Measurement of drift velocity and amplification coefficient in C2H2F4–isobutane mixtures for avalanche-operated resistive-plate counters

The knowledge of the transport and amplification parameters (drift velocity, first Townsend and attachment coefficients) of the gas mixtures used for avalanche-operated resistive-plate counters gives the possibility of better understanding the underlying physical processes of these detectors and of developing Montecarlo simulations of their behaviour. We present here a measurement of the drift velocity vd and of the amplification coefficient η in C2H2F4–isobutane mixtures. The ionization in the gas, contained in a 6×22 cm2, 2 mm gap RPC, is obtained by means of a pulsed N2 laser, via double-photon ionization. Results are presented for vd and η as a function of the applied electric field.