Positronium formation at low temperatures

In 1987, a slow increment of positronium (Ps) formation at low temperature in polyethylene had been obtained by Kindl et al. This phenomenon was successfully explained by the reaction of positrons and trapped electrons accumulated in the materials by previously injected positrons during PAL measurement. There are decrements of Ps formation observed near room temperature in many polymers. Quasi-stable polymer chain packing probably causes the decrement of Ps formation near room temperature. Rapid cooling from temperatures above Tg can give the quasi-stable polymer chain packing and hence the decrement of Ps formation was expected and found. (orig.)