Annihilation of o-Positronium in the Vicinity of the Critical Mixing Point

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Let us consider a liquid mixture of two-component mutually soluble in high temperatures and forming saturated phases in low ones. At the temperature and the concentration of maximum of the coexistence curve, the phase transition is classified as continuous and belongs to (3, 1) universality class. The critical anomaly of second derivatives of the Gibbs free energy (heat capacity, isothermal compressibility, osmotic compressibility, etc.) results in large and long-living concentration fluctuations impeding most of macroscopic properties of such binary mixtures. Some of the quantities, as light scattering [1-5], non-linear dielectric effect [6-8], absorption of ultrasonic waves [9-11] exhibit strong critical anomalies while others, as density [12] or electric permittivity [13, 14] — weak ones. Many critical properties could be qualitatively predicted when regarding a mixture in the vicinity of critical point as an inhomogeneous one. This attempt was successively used in understanding of critical phenomena of low frequency electric permittivity and conductivity [15-20], as well as in nonlinear dielectric effect [21]. It has to be pointed out that formal similarities between critical mixtures and real inhomogeneous mixtures (for example emulsions) are true only very close to the critical point, where concentration fluctuations are long-range and long-living. Positron annihilation measurements are a suitable experimental technique for investigation of solid minerals [22], polymers [23], as well as for investigation of clathrate-like structures in diluted aqueous mixtures of non-electrolytes [24-26].