engleski

Operation of low-pressure proportional counter filled with argon-based gas mixtures

Mixtures of argon with propane, DME (Dimethyl-ether) and isobutane at total pressures P from 2 kPa to 90 kPa were studied in a low-pressure proportional counter. Strong non-uniform electric fields were thus produced at the anode surface (Sa). Deviations from the exponential dependence of gas gain on high voltage above a certain value of gas gain, M0, were observed in all mixtures. The best energy resolution has been obtained at low gas gains, and at gas gains above M0 resolution degrades much faster in "thin" mixtures due to avalanche chain formation. The reduced ionization coefficient a/P under non-equilibrium conditions is a function of mixture composition and of gas pressure. Crossings of a/P curves for different mixture compositions at the same total pressure are observed. The statistics of the avalanche size distribution (relative variance f) was determined by measuring the single-electron (SE) spectra. In pure gases and in mixtures with relatively high content of the molecular gas, the value is f < 1 (peaked distribution). In "thin" mixtures the SE spectra change the shape toward the over-exponential distribution, f > 1. The gas gain where f ~ 1 is roughly equal to the value of M0, showing that both effects are caused by insufficient quenching of the molecular admixture.

proportional counter; gas mixtures; gas gain; ionization coefficient

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