Measurements of the first Townsend coefficient in gas mixtures at high fields

Summary form only. In order to understand the operating characteristics of drift chambers operating in proportional and streamer regimes, and to obtain better localization accuracy, the authors have measured the first Townsend coefficient α, for some argon-based mixtures, namely, with methane, ethane, isobutane, di-methylether, carbon-dioxide, and CF₄, and for similar helium-based mixtures, as a function of high field, at various pressures. The measurement has been performed by illuminating a drift chamber with a parallel-plate amplification gap by an FE55 source. To extend the useful range of the measurements, operation was at low pressure to reach high values of E/p