US3577027A

US3577027A - Low noise image intensifier

Info

Publication number: US3577027A
Application number: US756443A
Authority: US
Inventors: Constantin S Szegho
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1968-08-30
Filing date: 1968-08-30
Publication date: 1971-05-04
Anticipated expiration: 1988-05-04
Also published as: GB1219737A

Abstract

An image intensifier of otherwise conventional construction is improved by an inwardly directed extension of the envelope, projecting from one edge of the focus electrode and extending toward the anode. The focus electrode is a conductive coating on the internal surface of the envelope and is, in effect, extended by a conductive coating applied to the inwardly facing surface of the envelope extension. That extension terminates in a metal ring. With the structure, the surface separation or leakage path between the metal ring termination of the focus electrode and the anode is substantially greater than that otherwise existing between those electrodes. The increased separation of these electrode elements along the surface of the envelope minimizes the tendency toward field emission and its contribution to background noise. A high resistance coating or an electron shield protects the shoulder of the envelope from electron bombardment and eliminates that source of background noise.

Description

United States Patent [72] Inventor Constantin S. Szegho Chicago, Ill. [21 Appl. No. 756,443 [22] Filed Aug. 30, 1968 [45] Patented May 4, 1971 [73] Assignee Zenith Radio Corporation [54] LOW NOISE IMAGE INTENSIFIER 8 Claims, 4 Drawing Figs.

[52] US. Cl 313/101, 250/213, 313/65, 313/313 [51] Int. Cl ..H0lj 39/00, 1-l0lj H52 [50] Field of Search 313/64, 65, 67, 74, 82, 94, 101, 102, 58, 59, 204, 313; 250/213 [56] References Cited UNYTED STATES PATENTS 3,026,437 3/1962 Niklas 313/65 3,225,204 12/1965 Schagen 313/65 3,280,356 10/ 1966 Stoudenheimer 313/65 Primary ExaminerJames W. Lawrence Assistant Examiner-David OReilly Attorney-Francis W. Crotty ABSTRACT: An image intensifier of otherwise conventional construction is improved by an inwardly directed extension of the envelope, projecting from one edge of the focus electrode and extending toward the anode. The focus electrode is a conductive coating on the internal surface of the envelope and is, in effect, extended by a conductive coating applied to the inwardly facing surface of the envelope extension. That extension terminates in a metal ring. With the structure, the surface separation or leakage path between the metal ring termination of the focus electrode and the anode is substantially greater than that otherwise existing between those electrodes. The increased separation of these electrode elements along the sur' face of the envelope minimizes the tendency toward field emission and its contribution to background noise. A high resistance coating or an electron shield protects the shoulder of the envelope from electron bombardment and eliminates that source of background noise.

PATENTEU M 4m 3577.027

'00. HQ ||b Ob all Constantin S. Szegho Attorney LOW NOlSlE IMAGE HNTENSIFIER BACKGROUND OF THE INVENTION The present invention is directed to image intensifiers and is most particularly concerned with improvements in their noise properties. Obviously, any image converter or translating device benefits from improved noise properties so the invention may be utilized with any type of image intensifier whether it be arranged to respond to visible light, X-ray, infrared, gamma, neutron energy or any other form of excitation.

The invention is particularly valuable in the field of nuclear medical examination in which radioisotopes energize an image converter the response of which is in the form of scintillations of an input layer such as a cesium iodide crystal. To obtain a useful output it is necessary that the scintillations be integrated and the need for integration makes clear that a high noise ambient within the intensifier will destroy its usefulness.

The causes of background noise in an image converter are manifold but principal offending phenomena are undesired emissions within the device.

In the usual converter the focus electrode which, in conjunction with the anode, focuses and accelerates an electron image to impingeupon a fluorescent output screen is a conductive layer on the internal surface of the envelope. This is a very thin layer and the edge which faces the anode assembly is generally jagged. The anode, which also includes a conductive layer on the inner surface of the envelope, is at a very high potential relative to the focus electrode and consequently if the adjacent edge of the focus electrode is, in fact, jagged, conditions exist that are favorable to field or cold emission. Where fieldemission is experienced, the electrons released may find their way directly to the output screen and contribute noise to the image or they may bombard the glass envelope producing a fluorescence. The resulting light may reflect onto the photoemissive cathode, giving rise to further electron emission which represents additional contributions to background noise.

One previous effort to improve matters in these regards is the subject of U.S. Pat. No. 3,026,437, issued Mar. 20, 1962, in the name of W. F. Niklas and assigned to the assignee of the present invention. lt features the application of a high resistance semiconductive coating, such as chrome oxide, to the internal portion of the envelope which intervenes the focus electrode at one side and the wall coating of the anode structure on the other. The coating overlaps both of these electrode elements and establishes a uniform potential gradient therebetween which tends to suppress cold emission. It has been found, however, that if the conductive coating is carelessly applied it may crumble or flake and introduce undesired particulate matter within the tube envelope. lt is also found that if the resistance of the coating is not uniform throughout, or if it should age nonuniforrnly in the coated area, the electron optics of the intensifier may become impaired. The present invention is a distinct improvement, offering the possibility of greatly enhanced freedom from noise and freedom from unwanted particulate foreign matter within the envelope.

Accordingly, it is a principal object of the invention to improve the background properties of an electron discharge device, specifically, an image converter.

It is a very particular object of the invention to provide an image intensifier that has exceedingly low background noise and, therefore, is especially suited for nuclear medical systems.

SUMMARY OF THE lNVENTlON An image intensifier typically comprises a cylindrical envelope of insulating material having an electron-image source at one end, an image screen at the opposite end and a focus electrode on the internal surface of the envelope intermediate such ends. An anode structure is located adjacent the image screen and includes a conductive band on the envelope spaced from the focus electrode. in accordance with the invention such an image converter is improved by the provision of an inwardly directed extension of the envelope projecting from the edge of the focus electrode that faces the image screen in order to increase the surface separation or leakage path of the focus electrode and the conductive band of theanode structure.

In one aspect of the invention, the envelope extension projects toward the anode structure and terminates in a conductive ring. A conductive coating on the inwardly facing surface of the envelope extension causes this extension to be, in effect, a portion or continuation of the focus electrode.

In another aspect of the invention, the shoulder portion of the envelope which usually leads from the location of the focus electrode to a reduced diameter section which accommodates the image screen is coated with a thin film of calcium fluoride. This coating serves much the same purpose as the chrome oxide coating of the prior art described above but is not subject to crumbling and avoids the possibility of contributing unwanted particulate matter within the tube envelope.

DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing,-in the several FIGS. of-which like reference numerals identify like elements, and in which:

FIG. 1 is a view in cross section of an image intensifier constructed in accordance with the invention to exhibit improved noise properties; and

H65. 2, 3 and 4 are various modifications of the image intensifier.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS The image converter of FIG. 1 comprises an evacuated envelope formed of insulating material, usually glass, and cylindrical in shape having a uniform diameter throughout most of its length. The envelope may have any of a variety of shapes but, as shown, it has two

principal sections

10a and 10b which terminate in Kovar metallic flanges 11a and 11b which may be heliarc welded to join the envelope sections into an integrated enclosure. At one end, the envelope has a'closure 10c which, of course, must be transparent to the radiation to which the image converter is to respond. Typically, it is formed of the same material as the remainder of the envelope. At the other end the envelope is closed by a reentrant press 10d which is a reduced diameter section being connected with section 10b through a shoulder portion 10a.

An electron-image source is located at the large end of the envelope. This is a pickup screen 12 which approximates a sector of a sphere and is so oriented that its concave surface faces toward the reentrant portion 10d of the envelope. The pickup screen is a multilayer component having a sphericallyshaped support member of aluminum or other material that is transparent to the radiation for which a response is to be obtained, here assumed to be X-ray simply for convenience of illustration. Over the support is a layer which responds to that radiation and emits light to activate a final layer which is a photoemitter. These two layers are frequently separated by a barrier layer for the purpose of preventing undesired chemical interaction therebetween. The present invention does not concern the structure of thepickup screen and suitable arrangements of this component are well known in the art. Suffice it to say that for an X-ray converter, the X-ray image sensitive layer is usually phosphor, such as silver-activated zinc sulfide, but for a nuclear converter the first layer may be formed of cesium iodide. The barrier layer, if employed, may be aluminum oxide and the photoemissive top layer is generally an antimony-cesium composition. The diameter of the pickup screen is as large as practical considerations permit.

At the opposite end of the envelope, there is an image screen which usually is a fluorescent coating l3 applied to the glass closure plate of the reentrant envelope section d. The

pose of accelerating an electron image in the direction of screen 13 and focusing the image to reach that screen by way of an aperture in the anode structure. This electron system accordingly includes a focus electrode 114 in the form of a conductive metallic coating on the internal surface of envelope section 10b. It is usually a film of copper or aluminum. One end of focus electrode M extends to metal flange lllb and facilitates the application of a focus potential to that electrode. This is a-low potential of the order of a few hundred volts. The other end of the focus electrode extends to an inwardly directed extension of the envelope to be discussed more particularly hereafter.

' The anode structure comprises a metallic electrode lb which has a cylindrical portion lea having a spherical termination with a centrally disposed aperture Mb. The cylindrical section leads to a conically-shaped section which terminates in a skirt portion lltic of suitable dimension to be accommodated by mounting over the reentrant return portion 10d of the envelope. The convex end face of anode cap 16a faces the concave surface of pickup screen 112 and the anode, as apparent in FIG. 11, encloses viewing screen l3 and is electrically connected to the conductive backing layer I30 thereof so that the screen is maintained at the same electrical potential as the anode. A portion of reentrant section llla' has a conductive band or coating led which is in circuit connection with anode 116 by means of contact springs or fingers the. This facilitates the application of anode, potential from a lead shown schematically at l7 which penetrates the reduced diameter section of the envelope and connects with conductive band Md.

The structural details of the image converter as thus far described may be entirely conventional and will be familiar to those skilled in the art. In operation, an image of X-radiation is focused on pickup screen 112 where it is converted to an electron image that is accelerated toward anode to. Focus electrode M, in conjunction with anode l6, constitutes an immersion lens for the purpose of focusing an electron image originating at pickup section 112 through anode aperture lob onto viewing screen 113. While in prior structures the focus electrode coating Ml extends far enough on the internal surface of envelope llllb to overlapanode l6, it will be apparent from FIG. I that the wall coating 14 of the focus electrode need not extend that far with the structure under consideration.

This difference from the usual prior art converter is involved in the structural change introduced in the practice of preferred forms of the present invention for improving the noise background of the converter.

More specifically, the wall coating It extends to an inwardly directed extension 20 of envelope section 10b which projects in the direction of anode 16. It may be likened to an inner wall of envelope section I012 similar to and spaced inwardly of and concentric with the portion of shoulder Illa that leads to the uniform diameter section 10b of the envelope. Envelope extension 20 terminates in a conductive ring 21 which, as illustrated, has an axially extending portion which terminates in a flange disposed transversely of the tube axis. The surface of envelope extension 20 that faces inwardly is prov vided with a conductive coating 20a which overlaps or contacts wall coating Ml at one end and conductive ring 211 at the other. This, in effect, causes

elements

20 and 21 to constitute a continuation of focus electrode 141 so that the composite electrode, as in the case of prior art structures, reaches beyond and is spaced from cap portion lea of the anode. It is desirable that envelope extension 20 have a close spacing to the circumscribing section of the converter envelope in order to have the'least disturbance on the focusing field established between

electrodes

14 and 16. In one embodiment of the invention found to most significantly improve the noise background of an image intensifier, the structure had the following significant dimensions: 1 diameter of envelope section 10b 9-29/32 inches length of conductive coating on envelope section 10b 6- 13/16 inches shoulder radius of envelope section lltle 2-% inches radius of envelope extension 20 inches axial length of ring 211 inches separation of ring 21 from envelope section We diameter of image screen 13 2 inches With this construction, the separation from one edge of conductive band led of the anode structure along the internal surface of the envelope to the adjacent edge of the focus electrode, specifically to ring 21 of this electrode, is much greater than that found in prior structures. In the first place, wall coating 14 is foreshortened with respect to the previous practices in the art and terminates prior to and, therefore, in nonoverlapping relation with the spherical end 16a of anode 16 whereas usually the wall coating extends farther in the direction of the anode to be in encompassing but spaced relation thereto. Additionally, the uncoated and outwardly facing surface of envelope extension 20 adds to the surface separation of these electrodes providing an enlarged gap having the dimension indicated by arrow S. The increased surface separation affords a major improvement in suppressing the tendency to field emission for a given potential difference between anode l6 and focus electrode 14.

Further and significant improvement in this regard results from terminating the focus electrode in conductive ring 211 which, even in the form represented in FIG. Limproves matters with respect to field emission when compared with the edge otherwise presented by wall coating 14. Typically, such a conductive coating has a thickness of the order of 10,000 A. and also it is generally ragged. If ring 21 is formed of metal having a thickness of one-sixteenth inch, and has a flange radius of one-sixteenth inch, its edge dimension is large compared with that otherwise presented by wall coatingl l and it may readily be processed to have a smooth surface with no raggedness. This, too, is a substantial improvement in suppressing the tendency to field emission.

Still further improvement is obtained by the specifically different configuration of ring 21, shown for. example in FIG. 2 where the ring has a smoothly curved leading edge rather than the angular configuration of FIG. l.

While it is distinctly preferable, based on present experience, to terminate envelope extension 20 with conductive ring 2i, this may not be necessary and may be omitted in image intensifiers that do not have the severe background noise requirement of converters constructed for use in nuclear medical systems. By way of illustrating the difference in noise properties required in differing applications'of image converters, it may be noted that a background level of 0.001 foot Lamberts is considered not only acceptable but quite good for X-ray image intensifiers whereas an improvement of nearly I00 fold, which may be obtainedwith the structures of FIGS. 1 and 2 including conductive ring 21, is required for nuclear intensifiers. For the X-ray and similar applications where there is more tolerance for noise background, ring 21 may be omitted and the focus electrode continued simply by the coating 20a of envelope extension 20.

The discussion thus far has concerned itself principally with structural improvements which enhance the noise properties of the converter by suppressing tendencies toward field emission. As explained above, a contribution to noise background may be made if electrons bombard the envelope and produce fluorescence. This is most apt to occur by electrons impinging on shoulder portion We of the envelope and may be suppressed or eliminated by the use of a wall coating 25 extending A; inches from conductive ring 16d of the anode structure in the direction of the focus electrode although it will not, as a practical matter, reach the focus electrode structure. Coating 25 may be of chrome oxide as disclosed in the above-identified Niklas patent in which case it would be painted in place. Alternatively and preferably, in accordance with the present invention, it is a thin evaporative film of calcium fluoride. This not only protects against fluorescence but is beneficial in that it is not susceptible to flaking or peeling. Because of its controllable conductivity, it may also facilitate the discharge of surface charges that could otherwise be present on shoulder section l0e. Of course, the calcium fluoride coating may be substituted directly for the chrome oxide coating in image intensifier structures of the type shown in the Niklas patent.

The modification of FIG. 3 features the use of an electron shield 26 that is mechanically fastened to ring 21 by mounting screws 26a. It extends toward anode l6 and is conical in shape, serving to protect envelope section we from electron bombardment. Where such a shield is used, wall coating 25 may be omitted. Since the shield is removably attached to ring 21, it is not installed until all other processing steps for en velope section 10b have been completed. By installing the shield as the last step of the process, one has the advantage of greater access for inserting anode structure 16 into position.

As shown in FIG. 4, one may employ a conical conductive piece 28 as the counterpart of the coating 20a on envelope extension 20 and shield 26 of the embodiment of F l0. 3. In such case, the enlarged end of member 28 is slotted to form resilient fingers and its edge is configured to snap over and processing the section of the converter which includes image screen 13, the focus electrode and the anode structure. Of course, it is necessary that the coating 14 on the internal surface of envelope section 10b extend into the recess defined by the configuration to envelope section 20 in order that insert 28 may be in circuit connection with wall coating 14.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

lclaim:

1. in an image converter comprising a cylindrical envelope of insulating material,

an electron-image source at one end,

an image screen at the opposite end,

a focus electrode on the internal surface of said envelope, and an anode structure adjacent said screen including a conductive band spaced on the internal surface of said envelope from said focus electrode, the improvement which comprises an inwardly directed extension of said envelope projecting from the edge of said focus electrode facing said screen and between said fo'cus electrode and said opposite end, to increase the effective separation of said focus electrode and said anode band.

2. An image converter in accordance with claim 1 in which said envelope extension projects from the edge of said focus electrode toward but in spaced relation to said anode;

and in which the inwardly facing surface of said envelope extension has a conductive coating which connects with said focus electrode so that said envelope extension serves as a continuation of said focus electrode.

3. An image converter in accordance with claim 2 in which a conductive ring, having an edge dimension large compared with that of said focus electrode, is affixed to the free end of said envelope extension and is overlapped by said coating on said envelope extension.

4. An image converter in accordance with claim 3 in which said envelope has a reduced diameter section at said opposite end where said screenand said anode band are located and has a shoulder portion interconnecting said reduced diameter section to the portion of said envelope where said focus electrode is located, and in which said shoulder portion has a coating of high-resistance material to suppress undesired emission attributable to electron bombardment of said shoulder portion.

5. An image converter in accordance with claim 4 in which said coating on said shoulder portion is calcium fluoride.

6. An image converter in accordance with claim 3 in which said envelope has a reduced diameter section at said opposite end where said screen and said anode band are located and has a shoulder portion interconnecting said reduced diameter section to the portion of said envelope where said focus electrode is deposited, and in which a frustoconically-shaped conductive electron shield extends from said ring toward said anode to shield said shoulder portion from electron bombardment.

7. An image converter in accordance with claim 1 in which said envelope has a reduced diameter section at said opposite end where said screen and said anode band are located and has a shoulder portion interconnecting said reduced diameter sectionto the portion of said envelope where said focus electrode is located, and in which a frustoconically-shaped conductive member is secured to said envelope extension and extends toward said anode to constitute a continuation of said focus electrode and an electron shield for shielding said shoulder portion from electron bombardment.

8. An image converter in accordance with claim 3 in which the edge of said focus electrode closer to said anode structure is spaced from said anode structure in the direction of said electron-image source; and in which said envelope extension and said conductive ring are dimensioned so that at least a portion of said ring is spaced from but is in overlapping relation with respect to the end portion of said anode facing said electron-image source.

Claims

2. An image converter in accordance with claim 1 in which said envelope extension projects from the edge of said focus electrode toward but in spaced relation to said anode; and in which the inwardly facing surface of said envelope extension has a conductive coating which connects with said focus electrode so that said envelope extension serves as a continuation of said focus electrode.
3. An image converter in accordance with claim 2 in which a conductive ring, having an edge dimension large compared with that of said focus electrode, is affixed to the free end of said envelope extension and is overlapped by said coating on said envelope extension.
4. An image converter in accordance with claim 3 in which said envelope has a reduced diameter section at said opposite end where said screen and said anode band are located and has a shoulder portion interconnecting said reduced diameter section to the portion of said envelope where said focus electrode is located, and in which said shoulder portion has a coating of high-resistance material to suppress undesired emission attributable to electron bombardment of said shoulder portion.
5. An image converter in accordance with claim 4 in which said coating on said shoulder portion is calcium fluoride.
6. An image converter in accordance with claim 3 in which said envelope has a reduced diameter section at said opposite end where said screen and said anode band are located and has a shoulder portion interconnecting said reduced diameter section to the portion of said envelope where said focus electrode is deposited, and in which a frustoconically-shaped conductive electron shield extends from said ring toward said anode to shield said shoulder portion from electron bombardment.
7. An image converter in accordance with claim 1 in which said envelope has a reduced diameter section at said opposite end where said screen and said anode band are located and has a shoulder portion interconnecting said reduced diameter section to the portion of said envelope where said focus electrode is located, and in which a frustoconically-shaped conductive member is secured to said envelope extension and extends toward said anode to constitute a continuation of said focus electrode and an electron shield for shielding said shoulder portion from electron bombardment.
8. An image converter in accordance with claim 3 in which the edge of said focus electrode closer to said anode structure is spaced from said anode structure in the direction of said electron-image source; and in which said envelope extension and said conductive ring are dimensioned so that at least a portion of said ring is spaced from but is in overlapping relation with respect to the end portion of said anode facing said electron-image source.