The pad readout of the CERES RICH detectors

The CERES/NA45 electron spectrometer uses two Ring Imaging CHerenkov detectors with UV-sensitive gas counters of 2.84 m2 and 0.42 m2 size. The backplanes of these gas detectors consist of 53 800 (48 400) square pads in a 2.74 mm (7.62 mm) grid which pick up induced signals of ∼2 × 105 electrons from single-electron avalanches. Compact multi-channel preamplifier modules with low-noise VLSI chips are directly plugged onto the gas detector backplanes. The pad amplitudes are corrected for pedestal variations, digitized and transferred sequentially at 14 MHz in chains of up to 4096 pads to the control room. After zero suppression, the pad information is processed in a chain with pipeline organization, performing a transformation to xy-coordinates and branching part of the data to a trigger processor for fast ring finding. With a simple mechanical concept and a multiplexed data transfer, this readout method offers a good signal-to-noise ratio at gas gains of a few 105, high spatial resolution on large areas and unambiguous reconstruction of multiple hits at a cost of ≲6 DM/channel.