Positron annihilation in low-temperature rare gases. I. Helium

Abstract

The lifetime spectra of positrons annihilating in helium gas have been obtained over the temperature range 4.6-300 K and the density range 0.016-0.030 g/${\mathrm{cm}}^{3}$. The spectra are decomposed into slow-positron and orthopositronium annihilation components which are presented and discussed separately. At low temperatures, the density and temperature dependences of the data are found to depart markedly from the usual behavior observed at room temperature. Simple quantitative treatments are given of the cluster model for the slow-positron data and of the cavity model for the ortho-positronium results.