TEMPERATURE-DEPENDENCE OF POSITRON TRAPPING AT DEFECTS IN AN AL-LI ALLOY

The Al-3.7% Li alloy has been thermally treated in order to obtain sam ples containing vacancy-ii clusters and samples with both clusters and dislocation loops. The positron lifetime parameters have been measure d in both cases as a function of the temperature in the interval 10-29 5 K. For the samples containing only vacancy-ii complexes two lifetime s are observed in the whole temperature range. The average lifetime, t he long component and its associated intensity stay roughly constant f rom 100 up to 295 K and decrease in the interval 100-10 K. In the samp les containing two kinds of trap for positrons, two lifetimes are obse rved only in the temperature region 100-295 K, where the presence of l oops leads to a drastic decrease of the long lifetime and an increase in its associated intensity. In the temperature range 10-100 K the onl y resolved lifetime decreases as observed in the sample containing vac ancy-Li complexes. The results are interpreted in terms of a strong te mperature dependence of the trapping rate into loops and a reversible 'pinning' effect of the ii atoms on the open-volume defects.