Positron Annihilation in Condensed Gases

Abstract

The time distributions of positron annihilation in condensed ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$ have been measured. The free positron annihilation lifetime, ${\tau }_1$, for each substance shows a gross density dependence within a single phase and across the liquid-solid phase transition. For these materials studied ${\tau }_1$ is shorter than that predicted by the spin-averaged cross section for the $1S$ state annihilation pair and varies directly with $\frac{E_i}{{\rho }_e}$— the ratio of the first electron ionization potential to the outer shell electron density. Positronium is formed in all these substances. The orthopositronium pickoff lifetime, ${\tau }_2$, appears to be sensitive to the liquid-solid phases change in ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ but insensitive to the normal-superfluid state transition in He.