Positron lifetime spectra for gases

Recent observations on the lifetime spectra for gases are presented and discussed. There is little to report on the inert gases helium, neon, and argon and the spectra for these gases are thought to be understood. New lifetime data for krypton and xenon have revealed two fast components which, although probably connected with the low positronium fractions for these gases, have yet to receive a satisfactory interpretation.The polarized orbital calculations for Zeff's and momentum transfer cross sections for all the inert gases by the York group are now complete and are generally in good agreement with experiment.For the molecular gases some information on rotational excitation cross sections has been obtained from thermalization times for nitrogen, hydrogen, and deuterium. In general, the [Formula: see text] parameter is dependant on gas density and temperature with some gases showing a pronounced maximum in the density dependance. The gases C3H8, C4H10, and CH3Cl have very large values of [Formula: see text] which indicate localization or capture of the positron by one or more molecules. These three gases exhibit a maximum in the instantaneous decay rate of the "free" positron component at low densities from which a "capture" cross section can be estimated.The positronium fraction, F, generally increases with the gas density, usually approaching a limiting value asymptotically, but for nitrogen this density dependance has a maximum at ~ 140 amagat. No satisfactory explanation has been offered for this behaviour.Positronium formation cross sections for several gases have been deduced from measurements of the variation of F with the concentration of the gas in helium. These cross sections are similar in magnitude to those deduced from the total cross section beam measurements.