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

We have determined the positron mobility (mu(+)) in polyethylene sampl es (67.2% crystalline, glass transition temperature T-g = 151 K) in th e 64-400 K temperature range by Doppler shift measurements. A method b ased on the simultaneous observation of two gamma lines from Ba-133 an d Cs-137 radioactive sources together with the positron annihilation g amma line, was employed to measure the Doppler shift of the 511 keV ga mma line as a function of the electric field applied to the samples. W ith this method we were able to measure at the same time the drift vel ocity of positrons and the S parameter. This parameter is very importa nt in the interpretation of the mobility trend in samples where the po sitron states change with temperature. The positron mobility was corre cted 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 a gain up to 170 K (mu(+) = 26.9 +/- 0.8 cm(2) V-1 s(-1)). This sharp ch ange in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K, F rom 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 b y a simple model taking into account scattering and a thermally activa ted process (hopping mechanism).