XPS characterization of niobium for RF cavities

Niobium utilization for the realization of RF devices is nowadays very comm on; nevertheless there are still many problems concerning the achievement o f the critical magnetic field and the reduction of the surface resistance, produced principally by surface contamination. A first solution of these pr oblems is constituted by a series of surface treatments, among which there are the thermal treatments. The consequences of thermal treatments (in the range 30-1000 degrees C) on niobium surface exposed to different preparatio ns have been analysed by XPS and ARXPS. During the heating phase an irrever sible transition has been observed near T = 200-300 degrees C; it is charac terized by a progressive reduction of niobium oxides from Nb2O5 to NbO2 and finally to NbO. The oxygen signal disappears near T = 1000 degrees C and i t reappears below 900 degrees C, during the cooling phase. This can be inte rpreted as due to oxygen migration in the niobium matrix, raising T, and to oxygen diffusion towards the surface as the temperature decreases. Work function measurements have been performed and the Phi values have been correlated with the chemical surface composition. All these processes are of primary importance in understanding the correlat ion between the chemical composition of the surface and the secondary elect ron emission occurring inside superconducting cavities.