Photoelectron backscattering effects in photoemission from CsI into gas media

The photoemission from solid surfaces into gas is important in view of the application of solid photocathodes in fast-RICH devices. The photoemission from CsI into gas has been investigated in He-, Ar- and CH4-based gas mixtures as a function of the electric field at the photocathode surface. Measurements were made both in laboratory, with a UV source, and in a beam with a RICH detector. The results are interpreted in terms of current models of electron transport in gas. The electron collection efficiency, below gas ionization threshold, is reduced by backscattering. This phenomenon is particularly important in He-based gas mixtures. In CH4 and Ar/CH4 mixtures the backscattering is very low. At high electric field, under charge multiplication, a full collection efficiency, similar to that in vacuum, is obtained in all gases investigated. We discuss the parameters governing the choice of the gas mixture in this kind of photon detectors.