US2970219A - Use of thin film field emitters in luminographs and image intensifiers - Google Patents

Description

INVENTORS ATTORNEY William L. Roberts 8 William E. Newell.

WITNESSES @MA ma? United States Patent O USE F THIN FILM FIELD EMITTERS IN LUMINO- GRAPHS AND IMAGE INTENSIFIERS William L. Roberts, Monroeville, and William E. Newell, Penn Township, Allegheny County, Pa., assignors t0 Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 18, 1955, Ser. No. 529,267

Claims. (Cl. Z50-213) Our invention relates to radiation image reproducing devices, and, in particular relates to such devices in which an output light image is a greatly intensified replica of the intensity distribution in an incident radiation field. An example of devices of this general type is the tube for intensifying X-ray pictures described in the U.S. Patent 2,523,132 to Mason and Coltman. While the image intensifier of this patent has proven powerful enough to open a new field of commerce, there is need for a still more powerful X-ray picture intensifier; there are also needs for intensifiers of other types of radiation; and our present invention is applicable to improvements of radiation detectors of many types.

Briefly stated, our invention employs the properties of an aluminum oxide or similar film when made the cathode surface for an electron discharge in which the electron current reaching the luminescent `screen of the devices varies from point-to-point in dependence upon the variations of intensity from point-to-point of a radiation field projected upon the cathode (or grid) surface. Certain modifications of our invention utilize magnesium oxide cathode surfaces in conjunction with photoconductive layers to reproduce radiation images.

One object of our invention is accordingly to provide a novel and superior type of radiation-image reproducer.

Another object is to utilize the radiation-responsive properties of materials such as aluminum oxide toreproduce and intensify radiation images.

Another object is to utilize the variations in electron emission from aluminum oxide layers and similar substances in response to changes of incident radiation to control electron flow.

Another object is to utilize the sensitivity of electron emission from aluminum oxide and similar substances to incident light to produce intensified reproduction of light images.

Another object is to utilize the responsiveness of electron emission from aluminum oxide and similar surfaces to radiation fields to produce electrical potential distribution over the surfaces which are replicas of the radiation distribution.

Still another object is to combine magnesium oxide layers with photoconductive layers to produce current flows which reproduce variations of incident radiation.

Yet another object is to provide a novel type of cathode in which a magnesium oxide layer acts as a source of electrons which is self-sustaining for long periods after having received an initial stimulus.

Still another object is to provide a novel type of cathode in which magnesium oxide acts as a copious source of secondary electrons when stimulated by a local source of primary electrons.

Yet another object is to employ the radiation-responsive yproperties of secondary-electron ow from the cathode mentioned in the preceding paragraph in an electron-optical system.

The foregoing and other objects of our invention will be apparent upon reading the following description taken in connection with the drawings, in which:

Figure 1 is a diagrammatic view, partly in section, of a tube having a planar-type cathode comprising a layer of magnesium oxide or aluminum oxide and embodying certain principles of our invention;

Fig. 2 is a similar view of a planar-type cathode illustrating another modification of our invention;

Figs. 3 and 4 are similar views of tubes having planartype cathodes useful in third and fourth modifications of our invention;

Fig. 5 is a schematic view, partly in section, of a radiation-image reproducer or intensifier embodying another form of our invention; and

Fig. 6 is a similar view of a radiation-image reproducer embodying an aluminum oxide or similar cathode in still another form of our invention.

Referring in detail to Fig. 1, a planar cathode 1 comprises a glass backing plate 2 having its surface coated with a layer 3 of conductive material such as the conductive glass sold under the trade name NESA, which layer is coated with a layer 4, which may be about 10-4 cms. thick, of magnesium oxide or aluminum oxide.

An electron gun 5 is arranged in ways well known in the art to scan (or ood) the layer 4 with an electron beam. A collector electrode 6, which may be of screen type, is supported parallel to the surface 4. The electrodes 3, 5 and 6 are provided with suitable inleads through a container 7 for impressing appropriate potentials between them. When a voltage of 100 to 150 volts is impressed between electrodes 3 and 6 the layer 4 acts as a cathode furnishing a copious flow of secondary elec.` trons to collector electrode 6 started by the stimulus of primary electrons from gun 5. By proper adjustment of the voltage and current to electrode 6, this discharge can be rendered self-sustaining. However, operation at such voltage and current values that current flow is interrupted when the electron beam from gun 5 leaves a particular area will be preferred for some purposes.

An electron phosphor screen 8, e.g., of zinc sulphide may be supported behind the collector electrode 6, and if the scanning beam from gun 5 is modulated with picture signals similar to those of a television system, an image of the picture modulating these signals will appear on screen 8.

Fig. 2 shows a modification of the Fig. 1 cathode in which the electron gun 5 is rendered unnecessary by dcoating the magnesium oxide or aluminum oxide layer .i4 with a thin layer 9 of photoelectrically-emissive material such as cesium oxide. A thickness of 10 to 100 microns would be suitable for most purposes. Light is projected onto the layer 9 and primary electrons emitted therefrom generate secondary electrons in the layer 4 causing it to act as a cathode. The light projected onto layer 9` may be steady or be interrupted with results in performance of the cathode similar to those described in connection with the electron gun 5 in Fig. 1. If the light-field or other radiation-field projected on layer 9 is an image, a replica thereof of greatly enhanced intensity will appear on an electron-phosphor screen positioned like screen 8 in Fig. l.

Fig. 3 shows a type of image intensifier tube in which the radiation image passes through the cathode and controlselectron flow to an output screen via a grid. ln this case the radiation image is to be protected from the side of the cathode opposite to the control grid and, to permit such projection, the magnesium oxide or aluminum oxide is deposited on the conductive layer 3 by projecting it through a mask so that a mosaic of separate areas 12 is formed. The control-grid 13 may comprise a foraminated plate in which the perforations are rimmed or lined with a material 14 which emits electrons upon Patented Jan. 31, 1961.

incidence of the radiation image which is projected onto it through the cathode 1. The input radiation image, after passing through the cathode and the electrode 6, produces a charge image on the grid 13 covered by the photoemissive material 14. In the absence of such a charge image the electron How from the cathode to electrode 6 is substantially uniform and the grid 13 is biased to such a potential as to reduce or cut off the flow of electrons to the phosphor screen 8. However, due to the input radiation, the linings 14 with a potential distribution substantially duplicating the input radiation image control the ow of electrons from cathode 1, through electron 6 to the electron phosphor screen 8, producing an intensified replica of the input radiation-image on the latter. Copending application of W. L. Roberts, Serial No. 446,222, filed on July 28, 1954 (Patent 2,871,385, issued January 27, 1959, entitled Image Reproduction System) and assigned to the assignee of this application shows another type of image intensifier employing a control grid similar to that just described.

Fig, 4 shows another modification of our invention in which a photoconductive layer is employed and makes the use of a mosaic cathode structure unnecessary. Thus, the cathode comprises a layer 15 of photoconductive material such as selenium or antimony trisulphide, about microns thick, sandwiched between a conductive layer 3 and a layer 4 of magnesium oxide or aluminum oxide. A source of electrons 5 symbolized in this case as an electron gun, supplies primary electrons which generate secondary electrons in layer 4. A pair of meshtype electrodes 17 and 1S intervene between the cathode assembly 2, 3, 15 and 4 and the thin aluminum layer 19 covering phosphor layer 8. The electrode 17 is impressed with a constant voltage positive to layer 4, e.g., by 55 volts, and the electrode 18 is impressed with a voltage which has positive rectangular pulses of the general form indicated below the figure rising e.g. to 50 volts positive, with a trough of e.g. l() volts negative, all relative to cathode layer 3.

The mode of operation of Fig. 4 is substantially as follows. The radiation image which is to be reproduced is focussed through the glass backing plate 2 and transparent conductive layer 3 on the photoconductive layer 15. Before the image is so focussed the action of the scanning beam (or alternatively a photoelectric layer such as 9 in Fig. 2) in bombarding the magnesium oxide layer 4 with electrons causes secondary electron emission therefrom with the result that its surface rises to nearly 55 volts positive during the positive crest phase of the wave impressed on electrode 18 since the photoconductive layer 15 is substantially an insulator preventing any replenishing liow from layer 3 of electrons to replace the secondaries attracted to electrode 17. During the negative phase of the voltage on electrode 18, thc potential of the surface of layer 4 remains nearly at the same potential because of this isolating action of layer 15, and this situation continues for all elemental areas of the screen so long as they remain dark after the radiation image is projected. However, in those picture areas where the radiation intensity of the projected irnage is substantial, the photoconductive layer 15 becomes substantially conductive and permits electrons to flow through it from conductive layer 3 to magnesium oxide layer 4, and the potential at the surface of the latter falls substantially below the 55 volt positive potential of electrode 17. Thus, a potential image is formed on the surface of layer 4 which is a replica of the intensity distribution of the radiation image, and electrons attracted from the higher intensity areas when electrode 18 again becomes positive will have energy enough to pass through the meshes of electrode 1,7 to phosphor screen 3, while electrons from the darker areas of the images, since they emanate from areas of layer 4 over 55 volts in positive potential, will not have sufficient energy to pass ciccl trode 18. A light image duplicating the radiation image in distribution will thus appear at phosphor layer 8.

Fig. 5 shows a modification of our invention which comprises a series of superposed layers without intervening spaces. Thus, a glass plate 2, which may, or may not, be a portion of a vacuum-tight container, supports a conductive transparent layer 3 covered by a layer 1S of photoconductive material, which is coated in turn by a layer 4 of magnesium oxide and on this layer is deposited a thin layer 21 of aluminum or other suitable conductor, and a layer 8 of an electron phosphor such as zinc sulphide is deposited on the latter. When a radiation field to which the photoconductive layer 15 is susceptible is projected onto it through glass plate 2, the areas of the photoconductor 15 where the radiation eld is of high intensity lose their insulating power and the entire voltage impressed between the conductive layers 3 and 21 is concentrated across the magnesium oxide layer 4 thus causing a copious ow of electrons through it into impact with phosphor screen 8. On the other hand, those areas of photoconductive layer 15 where the radiation field is dark retain high insulating power so that the voltage gradient in the magnesium oxide layer 4 directly beneath them is low and almost no electrons are projected into impact with phosphor screen 8 at dark image areas. A light image, duplicating in distribution the radiation eld, thus appears on output screen 8.

While we have shown walls 7 enclosing the Fig. 5 layers, it may be desirable for some uses to omit these, and this may be done since the materials forming the layers are chemically inert to air.

Fig. 6 shows a form of our invention which makes use of the property shown by aluminum oxide and magnesium oxide of varying their emission of secondary electrons when the intensity of incident radiation on them changes. Thus, a glass backing plate 2 carries a layer of transparent conductive material 3, such as NESA, on which is deposited a layer 4 of aluminum oxide or magnesium oxide. The latter is coated in turn with a layer 9 of cesium oxide or other photoelectrically-emissive material. A first grid 22 and a second grid 23 are positioned parallel to the layer 9 and to phosphor layer 8 covered by a thin layer 21 of aluminum. The first grid is impressed with a potential V1 relative to conductive layer 3 and when the photoemissive layer is irradiated or bombarded with primary electrons of suitable energies, secondary electrons are drawn from the composite cathode structure to electrode 22 until the oxide layer 4 rises nearly to the potential V1 of the latter so long as no radiation is projected onto the layer 4 through glass plate 2. Since the oxide layer is normally of very high resistivity and very thin, a high voltage gradient tends to build up in it and this results in an avalanching of electrons which permits electrons to ow to grid 22 under substantially space-charge limited conditions. When, however, any elemental area of layer 4 is irradiated by such radiation the secondary emissivity of that elemental area of layer 4 decreases probably due to the fact that the oxide layer becomes more conducting, and consequently, the potential of said area becomes less positive. The surface of oxide layer 4 thus becomes a cathode with a potential distribution varying from point to point corresponding to the radiation image projected through glass plate 2. The grid 23 is given a potential intermediate between the maximum and minmum values present in this potential distribution. As a result, electrons emitted from elemental areas of the cathode where the radiation intensity is high will be accelerated to energies which enable them to pass through grid 23 and into incidence with phosphor layer 8, while electrons from radiation areas of lower intensity will be unable to pass electrode 23. A light image having a pattern like the radiation image thus appears on the output screen 8.

We claim as our invention:

1. An image display device comprising a large area electron source, an output screen on which the electrons from said source are projected and grid means positioned between said source and said output screen, said electron source comprising a first layer of electrical conductive material and la second layer deposited on said first layer on the side thereof facing said output screen of a material selected from the group consisting of magnesium oxide and aluminum oxide, means for impressing a uniform charge on the surface of said second layer to establish a high voltage gradient within said second layer to cause field emission of electrons from said source, means for projecting a radiation image onto said source to establish va charge pattern on the surface of said second layer corresponding to said light image and means for accelerating an electron image corresponding to said charge pattern from said source to said output screen, said grid means comprising a first grid and a second grid, said first grid disposed nearer to said electron source than said second grid and having a potential applied thereto more positive than that applied to said electron source, said second grid having a potential yapplied thereto between the potential applied to said rst grid and the potential assumed by said second `layer under substantial radiation.

2. An image display device comprising a large area electron source, an output screen on which the electrons from said source are projected land grid means positioned between said source land said output screen, said electron source comprising a first layer of electrical conductive material and a second layer deposited on said first layer on the side thereof facing said output screen of a material selected from the group consisting of magnesium oxide and aluminum oxide, means for impressing a uniform charge on the surface of -said second layer to establish a high voltage gradient within said second layer to cause field emission of electrons onto said source, means for projecting a `radiation image onto said source to establish a charge pattern on the surface of said second layer corresponding to said light image and means for accelerating an electron image corresponding to said charge pattern from said source to said output screen, said means for impressing a uniform voltage on the surface ofv said second layer comprising a layer of photoelectrically-emissive material superimposed on said second layer.

3. An image display device comprising a large area electron source, an output screen on which the electrons from said electron source are projected, said electron source comprising a layer of electrical conductive material and a layer of a material exhibiting the property of electron emission in response to an electric field across said layer; means for establishing a field across said electron emissive layer of said electron source comprising a coating of a photoelectric material provided on the exposed surface of said electron emissive layer and an auxiliary radiation source for directing radiations onto said photoelectric coating, a first grid member positioned between said electron source and said output screen and means for providing a potential on said first grid member positive with respect to the potential applied to said electrical conductive member such that the surface o-f said second layer of said electron source tends to seek the potential of said first grid member thereby establishing a field across said electron emissive layer such that said electron emissive layer will emit electrons, a second grid positioned between said first grid and said output screen and means for providing potential difference between said second -grid and said output screen to direct the electrons from said electron source into incidence with said output screen.

4. An image display device comprising an electron source, an output screen of a material which emits light in response to electron bombardment projected from said electron source, said electron source comprising a layer of electrical conductive material and an electron emissive layer of a material selected from a group consisting of magnesium oxide and aluminum oxide, means for establishing a uniform charge on the surface of said electron emissive layer to establish an electric field across said electron emissive layer to cause field emission of electrons from said source, said charge establishing means comprising an auxiliary electron source for directing elec'- trons onto the surface of said electron emissive layer and a iirst grid member positioned between said electron source and said output screen and maintained at a positive potential with respect to the potential applied to said electrical conductive layer of said electron source, a second grid positioned between said first grid and said output screen and means for providing a pulsating voltage on said second grid and means providing a positive potential on said output screen with respect to said electron source to accelerate electrons passing through said second grid into incidence with said output screen.

5, An image translation device comprising a large area electron source, an output screen which emits light in response to electron bombardment from said electron source, said electron source comprising a layer of an electrical conductive material and a layer of a material that exhibits the property of emission of electrons in response to an electric field established Within said electron emissive layer, means for establishing an electric field in said electron emissive layer, said field establishing mea-ns comprising a first grid member positioned between said output screen and said electron source, a potential source for providing a positive potential on said first grid with respect to said electrical conductive layer, and an auxiliary electron source for directing electrons onto the surface of said electron emissive layer, a second grid positioned between said first grid and said output screen, said second grid provided with a radiation sensitive coating responsive to an input radiation image to provide a charge image thereon corresponding to said radiation image and means for accelerating the electrons from said electron emissive layer modulated by the charge image on said second grid into incidence with said output screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,058,941 Arnhym Oct. 27, 1936 2,199,438 Lubszynski May 7, 1940 2,603,757 Sheldon July 15, 1952 2,699,511 Sheldon Jan. 11, 1955 2,708,726 Atherton May 17, 1955 OTHER REFERENCES Malter: Thin Film Field Emission, Physical Review, vol. 50, page 48.

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