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, Delta R, entering in the semi-empirical Eldrup equation, is proposed: Delta R = (0.191 +/- 0.007) nm; it explains some of the discrepancies previously found in crystalline solids. (C) 1998 Elsevier Science B.V. All rights reserved. [References: 15]
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