Effect of oxide layer on microwave surface resistance of superconducting niobium

We have measured the influence of oxide layers on the surface resistance of niobium resonant cavities at temperatures below 1.4°K. The 8.6 GHz cavity halves were drawn from high purity rolled sheet and electron beam welded together. We measured the resonance quality, Q, of several cavities after alternately removing and growing an oxide layer on the inside surface. Removal was accomplished by absorbing the oxygen into the bulk Nb, at 1100-1200°C, the required temperature having been determined by Auger spectroscopy. Oxide layers were grown by admitting pure oxygen into the cavity at reduced pressure. We found that these oxide layers contribute a residual resistance of .