Recombination mechanism of Ps formation

The preceding discussion reserves the unique possibility of Ps formation in molecular media. It is the recombination mechanism, which postulates that Ps is formed via combination of the deenergized positron with one of the electrons formed at the end part of its track. This process is not energetically forbidden in the sense discussed above. 

Over the last 30 years the recombination mechanism has become extremely widespread [16, 17]. It has been used to interpret extensive data on Ps chemistry, and explain variations of Ps yields from 0 to 0.7 in very different chemical substances where parameters of the Ore gap are practically the same. Variations of Ps formation probability under phase transitions have also received natural explanation. Experimentally observable monotonic inhibition of Ps yields (practically down to zero) in solutions of electron acceptors contradicts the Ore model, but is well incorporated in the recombination mechanism. It explains the anti-inhibition effect, including experiments on Ps formation in moderate electric fields in pure liquids and mixtures. 

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In this chapter we will briefly discuss mechanisms of the positron slowing down, the spatial structure of the end part of the fast positron track, and Ps formation in a liquid phase. Our discussion of the energetics of Ps formation will lead us to conclude that (1) the Ore mechanism is inefficient in the condensed phase, and (2) intratrack electrons created in ionization acts are precursors of Ps. This model, known as the recombination mechanism of Ps formation, is formulated in the framework of the blob model. Finally, as a particular example we consider Ps formation in aqueous solutions containing different types of scavengers. 

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