HELIUM BUBBLES IN TUNGSTEN STUDIED BY POSITRON-ANNIHILATION

Positron lifetime and Doppler broadening measurements were made on uniformly helium-implanted tungsten to study the evolution of defects and helium bubbles during post-irradiation isochronal annealing. Helium decoration of loops gets enhanced below 500-degrees-C. Above 550-degrees-C helium gets dissociated from loops and the loops anneal out upon further annealing. Coarsening of helium-decorated vacancy clusters results in the formation of helium-filled voids, which upon annealing above 900-degrees-C transform into helium bubbles. The bubble parameters deduced from the positron lifetime data using a model analysis indicate the presence of overpressurized bubbles.