ИСТИНА

In the terminal part of e+ track (in the positron blob) energetic positron releases up to 1keV of its energy because of ionizations and subsequent ion-electron recombinations [1]. Within less then 1 ps all this energy converts into heat. We call this as the local heating effect [2]. If the bulk temperature of the medium is below, but close enough to the melting temperature, the substance may melt, yielding a peculiar temperature dependence of the lifetime of the long-lived component in LT spectra. We have suggested a model describing temperature dependence of the ortho-positronium lifetime in solid substances (methanol, ethanol, butanol and water) close to the melting point. Our approach concerns the positron blob as a region where generation of heat takes place rather efficient because of ionization slowing down process. Temperature profiles can be calculated numerically, solving macroscopic heat transfer equation (spherical symmetry is assumed). Initial and boundary conditions include bulk medium temperature Tbulk. Phase transition is taken into account as an overshot in the temperature dependence of heat capacity of the medium. Calculated temperature profiles T(r,t) at different bulk temperatures allow to estimate the size R of the melted region. Then basing on the mechanism of Ps formation we estimate fraction f(Tbulk) of the Ps atoms which are localized in a melted area. Finally, temperature dependence of the ortho-positronium lifetime is calculated as τ3 (Tbulk) = τ liq * f + τ sol * (1-f), where τ sol and τ liq are found as the Ps lifetimes in a solid and liquid phases respectively. Our approach satisfactorily describes experimental data for investigated media.