Residual Oxygen on Nb Heated to 500 C

The superconducting accelerating cavities for the International Linear Collider will be constructed of high-residual resistivity ratio (RRR) niobium sheet. Excessive oxygen within the skin depth (several microns) will reduce the RRR and increase resistive losses. We measure the thickness of this oxide layer, following bakeout simulation, to be about 0.5 nm thick. The results suggest that this layer will very slowly disappear from the top five nm at 500 C.