Atomic-Scale Chemical-Analyses of Niobium for Superconducting Radio-Frequency Cavities

The key technology for the linear collider is the high gradient superconducting radio-frequency (SRF) cavity, approximately 20,000 of which will make up the accelerator. The preferred technology is to make the cavities from high-purity niobium-sheet. From the RF superconductivity point-of-view, the interface between the native niobium oxide on the surface of the cavity and near sub-surface region is the most important one. Superconducting properties of cavities depend on the chemistry and microstructure of the surface oxide and the concentration and location of impurity elements. Little is known, however, about this information and the effect of low-temperature baking on the surface region. Atom-probe tomography (APT) provides chemical information of the analysed materials on an atomic scale utilizing time-of-flight (TOF) mass spectrometry, with the field evaporation of materials permitting atom-by-atom dissection. We employ a 3-D local-electrode atom-probe (LEAP) tomography to analyse the chemistry of niobium tips, from the surface niobium oxide to underlying bulk niobium.