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.

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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 implantation profiles in various materials for22 Na isotope source are discussed. The spatial distribution of positrons in materials with densities from 1 to 21 g/cm 3 are simulated using GEANT4 code. The results indicate that depth distributions of thermalized positrons in any homogeneous medium can be described by the sum of two exponential functions. Contribution of the short-range exponent is about 11% in the materials. The both absorption coefficients can be well represented as a function of material density and atomic number. Proposed empirical model agrees well with available experimental data.

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A method for detection of carcinogenic properties of chemical substances based on the Ps formation mechanism is briefly discussed.

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Chemical carcinogens are often strong electrophiles. The variation of the Ps formation probability versus concentration of the substance being tested in a solvent (modelling intracellular milieu) may serve as an indicator of carcinogenic properties of the solute. Being effectivescavengers of track electrons such chemicals strongly inhibit Ps formation.Tests carried out with a number of well‐known chemical compounds indicate this expectation and indicate the possibility of using PAL spectroscopy in the search for carcinogenic and anticarcinogenic agents. It is further shown that carcinogenic activity of macro‐samples seemto correlate well with their work function. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)