Measurement of relative quantum efficiency of CsI photocathodes

The response of CsI photocathodes widely used in Fast RICH detectors is greatly affected by the environment in which the CsI is deposited. It has been shown in previous studies that the exposure of the CsI to air is detrimental to the CsI photocathode response. In this work we introduce a novel spraying technique where the whole chamber is exposed to nothing but a controlled dry argon atmosphere and the ambient gas environment in which measurements are taken, i.e., methane. All results concerning the CsI are obtained relative to copper. A relative quantum efficiency of 400 to 700 was measured in the wavelength range 170-180 nm. We also investigated the reproducibility of our measurements and estimated the CsI long wavelength cut-off to be approximately 213 nm. Finally, the deterioration of CsI photocathodes is studied when the chamber is sealed in a methane environment