Field-dependent photoelectron extraction from CsI in different gases

We present the results of our latest investigations, made in the laboratory and in a RICH detector, of the dependence of the UV quantum efficiency (QE) of CsI on the gas and the electric field. We confirm that in charge collection mode the QE in a gas is generally lower than in vacuum due to electron backscattering; in charge multiplication mode the QE increases with the field and reaches the vacuum value at high gas gains. The lowest photoemission under low electric field at the photocathode surface was observed in He-based gas mixtures; in Ar-hydrocarbon mixtures it approaches that of pure hydrocarbons. In CH4 and in CH4/i-C4H10 the QE value is practically independent of the field and is close to that in vacuum. The implications of CsI-based W-detectors, particularly to fast RICH devices, are discussed.