US 3688145 A
The faceplate of a light detector tube is provided with an internal photocathode having wedge-shaped projections and external lenses to concentrate the light from a source onto the projecting photosensitive surface areas. An accelerating grid having a corresponding configuration is spaced closely to the photocathode to increase the emission of electrons from the photosensitive surface. A potential source establishes an electric field between the photocathode and an anode, with the anode collecting the electrons.
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Description (OCR text may contain errors)
United States Patent Coles  LIGHT DETECTOR HAVING WEDGE- SHAPED PHOTOCATHODE AND ACCELERATING GRID STRUCTURE  Inventor: Donald K. Coles, 2505 Capital Ave.,
Fort Wayne, Ind. 46806  Filed: Oct. 8, 1970  Appl. No.: 79,102
 US. Cl ..313/102, 313/99  Int. Cl. ..H01j 39/02, l-lOlj 39/14, l-lOlj 39/04  Field of Search ..313/94, 95, 102, 96, 101
 References Cited UNITED STATES PATENTS 2,953,703 9/ 1920 Lempert ..313/95:X 3,107,313 10/1963 l-lechtel ..315/5.16 3,513,345 5/1970 Feaster ..313/95 3,558,967 1/1971 Miriam ..315/3.5
[ 1 Aug. 29, 1972 3,586,895 6/1971 Sowers etal ..313/94 Primary Examiner-Robert Segal Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson, Jr., Philip M. Bolton, Isidore Togut, Edward Goldberg and Menotti J. Lombardi, Jr.
 ABSTRACT The faceplate of a light detector tube is provided with an internal photocathode having wedge-shaped projections and external lenses to concentrate the light from a source onto the projecting photosensitive surface areas. An accelerating grid having a corresponding configuration is spaced closely to the photocathode to increase the emission of electrons from the photosensitive surface. A potential source establishes an electric field between the photocathode and an anode, with the anode collecting the electrons.
5 Claims, 3 Drawing Figures PATENTEDwszs m2 INVENTOR OONALD K. COLES BY 5 V v I Al rum-u Y OUTPUT v o 0 w 0 3 9 8 6 2 LIGHT DETECTOR HAVING WEDGE-SHAPED PHOTOCATHODE AND ACCELERATING GRID STRUCTURE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved sensitivity phototube and particularly to a novel photocathode and grid structure providing more efficient electron emission.
2. Description of the Prior Art Use of projecting photocathode surfaces in the form of prismatic gables or triangular ribs has been shown in the prior art, such as in US. Pat. No. 3,043,976, issued July 10, 1969, for the purpose of providing maximum light reflections within the angled surfaces of a glass faceplate and to obtain corresponding increased photoelectron emission. It has also been known to use minute external spherical lenses on faceplates of cathode ray tubes for various purposes, such as shown in US. Pat. No. 2,740,954, issued Apr. 3, 1956. The gabled photocathode surface is subjected to a strong electric field with respect to a spaced internal anode to emit electrons copiously from the extreme pointed ends of the triangular surfaces, but the field at the remote angled portions furthest from the anode is quite weak. Thus, photoelectrons emitted from an interior surface remote from the projecting end tend to travel in a straight perpendicular line across the space to the nearest portion of the next adjacent projecting surface and be lost, instead of being directed outwardly toward the anode. A gabled photocathode surface having very acute angles is thus best for reflecting and trapping the maximum amount of light but has the worst total electron emission properties. Another difficulty is that the conductivity of photocathode materials is often weak and may require enhancement by thin conducting strips spaced along the generally flat surface between an outer supporting ring connection and the center of the photocathode. This permits improved transverse electron flow through the photocathode to the point at which emission occurs by photoelectric action but blocks some of the sensitive surface areas. None of these prior art devices have been able to achieve satisfactory performance for many applications which require greater sensitivities and electron emission.
SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide a more efficient improved sensitivity light detector tube having a photocathode and grid structure which permit an increased emission of photoelectrons. This is accomplished by a novel gabled photocathode surface, an external lens arrangement on the tube faceplate which focuses incoming light close to the projecting peaks of the photocathode, and an adjacent correspondingly gabled accelerating grid which permits an increased electrical field to penetrate into the more remote, less emissive, areas as well as the peaks. An additional conductive coating may be deposited in the extreme remote angled areas to enhance the electron conductivity laterally through the photosensitive material without interfering with electron emission. The details of the invention will be more fully understood and other objects and advantages will become apparent in the following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of an electron multiplier tube incorporating the increased sensitivity photocathode;
FIG. 2 shows an enlarged view of the novel photocathode and grid structure; and
FIG. 3 shows a cross-sectional view of another variation of the light detector tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIGS. 1 and 2, an evacuated cylindrical tubular envelope 10 includes a light transparent faceplate 12 at 'one end. The inner surface of the faceplate has a coating 14 of a photosensitive electron emissive material of a suitable known composition forming a photocathode which is capable of emitting electrons upon exposure to light. The inner surface and coating have a plurality of wedge-shaped or triangular projections 16 having peaks 18, extending longitudinally toward the other end of the tube, and valleys 20 therebetween. The projections may be in the form of individual conical elements, parallel ribs along the faceplate surface in a lateral or transverse direction, concentric rings or a hexagonal configuration.
A plurality of external lenses 22 on the outer surface of the faceplate focuses the incoming light rays 24 onto the peak areas of the photocathode. An accelerating grid structure 26 is positioned in close proximity to the photocathode and has similar wedge-shaped projections thereon complementary to those of the photocathode. The action of the accelerating grid will be more fully described below. An anode 28, connected to a potential source, is positioned at the other end of the tube to collect electrons emitted by the photocathode. An intermediate electron multiplier structure 29, 30 in the form of slats or venetian blinds, is positioned between the anode and accelerating grid. The multiplier dynodes are coated with a known secondary emissive material which emits secondary electrons in response to the impingement thereon of primary electrons from the photocathode. This provides a multiplication or amplification of the photoelectrons from the photocathode in accordance with the intensity of the light source and a corresponding output signal is extracted at the anode which is coupled to a suitable utilization device. A second or intermediate mesh grid 31, electrically connected to the second set of multiplier plates 30, may be employed to aid in establishing proper field relationships between the multiplier sections and facilitate the flow of electron from the first set of plates 29.
An additional conductive coating 32, in the form of 200 strips, is deposited in the valley areas of the photocathode to improve the conductivity through the photocathode material in the lateral dimension. Since the potential on the photocathode is applied at the periphery of the tube by a connection to an external supporting ring 34, the central areas of some coating materials may have a weaker conductivity. This is enhanced by the strips which serve as feeders to conduct electrons to the central photocathode areas where emission is provided by photoelectric action. The strips, which may be in the form of parallel lines, concentric rings or hexagons, are placed in the valley areas -d.c. on the anode.
which have an inherently weaker emission due to the greater distance from the anode and lack of penetration of the electric field. A narrow conductive radial line, not shown, is preferably deposited laterally on the inner faceplate between the center and outer strips to connect all the strips together and to the source of photocathode potential. In addition, the focusing of the light upon the peak areas by the lenses avoids the conductive strips in the valleys and provides increased emission at the peaks. This reduces the total loss of electrons from the surface areas blocked by the strips.
The optimum photocathode geometry, for the purpose of trapping light and obtaining multiple reflections within and between adjacent projections to generate electrons, is generally an acute angle. However, this is not the most efficient for extraction of the electrons from the remote internal surfaces by the electric field from the anode. By using the accelerating grid or mesh 26 positioned closely adjacent to the photocathode and having complementary shaped triangular indentations substantially parallel to the emissive cathode surfaces, the electric field can be made to penetrate deeply into the remote areas to extract more electrons. The accelerating grid in cooperation with the lenses thus permit employment of smaller acute angles and sharper projections on the photocathode so that light trapping efficiency and electron emission are increased.
Typical dimensions that may be utilized in the configuration of FIG. 1 are for example: a tubular envelope outer diameter, 300 inches; the spacing between the cathode and accelerating grid, one-sixteenth inch; accelerating grid to first multiplier section, one-fourth inch; between multiplier sections including the second grid, one-fourth inch; and anodeto-multiplier, onefourth inch. The angles of the photocathode projections may be from 30 to 60 with about 45 being typical. The grids may be formed of a mesh of 0.005 inch diameter wire. The faceplate may be of three-eighths inch thick glass, with glass or plastic cylindrical or spherical lenses of one-fourth inch diameter. Typical voltages may be in the order of +200 Volts d.c. on both the accelerating grid and a first set of multiplier plates with respect to the photocathode which is at Volts d.c. or ground potential, +400 Volts do. on the second set of multiplier plates and second grid, and +600 Volts In another variation of a light detector tube employing the invention, as shown in FIG. 3, a curved faceplate 36 includes a photocathode coating 38 on the internal surface having a single triangular projection 40. An accelerating grid 42 is closely spaced from the photocathode and has only one matching projection. A series of cup-shaped multiplier dynodes 44, 46, 48 are spaced symmetrically about the grid and along the length of the tube to collect and multiply the electrons emitted from each angled side of the photocathode. The last dynode 48 also serves as an anode. The potenlight detector having a greater sensitivity and increased emission of electrons. While onl tw embodiments have been illustrated and descri ed, it is to be understood that many variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.
What is claimed is:
l. A light detector tube comprising:
an evacuated envelope;
a light transparent faceplate at one end of said envelope;
a photosensitive electron emissive coating on the internal surface of said faceplate, said internal surface including a plurality of wedge-shaped projection, having peaks and valleys, said projections extending toward the opposite end of said envelope;
an accelerating wire mesh grid including a like plurality of wedge-shaped projections having peaks and valleys complementary to and aligned with corresponding said internal surface projections, said grid being spaced in close proximity to and disposed in the path of electrons from said photosensitive coating, said light transparent faceplate including a plurality of lenses on the external surface thereof, said lenses focusing light upon the peak areas of said internal surface projections extending toward said opposite end an anode spaced from said mesh grid for collecting electrons emitted from said photosensitive coating and passing through said mesh grid; and
means applying stepped potentials to said anode, grid and photosensitive coating to provide an electric field causing electron emission from said photosensitive coating upon the impingement of light thereon.
2. The device of claim 1 including electron multiplier means positioned between said accelerating grid and anode and having a connection to said stepped potential means.
3. The device of claim 1, including a plurality of conductive strips disposed within said valleys and connected to said photosensitive coating and potential means for increasing the conductivity of said photosensitive coating.
4. The device of claim 2, wherein said multiplier means includes first and second sets of multiplier plates.
5. The device of claim 4, including a second mesh grid disposed between said sets of multiplier plates and connected to said second set.