Spatial resolution of RPC in streamer mode

In the framework of an R&D program on the ALICE dimuon trigger system, the spatial resolution of Resistive Plate Chambers working in streamer mode has been investigated during beam tests. Preliminary studies with cosmic rays had evidenced a dependence of the charge profile to the high voltage and to the gas mixture. However, the spatial resolution measured from the beam tests is only slightly varying with the high voltage and is close to the expected value w/ 12 where w is the strip pitch, for strips of 1 and 2 cm width. The probability to fire a strip as a function of the distance from the strip to the particle impact has been measured for various high voltages. A simple parametrisation of this probability has been achieved. This allows to predict, under various working condition, the cluster size distributions for RPCs with different strip widths and also to account for the effect of the particle incident angle.