CsI reflective photocathodes and their use for RICH detectors Experimental studies

This article first describes various tests on CsI reflective photocathodes (PC) performed in the laboratory to study the quantum efficiency QE (λ) dependence on the temperature and the ageing problems with the outgassing of the construction materials. Application to a fast RICH detector (0.5 m2 detector area) with cathode pad readout (5376 channels) is widely discussed. Detailed beam test results are reported concerning the QE dependence on the layer thickness, the choice of the CsI material (powder/microcrystals), the PC temperature and of the quencher used. A CsI QE of 18–19% is achievable at 180 nm, in rather close agreement with values already reported. However, it is shown that the avalanche gain in a MWC coupled to the PC is limited, making a full detection efficiency with the use of a fast readout impossible.