Measurements of Quenching of Ortho-positronium in Gases

Abstract

Three photon annihilations of ortho-positronium in gases as a function of pressure have been investigated using a scintillation spectrometer technique. Quenching rates per atmosphere were determined for argon (0 035λ0), neon (0.024λ0), helium (0.014λ0), oxygen (3λ0), nitric oxide (400λ0), and carbon dioxide (0.08λ0), the last three being only very approximate. λ0 is the natural annihilation rate of ortho-positronium, 0.7 × 107 sec-1. The high quenching rates in oxygen and nitric oxide are attributed to ortho-to-para conversion by electron exchange. The order of magnitude of the smaller quenching rates is understandable if the positronium atom annihilates with an electron of a gas molecule during collision (pick-off quenching).