Positronium lifetimes in porous Vycor glass

The ortho-positronium lifetimes in porous Vycor glass were measured in a wide range of pore radii. A good correlation of the most probable lifetime with the model accounting for the population of upper levels of a particle in the potential well was observed. For channel-like voids (capillaries in porous media) a new value of the empirical parameter, ΔR, entering in the semi-empirical Eldrup equation, is proposed: ΔR=(0.191±0.007) nm; it explains some of the discrepancies previously found in crystalline solids.