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 ofbetter understanding the underlying physical processes of these detectors and of developping Montecarlo simulations of their behaviour.We present here a measurement of the drift velocity $v_{d}$and of the amplification coefficient $\eta$ in $C_{2}H_{2}F_{4}$-Isobutanemixtures. The ionization in the gas, contained in a 6$\times$22~cm$^2$, 2~mm gap RPC, is obtained by means of a pulsed $N_{2}$ laser,via double photon ionization. Results are presented for $v_{d}$ and $\eta$ as a function of the applied electric field.