Positron annihilation and scanning electron microscopy studies of α-irradiated aluminium crystals

Abstract

Abstract Measurements of positron lifetime and annihilation γ lineshape have been made on 0–40 MeV α-irradiated Al single crystals and polycrystals to a total dose of about 5 × 1017 αcm−2. Scanning electron microscopy (SEM) observations were also made to study the surface morphology of the irradiated specimens. Although both sets of specimens were treated under the same irradiation conditions, significant differences were found between the defect structures of single crystals and polycrystals. A simple model is applied to the lifetime parameters to estimate the size and concentration of the He bubble. In polycrystals the size of bubbles formed is smaller and their concentration is higher than in single crystals. The effects of impurities and grain boundaries on bubble growth are discussed. The results of annihilation γ lineshape show evidence for positronium (Ps) formation in the single crystals. No Ps is formed in the polycrystals. A lower limit of about 36 A is estimated for the radius of the He bubble ...