GB487899A - Improvements in secondary emission electron multipliers - Google Patents

Abstract

487,899. Electron multipliers. GENERAL ELECTRIC CO., Ltd., SIMMS, C. H., and TRELOAR, L. R. G. Feb. 3, 1937, No. 3198. Drawings to Specification. [Class 39 (i)] In secondary emission electron multipliers of the type having a photo-electric primary cathode and in which the electrons are guided by co-operating electrostatic and magnetic fields, the cathodes are made of silver plates and the surface of the primary cathode is much less highly polished than the secondary cathodes before the oxidation process which precedes sensitization. To obtain uniformity of oxidation of the polished secondary cathode plates, it is preferred that they should be oxidized once and then heated to about 300‹ C., to decompose the oxide, the process being repeated one or more times; or they may be heated in oxygen at a pressure of about 1 m.m. at 400‹ C. for an hour. The treatment must not be so severe as to destroy the polish of the secondary cathode plates relatively to the primary cathode. In one embodiment, the plates are assembled as described in Specification 478,262. The primary plate is etched with 50 per cent solution of pure nitric acid and washed and the secondary plates are all highly polished. After assembly, the tube is evacuated at 400‹ C. Oxygen is then admitted at 1 m.m. pressure, and the device heated for 15 minutes at 400 C. The oxygen is evacuated and the multiplier cooled. Fresh oxygen is then introduced at a pressure of À7 m.m. and the surface of each electrode oxidized by electric discharge so that it assumes a green colour. The oxygen is then pumped away and the temperature allowed to reach 200‹ C. and maintained while caesium is introduced until the sensitization has reached the desired point.