An investigation on the distribution of counts in positron annihilation lifetime spectra

Positron annihilation lifetime spectra with a high number of counts give useful information for various purposes (like accurate determination of distributions of lifetimes or a comparison among competing models). This requisite, coupled to the need of a good resolution, implies long times of measurement; the consequent instrumental drifts can degrade the quality of the data. In this paper a statistical analysis of spectra of polyethylene terephthalate is presented, which reveals that the distribution of counts displays deviations from the expected Poissonian behaviour, as a consequence of the jitter of the time zero. By means of a simple method for the compensation of such fluctuations it is possible to recover almost ideal statistical properties.