Positronium formation in the noble gases

Positronium formation fractions have been determined from lifetime experiments for all the noble gases. Taken in conjunction with the positron beam total cross section measurements of Canter et al. (see abstr. A27547 of 1974), it is possible to estimate values for the positronium formation and positron excitation cross sections for helium and, with reduced accuracy, for neon and argon in the energy range from the positronium threshold E₀ to the ionization potential E₁. It is shown that these cross sections are approximately 0.2 pi a₀² at E₁ for helium and somewhat larger for neon and argon.