GB846911A - Improvements in or relating to photo-multipliers - Google Patents

Abstract

846,911. Electron multipliers; cathode-ray tubes; cathode materials. SCHLUMBERGER WELL SURVEYING CORPORATION. Dec. 13, 1956 [Dec. 26, 1955], No. 38118/56. Class 39(1). A photomultiplier comprises a photo cathode 11, an anode 13, a plurality of dynodes 20a, 20j, 20k, and an evacuated envelope 10, a plurality of insulating cylindrical spacers 24a, -24j enclosing the regions between adjacent dynodes, and a plurality of centering rings 22a-22j extending between adjacent spacers and being conductively secured to a corresponding annular dynode frame 37, 38, e.g. of nickel, for supporting the dynodes in spaced axial alignment. The dynodes have spaced apart strips 43 inclined with respect to the axis of the frame, e.g. at 45 degrees, and having secondary emissive surfaces 44, e.g. of a silver-magnesium alloy, confronting the photo-cathode 11, and opposite surfaces 45, composed of an adsorptive gettering, material, such as titanium, molybdenum, zirconium, tantalum or palladium, confronting the anode 13 to minimise feedback. Each dynode frame has an inwardly opening channel member 38, a wire grid 40, forming a plarw-convex lens, being arched over the dynodes 43 and being received by the channel member and retained thereby in conductive contact with the dynodes. The wire grid 40 can be of the same gettering material as the surfaces 45 of the dynodes. Each centering ring 22a, &c., comprises a flat annulus 26 of mica having a notched outer periphery 27 engaging the wall of the envelope 10 and an inner periphery 28 supported in an inwardly crimped portion 29 of a band 30 of aluminium. The notched periphery 27 allows the extension of a conductor 35, connected to member 38, and support rods, along the wall of the envelope. The spacers 24a, &c., can be of glass or ceramic having an inner layer 51 of evaporated gold or silver or of a getter material of appreciable conductivity. The resistance of the spacers can be of the order of 200 kilohms with about 150 to 200 volts between successive dynodes, the resistance having a low temperature coefficient. The spacers 24a, &c., are smaller in depth and the arched wire grids 40 shallower towards the base of the envelope. The photo-cathode 11 has a photo-sensitive layer 16 composed of caesium-antimony, sodium-antimony or other compounds of alkali or akaline earth metals and antimony, sulphur, selenum, tellurium, bismuth, thallium or lead. A layer 15, composed of a conductive material and transparent to the radiations passing through the window 14, is positioned between the envelope and the layer 16 to isolate the layer 16 from constituents, such as boric oxide, contained in the envelope. A conductor 17 is connected with the photocathode 11 through a band 18 of silver and is led out through the envelope for connection with the external circuitry. The wire grid anode 13 is disposed within a conductive cylinder 53 and a lead-in wire 55 passing through an aperture 52 in the spacer 24j and an aperture 54 in the cylinder 53, firmly supports the anode. The cylinder 53 can be secured to the cylinder 53 conductively. The last dynode 20k is formed as a circular plate under the anode 13 and its periphery fits within the bottom of the spacer 24j and is in contact with a conductive layer 51 thereon. A conductive lead 57 is in contact with the dynode 20k. In modifications the photo-cathode is disposed in front of the first dynode 20a of the focus of a Schmidt type optical system coupled to the source of radiation, the wire grid 40 is formed of wires woven in a criss-cross pattern, only the last five of the grids 40 and the surfaces 45 of the dynodes are of getter material, and a coating of low conductivity similar to the coatings 51 is applied on the inner side walls of the cylinder 53 to reduce ion feed-back. The circuit for operating the photo-multiplier comprises a D.C. source 60 connected across a potentiometer 61 having its -ve terminal connected to the conductor 17 and its +ve terminal connected through resistor 59 to the anode 13. The conductor 57 is connected to a tap 62 on the potentiometer so that the potential on the dynode 20k is less +ve than that on the anode 13. A capacitor 63 connected to the anode 13 couples the signal developed across the resistor 59 to an indicating circuit. The conductor 35 connects the first dynode 20a with a tap 65 on the potentiometer. Additional lead-in and supporting conductors may be connected to each of the dynodes 20b-20j in the same way as conductor 35 is connected to the dynode 20a. The spacers 24a-24j, Fig. 4, schematically represented as resistors connecting corresponding pairs of dynodes 20a-20k. A.C. voltage operation is also referred to.