Improvement of Thin Film Caesium Photo-Electric Tubes

The photo-electric sensitivity of thin films of caesium deposited on silver oxide with silver as a base metal was studied by L. R. Koller. Photoelectric tubes with sensitive surfaces made in this manner have a maximum sensitivity in the short-wave range and also a second maximum in the red, with the long wavelength limit moving out into the infra-red. We have found that if a thin film of silver or gold is deposited on the above sensitive cathode surface and afterwards baked, it becomes very much more sensitive and shows displacements of the long wavelength limit still further in the infra-red. By this new method, which will be described below, sensitivities as high as 40 and even 48 microamperes per lumen have been obtained. This is several times better than for the caesium on silver oxide tubes and about thirty times better than potassium hydride tubes. The new sensitive surface or cathode thus formed may be considered to have a thin film of caesium, over which is deposited, as a final layer, a thin film of silver. The sensitivity as reported was measured with a gas-filled tungsten lamp operating at 2700 deg. K.