Corrections to the prevalent bubble model of positronium annihilation in liquids

The prevalent bubble model to account for the medium dependent pick-off process for positronium (Ps) annihilation in liquids is based on the notion of a bubble (or cavity) in which Ps gets self-trapped. This description, however, suffers from several rather unrealistic features. The Ps atom is treated as a structureless point particle, the potential responsible for its entrapment (as well as the molecular density profile of the cavity) is taken to have a sharp and discontinuous boundary, and the expected change in the surface tension from its bulk value (due to the curvature effects in such microbubbles) is neglected. We demonstrate that all these ad hoc assumptions can be corrected for in a rather simple manner. without the introduction of any new free parameter. The finite size of the positronium atom taken in conjunction with the diffusivity of the bubble boundary plays a crucial role. As a consequence, the discrepancies in the prediction of the annihilation characteristics are removed and satisfactory agreement with observations is achieved.