Ionization coefficient in propane, propane-based tissue equivalent and dimethyl-ether in strong non-uniform electric fields

Gas amplification in a proportional counter filled with propane, propane-based tissue-equivalent gas mixture and dimethyl-ether is measured. By using low pressures (2-50 kPa) relatively strong reduced electric fields are produced, which enabled a study of non-equilibrium effects. The gas gain data are analysed according to the field-gradient model and the reduced ionization coefficient /P is thus obtained. The assumption of electrons being in equilibrium with the electric field fails to be valid in low-pressure gases, where the electric field strengths at the anode surface, S_a, above about 5 × 10⁵ V m^-1 kPa^-1 are obtained. Under non-equilibrium conditions /P is not a unique function of the reduced electric field strength, but depends on the gas pressure as well. Good agreement with other experimental or theoretical /P obtained mostly under equilibrium conditions is obtained at relatively weak electric field strengths. The presented /P in strong fields may be used for a more accurate modelling of low-pressure counters.