin of the rays. The rays being absorbed before they have spread out over av large area, the shield l5 need not be so large as when .it is attempted to absorb them at a comparatively long distance from their source,'which is frequently done. It will be noted that although an opening exists between the anode 4 and the shield 15 X-rays cannot make their escape therethrough since they travel only in straight lines. That portion of the X-rays which is directed towards the lower opening of the cylindrical anode 4 forms a cone directed towards the intended objective and is thereby utilized. llt will be appreciated that, by the arrangement here shown, the stray X-rays are absorbed not only by the shield 15 but alsoby the anode 4.

Attention is also called to another valuable feature of t-he arrangement of the electrodes herein shown, which consists in so disposing the cathode that substantially all of the high velocity electrons impinge on the surface which it is intended to employ as the X-raygenerating surface. This feature is intended as an improvementover prior X-ray tubes inasmuch as the latter have been so constructed that some of the electrons emanating from the cathode have impinged'on other surfaces than that intended as the Asurface of the target and have thereby resulted in the generation of stray X-rays which not only called for additional shielding means but detracted from the eiiciency of the tube.

It is also obvious that by reason of the peculiar disposition of the electrodes relative to eachother and to the glass walls of the envelope, bombardment by electrons of the walls ofthe tube is prevented, thus preventing ero-sion thereof, thus making thev present type eftube particularly suitable for higher capaclties.

In the preparation of the tube, the envelope shouldbe exhausted of gases and vapors, means for accomplishing which are well known'- in the art. The parts constituting the X-raydevice, particularly the electrodes, should also be freed from gases and vapors; thls may accomplished, for example, by heating the device to a high temperature inl a vacuum and removing the released 'gases and vapors. The degree of vacuum to which the tube is exhausted or the pressure of the gas admitted should be such" that, with electrodes of given form and spacing, no substantial or detrimental amount of positive ionization will occur during the operation of the tube. kBy detrimental positive ionization is meant that quantityof` ioniation which of the Naperian system o logarithms.

. drical eleetron-emittin ltem of logarithms.

' during tube operation will be evidenced by liuctuations or instability of the tube dis--` charge current at a given electrode spacing, gas pressure and applied voltage. l

While I have herein described a preferred embodiment of the several features of my lnventlon, it is to be understood that modificatlons of the same may be contrived without departing from the proper scope thereof.-

What is -claimed 1s:

`1. An X-raytube comprising a gas-tight envelope, an anode within said envelope, said anode having a passageway extending from one said of saidanode to another side thereof, a thermionic electron-emitting cathode adjacent one end of the passageway in said anode, means for absorbing X-rays emerging from the 'cathode end of said passageway, the other end of the passageway in said anode being free and unobstructed to permit the` vemission of a solid beam of X-rays therefrom. v Y

2. An X-ray tube comprising a gas-tight envelope, an anode within said envelope, said anode having a passageway extendin from one side of said anode to another side t ereof, a thermionic electron-emitting cathode pro- ]'ectln within `one end of the passageway, the ot er end of the passageway being free and unobstructed .to permit the emission of a solid beam of X-rays therefrom. v

3. A In an X-ray tube, the combination of a hollow cylindrlcal anode and a cylindrical' cathode positioned within said anode, the diameter of'sai cathode being substantiallyequal to the pro uct of the interior diameter of said anode by the reci rocal of the base 4. In an X-raytube, the combination ofV of a hollowcylindrical anodepand a -cylincathode positioned within said anode, the iameter of said cathod'e being substantially equal to the product vof the interior diameter of said anode by the reclprocal of the base of the Naperian sysenvelope, a hollow ano 5. An electrical discharge device comprislng an evacuated gas-tight envelope, an electron-emitting cathode within said envelope, an anode having a passageway therethrough and means for shielding one end of said passageway, said shielding means havingmeans ing from one side of said anode to another side thereof, means for intercepting X-vrays emerging from one end of the passageway, the other end of the passageway being free land. unobstructed, to permit a solid beam X-ray to freely emerge from the tube.

8. `An X-ray tube comdprising a gas-tight velope, 'means .for generating electrons, subi stantially all of the electrons falling upon the target face of `said anode for the produc- -within said ention of X-rays, means for preventing X-rays emerging from one. end of'said anodel from being projected outsideof said envelope, the

other end of said anode being open for the free and unobstructed movements of a solid beam of X-rays from the tube to a point outside of the same.

9. In an X-ray tube', the combinationof an evacuated envelope, an electron-emittmg cathode and an anode therein, said anode consisting of a hollow body formed of large radii of curvature and having a passageway extending from one side to another side of the anode, a metallic cup-shaped` shield adjacentone end of the passageway in said anodeto intercept X-rays emerging therefrom, theother end of the passagewayin e the anode bein free and unobstructed to permit a solid eam of X-rays to emerge therefrom andlfrom said tube.

10. A vacuum tube for the production of X-ray comprising an incandescent cathode,

an anti-cathode surrounding said cathode at a distance, an evacuatedgas-ti ht envelope l enclosing said cathode and ano' e, a portionat least 'of said envelope being permeable .to

X-rays, a shield for 4intercepting X-rays emerging from one end of said anti-cathode,

said shield beingremote from the end of the anti-cathode adj acentthat portion of the envelope permeable to- X-rays.

j 11. An .electrical-discharge device comprising anenvelope'havin afgas pressure therein greater than .6 oA a micron, electrodes having a gas pressure therein. greater than .(of a micron, velectrodes comprlsing a themionic electron-emitting cathode consistmg of a refractory metal wire and an anode Within said envelope, the electrodes being spaced apart a distance less than the electrode distance at which a detrimental amount of positive ionization will occur with a given gas pressure and applied voltage.

13. An electrical discharge device comprising an envelope havin a gas pressure greater than .6-of a micron, e ectrodes comprlsing a thermionic electron-emitting -,cathode con# I sisting of a refractory metal Wire and an anode within said envelope, said anode havl ing surfaces of large radii of curvature preventing excessively intense electrical ields at such surfaces, said electrodes being spaced apart a distance less than the electrode distance at which a detrimental amount of posi! tive ionization will .occur with a pressure and applied voltage.

14. An X-ray tube comprising an envelope having a gas pressure greater than .6 of a given gas micron, electrodes comprising a thermionic electron-emitting @cathode consisting of a refractory metal wire and an anode within said envelope, said anode having surfaces of large radiiof'curvature preventing excessively intense electrical fields at such surlfaces, said electrodes 0being' spaced apart a distance less than-the electrode. distancesat which a detrimental -amount of positive ionization will occur with a given gas pressure and a plied voltage.

15. an electron discharge device which during operation produces X-rays, the combination of an envelope, a thermionic electronX-emittingcathode and an anode having -ray part thereof integral with said anode and ggsed in said envelope for absorbing all of the -rays except a useful beam thereof. 16. In an X-ray tube, the combination of an envelope, a thermionic electronemtting cathode and an anode having an X-ray` enerating surface within said envelope, an X- ray sheldin means having a. part thereof integral wit said anode and disposed in said envelope for absorbing al1 of the X-rays f except a useful beam thereof. In testimony whereof, I have hereunto sub-v scribed my name this 5 day of May 1922. Y MoNTFonD MoRiusoN.

v neratin surface within saideny velope, an X-ray s 'elding means havin a

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