Positron annihilation lifetime studies of stainless steel under proton and Xe irradiation

Specimens of stainless steel reactor internals were irradiated with 240 keV protons and 6 MeV Xe ions up to 15 dpa. The positron annihilation lifetime spectroscopy (PLS) was carried out to study the development of vacancy-type defects. It is indicated that positrons are trapped by vacancies, vacancy clusters and other defects (precipitate, dislocations). The short lifetime parameter τ1 decreases when specimens are irradiated to 2 dpa by proton, whereas in the case of Xe irradiation, τ1 decreases when specimens are irradiated to 7 dpa. The positron annihilation at small cluster with H atoms (for H irradiation) and precipitated phase (for Xe irradiation) may explain this transition. The long lifetime parameter τ2 increases continuously with damage increase. Compared with Xe irradiated specimens, a larger τ2 is obtained in proton irradiated specimens at the same damage. The gas atoms-vacancies, which are expected to be the early state of H bubbles, will develop into small bubbles with the increase of irradiation damage and thus cause long lifetime parameter increase.