Abstract
Positron lifetimes and Doppler broadening of annihilation radiations have been measured on high-purity condensed methane at temperatures ranging from 30 K to 150 K. In the solid phase, the o-Ps lifetime and S parameter increase sigmoidally as a function of temperature. This variation is interpreted as being due to a Ps atom being trapped in thermally activated vacancy-type defects. A vacancy formation enthalpy of 0.10+or-0.02 eV is obtained in solid methane. In the liquid phase, large values for the o-Ps lifetime and intensity, and strong temperature variation of these values are observed. These results are interpreted as being due to the localization of the Ps atom in a bubble state. The temperature dependence of Ps bubble size and microscopic surface tension are evaluated using o-Ps lifetime data.
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