N.J. Pedersen

Temperature dependence of the inhibition of positronium by chlorine-substituted hydrocarbons in non-polar liquids

Abstract Positron annihilation lifetime spectra were measured for solutions of 1,2,3,5-C 6 H 2 Cl 4 in hexane, toluene, m -xylene and mesitylene, CCl 4 in hexane and toluene, and C 2 HCl 3 in n -hexane for concentrations below 1 M and at various temperatures between −30°C and 67°C. The Ps inhibition by C 6 H 2 Cl 4 was roughly 14 times stronger at −30°C than at 67°C in toluene, m -xylene, and mesitylene, while that of CCl 4 displayed hardly any temperature dependence in hexane and toluene. Previously, the Ps inhibition by C 6 H 2 Cl 4 in various liquids at 20°C was explained by a competition between electron pick off by the positron from C 6 H 2 Cl 4 − formed by electron scavenging in the positron spur, and dehalogenation of the same anion at picosecond times after the spur formation. Three effects might reasonably explain the strong temperature dependence of the Ps inhibition of C 6 H 2 Cl 4 : (1) The dechlorination time of C 6 H 2 Cl 4 − might be much shorter at low than at high temperatures. (2) The positron mobility might decrease very strongly with decreasing temperature. (3) Positrons possibly trapped on C 6 H 2 Cl 4 might thermally detach at high but not at low temperatures.

Positron annihilation in pivalic acid. Temperature dependence of angular correlation curves

Abstract Positron annihilation angular correlation curves have been measured as a function of temperature for trimethylacetic (pivalic) acid in both the brittle and plastic phases. A simple fitting of the data to a sum of three gaussians shows the presence of a narrow component due to para-positronium ( p -Ps) annihilation. In the brittle phase the intensity of the narrow component is inconsistent with previous positron lifetime data. A more detailed analysis, requiring consistency with the lifetime data, results in the determination of the shapes of the angular correlation components for free positron-, pick-off-, and p -Ps intrinsic annihilation. The p -Ps component has a width (fwhm) of 3.75 mrad in the brittle phase, probably due to Ps self-trapping or trapping in defects smaller than molecular vacancies. In the plastic phase the width (fwhm) is 3.25 mrad which is ascribed to Ps localization in vacancies and divacancies in accordance with positron lifetime data.