Positronium Decay in Condensed Matter

In earlier papers [Phys. Rev. 120, 1289 (1960); 142, 231 (1966)] the pickoff annihilation rate of positronium was calculated in a cell approximation for various cell geometries. The rate can be expressed in terms of two functions $F(S,v^{*})$ and $\xi (S,v^{*})$ that depend on a cell potential parameter $S$ and the reduced cell volume $v^{*}$. These functions are rederived and previous errors corrected. Extensive numerical tabulations are made available and analytical approximations presented.