| Summary: | The annihilation rate, λ[sub e] , of positrons in unsaturated, saturated and liquid argon has been measured over the temperature range of 100 K to 148 K with applied electric fields up to 40 V cm⁻¹ amagat⁻¹. The data was analysed in terms of the classical distribution for self-nucleating clusters which depends on the difference in chemical potential, Δg, between the gas state and metastable liquid state (at the same temperature and pressure). Results of this analysis suggest that the low temperature enhancement of the annihilation rate arises from interactions of the positron with clusters of argon atoms, the size of the clusters being surprisingly small, of the order of 10 atoms or less. In addition, experimental evidence supporting the value of the pre-exponential factor determined by Kikuchi (1969) was also obtained. A growth model, consisting of a free and localized positron population was developed which accurately describes the saturation characteristic of the results, in particular the transition in the annihilation rate from a density proportional dependence to a rate, λ*, linearly dependent on Ag. For the saturated gas, for example, where Δg = 0, λ[sub e]/D = 40.0±0.6 μs⁻¹ amagat⁻¹ and λ* = 1.2 ns⁻¹. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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