A multi-state model for the density dependence of ortho-positronium annihilation rate in gases

Abstract

We present a new model for the calculation of the annihilation rate of orthopositronium in gases. The measured positronium pick-off annihilation rate is identified with an expectation value over an ensemble of inhomogeneities of the fluid. A single density inhomogeneity is described with a predefined, suitably parametrised density profile and the expectation value is integrated over a reasonably selected volume in the parameter space. Comprehensive tests of the model are performed to determine the dependences on various model parameters and the results are compared with both experimental data and previously used theoretical models. Results for argon, xenon and methane are presented. The qualitative agreement with the experimental data is better throughout a wide density range than that obtained with previously used theoretical models.