Temperature and phase dependence of positron lifetimes in solid cyclohexane

Abstract

Abstract The temperature dependence of position lifetimes in both the brittle and plastic phases of cyclohaxane has been examined. Long-lived components in both phases are associated with the formation of positronium (Ps). Two long lifetimes attributable to ortho-Ps are resolvable in the plastic phase. The longer of these (≈ 2.5 ns), which is temperature dependent, is ascribed to ortho-Ps trapped at vacancies. The shorter lifetime (≈ 0.9 ns), shows little temperature dependence. In contrast to most other plastic crystals, no sigmoidal behaviour of the average ortho-Ps lifetime is observed. A possibility exists that some self-trapping of Ps may occur just above the phase transition. In the brittle phase only one long lifetime of ≈ 1.1 ns is resolvable. The presence of 100 ppm impurities reduces the ortho-Ps yield, especially in the brittle phase, but does not influence the lifetimes. The ratio between the apparent ortho-Ps and para-Ps, yields is close to 3 for the pure-crystals in the plastic phase, but smaller in the brittle phase. A correlation is established between the molecular packing coefficients for plastic and brittle crystals and the ortho-Ps lifetime in the crystals.