Laser beam characterization of the ATLAS RPC gas mixture

A measurement of the electrons drift velocity in C2H2F4-based gas mixture has been performed and results have been compared with calculations. Primary ionization is induced in the gas via double photon ionization process by mean of a pulsed Nitrogen laser. The results of the drift velocity, obtained at room temperature and normal pressure, are presented as a function of the electric field strength. To perform the measurements we used a small sized RPC prototype with a 2 mm gas gap delimited by 2mm-thick linseed-oil-treated bakelite plates with resistivity of about 1.71×1010Ω cm at 20°C.