Angular Correlation Studies of Positron and Positronium Annihilation in Gases

A series of angular correlation spectra for positrons annihilating in the noble gases and in gas mixtures has been obtained using the University of East Anglia two-dimensional angular correlation spectrometer.l Beta positrons from a sodium-22 source are guided by a O.8T magnetic field to the interaction region, which is first evacuated and then filled with gas at 1 bar pressure at room temperature. Gamma cameras placed at 3m on either side of the sample chamber record the two-photon annihilations of free positrons and positronium (Ps) atoms in a 9mm-diameter volume of gas at the centre of the cell. Events recorded include annihilation from the triplet (m = O) — singlet state, which at O.8T is 91% mixed and has a mean lifetime of 9.7ns. In contrast, a negligible number of two-photon decays are rec — orded from (triplet) o-Ps atoms quenched on the (distant) walls of the gas cell or through collisions with gas atoms within the viewed volume.