Abstract
Ortho-positronium lifetime and intensity were measured as a function of temperature for a homologous series of normal alkanes. Free volume shape and concentration of defects are different for various phases. The lifetimes observed were compared with model predictions for an empty gap in the crystalline structure (at low temperatures), finite cylinder or cuboid (rotator phase) and sphere (liquid). The role of intramolecular defects (nonplanar conformers) was discussed. A distinct difference in temperature dependence of lifetime spectra when melting is preceded by the rotator phase or C phase was found. Attention was paid to equality of lifetimes in the rotator phase and in liquid in a broad range of carbon chain lengths.