Complex dependence of the positron annihilation rate on methane gas density and temperature

Abstract

Annihilation rates have been measured for positrons in methane gas at temperatures of 22, 4.5, −28, and −50°C at densities in the range 6.6×10−4−1.1×10−1 g/cm3, in 22.5 and −28°C methane gas as a function of applied electric field strength and gas density, and in 22°C methane−argon gas mixtures as a function of argon concentration. A complex dependence of the slow positron annihilation rate on methane density and temperature was observed. The orthopositronium annihilation rate became temperature dependent at high densities. Both the positron−methane and the orthopositronium−methane annihilation rates were found to be independent of argon concentration in the gas mixtures. Electric fields produced a decrease in the slow positron annihilation rates, but had no measurable effect on the orthopositronium rates. The annihilation behavior of slow positrons is discussed in terms of several annihilation models, and empirical analyses of the data in terms of these models are presented. Only the single−stage formation of a positron−methane complex can account for all the observed phenomena.