Positron annihilation in xenon

Positron lifetime measurements in the temperature range from 150 to 340 K and pressures up to 120 bar show localisation of positronium in all the three phases of xenon. The density functional calculations at 300 K combined with the density fluctuations qualitatively agree with the experimental results. Clustering of Xe atoms around positrons is not quite evident in the experimental results but it is supported by the theory. Incomplete thermalisation of the positron seems to show in the experimental data. A new explanation is suggested for the low positronium fraction extracted from the lifetime spectra at low densities. (author)