Annihilation of Positrons in Methane at High Densities

Measurements of positron-lifetime spectra in methane gas at 23°C, previously reported at gas densities less than 1.5 × ${10}^{-2\mathrm{}}$ g/${\mathrm{cm}}^3$, have now been extended to a density of 5.2 × ${10}^{-2\mathrm{}}$ g/${\mathrm{cm}}^3$. The quenching rate of orthopositronium continues to be proportional to the gas density and corresponds to ${}_{}{}^1{}_{}{}^{}Z_{\mathrm{eff}}^{}{}_{}{}^{}=0.437\mathrm{}\pm 0.010$. The annihilation rates of positrons which do not form positronium deviate markedly from the linear density dependence observed at lower densities.