MEASUREMENT OF CHERENKOV ANGLES WITH A CSI PHOTOCATHODE IN A MULTIWIRE CHAMBER

Cherenkov rings caused by cosmic rays in 1 cm thick NaF and LiF radiat ors have been measured with a 10 X 10 cm2 multiwire chamber. One catho de plane of the multiwire chamber is divided into 1 X 1 cm2 pixels, wh ich are covered with a 500 nm thick layer of CsI. The distance between the CsI cathode plane and the anode wire plane is only 0.5 mm enablin g high, fast and narrow signals. The main features of the measured ang ular distribution are predicted by Monte Carlo simulation for beta = 1 particles. The measured rms resolution of the Cherenkov angle determi ned from a single photon is 51 mrad for NaF and 40 mrad for LiF with c ontributions from dispersion and photon detector position resolution b eing of the same order of magnitude.