Effect of Nitrogen and Oxygen Interstitials on Niobium SRF Cavity Performance

High-gradient and low-loss superconducting radiofrequency (SRF) cavities are key to enabling the next high-energy particle accelerator. Performance in SRF cavities can be altered by various surface treatments, such as nitrogen doping and in-situ baking, that diffuse beneficial impurities into bulk niobium. We seek to understand the precise role of nitrogen and oxygen in terms of cavity performance and material properties. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) of niobium cavity cutouts is used to quantify the absolute concentration of each impurity. The resulting impurity depth profiles are then correlated with cavity measurements and compared with BCS theory. We find that ten times more nitrogen oxygen is required to achieve the same improvement in BCS resistance as interstitial nitrogen.