Influence of defect-impurity complexes on slow positron yield of a tungsten moderator: Positron annihilation, Auger, and SIMS study

Abstract

Polycrystalline tungsten foil annealed at successively higher temperature up to $\ensuremath{\sim}2300\mathrm{K}$ has been investigated for slow positron moderation yield. LINAC-based intense positron beam lifetime studies have revealed that reemitted slow positron and positronium fractions gradually improve upon high-temperature annealing. So as to correlate the presence of defects and chemical impurities with the improvement in slow positron yield, positron annihilation, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS), studies have been carried out on virgin and high-temperature $(\ensuremath{\sim}2300\mathrm{K})$ annealed W foils. The positron beam S parameter shows a large value throughout the sample depth corresponding to the virgin sample, while it is lower for annealed samples. This indicates that, as compared to virgin sample, the annealed W sample has a lower concentration of vacancylike defects. Auger studies revealed that in virgin state the surface is fully contaminated with carbon, while the annealed foil shows prominent W peaks. Corroborative SIMS concentration profiles have indicated that the carbon content is much lower in an annealed sample over a large depth region. From these studies, it is concluded that improvement in slow positron yield upon high-temperature annealing is obtained due to the removal of the surface tungsten-carbide layer as well as carbon-vacancy complexes present throughout the sample depth.