Positron annihilation in low-temperature rare gases. II. Argon and neon

Lifetime measurements of slow-positron and ortho-positronium (o-Ps) annihilation have been made in argon and neon gases at room temperature and below. The argon experiments cover the temperature range 115-300 K and the density range 0.0356-0.0726 g/cm³ ({} 20-40 amagat). The slow-positron spectra in argon exhibit a departure from free-positron annihilation below 200 K. The departure becomes more marked as the temperature is lowered. No deviation from free o-Ps pickoff annihilation is observed in argon at low temperatures. The neon measurements cover the temperature range 30-300 K and the density range 0.032-0.89 g/cm³ ({} 35-980 amagat). No effect of temperature on the slow-positron spectra throughout the temperature and density ranges investigated in neon is observed. The spectra are very exponential with a corresponding decay rate which is temperature as well as time independent and is directly proportional to density over the ranges investigated. The o-Ps data are more eventful in that the o-Ps lifetime at near-liquid densities is approximately 20 nsec, a factor of nearly 4 greater than the value obtained using the pickoff-annihilation coefficient obtained at lower densities. This is evidence for positronium-induced cavities in low-temperature neon. A brief discussion of the argon and neon results is given in the context of the explanations offered for the low-temperature effects observed in helium gas.