Lifetime Effects of Positronium in Powders

Abstract

A recently reported precision lifetime measurement of orthopositronium in ultrafine Si${\mathrm{O}}_2$ powders gives a decay rate, extrapolated to zero powder density, which is 2% below the theoretical free-space rate. In this Comment the influence of the powder environment is discussed, including effects of surface bound states, collisional interactions with the powder grains, and Stark shifts of the decay rate. The conclusion is that none of these effects can explain the observed discrepancy in decay rate.