3. Results and discussion
3.1. PALS results
A simple relation developed by Nakanishi et al. [42] relates o-Ps
lifetime s3 to the average free volume hole radius R. In this model,
the positronium atom is assumed to be localized in a spherical potential
well having an infinite potential barrier of radius R0 with an
electron layer in the region R < r < R0. According to this model, the
relation between s3 and R is given by