AES depth profile measurements of niobium for superconducting cavities

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Abstract
The paper reports on AES measurements taken on eight niobium samples which were treated according to typical procedures used in the preparation of superconducting cavities such as ultrahigh vacuum degassing, chemical polishing, electropolishing, and anodic oxidizing. The main objective of the present investigation was the determination of the oxygen depth profile of niobium surfaces. It was concluded that the surface treatments typically applied to superconducting rf cavities result in surfaces covered by layers of lower oxides on the order of 100 Å thick. In anodized niobium, the thickness of the pentoxide‐metal transition is of the same order. The presence of a layer of a two‐phase mixture NbO+NbO2, up to 50 Å thick, as well as a layer of two‐phase NbO+Nb, about 50 Å thick, can be inferred. It is suggested that the performance of superconducting cavities could be limited by these low‐Tc layers.