Alpha-Particle Ionization in Argon-Methane Mixtures and the Energy Dependence of the Ion Pair-Formation Energy

Abstract

Experiments have been performed to investigate a long-standing discrepancy in the measurement of $\alpha$-particle energies by the ionization method. The work of Jesse and his collaborators, carried out in pure argon with a total collection of ions, indicates a linear relation between ionization and $\alpha$-particle energy for energies between 1 and 9 MeV. A different relation between ionization and $\alpha$ energy is observed in a large number of experiments with gridded pulse chambers; here, with collection of only the electronic component of the ionization in argon-methane (or argon-carbon dioxide) mixtures, an appreciable relative decrease of ionization is observed with decreasing $\alpha$ energy. Discussions in the literature have often attributed this difference between the two methods to the mode of collection of ions, whether total or electronic collection. The present experiments have investigated the possibility that the discrepancy in the two types of experiment is due to the different media used - whether pure argon or argon-methane mixtures. To this end, a series of measurements was carried out for four $\alpha$-particle energies, ranging from 1.58 to 5.3 MeV. For each of these particles, the ionization, relative to that in pure argon, was measured by a total collection method for a wide range of argon-methane mixtures. The results indicate a dependence with $\alpha$ energy in argon-methane mixtures considerably in excess of any possible dependence which may exist in pure argon. The variation with energy, moreover, in a mixture of argon with 6% methane is in good accord with past results obtained in gridded pulse chambers for this mixture. The present experiments would suggest therefore that the observed difference between the two methods would be the result, not of the method of ion collection, but of the different gaseous media used.