Performance of the CAPRICE98 balloon-borne gas-RICH detector

A RICK counter using a gas radiator of C4F10 and a photosensitive MWPC with pad readout has been developed, tested in particle beam at CERN and used i n 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 of the Cherenkov light. The induced signals in t he pad plane were read out using the AMPLEX chip and CRAMS. The good effici ency of the Cherenkov light collection, the efficient detection of the weak signal from single UV photons together with a low noise level in the elect ronics of the RICH detector, resulted in a large number of detected photoel ectrons per event. For beta similar or equal to 1 charge one particles, an average of 12 photoelectrons per event were detected. The reconstructed Che renkov angle of 50 mrad for a beta similar or equal to 1 particle had a res olution of 1.2 mrad (rms). The RICH was flown with the CAPRICE98 magnetic s pectrometer and was the first RICH counter ever used in a balloon-borne exp eriment capable of identifying charge one particles at energies above 5 GeV . The RICH provided an identification of cosmic ray antiprotons up to the h ighest energies ever studied (about 50 GeV of total energy). The spectromet er was flown on 28-29 May 1998 from Fort Summer, New Mexico, USA. (C) 2001 Elsevier Science B.V. All rights reserved.