Effect of epithermal -Ps on the positron lifetime spectra of silica glass and polytetrafluoroethylene (PTFE)

Kobayashi, Yoshinori*; Sato, Kiminori*; Yamawaki, Masato*; Michishio, Koji*; Oka, Toshitaka   ; Washio, Masakazu*; Kuriplach, J.*

Our recent observation that -Ps is not thermalized in silica glass and fluorinated polymers such as PTFE states that epithermal -Ps needs to be taken into consideration in the interpretation of positron lifetime data. This problem was taken up by Dauwe et al. for polymers many years ago, regarding that Ps behaves as a classical particle. They assumed Ps to continuously lose energy and in line with this the pick-off annihilation lifetime gradually elongates with time. The model predicts a continuous distribution of -Ps lifetimes, apart from that due to the free volume size distribution. However, Ps trapped in the free volume, approximated by the Tao-Eldrup potential, occupies discrete energy levels, hence the treatment by Dauwe et al. disregarding the quantum mechanical nature of Ps trapped in the free volume may not be adequate. In this presentation, we discuss the influence of epithermal -Ps on the positron lifetime spectra of silica glass and PTFE. Our approach is based on the Tao-Eldrup model that allows a single particle Ps to occupy not only the ground state but also excited states and a simple assumption that Ps occupies a single excited state at time zero. The lifetime spectra of -Ps are predicted to be a multi-component exponentially decaying function, not the continuous lifetime distribution predicted by Dauwe et al. The prediction of the model is compared with the available experimental data on silica glass and PTFE.