Positron lifetime distributions in polymers determined with the maximum entropy method

A new evaluation method using Bayes' theorem and the maximum entropy principle has been used to evaluate positron annihilation lifetime (PAL) spectra of polymers. The purpose of this work was to examine the possibilities for extracting ortho-positronium (o-Ps) lifetime distributions. The program used for the calculations is called MELT (maximum entropy lifetime analysis) and was introduced by Shukla et al. [1,2] for the evaluation of short lifetime spectra from annihilation processes in metals and semi-conductors. In our study, experimental spectra containing between 1 and 50 million counts were used. MELT is sufficiently powerful to give a reliable o-Ps distribution of a PAL spectrum containing 2.5 M counts, which means that long measuring times can be avoided and thereby zero time fluctuations can be minimized during raw data collection. A comparison of o-Ps lifetimes and o-Ps intensities resulting from MELT and POSITRONFIT evaluations is given and shows good agreement between the results of the two methods over a large interval of relative intensities and lifetimes.