Fast RICH Detector with a Caesium Iodide Photocathode at Atmospheric Pressure

CsI photocathodes of 30 30 cm2 size have been operated in a fast RICH detector composed of a NaF radiator and a multiwire proportional chamber with a cathode-pad readout. Results are presented from tests with a 3 GeV / c proton beam. A Cherenkov angular resolution of 8 mrad is obtained by detecting a mean of 8 photoelectrons per ring. Methods are described for counting the Cherenkov photon and evaluating the differential quantum efficiency of the CsI photocathode. The results obtained with photons impinging at a large angle on the CsI layer are presented and discussed in comparison with higher quantum efficiencies observed under different experimental conditions.