Determination of Positronium – Atom Collision Cross Section

We have determined the collision cross section of positronium in He, Ne, Ar, Kr, Xe, N2, and isobutane gases at thermal energies by studying the diffusion of positronium to the walls of a specially designed chamber. The time distribution of annihilations is measured as a function of pressure for each gas. The presence of an admixture of 20 Torr of isobutane in the chamber quenches the free positron component so that the orthopositronium component is the only exponential decay in the time spectrum of annihilations. Diffusion theory is used to obtain theoretical values of the orthopositronium decay constant λ2. These values are a function of three parameters, two of which are common to all results; the third one is σc, the collision cross section. Monte Carlo simulations of the isobutane experiments verify the theoretical results and determine the wall annihilation parameter which is common to all experiments. The Monte Carlo calculations also enable the determination of the second parameter common to all the experiments, which occurs in a correction term in the diffusion theory. The entire analysis has been carried out twice, one in each of the following extreme cases: (1) assuming that no triplet to singlet conversion takes place at the walls of the cells, and (2) assuming that the maximum conversion takes place at the walls.The cross sections in case (1) are, in units of πa02: He, 0.0166 ± 0.0017; Ne, 0.042 ± 0.003; Ar, 0.086 ± 0.005; Kr, 0.099 ± 0.006; Xe, 0.128 ± 0.008; N2, 0.238 ± 0.014; and isobutane, 0.534 ± 0.016. The implications for other types of experiment and theory are briefly discussed, in particular the bubble model for positronium in liquids.