The CERES RICH detector system

We describe the two RICH detectors of the CERES electron pair spectrometer at the CERN SPS which are used for electron identification and, in conjunction with a novel silicon drift detector, for tracking in pp, pA and AA collisions. The RICH detectors are operated at a high γ_th {reversed tilde equals} 32 (CH₄ at 1 atm.) and are thus rather insensitive to hadrons. The UV-detectors are multistep counters with a multiwire chamber as the last stage. They are operated at gains of 2-4 × 10⁵ using a mixture of He + 6% C₂H₆ (or CH₄) + TMAE. The two UV-detectors are equipped with 53 800 (48 400) square pads. The front end electronics consists of modules with 256 (121) channels, based on a 64-channel charge-sensitive preamplifier VLSI chip. The total readout time is 280 (1600) μs per event. Subsets of the pad data are used as input to a fast trigger processor selecting events with at least two separated electron rings. The trigger achieved an enrichment factor of ∼ 100 in proton-induced interactions, and a factor of ∼3 in ³²SAu collisions. The RICH detectors perform very close to their design values. We observe clean Cherenkov rings with an average number of 11.2 (12.5) photons/ring. This is consistent with the calculated value N₀ = 131 (75) cm^-1 within the systematical error of about 10%. The spatial resolutions of 1.0 and 0.8 mrad (rms) for individual photons are dominated by chromatic aberration and in very good agreement with theoretical expectations.