Resistive Plate Chambers for the ALICE muon arm

The muon arm of the ALICE experiment at the LHC will study heavy flavour production in heavy-ion, p-A and p-p collisions, through their muonic decays, within the frame of the expected Quark Gluon Plasma formation. The trigger system of the muon arm consists of four plans of 18 single gap Resistive Plate Chambers (RPC), arranged in two stations covering an area of about 5.5x6.5 m2 each. The choice of the trigger detectors is the result of an intense R&D program. Beam tests have shown that low resistivity (~10 Ohm cm) RPCs meet the ALICE requirements in terms of efficiency, rate capability, position and time resolution. Two different gas mixtures have been studied: a streamer mixture for heavy ion collisions and a highly saturated avalanche one for p-p collisions. Ageing effects on the detector have been investigated at the CERN Gamma Irradiation Facility for both the streamer and highly staurated avalanche modes. RPCs have been assembled in Torino; all chambers have been tested with cosmic rays to select the 72 final detectors (plus a number of spares). All RPCs have been characterised and results stored in a database: these include efficiency, cluster size, noise, dark and leakage current. An overview on the tests of the final RPCs is given, together with the preliminary results of the ageing test performed on two small prototypes operated in highly saturated avalanche mode.