This chapter reviews the following items: 1. Energy deposition and track structure of fast positrons: ionization slowing down, number of ion-electron pairs, typical sizes, thermalization, electrostatic interaction between e + and its blob, effect of local heating;2. Positronium formation in condensed media: the Ore model, quasifree Ps state, intratrack mechanism of Ps formation;3. Fast intratrack diffusion-controlled reactions: Ps oxidation and ortho-para conversion by radiolytic products, reaction rate constants, interpretation of the PAL spectra in water at different temperatures; 4. Ps bubble models. "Non-point" positronium: wave function, energy contributions, relationship between the pick-off annihilation rate and the bubble radius.

Abstract. In application to positron annihilation spectroscopy, Ps atom is considered not as a point particle, but as a finite size e + e − pair localized in a bubble-state in a medium. Variation of the internal Coulombic e + -e − attraction vs. the bubble radius is estimated.Introduction Typical lifetimes (up to annihilation) of a para-positronium atom(p-Ps; spin = 0) 1 in condensed medium are about 130-180 ps. They are close to the p-Ps lifetime in vacuum (125 ps). The ortho-positronium lifetime in a medium is considerably shorter (about 100 times; some ns) in comparison with that in vacuum. This is due to the so-called pick-off processprompt 2γ-annihilation of the e + , composing Ps atom, with one of the nearest e − of surrounding molecules, whose spin is antiparallel to the e + spin. Just this property turns Ps into a nanoscale structural probe of matter. The theoretical task consists in calculating the pick-off annihilation rate λ po , i.e. in relating λ po with such properties of the medium like surface tension, viscosity, external pressure and size of the Ps trap.Originally, to explain the unexpectedly long lifetime of the ortho-Ps atom in liquid helium R.Ferrel [1] suggested that the Ps atom forms a nanobubble around itself. This is caused by a strong exchange repulsion between the o-Ps electron and electrons of the surrounding He atoms. Ferrel approximated this repulsion by a spherically symmetric potential barrier of radius R ∞ . To estimate the equilibrium radius of the Ps bubble he minimized the sum of the Ps energy in a spherically symmetric potential well, i.e. π 2 2 /4mR

A self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given. By using the blob model of Ps for-mation we have obtained the contact density in the positronium atom in water, which is in agree-ment with the previous measurements.

Positron annihilation lifetime (PAL) spectra are measured in liquid water in the temperature range 2 – 930C. The spectra are treated by taking into account intratrack reactions and assuming that radical reactions with Ps are diffusion-controlled (the respective temperature dependences obeying the Stokes-Einstein law). Equilibrium Ps bubble parameters are obtained.

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