Evaluation of quasi-closed cavities connected with air through narrow necks
is discussed. These cavities behave as closed pores when they are studied by
Positron Annihilation Lifetime Spectroscopy (PALS). The reason is a short
lifetime of o-positronium (Ps) and energy barrier that exist for Ps
diffusion from large pores (d>3 nm) to small ones (d<3 nm). It is
shown that more comprehensive information can be obtained using adsorption
porosimetry. Standard adsorptives used in adsorption porosimetry have
infinite lifetime allowing complete penetration and filling all the cavities
during the measurement. Calculation of the neck and cavity sizes is based on
the theory of metastable adsorption phases developed by Derjagin, Broekhoff
and de Boer (DBdB). Results of evaluation are in good agreement with data
obtained by SEM and TEM.