The Quenching of Ortho-Positronium in Helium

The lifetime of ortho-positronium has been m easured in helium at high pressures, yielding a mean quenching rate per atmosphere of 0.0123 A0. (A0 is the natural annihilation rate of ortho-positronium .) This small quenching rate can be explained qualitatively by a pick-off process, in which the positron of the positronium atom annihilates with an electron in the helium atom during a collision. Lifetimes were also measured in argon at a range of pressures and in freon at subatmospheric pressures.