Dependence of ortho-positronium pick-off annihilation lifetime on free volume in water-methanol mixtures,and positronium bubble formation

Sizes of free volume were quantitatively calculated for water-methanol mixtures by means of the Kincaid and Eyring method. The results were discussed in relation to the pick-off annihilation lifetime of ortho-positronium in these mixtures. The existence of two ranges of methanol METH concentration in water was observed, within which the dependences of lifetime of o-Ps annihilating by the pick-off process τ₂ on the calculated free volume size are quite different. The problem that the calculated free volume was too small for the existence of o-Ps could be eliminated by the equilibristic formation of a bubble around o-Ps. Correlation was shown between the free volume radii and bubble radii. The obtained results testify to the existence of the strong dependence of τ₂ on the free volume radii. The peculiar shape of this dependence confirms the existence of the clathrate structure in METH-H₂O mixture.