EVIDENCE OF PREFERENTIAL DIFFUSION OF IMPURITIES ALONG GRAIN-BOUNDARIES IN VERY PURE NIOBIUM USED FOR RADIO-FREQUENCY CAVITIES

In order to overcome dissipation due to impurity segregation at grain boundaries, niobium cavities are submitted to a purification annealing (1300 degrees C+/-200 degrees C under vacuum) during which titanium i s evaporated onto the Nb surface. The resulting titanium layer acts as a solid state getter reacting with light impurities (H, C, N, O), the reby removing these impurities from the bulk of the niobium. Preferent ial titanium diffusion along grain boundaries have been studied using proton-induced x-ray emission. (C) 1997 American Institute of Physics.