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 gamma(th) congruent-to = 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 x 10(5) using a mixture of He + 6% C2H6 (or CH4)+TMAE. The two UV-detectors are equipped with 53800 (48400) 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) mus 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 approximately 100 in proton-induced interactions, and a factor of approximately 3 in S-32-Au 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.