Polycrystalline W foil annealed at successively higher temperature up to ~2300K was investigated for slow positron moderation yield. LINAC-based intense positron beam lifetime studies have revealed that re-emitted 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, studies were carried out on virgin and high-temperature (~2300K) annealed W foils. The positron beam S parameter showed a large value throughout the sample depth corresponding to the virgin sample, while it was lower for annealed samples. This indicates that, as compared to virgin sample, the annealed W sample has a lower concentration of vacancy-like defects. Auger studies revealed that in virgin state the surface was fully contaminated with C, while the annealed foil showed prominent W peaks. Corroborative secondary ion mass spectrometry concentration profiles have indicated that the C content was much lower in an annealed sample over a large depth region. From these studies, it was concluded that improvement in slow positron yield upon high-temperature annealing was obtained due to the removal of the surface tungsten-carbide layer as well as carbon-vacancy complexes present throughout the sample depth.

Influence of Defect-Impurity Complexes on Slow Positron Yield of a Tungsten Moderator - Positron Annihilation, Auger and SIMS Study. G.Amarendra, R.Rajaraman, S.Rajagopalan, R.Suzuki, T.Ohdaira: Physical Review B, 2004, 69[9], 094105 (5pp)