THE CERES RICH DETECTOR SYSTEM

We describe the two RICH detectors of the CERES electron pair spectrom eter at the CERN SPS which are used for electron identification and, i n 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) u sing a mixture of He + 6% C2H6 (or CH4)+TMAE. The two UV-detectors are equipped with 53800 (48400) square pads. The front end electronics co nsists of modules with 256 (121) channels, based on a 64-channel charg e-sensitive preamplifier VLSI chip. The total readout time is 280 (160 0) mus per event. Subsets of the pad data are used as input to a fast trigger processor selecting events with at least two separated electro n rings. The trigger achieved an enrichment factor of approximately 10 0 in proton-induced interactions, and a factor of approximately 3 in S -32-Au collisions. The RICH detectors perform very close to their desi gn 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 sp atial resolutions of 1.0 and 0.8 mrad (rms) for individual photons are dominated by chromatic aberration and in very good agreement with the oretical expectations.