Annihilation of Positrons in Liquid Helium

It has been shown experimentally that positron annihilation in liquid helium exhibits three components of lifetime. At 4.2°K and one atmosphere pressure the longest lived component has a mean life of (9.1±0.5)×${10}^{-8\mathrm{}}$ sec, an intensity ∼13%, and has been shown experimentally to arise from $1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ positronium. The predominant intermediate component has a mean life of (1.83±0.15)×${10}^{-9\mathrm{}}$ sec at the same temperature and pressure and displays approximately the same fraction of three-quantum annihilations which one expects from free positron annihilation. The shortest component (∼2×${10}^{-10\mathrm{}}$ sec) is weak and is partially masked experimentally by annihilation in source materials. It would appear to arise from $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ positronium.