POSITRON MOBILITY IN POLYETHYLENE IN THE 60-400 K TEMPERATURE-RANGE

We have determined the positron mobility (mu(+)) in polyethylene samples (67.2% crystalline, glass transition temperature T-g = 151 K) in the 64-400 K temperature range by Doppler shift measurements. A method based on the simultaneous observation of two gamma lines from Ba-133 and Cs-137 radioactive sources together with the positron annihilation gamma line, was employed to measure the Doppler shift of the 511 keV gamma line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and the S parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation, mu(+) at 64 K is 31.7 +/- 0.8 cm(2) V-1 s(-1) then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (mu(+) = 26.9 +/- 0.8 cm(2) V-1 s(-1)). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K, From 250 K to 377K, mu(+) increases and reaches the value of 38.4 +/- 1.0 cm(2) V-1 s(-1). The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).