Performance of glass RPC operated in streamer mode with four-fold gas mixtures containing SF6

We have studied the performance of a glass Resistive Plate Chambers (RPC), operated in streamer mode, with several gas mixtures composed by argon, C2H2F4, ISO-C4H10 and SF6. A small percentage of SF6 strongly reduces the streamer size, increasing the RPC rate capability. The UV photons involved in the streamer process are also decreased, allowing their absorption with an isobutane percentage well below the flammability limit. Besides the streamer reduction, the SF6 increases the size of the avalanche signal preceding the streamer. By exploiting the relatively large amplitude of the prompt precursor, a time resolution of about 1 ns (r.m.s.) has been obtained over the whole full efficiency region. Our measurements suggest that the SF6 cannot replace the C2H2F4, which must have a level of about 50%, in order to minimize the amount of afterpulses