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 wi th methane at room temperature and atmospheric pressure and a gas gain of about 10(5). Two different methods are used to determine the quant um efficiency (QE), The first uses Cherenkov radiation of beta-particl es in a 1 cm thick NaF window of the CsI-MWPC and measures the average ratio of the CsI quantum efficiency to a standard QE(s) (lambda) used in simulation. The second method uses a monochromator and compares th e count rates of the CsI-MWPC with the count rates of a Philips XP2020 Q photomultiplier, Results are presented for the dependence of the qua ntum efficiency on the photocathode production procedure, on the subst rate material, on the thickness of the CsI layer, on exposure time to ambient air, on heating of the photocathodes, on flushing them with me thane and on the cumulative charge collected.