A. Schön

The CERES RICH detector system

Abstract 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 ⋍ 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 32S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.

A highly efficient low-pressure UV-rich detector with optical avalanche recording

Abstract UV photons from a Cherenkov radiator are multiplied in a multistep avalanche chamber operating in a gated mode at low gas pressure (40 Torr). The gas mixture is C 2 H 6 -argon ( 80 20 ) and TMAE at 34°C. Visible light emitted from single photoelectron avalanches is detected by a CCD camera coupled to an image intensifier system. The detector was tested with 5 GeV c electrons, using a CH 4 radiator gas at 1 atm. Cherenkov rings essentially free of particle background and of secondary photon feedback were obtained in this mode of operation with a mean number n ≅ 11.5 ( N 0 ≅ 76 cm −1 ). We present this new method and discuss its performance.

Notice of Violation of IEEE Publication Principles Pad readout for gas detectors using 128-channel integrated preamplifiers

A novel two-dimensional readout scheme for gas detectors is presented that uses small metal pads with 2.54-mm pitch as an anode. The pads are read out via 128-channel VLSI low-noise preamplifier/multiplexer chips. These chips are mounted on 2.8-cm*2.8-cm modules, which are directly plugged onto the detector backplane, delay-chained with jumpers, and read out sequentially. The readout has been successfully tested with a low-pressure, two-step, TMAE-filled, ultraviolet-sensitive ring imaging Cerenkov counter. A single-electron efficiency of >90% was observed at moderate chamber gains (<10/sup 6/). The method offers high electronic amplification, low noise, and high readout speed with a very flexible and compact design that is suited for space-limited applications.<<ETX>>

In beam performance of a low pressure UV rich detector

A low-pressure multistep, TMAE-filled, UV-sensitive detector has been developed for Cerenkov ring imaging for ultrarelativistic heavy-ion experiments (HELIOS-CERN). A description is given of the detector structure and its basic properties, and the experimental setup and the detector performance in tests with high-energy electrons are presented. Three modes of readout are used: three-wire coordinates and flash analog-to-digital converters; pad readout; and optical recording of the avalanches. Single photons can be localized with an accuracy of sigma approximately=2 mm, mainly due to single-photoelectron diffusion and to chromatic dispersion in the radiator gas. The experimental N/sub 0/ values obtained by the three methods are close to those expected, taking into account the various losses due to the test conditions. >

The pad readout of the CERES RICH detectors

Abstract The CERES/NA45 electron spectrometer uses two Ring Imaging CHerenkov detectors with UV-sensitive gas counters of 2.84 m 2 and 0.42 m 2 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 × 10 5 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 10 5 , high spatial resolution on large areas and unambiguous reconstruction of multiple hits at a cost of ≲6 DM/channel.

In-beam experience from the CERES UV-detectors: prohibitive spark breakdown in multi-step parallel-plate chambers as compared to wire chambers

Abstract The UV-detectors of the CERES/NA45 experiment were originally conceived as parallel-plate counters with two-step amplification, an intermediate gate-electrode pair, and a final drift stage towards a pad electrode. Operated in beams of a few 106/burst protons or 32S-nuclei at 200 GeV/u, this scheme was found to suffer from excessive spark rates, even in the gated mode. The origin of the sparks is quantitatively explained by event-correlated slow secondary particles, creating avalanches above the critical threshold of ∼ 108 charges at the required gain of a few 105. This lack of sufficient dynamic range is also present in other schemes proposed in the literature; it severely limits the use of large-area parallel-plate counters for single-electron detection in a realistic high-energy physics environment. Possible improvements resulting from resistive anodes instead of metal meshes are also discussed. The spark problem of the CERES UV-detectors was solved by introducing wire amplification as the last stage. Laboratory tests showed a large increase in the dynamic range, due to a strong reduction (by factors of at least 20–30) of the net gain of the wire plane for high input charges via space-charge limitation. The residual spark rates of this scheme are lower by orders of magnitude and quite acceptable even for 32S-beams; gating was found to be unnecessary.

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p⊥<2.0 GeV/c the yield andp⊥-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.

First results of the CERES electron pair spectrometer from p + Be, p + Au and S + Au collisions

Abstract The CERES experiment (CErenkov Ring Electron Spectrometer) studies the production of low mass e + e − pairs in proton-proton, proton-nucleus and nucleus-nucleus interactions at the CERN SPS. The CERES spectrometer, has a novel design based on two Ring Imaging Cherenkov (RICH) counters, and it operates close to its design specifications. Data were recorded with 200 GeV u sulfur beam and 450 GeV proton beam. The analysis is in progress. We have extracted first e + − -pairs samples for p+Be, p+Au and S+Au collisions. In addition other physics topics were addressed. Inclusive photon spectra were measured in S+Au interactions. No excess over known hadronic sources was found within our present systematic error of 11%. Results on high p i charged pion spectra are presented up to 4 GeV c . We also studied the production of e + e − -pairs m the strong electromagnetic fields of very peripheral S+Pt collisions. The data are well described by a first-order perturbative QED-calculation.