TEMPERATURE EFFECTS IN POSITRONIUM FORMATION AND LIFETIME IN SOLUTIONS OF NONPOLAR ORGANIC-SOLVENTS

The temperature dependence of the positron annihilation lifetime (PAL) spectra of several solutions are examined. Four solvents are investigated: n-hexane, cyclohexane, squalane, and an equivolume mixture of n-hexane and squalene. Solutes in the study are nitrobenzene, nitrocyclohexane, and hexafluorobenzene. Arrhenius plots of lifetimes are made from 10-degrees-C to 50-degrees-C. The temperature dependence of the PAL spectra of nitrobenzene solutions in a magnetic field varying up to 15 kG are also presented. Anomalous behavior is observed for the solvents n-hexane and squalane, both neat and with nitrobenzene as a solute. In addition, it is found that the behavior of neat and nitrobenzene solutions of the n-hexane-squalane mixture is not intermediate between those involving n-hexane and squalane separately. We conclude that there is a change in the mechanism for positronium formation from n-hexane to squalane which is not accounted for by changes in free volume. Nitrocyclohexane is found to enhance positronium formation in squalane. This is the first example of antiinhibition by an aliphatic solute known to the authors.