Tests of a large area MWPC with a CsI photocathode

Abstract The relative quantum efficiency of several CsI photocathodes in an asymmetric multiwire proportional chamber has been measured. The photocathode surface of 24 × 24 cm 2 is divided into 7.5 × 7.5 mm 2 pads and the chamber is operated with methane at normal temperature and pressure and a gas gain of about 10 5 . During tests of the high rate performance (∼2 MHz/cm 2 ) of a smaller chamber (5 × 5 cm 2 ), an interesting time variation of the count rate has been observed when illuminating the chamber with constant UV light.