Three-Photon Annihilation of Ortho-Positronium in Argon Gas

Marked deviations from simple exponential decay are known to characterize the two-photon annihilation of positrons in the noble gases. In this experiment, a quadruple coincidence system was used to examine the detailed shape of three-photon positron lifetime spectra in argon. No deviation from a simple exponential was observed over the density range 2.57 to 33 amagat, except when a high electric field was applied. The ability to distinguish three-photon ortho-positronium annihilation from other modes of positron annihilation permitted, for the first time, a detailed investigation of the density dependence of the annihilation of ortho-positronium in a simple monatomic gas. Over the whole density range, the ortho-positronium annihilation rate was found to depend linearly on density, with a value of 7.15 ± 0.10 μs−1 for the vacuum rate of ortho-positronium annihilation and 0.250 ± 015 μs−1 amagat−1 for the quenching rate of positronium in argon. With a field of 125 V cm−1 amagat−1 (at 2.57 amagat), a deviation from simple exponential decay was observed, a deviation whose shape is shown to be consistent with an interpretation in terms of time-delayed enhanced positronium production from the equilibrium free positron population.