Annealing of radiation-induced defects in tungsten: Positron annihilation spectroscopy study

Positron annihilation lifetime spectroscopy (PALS) was applied to study the annealing of radiationinduced defects in polycrystalline tungsten (W) irradiated with 21.6 MeV protons at 100 C up to a fluence of 5 1015 p/cm2. Three components were observed in the measured spectra: short-lifetime of 100 e120 ps (positron annihilation in the defect-free W lattice), medium-lifetime of ~190e330 ps (annihilation at mono-vacancies and small vacancy cluster containing ~ 2e4 vacancies) and long-lifetime of ~500 ps (annihilation in large vacancy clusters containing more than 10 vacancies). The irradiation of W with protons at 100 C, primary, led to the formation of mono-vacancies, self-interstitial defects were created as well but migrated towards sinks during the irradiation. Onset of vacancy diffusion in W starts already at 200 C before defect recovery stage III. After annealing at ~400 C, a sharp drop in the intensity of the positron medium-life component together with a simultaneous increase in positron lifetime from ~220 to ~280 ps is observed, and a long-life component appears. This indicates migration and annealing of vacancies and their agglomeration in large vacancy clusters. After annealing at 500e700C, the intensity of long-life component increases indicating the growth of large vacancy clusters but at 900 C they anneal completely as the mean lifetime recovers nearly to the value measured in the un-irradiated material.