Recent Work on Photoemission and Dark Emission Problems

Recent developments have resulted in multiplier phototubes having wider spectral responses from the ultraviolet to the infrared, having higher absolute sensitivity, and having lower dark emission. The multialkali photocathode discovered by Sommer is now being used in the development type multiplier phototube, C7237. Despite the very high red sensitivity of this cathode, the dark emission is a factor 10 lower than that of antimonycesium cathodes used in typical scintillation counters. Output dark current is basically due to the photocathode dark emission, but this is sometimes increased by regenerative processes. In a special sixteen-stage developmental type with a sealed-in optical shield, the dark current was kept to a minimum for gains as high as 109 to 1010. Photocathodes of very high quantum efficiency have been achieved on the 7029 multiplier phototube type of means of an internally evaporated-front-surface photocathode of the antimony-cesium type. Absolute comparisons of special characteristics are presented for the photocathodes mentioned above, as well as for the ultraviolet-sensitive 6903 and for the 7102 multiplier phototube which has red and infrared sensitivity.