This chapter reviews the following items: 1. Energy deposition and trackstructure of fast positrons: ionization slowing down, number of ion-electronpairs, typical sizes, thermalization, electrostatic interaction between e+ andits 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-paraconversion by radiolytic products, reaction rate constants, interpretation ofthe PAL spectra in water at different temperatures; 4. Ps bubble models."Non-point" positronium: wave function, energy contributions, relationshipbetween the pick-off annihilation rate and the bubble radius.
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