Decay of S-20 Photocathode Sensitivity Due to Ambient Gases

Abstract

This chapter focuses on decay of S-20 photocathode sensitivity because of ambient gases.All photosensitive surfaces that are responsive to low-energy photons in the visible spectrum are very reactive. In this study it was assumed that if a good photocathode is formed and remains stable, the effect of substrate contamination is negligible. The materials tested were processed in the manner appropriate to their operation in the camera, and the outgassing was measured with a residual gas analyzer. Stainless steel, Viton-A, Teflon, Mylar, and Kodak SO-159 film were studied and the predominant gases found were hydrogen, nitrogen, methane, oxygen, carbon dioxide, carbon monoxide, and water vapor; those having the highest partial pressures being hydrogen and water vapor. The most significant result of this study is that the sensitivity of a photocathode is selectively degraded by contaminants and is most degraded in the longer wavelengths. Hence contaminants may be only a small problem for images in a particular region of the visible spectrum but a much larger problem in other regions.