Laser beam studies of RPC behaviour in avalanche mode

Primary ionization is created inside the gas gap of a small size (10×20 cm2) RPC, analogous to the RPC of the ATLAS muon detector, by focusing a pulsed (width ⩽0.5 ns) nitrogen laser beam (transverse diameter ≈15 μm). 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% C2H2F4, 3% isobutane and a small addition of SF6), we have evidence of a space charge influence on RPC rate capability.