Reactions of free positrons in low temperature polyethylene; Delayed Ps formation and positron localization

Hirade, Tetsuya  ; Suzuki, Naoki*; Saito, Fuminori*; Hyodo, Toshio*

Positronium (Ps) can be formed by a reaction of trapped electrons and free positrons at low temperatures in molecular solids. While Ps formation by Ore and spur process must be fast, Ps formation by trapped electrons and free positrons is possible even at later positron age. The evidence of this delayed Ps formation was obtained by age-momentum correlation measurement of electron-positron pair annihilation -rays. In this work the parameter curves measured with trapped electrons and no trapped electrons in polyethylene are interpreted by introducing possible positron reactions. Positrons that escape from Ps formation by the Ore and the spur processes have chance to form Ps with trapped electrons. This Ps formation competes with the positron localization resulting from the trapping between molecules or the weakly binding on molecules, ions, or radicals. Assuming the possibility of both the delayed Ps formation and delayed positron trapping and that S-parameters for ortho-Ps and free positrons are the same, we obtain a good fit to the observed time dependent S-parameter curves .