GB1163499A - Method of Making a Multi-Alkali Photo-Cathode - Google Patents

Abstract

1,163,499. Cathode materials and processing. RADIO CORPORATION OF AMERICA. 5 July, 1967 [6 July, 1966], No. 30871/67. Heading H1D. [Also in Division C7] A method of forming a multi-alkali photocathode comprises the steps of evaporating a layer of antimony on a substrate, e.g. of glass, co-evaporating a pair of alkali metals such as sodium and potassium from a common source over the antimony layer, and evaporating caesium on to the sodium-potassium deposit. In one example of making an evacuated photo-cell the antimony is deposited by heating a filament coated with an alloy of 40% antimony and 60% platinum until light transmission is reduced to 65%. The tube envelope is then raised to a temperature of 160-175‹ C., preferably 165‹ C., and sodium-potassium is deposited by electrically heating a channel member containing 45% zirconium, 7% potassium chromate, 7À5% hydrated sodium tungstate and 40À5% tungsten. The proportions of sodium and potassium in the mixture are such as to produce a layer having the composition Na 2 KSb, and the heating current is gradually increased in stages until the photo-sensitivity reaches a stable value; the tube is then baked for 65 minutes and the deposition resumed until the sensitivity reaches a peak value. Caesium is then evaporated in a similar way from a mixture of caesium chromate, zirconium and tungsten powder. The tube is then cooled slowly at a decreasing rate to room temperature and sealed off. In an alternative method, the sodium-potassium deposition may be carried out whilst the tube is held at a temperature of 215‹ C. As alternatives to sodiumpotassium the pairs potassium-caesium and rubidium-caesium are mentioned.