Search

The trigger detectors for the Muon Forward Spectrometer of the ALICE experiment will be single gap RPCs with low-resistivity bakelite electrodes. The detector will take data for different colliding systems resulting in a wide range of working conditions and requirements, especially concerning position resolution and lifetime. To this aim we have investigated the possibility to operate the RPCs both in streamer and in saturated avalanche mode with the same FE electronics. The study of the detector performance after long irradiation periods has been carried out at the Gamma Irradiation Facility at CERN with different gas mixtures: a quenched streamer gas mixture (51.5% Ar, 41.3% forane, 7.2% C/sub 2/H/sub 10/ and 1% SF/sub 6/) and a saturated avalanche mixture (88% forane, 10% C/sub 2/H/sub 10/ and 2% SF/sub 6/). We present the results of the ageing test of a final RPC in "ALICE-like" irradiation conditions using the streamer gas mixture and the results of the ageing test of a small prototype with the avalanche gas mixture.

ALICE (a large ion collider experiment) is the heavy-ion dedicated experiment at LHC. The resistive plate chamber detector (RPC) with low-resistivity bakelite electrodes was selected for the trigger system of the muon spectrometer. Although the main goal of ALICE is the study of nucleus-nucleus collisions, reference data in p-p interactions will be collected as well. If, on one hand, RPC operation in streamer mode is adequate for data taking with ion beams, on the other hand, the more severe ageing requirements for p-p data taking lead us to explore the possibility of operating the detector in "highly-saturated avalanche" regime. A detailed study of the signal was carried out with cosmic rays to get a more precise view of the intrinsic properties of the gas mixture (among others the streamer fraction vs. HV). The possibility of detecting avalanche signals with our FEE designed for the streamer mode was successfully investigated. Furthermore, tests with muon beam at CERN were carried out with satisfactory results in terms of efficiency, time resolution and cluster size. An analysis of the exhaust gas showed a low HF content: this is an encouraging result in view of long term stability of the detector which is at present under test.