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

Positron annihilation lifetime spectroscopy (PALS) was applied to study the annealing of radiation-induced 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 100–120 ps (positron annihilation in the defect-free W lattice), medium-lifetime of ∼190–330 ps (annihilation at mono-vacancies and small vacancy cluster containing ∼ 2–4 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 500–700C, 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.