Positron and orthopositronium decay rates in helium at high densities

Positron and orthopositronium ($o$-Ps) decay rates have been obtained in helium gas over the density ranges 72-200 and 80-520 amagat at gas temperatures 300 and 77°K, respectively. The 77°K $o$-Ps results exhibit the onset of bubblelike behavior observed by other investigators. The 300°K $o$-Ps decay rates are in agreement with a linear extrapolation of the high-precision low-density results obtained by Coleman et al. and are in marked disagreement with the density-dependent pickoff annihilation coefficient proposed by Ferrell. The free-positron decay rates are the first to be obtained above 200 amagat at well above saturated vapor temperatures. Above 200 amagat, the free-positron $Z_{\mathrm{eff}}$ is found to exhibit an unambiguous decrease with increasing density. At 520 amagat, for example, $Z_{\mathrm{eff}}=3.71\mathrm{}\pm 0.02$, as opposed to the density-independent value, $Z_{\mathrm{eff}}=3.89\mathrm{}\pm 0.04$ observed in the low-density regime.