Ageing test of RPC for the Muon Trigger System for the ALICE experiment

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.