Trigger detectors for the ALICE muon spectrometer

The muon spectrometer of the ALICE experiment will study the production of heavy quarkonia through their muonic decay, both in heavy ion and p-p collisions. The trigger system of the spectrometer will perform an event selection based on a cut on the muon transverse momentum. It consists of 72 single gap Resistive Plate Chambers (RPC), arranged in two stations of two detector planes each, covering an area of 140 m². The detectors are read out by means of 21k front-end channels, the signals are processed by fast electronics providing a trigger decision each 25 ns. The RPCs have been selected after a complete test in the INFN Technological Laboratory in Torino and now the full system is installed and operational at CERN. The system performances have been checked in commissioning runs with cosmic rays performed during the first months of 2008. Besides a description of the trigger system, results of the commissioning runs will be presented with particular regard to the detector performance