XPS analysis of the surface composition of niobium for superconducting RF cavities

The surface chemical composition of Nb samples prepared by following procedures commonly employed in the preparation of RF cavities has been studied by XPS and ARXPS. In order to understand the process occurring at surface, the thermal evolution of the surface has been studied in the 30 degrees C divided by 1000 degrees C range of temperature both in the heating and cooling phases. During the heating phase, an irreversible transition has been observed near T congruent to 200 divided by 300 degrees C; it is characterized by a progressive reduction of Nb oxides from Nb2O5 to NbO2 and finally to NbO. The O signal disappears near T congruent to 1000 degrees C and it reappears below 900 degrees C, during the cooling phase. This can be interpreted as due to O migration in the Nb matrix, raising T, and to O diffusion towards the surface as the temperature decreases. Work function measurements show that the values of Phi are strongly con elated with the chemical composition of the surface. (C) 1998 Elsevier Science B.V.