Temperature and radiation effects on positronium formation

During positron annihilation lifetime spectroscopy (PALS) experiments of po lymers, positrons emitted from the positron source induce radiation effects in the sample. Since these radiation effects can affect positronium (Ps) f ormation probability, PALS has some potential to be used as a new tool to s tudy radiation effects. As a function of the contact time of the sample wit h the position source, the Ps formation probability increases or decreases at low temperature or at room temperature respectively. The decrease of the Ps formation probability at room temperature may be attributed to the accu mulation of free radicals that can scavenge the electrons which could other wise have combined with positron to form Ps. The increase at low temperatur es may be attributed to the accumulation of trapped electrons that can be p icked up bq positrons to form Ps. (C) 2001 Elsevier Science Ltd. All rights reserved.