Abstract
The overlap of the positron component of the wave function of positronium (Ps) in dense molecular substances with the lattice wave function determines the annihilation rate of positrons bound in orthopositronium via electron pickup from the lattice ( decay), and in part the angular correlation. Ps acts as a probe for the dependence of one-particle wave functions in lattices on structural lattice parameters, and of radical-molecule interactions on atomic lattice parameters. To explore the sensitivity of this probe Ps wave functions are calculated for different lattice structures in the Wigner-Seitz approximation, and the dependence of decay on lattice parameters is derived. This dependence was tested experimentally by measurements on carefully characterized molecular substances in different physical states. It is confirmed that the temperature and state dependence is primarily a "free volume" effect in the sense that the overlap between the Ps and lattice wave functions decreases with increasing lattice spacing. The effects of a spherical confinement on the self-annihilation rates and stability of Ps are shown to be small for radii relevant to molecular lattices.
An anomalous behavior is found for the ice-water transition, where increases significantly despite the contraction of water between the melting point and 4°C.
- Received 16 March 1960
DOI:https://doi.org/10.1103/PhysRev.120.1289
©1960 American Physical Society