On the consistency between positron annihilation lifetime and Doppler broadening results in polypropylene

Polypropylene samples were measured as a function of the source exposure time at 20 K and as a function of the temperature (20-400 K) simultaneously by Positron Annihilation Lifetime (PAL) and Doppler Broadening (DB) spectroscopy. The peak shape parameter (S) measured by DB can be calculated as a sum of contributions of specific parameters corresponding to each positron state. The contributions are obtained by analysis of the PAL spectra. The confinement of positronium (Ps) in a free volume hole (FVH) determines the self annihilation shape parameter S-sa of the para-Ps, the pick-off annihilation lifetime iota(po), and supposes FVH size-dependence of the pick-off shape parameter S-po. The ortho-Ps lifetime and shape parameters are used to compute S,,. In this way, the temperature and time dependences of the S parameter are modelled. The fit of this model to the experimental S parameter shows a stronger dependence of the S-po on the FVH size (R) than the one as expected by taking into account only the Ps momentum. This result suggests that the penetration of Ps into the walls of the FVH must be properly considered and that the penetration depth depends on R itself. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.