Helium gas mixtures for ring imaging Cherenkov detectors with Csl photocathodes

In the past few years a readout for Ring-Imaging Cherenkov (RICH) detectors [1] based on CsI photocathodes has been under intensive development [2-15] . The detector is a parallel-plate avalanche chamber (PPAC) with a UV-sensitive solid-CsI photocathode separated by a narrow gap (1-2 mm) from a wire-mesh anode plane (or wire anode plane as in ref. [4]) . The Cherenkov light generated in a radiator enters this PPAC through a UV window, passes through the anode mesh, and is detected by the CsI photocathode . It has been demonstrated that under low pressure (20 Torr of methane, ethane or isobutane) this kind of PPAC can be operated free of photon-feedback breakdown up to very high gas gain [16] . Its simple mechanical structure and easy-to-prepare solid photocathode (CsI does not oxidize [17]) would be very attractive for the high-energy-physics community. A bonus is that the PPAC itself is nearly blind to minimum-ionizing particles because of the low density of the gas [23] . However, it is not easy to construct a practical large-scale PPAC that has low mass, is leak tight and is rigid enough to maintain good gap uniformity . If the PPAC can be operated at atmospheric pressure with stable, high gas gain while remaining blind to minimum-ionizing particles, a workable device should be obtained . According to this guideline helium is the most promising candidate for the primary chamber gas. First,