Commissioning and first performance of the resistive plate chambers for the ALICE muon arm

The trigger for the ALICE muon spectrometer is issued by 72 single-gap, low-resistivity bakelite RPCs, arranged in two stations of two planes each. The area covered by one plane is 5.5×6.5m2. In order to operate RPCs both in streamer and avalanche mode, two different gas mixtures have been developed. After installation in the ALICE experimental hall the detectors have undergone a two year commissioning phase, both with cosmic rays and during early LHC operation. The results of the commissioning both in streamer and avalanche mode are presented, with particular regard to the RPC performance in terms of current, counting rate and efficiency. The detector behaviour during the first physics run (December 2009) is also highlighted.