US2955219A - Electron discharge device - Google Patents

Description

Oct. 4, 1960 w. F; NIKLAS ELECTRON DISCHARGE DEVICE Filed Feb. 16. 1959 United States Patent M ELECTRON DISCHARGE DEVICE Wilfrid F. Niklas, Chicago, Ill., assignor to The Rauland Corporation, a corporation of Illinois Filed Feb. 16, 1959, Ser. No. 793,431

l4 Claims. (Cl. 313-65) This invention relates to electron discharge devices of the image converter type and more particularly to the object or pickup Window of an X-ray image converter envelope.

Conventional image converters are contained in an evacuated envelope, the body of which is substantially cylindrical in shape. One end section of the cylinder is vclosed and contains provisions for ophthalmic viewing of an image. The opposite end of the cylindrically shaped envelope is closed and customarily includes a spherically shaped object or pickup window area of a material transparent to a particular incident radiation, such as X-rays.

Within the radiationtransparent window area and in close proximity to it, or some instances actually applied on the inner surface of the window area, is a multiple layer pickup screen including a photoemissive cathode. 'I'he opposite end of the evacuated enclosure generally contains a fluorescent viewing screen and interposed between the photoemissive cathode and the iluorescent viewing screen is an electron optical system for the purpose of focusing and accelerating an electron image projected from the photoemissive cathode in response to the excitation of incident radiation from a source or object image. It has been noted that the pickup window must be transparent to the incident radiationand in the 'case of X-ray image converters the window is generally composed of glass. Materials such as aluminum or beryllium may also be used but ditculties of procurement as well as application usually limit their use.

It is 4essential for the proper operation of an image converter that the sensitivity of the device be as high as possible and that the brightness distribution in the image be as true a reproduction of the X-ray attenuation in the object under observation as it is possible to obtain. Existing image converter devices using an X-ray transparent pickup window of uniform thickness throughout suffer from a deficiency in both of the above elements due principally to the variation in transparency of the uniformly radiation-transparent window. Because of the divergent character of X-rays, it can beV seen that the X-ray path length through a uniformly thick pickup window, measured at the axis, is substantially less than the X-ray path length through the glass measured at any point which is off the axis and the diiference in path length increases with the distance from the axis. This change in path length becomes increasingly pronounced when `the pickup window is hemispherical in shape which is the common practice in image converters of the X-ray type.

The transparent window introduces a loss or attenuation to the incident radiation that may be assumed to be constant per unit length in the material and, consequently, with a divergent beam the loss experienced is a maximum at the periphery of the window. The loss in X-ray transmission in the peripheral area of a uniformly thick pickup window in response to a divergent X-ray beam is manifest in an unwanted reduction in brightness in the peripheral area of the reproduced image.

2,955,219 Patented Oct. v4, 1960 It is, therefore, a principal object of this invention to provide a new and improved image converter envelope in which one or more of the aforementioned difficulties encountered with prior art devices are eitheralleviated or substantially eliminated. y

It is another object of the kinvention to provide an image converter envelope structure having a pickup win',- dow in which X-ray transparency is substantially improved.

Itis still Ia further objectl of the invention to provide an image converter envelope having a structure which Y permits large image areas to be viewed without suffering a deterioration in sensitivity or undesirable contrast differences.

An image converter envelope, constructed in accordance with the present invention, comprises a substantially cylindrical section defining a transmission path for an electron image. One end of the cylindrical section is enclosed and contains provisions for viewing an image. The opposite end of the cylindrical section is likewise closed and includes a window area of a radiation-transparent material having a thickness which varies inversely with the distance measured radially from the axis of the cylindrical section and thereby provides a constant transmission path length for incident rays issuing from an external source along the axis of the cylindrically shaped section.

The features of the present invention which are believed to be novel are set forth with, particularity in the ap- `van evacuated envelope having a cylindrically shaped central section, fa s'huldervportion atf one end of the cylindrical portion yextendingradially inwardly toward the axis thereof and including provisions for viewing an image, and a'closure portion at lthe opposite end of the cylindrical section including an X-ray transparent window portion. `Morefspeciiically, there is an envelope section 10 which, for most of its length, is substantially a cylinder vand this section of the envelope is closed at one end by a re-entrant press 11. The aforementioned vshoulder lportion of the envelope is that Ywhich interconnects the cylindrical section with the re-entrant press. The =re mainder of. the glass envelope comprises a substantially spherical glass section 12 having a diameter approximately lequal to that of the cylindrical portion of envelope section 10. The sections 10 and 12 are pre-sealed around their perimeters to respective metal iianges 13a` and 13b which, in turn,` are sealed together by Heliarc welding or been separately processed. t

Suitably mounted' within envelope section 12 is an electron source in the form of a large diameter multilayer pickup screen'14 which approximates a sector of a the `like after the two envelope sections 10 and 12 have vsphere oriented so-that its concave surface faces toward .support member 14a. superimposed on the phosphor layer is the usual barrier layer 14e which may be of Y aluminum oxide or the like,V and on the surface of which is applied a photoemissive cathode layer 14d. Photoemissive cathode Y14a! may be of conventionalt antimonycesium composition. f L

A small 'diameter fluorescent screenis positioned V'within the; envelope within the area encompassed by the'shoulder portion thereof and facing the pickupk screen. More specically, re-entrant section 11 is closed by a flat glass plate 15,- having transverse dirmensionsY small with respect .to those of the cathode and. bearing Aon its inner face a suitable uorescent coating'to constitute a viewing screen 15a. Silver activated zinc-cadmium suliide or the like is thetiuorescent material generally employed andY the screen is preferably aluminizedor Votherwise provided with arnetallic backing layerj 15b.

A focusing and accelerating electrode system is. interposed intermediate the photoemissive cathode and viewing v screenV and it comprises a conductive focus. electrode 19 adjacent the inner'surface of the envelope and an anode 16 encompassing viewing screen 15a. The focus electrode spans'substantially'the entire cylindrical portion of the envelope and may conveniently take the form of a conductive wall coating having one end terminated at and electrically'connectedrto metal ViiangesV 13a and 13b. The anode 16 is a metallic electrode structure which is partially cylindrical and partially conicalV in shape -termin atcd atrits large end with Va skirt portion Which may b e accommodated by mounting over Vthe Ive-entrant, or returned portion of envelope section-11.

The opposite end Vof theeanode terminates in a substantially 'spherical' shaped cap117- having anaxial and circular aperture 18 which -provides access for electrons originating at the photoemissive cathode, admitting such electrons for impingementV against viewing screen 15. Likefscreen 15, the anode also has small transverse -dimensions relative to those of the cathode.Y This, it will ing screen having a much smaller diameter than the photo'- cathode in order to obtain a'marked intensiication of the visible image. The convex face of anode cap 1 7-faces the concave surface of; pickup screen'14. Theanode encloses viewing screen 15 and-isY electrically connected thereto so that the screen is maintained at the same electrical potential as theranode. lThe structural details. of the vimage converter'as thus far described may be entirely conventional and will, be familiar to those skilled in the art'.,

In the operation of the. described de'vice,m`ultiple layer pickupscreen Y14 is operated at or nearV ground potential Vthrough a lead extending through envelope section 12 as indicated'at 24. The anode structure connects through Y a'lead 25 to a high voltage Ysource (not shown). which establishes an anode potentialy of between and :35' kilovolts. Focusing potential is applied to electrode`19 by connection through iiangesV 13a, V 13brand the'focus potential is usually in the order of`several hundred volts.

VHaving established the Voperatingl potentials,` an X-ray Vimagernay be admitted through X-.ray transparent .window be recognized, is usual practice, employing a circular view- Y da=d cos are tan between thel X-ray source and X-ray transparent window 12V isobject 30 through which theV diverging' X-rays pass,` causing an X-ray image to'pass through transparent window 12 and impinge upon pickup screen 14'.V Aconversion is'here; accomplished in that'the 4X=r`ay image, by exciting pickup screen 14, causes a corresponding electron image to be developed. Thisimage, in turn, ist accelerated along the'electron pathY of the tube Vtoimpin'ge upon screen 15 where a further-` imageY conversionrtakes place,V

12m. i. .Cuge'alvisible imagejon this screen in the. usual lway.

. Wness is minimized. v

In prior art devices transparent window 12 is of uniform thickness throughout and the effective length of the X-ray path through the window increases with increasing radial spacing which results in an undesirable reduction in X-ray transparency which is most pronounced at the peripheral area of the window. The reduction in X-ray transparency with the distance from Ythe axseof the image converter results in a corresponding reduction in X-ray intensity on pickup screen 14 and produces an undesirable brightness variation in thek image reproducedon uorescent screen 15a. in particular, the brightness. of this image falls away or decreases in a manner which is essentially the same as the change in effect-ive X-ray transparency of the transparent window due to the efective increase'in X-ray path length with radial distance from the tube axis. it can be shown that for a hemispherically shaped pickup Window with a radius of 8 inches and an actual uniform window thickness of .080V inch and with a point X-ray source at an axial distance of 24 inchesY outside thewindow, the edective thickness of the window increases rapidly to a degree Where it may suer in excess of 20 percent loss of X-ray transmission value within a radial spacing of 5 inches from the axis of the system.l It can likewise be shown that similar transmissionV losses occur across the area of a dat window as occur in the case of a hemispher-ical window;

The improvementsrcontributed by the present inventionceuter particularly about a preferred form of window 12 whichovercomes many deficiencies of prior art structures and imparts to the device under consideration-very desirable-properties. In accordancewith the invention, the thickness of Vwindow 12 varies inversely Vwith distance measured radially from axis a-a of cylindrical portion 1t). in order toV provide a constant. transmission path length Vfor any incident rays issuing from an external source X disposed along axis a--a. The thickness of the window varies such that the divergent X-raysV originating from source X see equal glass thickness across the entire effective window diameter. Thus the physical thickness of the window decreases with increasing distance from the axis of the window. The preferred window coniiguration can beexpressed by the following geometric function:

Vda=actualV window Vthickness d=window thickness at-the axis of the tube R=radius of curvature of outer surface of Window r=radial spacing of the point of incidence for X-rays from the axis of system s L=distance from the X-ray source to outer window surface along axis. Y

A Vpickup window constructed in the manner of this invention satisfies the foregoing equation and provides an approximately constant transmission value for all incident X-rays. Thus, the brightness distortion experienced with prior structures, featuring a wiudowof uniform thick- While a particular embodiment of the invention has been shown and described, it is apparent that various .changes and modifications maybe made, and it is therefore intended in the following claims to cover all such `modificationsand changes as may fall within vthe trueV yspirit andi scope of this invention.`

I claim:

1. An envelope for an image converter comprising: a substantially cylindrical section defining a transmission path for an electron image; an end section aixed to and closing one end of said cylindrical section and having provisions for viewing an image; another end section axed to and closing the opposite end of said cylindrical section including a window portion of a material which is transparent to invisible radiations, which has the configuration of a sector of a sphere, and which has a thickness varying inversely with distance measured radially from the axis of said cylinder to provide a constant transmission path length for'incident rays issuing from an external source disposed on said axis.

2. An envelope for an image converter comprising: a substantially cylindrical section delining a transmission path for an electron image; an end section aixed to and enclosing one end of said cylindrical section and having provisions for Viewing an image; another end section aixed to and closing the opposite end of said cylindrical section including a window portion of a material opaque to visible radiation but transparent to invisible radiations, which has the configuration of a sector of a sphere, and which has a thickness varying inversely with distance measured radially from the axis of said cylinder to provide a constant transmission path length for incident rays issuing from an external source disposed on said axis.

3. An envelope for an image converter comprising: a substantially cylindrical section defining a transmission path for an electron image; an end section atiixed to and closing one end of said cylindrical section and having provisions for viewing an image; another end section aiixed to and closing the opposite end of said cylindrical section including a window portion of glass which is transparent to X-rays, which has the configuration of a sector of a sphere, and which has a thickness varying inversely with distance measured radially from the axis of said cylinder to provide a constant transmission path length for incident rays issuing from an external source disposed on said axis.

any@ X da=d cos arc tan where dg=actual window thickness d=window thickness at the axis of the tube R=radius of curvature of outer surface of window r=radial spacing of the point of incidence for X-rays from the axis of system L=distance from the X-ray source to outer window surface along axis.

References Cited in the le of this patent UNITED STATES PATENTS 2,060,977 De Boer et al Nov. 17, 1936 2,153,614 Coeterier et al. Apr. 1l, 1939 2,198,479 Langmuir Apr. 23, 1940 2,247,409 Roper July l, 1941 2,641,723 Farnsworth June 9, 1953 2,807,737 Wright Sept. 24, 1957 2,926,239 Skellett Feb. 23, 1960 UNITED STATES PA'IEMPV omer CTIFTE CORRECTINl Patent Noc 299551219 @otober 4W" i960 wurm@ e, Niklas It is hereby certified that error appears in the above numbered pat ent requiring correction and that the -sad Letters Patent shouldread as corrected below.

Column 2,I line ZOX1 for Vsiconv reed f Silicone Signed and Sealed this 29th day of May 19620 (SEAL) Attea:

DAVID L. LADD ERNEST w, swIDER ttesting Gfcer Commissioner of Patents

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