ALPHA PARTICLE IONIZATION OF ARGON MIXTURES--FURTHER STUDY OF THE ROLE OF EXCITED STATES

BS>The average energy required to form an ion pair, W, when alpha particles are absorbed in mixtures of argon with other gases is studied. The other gases were selected on the basis of their ionization potentials. One group consists of representative gases that have ionization potentials below that of Ar (15.77 ev) and above the doublet metastable state of Ar (11.49 and 11.66 ev). This list includes methane, carbon dioxide, nitrous oxide, xenon, and krypton. The other group of special interest consists of some gases with ionization potentials below the metastable state of Ar. All gases tested in these two groups cause an increase in ionization, i e., a decrease in W, when added to argon. As an illustration of this effect, the addition of 1/2% of acetylene to Ar will increase the ionization by 23% The experimental data, i.e., W as a function of relative concentration of the two gases, were fitted to a model in which it is assumed that energy is transferred from two excited levels in argon to the additive gases in collision processes. Good agreement between the experimental data and calculations based on the model is taken as additional evidence that the excited state notion is correct. A quantity of particular interest, the ionization efficiencies for the additive gases, was derived from the data fitting procedure and compared with those found in photoionization studies in a few of the cases. (auth