Performance of the CAPRICE98 balloon borne gas-RICH detector

A RICH counter using a gas radiator ofC 4F10 and a photosensitive MWPC with pad readout has been developed, tested in particle beam at CERN and used in the CAPRICE98 balloon-borne experiment. The MWPC was operated with a TMAE and ethane mixture at atmospheric pressure and used a cathode pad plane to give an unambiguous image ofthe Cherenkov light. The induced signals in the pad plane were read out using the AMPLEX chip and CRAMS. The good efficiency ofthe Cherenkov light collection, the efficient detection ofthe weak signal f rom single UV photons together with a low noise level in the electronics ofthe RICH detector, resulted in a large number ofdetected photoelectrons per event. For b ’ 1 charge one particles, an average of12 photoelectrons per event were detected. The reconstructed Cherenkov angle of50 mrad f a b ’ 1 particle had a resolution of1 :2 mrad (rms). The RICH was flown with the CAPRICE98 magnetic spectrometer and was the first RICH counter ever used in a balloon-borne experiment capable ofidentifying charge one particles at energies above 5 GeV. The RICH provided an identification of