Measurements of the quantum efficiency of CsI photocathodes in a multiwire proportional chamber

The quantum efficiency of reflective CsI photocathodes in a multiwire proportional chamber (MWPC) has been measured. The MWPC is operated with methane at room temperature and atmospheric pressure and a gas gain of about 105. Two different methods are used to determine the quantum efficiency (QE). The first uses Cherenkov radiation of β-particles in a 1 cm thick NaF window of the CsI-MWPC and measures the average ratio of the CsI quantum efficiency to a standard QEs (λ) used in simulation. The second method uses a monochromator and compares the count rates of the CsI-MWPC with the count rates of a Philips XP2020Q photomultiplier. Results are presented for the dependence of the quantum efficiency on the photocathode production procedure, on the substrate material, on the thickness of the CsI layer, on exposure time to ambient air, on healing of the photocathodes, on flushing them with methane and on the cumulative charge collected.