Positronium lifetimes in helium-oxide-powder mixtures

The effects of a high-density interstitial helium gas on the ortho-positronium (o-Ps) decay rate in the metallic-oxide powders, SiO2 and MgO, have been investigated. Measurements were made at gas densities 0-170 Amagat (at 293K) and 0-530 Amagat (at 77K). A model is presented in which the annihilation probability is expressed in terms of an effective barrier height against o-Ps penetration into the powder. A lower limit of 0.44 eV for the barrier height is obtained from the data. Indications of an energy loss for Ps moderation in helium gas significantly larger than that in evacuated oxide powders are also discussed. (author)