Positronium Formation an Diffusion in Polymers

@ High-energy positrons injected into condensed matter quickly lose their energy, become thermalized and annihilate with one of the electrons of the medium into gamma-rays. In polymers, some of the positrons form a hydrogen-like bound state between a positron and an electron prior to the annihilation. This bound state is called positronium (Ps). Ps is formed by recombination of a positron and one of the electrons released from the molecules in terminal positron spur. Positron-electron recombination in the positron spur competes with other reactions, so that the Ps formation probability depends on various factors. In this paper, we describe influence of additives, polar groups, and external field on Ps formation in polymers. A possibility of using Ps as a radiation chemical probe for polymers is described. Ps formed as a result of positron-electron recombination interacts with a subnanometer-size free volume hole, and diffuses over a certain distance before annihilation. We determined the diffusion coefficient of Ps using a chemical reaction between Ps and an additive, and its diffusion length in amorphous polymers was estimated to be ca.1 nm. This result is of fundamental importance to apply Ps as a probe for studying free volume in polymers.