The present approach describes the fate since its injection into a liquid until its annihilation. Several stages of the evolution are discussed: (1) energy deposition and track structure of fast positrons: ionization slowing down, number of ion-electron pairs, typical sizes, thermalization, electrostatic interaction between and the constituents of its blob, and 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, and interpretation of the PAL spectra in water at different temperatures; (4) Ps bubble models. Inner structure of positronium (wave function, energy contributions, relationship between the pick-off annihilation rate and the bubble radius). 1. Introduction Positrons ( ) as well as positronium atoms (Ps) are recognized as nanoscale probes of the local structure in a condensed phase (liquid or solid) and of the early radiolytic physicochemical processes occurring therein. The parameters of positron annihilation spectra determined experimentally (e.g., positron and Ps lifetimes, angular and energetic widths of the spectra, and Ps formation probability) are highly sensitive to the chemical composition, the local molecular environment of Ps (free volume size), and the presence of structural defects. They are also sensitive to variation of temperature, pressure, external electric and magnetic fields, and phase transitions. The informative potentiality of positron spectroscopy strongly depends on the reliability of any theory describing the behavior of positrons in matter, since it should help decipher the information coded in the annihilation spectra. So, realistic models are needed for track structure, energy losses, ionization slowing down and thermalization, intratrack reactions (ion-electron recombination, solvation, and interaction with scavengers), Ps formation process, Ps interaction with chemically active radiolytic species, and /Ps trapping by structural defects. Usually, treatment of the measured annihilation spectra is reduced to their resolution into a set of simple trial functions: sums of decaying time exponentials in the case of PALS (positron annihilation lifetime spectroscopy) and of Gaussians in the case of ACAR (angular correlation of annihilation radiation) and DBAR (doppler broadening of annihilation radiation). The outcome of such conventional analyses of positron annihilation data is the intensities of
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