US3657596A

US3657596A - Electron image device having target comprising porous region adjacent conductive layer and outer, denser region

Info

Publication number: US3657596A
Application number: US457430A
Authority: US
Inventors: Gerhard W Goetze; Alvin H Boerio
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1965-05-20
Filing date: 1965-05-20
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18
Also published as: BE681355A; DE1295614B; FR1480636A; NL6606827A; GB1137910A

Abstract

This invention relates to such electron image devices as television camera tubes and image intensifier tubes and includes in one illustrative embodiment an electrically conductive member upon which there is disposed a first layer or region of a secondary emissive material deposited in a porous form to allow conduction of the secondary electrons through the voids of the porous material, and a second layer or region of greater density than the first layer disposed upon the first region to inhibit the escape of the secondary electrons emitted within the volume of the first layer.

Description

United States Patent Goetze et al.

11 1 3,657,596 1 Apr. 18, 1972 ELECTRON IMAGE DEVICE HAVING TARGET COMPRISING POROUS REGION ADJACENT CONDUCTIVE LAYER AND OUTER, DENSER REGION Gerhard W. Goetze, Elmira; Alvin H.

[72] inventors:

Boerio, Horseheads Township, Elmira,

both of NY.

[73] Assignee:

sburgh, Pa.

May 20, 1965 Filed:

Appl. No.:

References Cited UNITED STATES PATENTS Westinghouse Electric Corporation, Pitt- I 2,927,254 3/1960 Kazan ..3 13/65 A 3,001,098 9/1961 Sc'hneeberger ....313/l03 X 3,002,124 9/1961 Schneeberger ....313/103 X 3,128,406 4/1964 Goetze et al....,... ..3l3/65 3,197,662 7/1965 Schneeberger ..313/104 3,213,315 10/1965 Lempert ..313/65 A X FOREIGN PATENTS OR APPLICATIONS 715,447 9/ i 954 Great Britain ..3 13/66 879,569 11/1961 Great Britain .;...3l3/65 Primary Examiner-Robert Sega] Attorney-F.1d. Henson and C. F. Renz ABSTRACT This invention relates to such electron image devices as television camera tubes and image intensifier tubes and includes in one illustrative embodiment an electrically conductive member upon which there is disposed a first layer or region of a secondary emissive material deposited in a porous form to allow conduction of the secondary electrons through the voids of the porous material, and a second layer or region of greater density than the first layer disposed upon the first region to inhibit the escape of the secondary electrons emitted within the 2,678,400 5/ 1 954 McKay ..3 13/89 X vomme f the fi t layer 2,757,233 7/1956 Webley ....3l3/65 A 2,905,843 9/1959 Lubszynski ..3 13/65 A 12 Claims, 5 Drawing Figures ELECTRON IMAGE DEVICE HAVING TARGET COMPRISING POROUS REGION ADJACENT CONDUCTIVE LAYER AND OUTER, DENSER REGION Goetze and Boerio, titled Image Storage System, and assigned to the assignee of this invention. The target member therein described comprises a conductive plate made of a suitable material such as aluminum onto which there is deposited a layer of porous material capable of generating secondary electrons in response to a bombardment of high energy primary electrons. The above-mentioned copending application further describes an application of this target member as a storage element in a television camera tube, in which an input radiation image is converted by a 'photocathode element into an electron image and directed onto the target member. The resulting stored image on the target member is read out in the form of a video signal representative of the radiation image directed onto the photocathode element. The video signal may then be displayed on a conventional display device. As described in the above-mentioned copending application, the readout of the information recorded upon the target member is achieved by directing a low energy electron beam upon the target member. Due to the erratic motion of the low energy reading beam, it has been often found necessary to dispose a grid electrode adjacent to the surface of the target member for collimating the electrons into a path substantially normal to the surface of the target member. Typically, a voltage in the range of 200 to 400 volts may be applied to this grid electrode.

in operation, the target member is polarized by applying a potential to the conductive plate and scanning the porous layer with an electron beam emitted by a suitable electron gun. The potential applied to the conductive plate is set at a suitable value of approximately volts positive with respect to the cathode element of the electron gun which is normally placed at ground potential. The electron image (of primary electrons) emitted by the photocathode element is accelerated to an energy (about 10 keV) sufficient to penetrate the conductive plate and to be directed within the porous layer, thereby, dissipating the energy of the primary electrons and creating many low energy secondary electrons. Most of the low energy secondary electrons are directed under the influence of the electric field established by the polarization of the porous layer through the voids of the porous layer and are collected by the conductive plate. The flow of secondary electrons establishes a secondary electron conduction current (i.e., SEC current), which drives the exposed surface of the porous layer progressively positive until the potential of this surface is essentially equal to that of the conductive plate. A pattern of charges may thus be established on the exit surface due to the secondary electron conduction. The intensity of the charges is a function of the number and energy of the primary electrons, the strength of the electric field established by polarization, and the capacity of the storage layer. Further, the resultant pattern of charges corresponds to the image of the primary electrons.

The target member has the further property of emitting from its exit surface secondary electrons which are accelerated by the external electric field established by the collimating grid electrode of the electron gun. The secondary elec trons which are emitted from the target member are designated as transmitted secondary electrons (i.e., TSE). The transmitted secondary electrons serve to increase the potential of the exit surface beyond that potential established upon the conductive plate to an equilibrium value between the potential of the grid electrode and the conductive plate. This equilibrium potential establishes an electric field acrossthe porous layer, opposite in polarity to that of the initial polarization, which is normally great enough to cause breakdown of the porous layer of the target member. Even if the intensity or duration of the bombarding primary electrons is limited to halt this process before breakdown potential of the porous layer is exceeded, the target is still not safe from destruction if the exit surface of the target member is permitted to charge to a value greater than the reflective first crossover of the storage material of the porous layer. In the process of reading out the pattern of charges established on the target member, a low energy electron beam emitted by the electron gun will cause reflective secondary emission from the target member. When this phenomena occurs, the emitted secondary electrons generated by the low energy beam of electrons will be attracted by the high potential placed upon the grid electrode, and the surface of the target member will become increasingly positive due to the loss or subtraction of electrons. The potential of the exit surface of the target member will continue to be driven positive until it is essentially at the potential of the grid electrode disposed adjacent to the target member. However, the high potential applied to the grid electrode and established upon the surface of the target member is normally sufficient to cause breakdown of the storage material of the target member. As disclosed in the above-mentioned copending application, the destruction of the target member may be prevented by the insertion of a second or auxiliary grid between the first grid electrode for collimating the reading electron beam and the target member. A positive potential below the first crossover of the storage material of the target member is applied to the auxiliary grid electrode to thereby prevent the surface of the target member from assuming a voltage which would cause the breakdown of this member.

Because of the relatively low first crossover potential for most practical materials, it is desirable to maintain the auxiliary grid potential at a low potential with respect to the conductive plate of the target member. Under these circumstances, the electric field established between the conductive plate and the auxiliary grid is extremely low so that substantially no transmission secondary electrons (TSE) are emitted from the exit surface of the porous layer, and that the target member is charged essentially due to the operation of secondary electron conduction current (SEC).

Another application of this target member is disclosed in U.S. Pat. No. 3,128,406 by Goetze and Kanter. in this application, the target member is employed in a direct view imaging tube in which an input radiation image such as light is directed onto an input screen and an output image is displayed on an output screen with increased brightness and/or contrast. In the above-mentioned US. patent, the target members (or dynodes) are inserted between the input and output screens to thereby multiply'the electrons emitted by the input screen in response to the input radiation. An auxiliary grid may be inserted in such an image tube to accelerate the emitted secon dary electrons and also to limit the potential to which the surface of the porous layer of the target member may rise. As disclosed in the above-mentioned U.S. patent, a plurality of the target members may be inserted between the input and output screens to thereby repeatedly multiply the electrons emitted by the input screen. In order to direct the electrons from one target member to the next, an increasingly higher or more positive potential is applied to successive target members. It may be understood that unless an auxiliary grid is employed, the target member may be destroyed due to the increasing emission of secondary electrons and the resultant rise of the exit surface of the porous layer toward the potential of the adjacent electrode. It is noted that the establishment of the potential of the exit surface of the porous layer is due primarily to the emission of transmission secondary electrons (TSE). An auxiliary grid is required to limit the potential to which the exit surface of the porous layer may rise. in order to prevent the destruction of the porous layer of the target member, the

auxiliary grid should be held at a voltage such that the electric field thus established between the target member and the auxiliary grid electrode does not exceed about 150 volts per cm.

However, the insertion of an auxiliary grid electrode in either a direct view imaging tube or in a television camera tube has been found objectionable for the following reasons. First, the insertion of such a grid electrode requires not only the incorporation of additional elements and their mounting within the tube, but also the provision of the associated equipment for establishing the grid electrode at the correct potential. Second, the pattern of information to be derived from a target member of such a device will be degraded by the incorporation of such a grid electrode.

Accordingly, it is an object of the present invention to provide an improved image device and target member therefor.

A further object of this invention is to provide an improved electron image device wherein the requirement of an auxiliary grid electrode to maintain the surface potential of the target member of this device below the breakdown and/or first crossover level is eliminated.

A still further object of this invention is to provide an improved target member for an electron image device wherein a surface potential established upon the target member may be controlled by the nature of the structure of the target member itself.

A further object of this invention is to provide a target member for an electron image device which may be produced by a simple manufacturing procedure at a low cost.

Briefly, the objects of this invention are accomplished by providing an electron image device in which an improved target member may be incorporated having the characteristic that the surface potential thereof may be controlled by the structure of the target member itself. More specifically, the target member is constructed having a first porous layer or region of a material having the property of emitting copious secondary electrons and a second region thereon of a denser consistency for reducing the escape probability of transmission secondary electrons from the surface of the target member. In one embodiment of this invention, the regions or layers of the target member may be established as discrete layers; whereas in a second embodiment of this invention, the target member may be constructed as a unitary layer wherein the density of the layer varies from a porous region upon one surface to a denser region upon the other surface.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a camera device embodying this invention;

FIG. 2 is a diagrammatic view of a direct view imaging device embodying this invention; and

FIGS. 3, 4, and 5 are enlarged sectional views of the target members which may be utilized in either of the devices shown in FIGS. land 2.

Referring in detail to the drawings and in particular to FIG. 1, an illustrative embodiment is shown wherein the teachings of this invention may be incorporated within a television camera tube 40. The camera tube 40 comprises an envelope 42 made of a suitable insulating material having one end thereof enclosed by a faceplate 44. The faceplate 44 is designed to be transmissive to the desired radiation from a scene 62 and is made of a suitable material such as glass in the case of a visible light input. A photocathode 45 is provided on the interior of the faceplate 44 and is made of a photoemissive material sensitive to the input radiation such as cesium antimony for a visible light input. An electron gun 50 is provided at the opposite end of the envelope 42 for generating and forming a pencil type electron beam which is directed upon a target member 24. The electron gun 50 is of any suitable type for producing a low velocity pencil-like electron beam and may consist of a cathode element 52, a control electrode 54, and an accelerating electrode 55. The

electrodes

52, 54, and 55 of the electron gun 50 along with a field electrode 56 formed as a coating upon the interior of the envelope 42 provide a focused electron beam which is directed upon the target member 24. Deflection means 58 illustrated as a coil is provided around the envelope 42 for deflecting the electron beam. By application of a suitable potential, the low energy electron beam emitted by the electron gun 50 is scanned over the surface of the target member 24 in a conventional manner. A focusing means 60 illustrated as a coil is also provided about the envelope 42 to provide focusing of the electron beam emitted from the electron gun 50 onto the target member 24. In addition, the focusing means 60 also focuses the photoelectrons emitted from the photocathode 45 onto the target member 24.

The target member 24 is disposed within the envelope 42 between the electron gun 50 and the photocathode element 45. Between the target member 24 and the photocathode element 45, there are provided a plurality of electrodes illustrated as 46 and 48 with suitable potentials provided thereon for accelerating and focusing the electrons emitted by the photocathode element 45 onto the target member 24. Positioned between the target member 24 and the electron gun 50, there is provided a mesh or grid 64 adjacent to and parallel to the target member 24. In one illustrative embodiment of the invention, the grid 64 is made of a fine wire mesh of an electrically conductive material such as nickel and is placed approximately 0.050 inches from the surface of the target member 24. A potential of between 200 to 400 volts is applied to the grid 64 for collimating the electrons emitted by the electron gun 50 into a path substantially normal to the surface of the target member 24.

Further, the target member 24 is mounted within the envelope 42 by a support ring 66 made of a suitable material such as Kovar (a trademark of the Westinghouse Electric Corporation for an alloy of nickel, iron and cobalt). The structure of one suitable target member 24 is further illustrated in FIG. 3. The target member 24 includes a support layer 27 of a suitable insulating material such as aluminum oxide with an electrically conductive member or layer 26 deposited thereon of a suitable material such as aluminum. A continuous, porous layer or region 28 of a suitable insulating, secondary emissive material such as potassium chloride is provided on the conductive layer 26. Other suitable materials for use in accordance with the present invention are barium fluoride, lithium fluoride, magnesium fluoride, magnesium oxide, cesium iodide, and sodium chloride. The layer 28 as will be described in detail later is deposited as a smoke or porous type layer. A second layer or region 30 is disposed on layer 28 and is characterized by the fact that its density is greater than that of the layer 28. Typically, the layer 30 may be made of the same material as that of layer 28; however, other suitable insulating materials may be used.

In the operation of the camera tube shown in FIG. 1, a potential of approximately 15 volts with respect to the cathode element 52 is applied to the conductive layer 26 of the target member 24. The low energy electrons emitted by the electron gun 50 are focused by the field electrode 56 and the focusing means 60 onto the surface of layer 30 of the target member 24. As a result, the surface of the layer 30 is established at substantially ground potential by means of the scanning electrons emitted by the electron gun 50. Further, a radiation image from the scene 62 is projected onto the photocathode element 45 and photoelectrons are emitted from each portion of the photocathode element 45 corresponding to the amount of radiation directed thereon. The photoelectrons emitted by the photocathode element 45 are focused upon the target member 24 by the focus means 60 and are accelerated to a sufficiently high energy by the accelerating

electrodes

46 and 48 to penetrate the insulating support layer 27 and the conductive layer 26. The incident primary electrons emitted from the photocathode element 45 penetrate into the layer 28 thereby generating copious quantities of low energy, secondary electrons within the voids of the porous layer 28. The low energy electrons generated within the porous layer 28 cause the exposed surface of the target member 24 to change its potential locally due primarily to secondary electron conduction across the layer 28 to the conductive layer 26 and due to a finite, but much less significant, emission of transmission secondary electrons from the exposed surface of the layer 30 which are then collected by the electrode 64. Thus, as explained above, a pattern of discrete charges of potential on the exit surface of the layer 30 has been established, which may be sensed or read out by any of the several well known read out techniques. In FIG. 1, there is illustrated a typical vidicon type read out assembly.

Referring now to FIG. 2, there is illustrated an electron image device such as a direct view imaging tube incorporating the teachings of this invention. The electron image device 10 comprises a vacuum tight evacuated envelope 12 made of a suitable material such as glass. The evacuated envelope 12 includes an elongated tubular portion 13 with an input window 14 closing off one end of the tubular portion 13 and an output window 16 closing off the other end of the tubular portion 13. The

windows

14 and 16 are also of a suitable material such as glass capable of transmission of the input and output radiations. On the inner surface of the input window 14 there is provided an electrically conductive coating 18. The coating 18 is transmissive to the input radiation and may be of a suitable material such as tin oxide. A layer 20 of a suitable photoemissive material such as cesium antimony is provided on the coating 18. The inner surface of the output window 16 is provided with a suitable light transmissive electrically conductive coating 22 of a suitable material such as tin oxide and a coating 23 of a suitable fluorescent material such as zinc cadmium sulfide deposited on the coating 22. The photoemissive layer 20 is responsive to a radiation image directed thereon from a scene and emits an electron image corresponding to the radiation image. The fluorescent material of layer 23 emits light in the visible region in response to the electron bombardment.

Positioned between the input window 14 and the output window 16 are a plurality of target members 24. The number of target members 24 desired depends on the intensification and/or enhanced contrast desired and only one may be required in some applications as for simple storage applications. The structure of the target member 24 has been described above with respect to FIGS. 1 and 3.

A suitable accelerating voltage is provided between the photoemissive layer and the first target member 24 by means of a potential source 31 connected between the conductive coatings l8 and conductive layer 26. An accelerating voltage is also provided between the adjacent target members 24 by a suitable potential source 33 connected between the conductive layer 26. A potential source 35 connected between the conductive layer 26 of the last target member 24 and the conductive coating 22 provides the necessary acceleration voltage between the last target member 24 and the output fluorescent layer 23. Further, a suitable focusing system may be provided around the envelope 12 for focusing the electron beams between the electrodes within the envelope 12. In a specific embodiment, the focusing means is shown as a permanent magnet 34 to provide a longitudinal field within the envelope 12.

In the operation of the device as shown in FIG. 2, an image from the scene 15 is.projected onto the photoemissive layer 20 which in turn generates an electron image corresponding to the scene image and the electrons are accelerated by a positive potential of about 4 kilovolts provided by the source 31 to bombard the first target member 24. The electrons emitted from the photoemissive layer 20 are accelerated with suffcient energy to penetrate the insulating support layer 27 and the conductive layer 26 into the region of layer 28 where the incident primary electrons generate copious numbers of secondary electrons. As is set out in the aforementioned U.S. Pat. No. 3,128,406, the porous nature of the layer 28 is a significant factor in obtaining a high gain target member. The electrons emitted from the first target member 24 are then accelerated to the second target member 24 where the electrons forming the electron image are again multiplied. The electrons emitted from the last target member 24 are accelerated by the potential source 35 of about 5 kilovolts to the surface of the layer 23, which in turn emits light corresponding to the radiation image directed onto the input window 14.

A specific example of a suitable target member 24 as shown in FIGS. 1, 2, and 3 and a method of forming such a structure will now be described. In an illustrative method of manufacture, the insulating support layer 27 may be formed by first oxidizing a plate of aluminum and then etching the aluminum plate away to leave the support layer 27 ofa suitable thickness of about 1,000 angstroms. Next, the conductive layer 26 is formed by evaporating aluminum onto the support layer 27 to a depth of approximately 1,000 angstroms. The conductive coating 26 and the support layer 27 are then placed in a bell jar having an atmosphere of approximately I millimeter of a suitable inert gas such as argon or nitrogen. A predetermined amount (such as 25 milligrams) of a suitable material such as potassium chloride is evaporated at a distance of approximately 3 inches onto the conductive layer 27. The evaporation process is carried out at a temperature slightly in excess of the melting point of the potassium chloride. The potassium chloride is evaporated to completion and it is found that the density of the layer 28 is approximately 1 to 10 percent of its bulk density and has a thickness of approximately 20 microns. In order to produce the desired secondary emission of electrons within the voids of the material comprising layer 28, layer 28 should have a mass per unit area in the approximate range of from 25 micrograms per square centimeter to 200 micrograms per square centimeter and a thickness of from 10 to 30 microns. Next, a second layer 30 of a suitable material such as potassium chloride may be deposited by evaporation in an inert atmosphere and at a pressure of approximately 01 millimeters of mercury or less. It is a significant aspect of this invention that a second region or layer 30 of the target member 24 have a denser structure than the region or layer 28 adjacent to the conductive layer 26. In the illustrative method described above, this may be achieved by evaporating the potassium chloride in an atmosphere of substantially reduced pressure as compared to that pressure at which layer 28 was deposited. In order to effectively control the equilibrium potential which may be established on the surface of layer 30 without seriously reducing the performance characteristics of the target, layer 30 should have a mass per unit area in the approximate range of 5 to 25 micrograms per square centimeter and a thickness in the approximate range of 0.5 to 5.0 microns. The density of the layer 30 lies in the approximate range of 10 to 50 percent of its bulk density.

As explained above, the primary electrons that penetrate into the porous or spongy layer 28 create a number of free electrons with low energy. Part of the low energy secondary electrons generated by primary electrons in the porous layer 28 are able to escape from the target member 24 thereby leaving a positive charge on the surface of the layer 30. As this process continues, the surface charge (Q) will continue to rise due to the very high resistivity of the

layers

28 and 30. In addition, a potential difference (V) is developed across the

layers

28 and 30 according to the relationship: V= Q/C, where C is the aggregate capacity of the

layers

28 and 30 relative to the conductive layer 26. It has been observed that increases in the potential difference (V) increases the amount of secondary electron current by enhancing the escape probability for free electrons formed within the layer 28. When a potential difference (V) above a predetermined value is established across the

layers

28 and 30 by the effect described above, conduction through the

layers

28 and 30 takes place since free electrons and/or conduction electrons are made available by the impact of the high energy (5 to l0 KV) write electron beam. This mun-s conduction limits the amotirTci positive charge that can be established upon the surface of the target member 24. Equilibrium between the charging of the surface by secondary emission and discharge by conduction is obtained whenever the secondary emission conduction current equals the transmission secondary emission current. This condition corresponds to a voltage across the

layers

28 and 30 which may be defined as the equilibrium voltage (V,,,,,,).

The equilibrium potential (V of typical target members is found to be higher than the voltage required to cause breakdown of the

layers

28 and 30. As a result, the exit surface of the target member 24 may continue to go positive by the process described above and attain a level exceeding the breakdown of the target member 24. However, the equilibrium potential (VF may be lowered by decreasing the transmission secondary emission current and/or increasing the secondary emission conduction current. According to this invention, this may be easily accomplished by providing a second layer or region of a denser consistency over the very porous layer or region in which the secondary electrons are generated. In terms of the physical phenomena, it may be understood that a denser layer or region may effectively decrease the probability of secondary electrons escaping from the exit surface of the target member and, at the same time, increase the solid state conduction through the target member. This reduces the dependence of the equilibrium potential on the external electric field so that an auxiliary grid to limit the external electric field may be eliminated.

It is noted that it is not necessary to form a target member having two discrete layers as shown in FIG. 3. Instead, a target member 74 such as is shown in FIG. 4 may be provided having a single, secondary emissive layer 72 wherein that portion or region adjacent to a surface 78 abutting an electrically conductive layer 76 has a density of approximately 1 to percent of the bulk density and that region close to an exit surface 80 of the layer 72 has a density of about 30 percent of the bulk density. This type of target structure will not only reduce the amount of transmission secondary electrons escaping from the exit surface of the target member but it will also increase the secondary emission conduction current through the layer 72 which further tends to reduce the equilibrium potential V A specific example of the target member 74 is shown in FIG. 4 and a method of forming such a structure will now be described. An insulating support layer 77 is formed of aluminum oxide and the conductive layer 76 is evaporated thereon as described above. Then the secondary emissive layer 72 of a suitable material such as potassium chloride is evaporated upon the conductive layer 76. Specifically, the material is evaporated at a distance of approximately 3 inches onto the conductive layer 76. The evaporation process is conducted at a temperature slightly in excess of the melting temperature of potassium chloride in a suitable inert gas atmosphere such as argon. Initially, the evaporation is conducted at a pressure of approximately 2 to 3 millimeters of mercury. As the evaporation proceeds, the pressure of the inert atmosphere is continually reduced by pumping out the inert atmosphere so that at the end of the evacuation process, the remaining pressure is in the order of a tenth ofa millimeter of mercury or less. Thus, it may be understood that those portions of the layer 72 initially deposited upon the conductive layer 78 are very porous and that as the process of evaporation continues that the regions of the layer 72 become increasingly more dense.

In addition, it is not necessary to form the second layer of the target member of the same material as the first layer deposited directly upon the conductive layer. In a particular application related to camera tubes as shown in FIG. 1, it may be desired to select a different material having less gain and a first crossover potential (V in reflection greater than that of the material deposited directly upon the conductive layer. Referring now to FIG. 5, a target member 84 is formed with an insulating support layer 87 and a conductive layer 90 deposited thereon. Further, a secondary emissive material similar to those materials described may be evaporated to form a porous layer 88 upon the conductive layer 90. Next, a layer 86 of a suitable electrically conductive material such as aluminum and having substantially percent of its bulk density may be evaporated thereon in a near vacuum condition to form a thin discontinuous coating to a depth of only a few molecules (Le, 10 to 20 angstroms) upon the particles of layer 88. It is noted that the conductive material not only coats the surface of layer 88 but tends to penetrate within the porous layer 88 and coat the voids to thereby enhance the conduction across ,this layer. Such a target member 84 would not only have the characteristic that the equilibrium potential is below the breakdown potential, but may also exhibit the characteristic that the equilibrium potential is less than the first crossover potential in reflection.

Thus, it may be seen that there has been disclosed a target member for an electron image device which has an equilibrium potential below that potential which would cause the breakdown of the target member. Further, it is obvious that the auxiliary grid electrodes required by the devices of the prior art to limit the potential of the exit surface of the target member may according to this invention be eliminated. In addition, there has been shown a target member which may be easily manufactured and incorporated within electron imaging devices without the necessity of incorporating additional potential sources associated with the above-described auxiliary grids.

While there has been shown and described what are presently considered to be the preferred embodiments of this invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. A target member for an electron image device comprising an electrically conductive member, a first region disposed on said member and comprised of a porous insulator material having the property of generating secondary electrons within the volume of said material in response to a bombardment of primary electrons and of conducting said secondary electrons through the voids of said porous insulator material, and a second region disposed on said first region having a density greater than that of said first region but not in excess of 50 percent of the density of said second region material in the bulk form.

2. A target member for an electron image device comprising a conductive member, a first layer being disposed on said member in a porous form and having the property of generating secondary electrons within the volume of said first layer in response to a bombardment of primary electrons, said first layer having a density in the range of l to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the range of 10 to 50 percent of the density of said second layer material in the bulk form to impede the escape of secondary electrons from said target member.

3. A target member for an electron image device comprising an electrically conductive member, a first layer disposed on said member in a porous form with a mass per unit area in the approximate range of 25 to 200 micrograms per square centimeter and a thickness in the approximate range of 10 to 30 microns and having the property of generating secondary electrons within the voids of said first layer in response to a bombardment of primary electrons, and a second layer disposed on said first layer and having a mass per unit area in the approximate range of 5 to 25 micrograms per square centimeter, and a thickness in the approximate range of 0.5 to 5.0 microns, said second layer limiting the probability of escape of said secondary electrons from said first layer to thereby lower the equilibrium potential established upon the surface of said second layer.

lOl032 054l 4. A target member for an electron image device comprising an electrically conductive member, and a layer disposed on said member, said layer having a first surface adjacent said member and a second surface remote from said member, said layer having a density of a value less than 10 percent of the density of the material of said layer in the bulk form at said first surface and the density of said layer becoming progressively greater toward said second surface said layer made of an insulating material having the property of generating electrons in response to electron bombardment and of supporting the conduction of electrons through the voids of said layer.

5. A target member for an electron image device comprising an electrically conductive member, and a layer of insulating material having the characteristic of generating secondary electrons within the volume of said layer in response to the bombardment of primary electrons, said layer disposed on said member in a manner that the density of said layer progressively increases across the thickness of said layer, a first region of said layer adjacent said member having a densi ty in the range of l to 10 percent of the density of said material in the bulk form, a second region of said layer having a density in excess of 10 percent of the density of said material in the bulk form sufficient to thereby impede but not totally suppress the escape of secondary electrons from the layer and to increase the solid state conduction of electrons across said layer.

6. An electrical device comprising a target member including an electrically conductive element, and a layer of secondary emissive material deposited on said conductive element, said layer having a first surface contiguous to said conductive element and a second surface remote from said conductive element, said layer having a density that progressively varies from a minimum value in the range of l to 10 percent of the density of said material in the bulk form at said first surface to a maximum value at said second surface, said maximum value being several times greater than said minimum value of said density; means for directing an electron image onto the exposed surface of said conductive element; and a display means disposed to receive the secondary electrons emitted by said target member and to provide an intensified visible image of said electron image.

7. A target member for an electronic image device comprising an electrically conductive member, a porous first layer disposed on said conductive member and made of an insulating material for generating secondary electrons within the volume of said material in response to a bombardment of primary electrons, said first layer having a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, and a second discontinuous layer disposed on said first layer of an electrically conductive material, said conductive material having a density substantially that of the density of said conductive material in the bulk form to thereby increase the first crossover potential and reduce the equilibrium potential of said target member.

8. An electron image device comprising a target element including a conductive member, a first layer disposed on said conductive member in porous form having a density less than 10 percent of the density of the first layer material in the bulk form and having the property of generating and conducting secondary electrons through the voids of said first layer, and a second layer disposed on said first layer having a density substantially greater than first layer in the approximate range of 10 to percent of the density of the second layer material in the bulk form for limiting the escape of said secondary electrons from said first layer; and means for directing electron through said conductive member and into the voids of said first layer.

9. An electronic camera device comprising a target member including an electrically conductive member, a first layer of a material having the property of generating secondary electrons within the volume of said material in response to a bombardment of primary electrons and being deposited on said conductive member in a porous form having a density less than 10 percent of the density of said material in the bulk form, and a second la er being disposed on said first layer and having a density subs antially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form; a first source for directing primary electrons on said conductive member and into the volume of said first layer to establish a charge pattern representative of said primary electrons; and a second source for directing electrons onto said exposed surface of said second layer to derive an electrical signal therefrom representative of said charge pattern.

10. An image intensifying device comprising at least one target member including an electrically conductive member, a first layer of an insulating material having the property of generating secondary electrons within the volume of said material and being deposited on said conductive member in a porous form having a density in the approximate range of l to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of between 10 to 50 percent of the density of said second layer material in the bulk form; means for directing an image of primary electrons through said conductive member and into said first layer; and display means for providing a visible image in response to secondary electrons received from said target member.

11. A target element for an electron image device comprising an electrically conductive member, and a storage member including a first layer of an insulating material deposited on said conductive member in a porous form with a density in the approximate range of l to 10 percent of the density of said material in the bulk form, said insulating material having the properties of generating secondary electrons within the voids of said material in response to a bombardment of primary electrons and of conducting said secondary electrons through the voids of said insulating material, and means for impeding the escape of said secondary electrons from said storage member and for enhancing the solid state conduction of electrons through said storage member including a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form.

12. A target member for an electron image device comprising an electrically conductive member, and a storage member including a first layer of a porous material deposited on said conductive member with a density in the: approximate range of l to 10 percent of the density of said material in its bulk form and having the property of generating secondary electrons within the voids of said porous material in response to a bombardment of primary electrons, and means for impeding the escape of secondary electrons from said storage member and for increasing the first crossover potential of said storage member including a second, discontinuous layer of a conductive material disposed to depth of a few molecules on said first layer.

Claims

2. A target member for an electron image device comprising a conductive member, a first layer being disposed on said member in a porous form and having the property of generating secondary electrons within the volume of said first layer in response to a bombardment of primary electrons, said first layer having a density in the range of 1 to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the range of 10 to 50 percent of the density of said second layer material in the bulk form to impede the escape of secondary electrons from said target member.

4. A target member for an electron image device comprising an electrically conductive member, and a layer disposed on said member, said layer having a first surface adjacent said member and a second surface remote from said member, said layer having a density of a value less than 10 percent of the density of the material of said layer in the bulk form at said first surface and the density of said layer becoming progressively greater toward said second surface said layer made of an insulating material having the property of generating electrons in response to electron bombardment and of supporting the conduction of electrons through the voids of said layer.

5. A target member for an electron image device comprising an electrically conductive member, and a layer of insulating material having the characteristic of generating secondary electrons within the volume of said layer in response to the bombardment of primary electrons, said layer disposed on said member in a manner that the density of said layer progressively increases across the thickness of said layer, a first region of said layer adjacent said member having a density in the range of 1 to 10 percent of the density of said material in the bulk form, a second region of said layer having a density in excess of 10 percent of the density of said material in the bulk form sufficient to thereby impede but not totally suppress the escape of secondary electrons from the layer and to increase the solid state conduction of electrons across said layer.

6. An electrical device comprising a target member including an electrically conductive element, and a layer of secondary emissive material deposited on said conductive element, said layer having a first surface contiguous to said conductive element and a second surface remote from said conductive element, said layer having a density that progressively varies from a minimum value in the range of 1 to 10 percent of the density of said material in the bulk form at said first surface to a maximum value at said second surface, said maximum value being several times greater than said minimum value of said density; means for directing an electron image onto the exposed surface of said conductive element; and a display means disposed to receive the secondary electrons emitted by said target member and to provide an intensified visible image of said electron image.

8. An electron image device comprising a target element including a conductive member, a first layer disposed on said conductive member in porous form having a density less than 10 percent of the density of the first layer material in the bulk form and having the property of generating and conducting secondary electrons through the voids of said first layer, and a second layer disposed on said first layer having a density substantially greater than first layer in the approximate range of 10 to 25 percent of the density of the second layer material in the bulk form for limiting the escape of said secondary electrons from said first layer; and Means for directing electron through said conductive member and into the voids of said first layer.

9. An electronic camera device comprising a target member including an electrically conductive member, a first layer of a material having the property of generating secondary electrons within the volume of said material in response to a bombardment of primary electrons and being deposited on said conductive member in a porous form having a density less than 10 percent of the density of said material in the bulk form, and a second layer being disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form; a first source for directing primary electrons on said conductive member and into the volume of said first layer to establish a charge pattern representative of said primary electrons; and a second source for directing electrons onto said exposed surface of said second layer to derive an electrical signal therefrom representative of said charge pattern.

10. An image intensifying device comprising at least one target member including an electrically conductive member, a first layer of an insulating material having the property of generating secondary electrons within the volume of said material and being deposited on said conductive member in a porous form having a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of between 10 to 50 percent of the density of said second layer material in the bulk form; means for directing an image of primary electrons through said conductive member and into said first layer; and display means for providing a visible image in response to secondary electrons received from said target member.

11. A target element for an electron image device comprising an electrically conductive member, and a storage member including a first layer of an insulating material deposited on said conductive member in a porous form with a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, said insulating material having the properties of generating secondary electrons within the voids of said material in response to a bombardment of primary electrons and of conducting said secondary electrons through the voids of said insulating material, and means for impeding the escape of said secondary electrons from said storage member and for enhancing the solid state conduction of electrons through said storage member including a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form.

12. A target member for an electron image device comprising an electrically conductive member, and a storage member including a first layer of a porous material deposited on said conductive member with a density in the approximate range of 1 to 10 percent of the density of said material in its bulk form and having the property of generating secondary electrons within the voids of said porous material in response to a bombardment of primary electrons, and means for impeding the escape of secondary electrons from said storage member and for increasing the first crossover potential of said storage member including a second, discontinuous layer of a conductive material disposed to depth of a few molecules on said first layer.