US3562516A

US3562516A - Image pickup tube with screen and field grids

Info

Publication number: US3562516A
Application number: US759669A
Authority: US
Inventors: Lucien Francis Guyot
Original assignee: THOMSON HOTCHKICS BRANDT COMP
Current assignee: FRANCAISE THOMSON-HOTCHKICS BRANDT Cie; THOMSON HOTCHKICS BRANDT COMP
Priority date: 1967-09-28
Filing date: 1968-09-13
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09
Also published as: DE1762900B2; DE1762900A1; FR1544839A

Abstract

The acceleration and focusing means located between a photocathode and a secondary emission induced conductivity target include a grid located and electrically energized to reduce the speed of electrons impinging on the target to a value providing for optimum secondary emission from the target, thus assuring low electron transit times and high resolution.

Description

United States Patent Inventor Lucien Francis Guyot Paris, France Appl. No. 759,669 Filed Sept; 13, 1968 Patented Feb. 9, 1971 Assignee Compagnie Francaise Thomson-Hotchkiss Brandt Paris, France a corporation of France Priority Sept. 28, 1967 France IMAGE PICK-UP TUBE WITH SCREEN AND FIELD GRIDS 1 Claim, 3 Drawing Figs.

US. Cl 313/65, 315/1 1 Int. Cl ..H0lj 31/28, H01 j 3 1/38 Field of Search 313/65 [56] References Cited UNITED STATES PATENTS 2,452,619 11/1948 Weimer 313/65 2,460,093 1/1949 Law 313/65 2,723,360 11/1955 Rotow 313/65 2,871,368 1/1959 Bain 313/65X 2,983,836 5/1961 Rudnick et al.... 3 l3/65X 3,303,373 2/1967 Alting-Mees 313/65 FOREIGN PATENTS 1,137,910 12/1968 Great Britain. 1,276,084 8/1968 Germany.

Primary Examiner-Robert Sega] Attorney-Stephen H. Frishauf ABSTRACT: The acceleration and focusing means located between a photocathode and a secondary emission induced conductivity target include a grid located and electrically energized to reduce the speed of electrons impinging on the target to a value providing for optimum secondary emission from the target, thus assuring low electron transit times and high resolution.

PATENTEUFEB sum 35 21515 sum 2 UF 2 IMAGE PICK-UP TUBE WITH SCREEN AND FIELD GRllDS The present invention relates to image pickup tubes, and more particularly to tubes having targets of the secondary emission induced conductivity type, which are read out by means of a slow electron beam, and which have high resolution.

Image pickup tubes of the type with which the present invention is concerned, usually are arranged, in the order of the longitudinal axis of the tube, to have first an emissive cathode, emitting electrons as a function of the local density of impinging electromagnetic or corpuscular radiation, and forming the primary image to be scanned. If the radiation is not of the type which provides for direct excitation of the cathode, an inter mediate luminescent image converter may be provided, for example by means of a fluorescent screen joined to the cathode. Further, the tube includes acceleration and focusing means, usually forming electrostatic lenses, in order to focus the electrons on a target. The target, of the secondary emission induced conductivity type, is usually formed by an insulating material having a high secondary emission coefficient, located on a metallic membrane which is permeable to highly accelerated electrons. The membrane is oriented to be directed towards the cathode. It has a high potential. with respect to the cathode. It has a high potential, with respect to the cathode, connected thereto, usually in the range of from to kilovolts. The membrane, itself, may be supported at the side facing the cathode, by an insulating member which is also permeable to fast electrons.

in addition, the readout tube comprises an electron gun providing a scanning electron beam, directed towards the porous layer of the target, and scanning thereover.

in operation, the electrons emitted from the cathode cross the metallic membrance of the target and liberate, within the porous layer, a large number of secondary electrons which are directed towards the membrane, leaving the porous layer with a charge image, thus fomiing an electrical image corresponding to the primary image reaching the photocathode. The scanning electron beam, formed of slow electrons, restores the potential and charge gradients formed on the target, thus causing within the supply conductor to the membrane the output signal current.

Tubes of the type described function satisfactorily so long as the acceleration and focusing system is comparatively short, for example a few centimeters. The diameter of the cathode is thus limited to values of the same order of magnitude. It is desirable, however, to increase the diameter of the cathode in order to improve resolution and sensitivity of the tube, or to increase its field of vision, a feature which is particularly important in X-ray luminescent amplifiers. It has been found, however, when the diameter of the cathode is increased, thus necessitating increase of the overall dimensions of the acceleration and focusing system, the resolution of the tube deteriorates. it is believed that this is due to the fact that the electrons coming from one spot of the cathode produce as an image at that point, a spot on the target which is larger than the spot from which they were derived. This phenomenon appears to be due to the fact that the initial speed of the electron is distributed at random in all spatial directions, so that radial components of speed will be present, thus increasing elemental areas of electron bundles directed towards the target, during the transit time of the electrons. The increase in transit time of the electrons, as a result of the increase in distances, may theoretically, be compensated by increase in the accelerating potential. In electrostatic systems, however, even using magnetic focusing, the transit time varies as the inverse of the square root of potential. Thus, in a system operating, for example, at an ordinary potential of 7 kv., doubling of the geometric dimensions requires an increase in voltage to 28 kv. in order to retain the same transit time for the electrons. lf magnetic focusing is then used, the weight and auxiliary equipment of the entire tube assembly will become prohibitive.

In addition, increase of the accelerating potential has as a result a decrease in the efficicncy of the target, which may be so great as to be inadmissible in actual practice. ln effect. the materials which can be used for the porous layer of the target have a maximum net coefficient of secondary emission with respect to incident energies of electrons which are much less than l0 Kev.

It is an object of the present invention to provide an image pickup tube of the type described which permits use ofa large area photocathode without requiring heavy auxiliary equipment or focusing structures and providing images of high resolution.

Subject matter of the present invention: Briefly, the acceleration and focusing arrangement of the tubes include, as the last electrode, a grid, hereinafter termed a field grid, having a transparency of at least 60 percent, and presenting a structure which is sufficiently fine and so located that it is at a distance sufficiently small from the target so that when a dif' ference of potential between the grid and target is applied. an essentially homogeneous field is produced between target and grid.

In operation of the tube, the accelerating potential of a value desired for the accelerating and focusing system is applied to the grid, whereas the target is carried with respect to the cathode at a potential chosen to provide optimum secondary emission from the porous layer.

in accordance with an embodiment of the invention, an additional grid, termed a screen grid, can be placed between the first mentioned, field grid and the target, and spaced closely from the target. it is connected to a potential which is similar to that of the target, or close thereto; it protects the target from the electrical field resulting from the difference in potential between grid and target itself.

The structure, organization, and operation of the invention will now be described more specifically with reference to the accompanying drawings, wherein:

FIG. 1 is a longitudinal cross section, somewhat schematic view of an entire tube in accordance with the present invention;

FIG. 2 is a greatly enlarged schematic view of the field gridtarget assembly of the tube of HO. 1; and

FIG. 3 is a different embodiment of the assembly of F K}. 2.

The tube generally designated with I has an image pickup section 2 and a readout section 3, the entire assembly being placed in an evacuated envelope 4.

The image pickup area forming face 5 of the envelope is connected to the remainder of the tube, which may be of glass, by a pair of annular metallic seals, 6,7, to support a photocathode 8. Photocathode 8 emits photoelectrons having a localized density corresponding to localized brightness of an image projected thereonto. An assembly of electrodes 9,10,11, accelerates and focuses the image on a secondary emission induced conductivity type target 12. Target 12 is supported by a ceramic collar 13. Collar 13 is, itself, maintained in position within the envelope 4 by means of a group of support stems, of which two, 14 and 15, can be seen on FIG. 1. Target 12, and the far end of electrode 11 adjacent thereto are shown in greater detail in FIG. 2, where the representation is schematic and without considering the relative sizes and proportions of the various elements. As an example, target 12 may be formed by a support membrane 16 made of alumina of a thickness of from 0.05 to 0.1 microns, an aluminum layer 17 of, for example, 0.03 microns in thickness, and a layer 18 of potassium chloride which is very porous, and having a thickness of from 15 to 20 microns. Layer 17 is the signal electrode of the tube. its supply conductor is one of the support stems, for example stem 14 as shown.

Electrode 11 carries, at its side closest to the target, a diaphragm the opening of which is covered by a mesh grid 19. Mesh grid 19 is formed of wires 20, of copper or nickel, and very fine, for example having a diameter of from 5 to 8 microns with a spacing p of from 25 to 50 microns to provide a mesh of from 20 to 40 mesh per millimeter. The transparency of such a grid will be greater than 60 percent. Grid 19, which forms the field grid of the system is located from target 12 by a distance d of from 5 to l millimeters.

As seen in FIG. 1, cathode 8, to which the first electrode 9 is connected, electrodes 10 and l l as well as electrode 17 of the target 12 are connected to tap points of a potentiometer 21, supplied by a source of potential 22. A resistance 23, inserted in the supply line from electrode 17 to its connection to the potentiometer 21, provides a dropping resistor for a signal output, taken off lead 24. Electrodes 10, 11 and signal electrode 17 are placed, for example, with respect to the potential of the cathode 8, at voltages of +2.5 kv +25 kv, and +10 kv.

Section 3 of the tube contains an electron gun, formed of a thermoemissive cathode 25, followed by a control electrode 26 and positively connected electrodes 27, 28. Electrode 28 extends in form of a hollow cylinder and terminates at a fine mesh 29 forming a grid, and located close to the target 12. A magnetic coil 30, located around the exterior of envelope 4 provides a magnetic field in an axial direction, cooperating with the electrostatic lens elements formed by electrodes 26,27,28 so that the electrons emitted from the cathode 25 form a very narrow, fine pencil beam which is directed almost perpendicularly onto the target 12. A coil 31 is arranged to provide for alignment of the electron beam with the axis of the assembly. The electrodes are connected to a potentiometer 32, which is supplied by a source 33. An intermediate point 39 on potentiometer 32 is connected to a junction 34 which also forms the other terminal of resistance 23, further connected to ground, across which the output potential appears. The ground point interconnects the supply of the input section 2 and the readout section 3. For example, cathode 25 may be carried at a potential of l0 v with respect to the target, and electrodes 26, 27, 28 may have, respectively, potentials of 60 v, +270 v, and +300 v with respect to cathode 25. Two pairs of deflection coils, of which 35, 36 only are seen on the drawing provide for scanning and deflection of the electron beam readout across target 12.

The electrons emitted by the photocathode are thus accelerated into the region of the target by a very intense electric field. A powerful acceleration system may be used, and thus the diameter of the photocathode may be substantial, for example in excess of 20 cm. without deterioration of quality of the secondary image on the target due to transit time effects. The electrons, after having received an energy of from 20 to 30 Kev. are, however, slowed upon passing through the field grid 19 and reach the target 12 with an energy appropriate to provide maximum secondary emission therefrom, that is, with an energy of from about to Kev. The target may thus present the usual, customary diameter of several centimeters only, the size of which is determined by its fragile structure. The deceleration field between grid and target is substantial, and practically homogeneous, due to the fine structure of the mesh grid and the small distance form the target itself. Each elementary bundle of electrons emitted from any one point of the photocathode is thus focused.

Little experimentation or adjustment is necessary in order to determine the optimum relative values of distance and potential differences between the mesh grid and the target so that a sharp image of high resolution is projected on the target. it also appears that the strong decelerating field between grid 19 and target 12 improves the emission of secondary electrons from the face of target 12 impinged by the electrons.

Referring now to H6. 3, an additional grid 37 which may be termed a screen grid may be located between the field grid 19 and target 12. Grid 37 is supported by a metallic collar or ring, located in a support ring 13 of the target and spaced by a distance d. of from about 0.2 to 1 mm. from the target. The current supply conductor may be one of the support stems, for example, stem 15. Just like the field grid 19, screen grid 37 may be a mesh grid made of fine wires from 5 to 8 microns in diameter, but located at a wider distance apart, for example between 50 to microns so that a mesh of from l2 to 20 mesh per millimeter will be formed, resulting in a greater transparency, preferably in excess of 80 percent. It is not necessary that the electric field between grid 37 and target 12 is as even and homogeneous as that caused by the field grid, since the screen grid is connected to a potential which is close to that of the target, or even the same; the trajectory of the electrodes which pass the grid thus are not substantially modified.

Screen grid 37 places target 12 into the shadow of an electrostatic field if there is a potential difference therebetween. lntroduction of the screen grid is advantageous if the field between field grid 19 and target is high, and particularly if the field has a physical effect on the target. which may eventually cause an internal short circuit or arc-over or a mechanical deformation of the target which may cause its destruction, particularly when, during use of the tube, shocks or mechanical vibrations may be expected. The screen grid further collects secondary electrons emitted from the target surface sub ject to impingement by the electrons from the photocathode. Additionally, the screen grid permits construction of a thinner target structure-as seen in FIG. 3in which target 12 merely consists of a signal pickup plate 17 fonned of an aluminum membrane of 0.07 micron thickness, which supports the porous layer 18 of potassium chloride.

Placing a fine mesh grid thus permits manufacture of tubes having large diameter while maintaining high resolution of readout.

The invention has been described particularly with respect to an electron tube in which a cathode emits electrons as a function of localized brightness and density patterns of electromagnetic, or corpuscular radiation impinging on the target face. It is, however, understood that the present invention is equally applicable to tubes responsive to other types of radiation, and that various changes and modifications may be made in the structure and arrangement of the tube in accordance with specific uses, within the inventive concept.

I claim:

1. An image pickup tube comprising a photoelectric cathode (8) emitting electrons as a function of impinging radiation in accordance with the image;

a porous storage target (12) of the secondary emission induced conductivity type;

means scanning reading electrons over said target;

means to accelerate and focus the electrons including:

a fine mesh field grid (19) having a transparency of at least Q0 percent and closely spaced from said target (12), said field grid being connected to a source of potential of such magnitude relative to the spacing of the grid that the electrical field between grid (19) and target (12) is substantially uniform and a screen grid (FIG. 3:37) having a transparency of more than 80 percent located between the target (12) and said first grid (19), said screen grid being located close to said target, said field grid (19) being formed of wires of from 5 to 8 microns diameter, having a mesh spacing of from 25 to 30 mesh per millimeter and located from 5 to 10 millimeters from the target; said screen grid being located from the target by a distance of from 0.2 to l millimeter, and formed of wire of from 5 to 8 microns diameter, having a mesh spacing of from 12 to 20 mesh per millimeter.

Claims

1. An image pickup tube comprising a photoelectric cathode (8) emitting electrons as a function of impinging radiation in accordance with the image; a porous storage target (12) of the secondary emission induced conductivity type; means scanning reading electrons over said target; means to accelerate and focus the electrons including: a fine mesh field grid (19) having a transparency of at least 60 percent and closely spaced from said target (12), said field grid being connected to a source of potential of such magnitude relative to the spacing of the grid that the electrical field between grid (19) and target (12) is substantially uniform and a screen grid (FIG. 3:37) having a transparency of more than 80 percent located between the target (12) and said first grid (19), said screen grid being located close to said target, said field grid (19) being formed of wires of from 5 to 8 microns diameter, having a mesh spacing of from 25 to 30 mesh per millimeter and located from 5 to 10 millimeters from the target; said screen grid being located from the target by a distance of from 0.2 to 1 millimeter, and formed of wire of from 5 to 8 microns diameter, having a mesh spacing of from 12 to 20 mesh per millimeter.