Laser beam studies of RPC behaviour in avalanche mode

Primary ionization is created inside the gas gap of a small size (10 multiplied by 20 cm**2) RPC, analogous to the RPC of the ATLAS muon detector, by focusing a pulsed (width less than approximately equals 0.5 ns) nitrogen laser beam (transverse diameter approximately equals 15 mum). With this set-up we can characterize the behaviour of the RPC by measuring such parameters as drift velocity and gas amplification. For an ATLAS-like gas mixture (97% C//2H//2F//4, 3% isobutane and a small addition of SF//6), we have evidence of a space charge influence on RPC rate capability.