Positron Annihilation in Helium

Positron mean lives were measured in helium gas over a wide temperature range at densities from 4.6 to 534 times the STP density and in liquid helium at 4.2 and 5.1°K. The orthopositronium mean life in the liquid at 5.1°K was found to be (9.4±0.6)×${10}^{-8\mathrm{}}$ sec which differs little from the lifetime in liquid at 4.2°K. Except at very low temperatures, the pick-off rate in gas agreed moderately well with theory. The low pick-off rate associated with orthopositronium in liquid was observed also in the gas at temperatures below 9°K for densities greater than 250 times the STP density.