Study of argon-based penning gas mixtures for use in proportional counters

Abstract

Results from an experimental investigation of three Penning gas mixtures, namely argon-acetylene (Ar-C₂H₂), argon-xenon (Ar-Xe) and argon-xenon-trimethylamine (Ar-Xe-TMA), are reported. The measurements, carried out in cylindrical geometry as well as parallel plate geometry detectors, demonstrate that the Ar-C₂H₂ mixtures show a significant Penning effect even at an acetylene concentration of 10% and provide the best energy resolution among all the argon-based gas mixtures (≤13% FWHM at 5.9 keV and 6.7% at 22.2 keV). In the parallel plate detector the Ar-C₂H₂ fillings provide a resolution of ≃ 7% FWHM at 22.2 keV up to a gas gain of at least ≃ 104. The nonmetastable Penning mixture Ar-Xe provides the highest gas gain among all the argon-based gas mixtures and is well suited for use in long-duration space-based experiments. Best results are obtained with 5% and 20% Xe in Ar, the energy resolution being ≃7% FWHM at 22.2 keV and ≃ 45% at 59.6 keV for gas gain <~103. Addition of ≥1% TMA to an 80% Ar-20% Xe mixture produces a dramatic increase in gas gain but the energy resolution remains unaffected (≃ 7% FWHM at 22.2 keV). This increase in gas gain is attributed to the occurrence of a Penning effect between Xe and TMA, the ionization potential of TMA being 8.3 eV, just below the xenon metastable potential of 8.39 eV.