EXPERIMENTAL STUDIES OF THE ANNIHILATION OF POSITRONS IN THE NOBLE GASES

Abstract

The direct annihilation rate of positrons in He, A, and Kr has been studied. Deviation from an exponential for the initial portion of the time distribution of these events is observed and is interpreted as a velocity dependence of the direct annihilation rate. The product of the gas pressure and the width of the non-exponential portion of the time distribution has a value of 270 nsec atm and is approximately independent of the pressure. An estimate of the fraction of positrons forming positronium in argon yields a value of 0.37 ± 0.03. Using this result, a check of the statistical ratio of ortho- to para-positronium formation indicates that 69 ± 7% of the positronium is formed in the ortho state.