Experimental studies of the annihilation of positrons in xenon show an anomalously large decay rate coupled with unusual nonlinear dependence on the density. In this work we use the path-integral Monte Carlo technique to study the equilibrium properties of a positron that has thermalized in xenon fluid. The decay rate and the correlation functions that describe the equilibrium structure of the interacting positron-xenon system are calculated on the isotherm T=340 K for the range of xenon densities studied experimentally. In contrast to density-functional theory, the most successful theory to date, path-integral Monte Carlo results give an accurate description of the positron-fluid system over a wide range of thermodynamic variables. In direct agreement with experiment, the calculations show that the decay rate increases nonlinearly with xenon density and that the degree of localization of the positron is a continuous function of the fluid density. The anomalous behavior of the decay rate with fluid density, strongly nonlinear at low density and then saturating at higher densities, is demonstrated to be a manifestation of the clustering of xenon atoms around the positron at low to moderate densities. The calculations clarify the connection between the localization of a positron and its annihilation in a highly polarizable fluid.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics