US3441787A

US3441787A - Secondary electron conduction storage system

Info

Publication number: US3441787A
Application number: US634304A
Authority: US
Inventors: Gerhard W Goetze; Alvin H Boerio
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1967-04-27
Filing date: 1967-04-27
Publication date: 1969-04-29
Anticipated expiration: 1986-04-29
Also published as: FR1569134A; DE1639074A1; GB1149997A; NL6803547A

Description

April 29, 1969 G. w. GOETZE ETAL 3,441,787

SECONDARY ELECTRON CONDUCTION STORAGE SYSTEM Filed April 27. 1967 F|G.l.

WITNESSES INVENTORS WW Gerhard W. Goetze 0nd Alvin H. Boerio U.S. Cl. 31512 4 Claims ABSTRACT 9F THE DISCLOSURE A secondary electron conduction target storage tube in which the secondary electron conduction material is disposed as a porous material upon a mesh support member and which fills the interstices of the mesh and covers both sides of the mesh.

Background of the' invention This invention relates to secondary electron conduction storage systems and more particularly to those which incorporate a secondary electron conduction target.

The particular application of this invention is to that type of device described in U.S. Patent 3,213,316 and which is assigned to the same inventors and assignee as this invention. The above-mentioned patent describes a secondary electron conduction tube including a target structure which consists of a thin film of aluminum oxide as a support layer stretched across a metal ring. A thin evaporated layer of aluminum which serves as a signal electrode is disposed upon this support layer and a layer of a suitable porous secondary electron conduction material is disposed on the signal electrode. A writing electron beam is directed into the secondary electron conduction layer through the aluminum oxide and aluminum layers and a reading beam is directed onto the exposed surface of the secondary electron conduction layer. It is also necessary in most devices to provide a conductive mesh in front of the exposed surface of the secondary electron conduction layer which serves as a suppressor grid and prevents destruction of the target under certain operation conditions.

It has been found that considerable skill and time are required in fabricating this multi-layer thin film structure. There are several problems inherent in manufacturing and operating the thin film structure as described in the above-mentioned patent. The diameter of the targets are limited to less than two inches. It is difficult to recognize defects found in the base supporting material of aluminum oxide which results in target blemishes. The target is not a rugged structure particularly in the larger type tubes. The porous secondary electron conduction layer must be evaporated in an inert gas atmosphere and time varying convection currents induced by the evaporation make it extremely difiicult to reproduce desired target structures. The non-uniformity of deposit particularly affects the target capacity and the lag characteristics of the structure. The storage capacity of the target depends on the target voltage and those cases where automatic target gain control is used, this dependence is highly undesirable.

It is accordingly an object of this invention to provide an improved secondary electron conduction target.

It is another object to provide an improved secondary electron conduction tube.

3,441,787 Patented Apr. 29, 1969 Summary of the invention Briefly, the present invention accomplishes the abovecited objects by providing a secondary electron conduction target utilizing a conductive mesh as the support for the second electron conduction material and in which the conduction material is deposited within the interstices of the mesh and on both sides thereof.

These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

Brief description of the drawing FIGURE 1 is an elevational view in section, schematically representing the pickup tube and associated system in accordance with the teachings of this invention; and

FIG. 2 is an enlarged elevational view in section illustrating the electrode target assembly in FIG. 1.

Descriptions of the preferred embodiment Referring in detail to FIGS. 1 and 2, there is illustrated a pickup tube in FIG. 1 incorporating the teachings of our invention. The tube comprises an envelope 10. A face plate 12 is provided in the envelope 10 and is transmissive to the input scene radiations. The face plate 12 is of a suitable material such as glass in the case of visible light input. A coating 14 of a suitable photoemissive material which is sensitive to the input radiation is provided on the inner surface of the face plate 12. The coating 14 emits photoelectrons in response to the input radiation. A suitable material for the coating 14 in the case of visible light input would be cesium antimony.

An electron gun 20 is provided at the opposite end of the envelope 10 for generating and forming a pencil-like electron beam which is directed onto a target member 30. The target member 30 is positioned between the electron gun 20 and the photocathode 14. Between the target member 30 and the photocathode 14, there are provided a plurality of electrodes illustrated as 16 and 18 with suitable potentials provided thereon for accelerating and focusing of the photoelectrons emitted from the photocathode 14 onto the target member 30. The electron gun 20 provides what is known in the art as a reading electron beam and the photocathode 14- provides what is referred to in the art as the writing electron beam. In the specific examples shown herein a large area image input photocathode is utilized. It is obvious that modifications could be made herein so as to make the photosurface sensitive to various input radiations. This could be accomplished by a radiation converter in which the input radiation is directed onto a phosphor and the light output of the phosphor is directed onto the photocathode 14. In addition, a conventional scanning electron gun could be utilized for directing an electron beam over the target surface in a similar manner as the reading electron gun 20. The video information in this case would be applied to the writing electron gun in conventional manner.

The electron gun 20 is of any suitable type for producing a low velocity pencillike electron beam to be scanned over the surface of the target electrode 31). The electron gun may consist of a cathode 22, a control grid 24 and accelerating grid 26. The

gun electrodes

22, 24 and 26 along with a coating 44 provided on the inner wall of the envelope provide a focused electron beam which is directed onto the target member 30. It may be desirable in some applications to utilize a mesh screen 40 in front of the target in a well known manner for maintaining a uniform electric field in front of the target 30. Deflection means illustrated as a coil 50 is provided around the envelope 10 for deflection of the electron beam generated by the electron gun 2t) and by application of suitable potentials scans the electron beam over the surface of the target 30 in a suitable manner. A magnetic coil 52 is also provided around the envelope 10 to provide additional focusing of the electron beam from the read gun 20 onto the target 30 as well as for focusing electrons from the photocathode 14 onto the target 30.

The target member 30 is supported upon a ring member 32 of a suitable material such as a Kovar alloy, a Westinghouse Electric Corporation trademark for an alloy of nickel, iron and cobalt. The ring 32 is supported within the envelope by any suitable means such as pins projecting through the glass walls. Further, the storage target 30 is comprised of a very fine mesh 34 which is made of a suitable electrically conductive material such as copper or nickel and which is secured to the ring 32 by an annular support member 36 which may be spot welded to the ring 32. The mesh 34 may be of the woven type or may be made from a solid sheet which has been etched to provide a perforated structure. In the specific embodiment illustrated, the mesh 34 has about 1000 openings per inch and an open area of 50 percent or more. The openings in the mesh are about 12 micrometers by 12 micrometers. The individual mesh element width is about 6 to 12 micrometers. The thickness of the mesh 34 may be about 6 to 20 micrometers. A layer 38 of a material exhibiting secondary electron conduction is deposited directly onto the mesh 34. Suitable examples of materials which exhibit this secondary electron conduction property includes potassium chloride, barium fluoride, sodium bromide and magnesium oxide. The layer 38 is a spongy or porous deposit having a density of less than 10 percent of the density of the material in its normal state. The porous layer 38 is formed by the evaporation of the secondary electron conduction material onto the mesh 34. The material to be deposited is heated to its evaporation temperature in the presence of an inert atmosphere, for example, helium or argon. The evaporation takes place at a distance in the order of a few inches in an atmospheric pressure of about 1 to torr. It is an important aspect of this invention that portions of the elements of the porous material are disposed within and over the surfaces of the mesh 34 to provide the layer 38. The material may be evaporated from both sides of the mesh simultaneously or may be evaporated sequentially in any suitable manner. The deposit Will grow sideways from the interstices of the mesh 34 to thereby fill in the interstices and then cover the entire area of the mesh and provide a continuous layer 30 of the porous material on both surfaces of the mesh 34. If the thickness of the mesh 34 is about 12 micrometers then the thickness of the target after the coating 38 is deposited is about 20 micrometers. This provides a porous coating of about 4 micrometers on each side of the mesh. The porous layer on each side of mesh may be of 4 to 20 micrometers. The active portion of the mesh 34 is completely embedded within the layer 38.

The values of representative potentials applied to the electrodes are illustrated in FIG. 1. The photocathode 14 is operated at a potential of about 8,000 volts negative with respect to the conductive mesh 34 to provide acceleration of the electrons from the photocathode 14 onto the target 30. The conductive mesh 34 may be operated at a potential of about 15 volts positive with respect to the electron gun cathode 22. The cathode 22 operates at about ground potential. The surface of the porous storage coat ing 38 is stabilized on the read side to an equilibrium potential which may be substantially ground potential by means of the scanning electron beam from the gun 20. If the grid 40 is utilized in front of the target 30- and between the electron gun 20 and the target 30, a retarding field will exist between the target 30 and the grid 40. Such a grid 40 would be operated at a potential of about 450 volts positive with respect to ground. In addition a suppressor grid 41 may be provided between the grid 48 and the target 30. The mesh 41 may operate at a potential of positive 50 volts.

The radiations from a scene are focused onto the pho tocathode 14 and photoelectrons are emitted from each portion of the photocathode 14 corresponding to the amount of light directed thereon. The photoelectrons are focused upon the target member 30. The photoelectrons are accelerated to sufficiently high energy of about 5000 electron volts so that they penetrate into the coating 38. The accelerating voltage should be adjusted such that substantially all of the primary electrons from the photocathode 14 almost completely penetrate the layer 38 but do not substantially pass on through the structure. The primary electrons from the photocathode 14 create a certain number of low energy or free electrons within the layer 38. The number of low energy electrons generated are orders of magnitude higher than the number of primary electrons. For example, the number of free electrons generated may be about 200 for each incident primary. The target 30 is polarized prior to the impact of the signal or writing electrons from photocathode 14. This is done by applying a positive potential of about 15 volts to the conductive mesh 34 and stabilizing the exposed surface or read side of the target at ground potential and the free electrons generated in the layer 38 flow within the voids of the layer 38. This causes the read surface to change its potential from ground due to flow of the free electrons in the layer 38 through the vacuum space or voids between the particles of the very porous layer 38 to the positive backplate or mesh 34. This local change of the exit surface potential that is on the read side can be employed to generate a video signal using any of the several well known readout techniques. In FIG. 1 there is illustrated a typical vidicon type readout assembly.

The storage capacity of this novel target 30 is determined by the geometry of the mesh 34 chosen and does not therefore depend as strongly on evaporation parameters and the time variance convection currents. This feat-ure makes it possible to predetermine and accurately control the target storage capacity required by the target format. Since no well defined plane serves as the signal plate, the undesired dependence of storage capacity on target voltage is in the first order eliminated. This will make it possible to use the target voltage as a parameter in achieving gain control.

Various modifications may be made within the spirit of the invention.

We claim as our invention:

1. An electron discharge device comprising a storage electrode including an electrically conductive mesh support member, a porous film deposit of less than 10 percent of its normal bulk density of high resistive material deposited in the interstices of said mesh and upon the surfaces of both sides of said mesh and having the property of generating free electrons in response to electron bombardment, means for directing a writing electron beam at one side of said target at a predetermined energy to penetrate said porous film to generate secondary elec trons within said porous film, means for establishing a field across said porous film to collect said secondary electrons emitted into the vacuum spaces within the particles of said porous layer but inadequate to collect charge carriers through said solid material and means for directing a reading electron beam below said predetermined energy at the other surface of said storage target to restore said bombarded surface to an equilibrium potential while deriving a signal corresponding to the signal written on said storage target by said writing beam.

2. The electron discharge device described in claim 1 in which the interstices and said storage mesh are of similar dimensions as the thickness of said porous coating on one surface of said conductive mesh.

3. The electron discharge device described in claim 1 in which said conductive mesh has openings of 500 to 1000 per inch constituting an open area of 50% or more and the thickness of said porous coatings on each surface of said mesh is about 10 micrometers.

4. The electron discharge device described in claim 1 in which said conductive mesh has a thickness of about 20 micrometers with about 1000 openings per inch, each opening of a dimension of about 12 micrometers by 12 micrometers with the porous coating filling the mesh openings and extending beyond the mesh on the reading side of the target.

6 References Cited UNITED STATES PATENTS RODNEY D. BENNETT, JR., Primary Examiner.

CHARLES L. WHITHAM, Assistant Examiner.

US. Cl. X.R.