Analysis of Interstitial Elements in Niobium with Secondary Ion Mass Spectrometry (SIMS)

Abstract

Superconducting Radio Frequency (SRF) cavities provide enhanced efficiency and reduced energy utilization in present day particle accelerators. Niobium (Nb) is the material of choice for these cavities due to its high critical temperature and critical magnetic field. In order to understand why certain treatments, especially a low temperature bake, improve performance, it is important to study Nb surface characteristics and identify elemental contamination that can affect the performance of the cavity. H, C, O, and N are of interest because they are interstitial impurities in Nb. In earlier work, SIMS analysis using a CAMECA IMS‐6F with Cs+ primary beam showed that C and N were probably not significant factors impacting performance but there was a very high level of H in the Nb. Ion implants of C, N, O, and D into Nb provided quantification of C, N, O and indicated that D is very mobile in the Nb. Further analyses showed that heat treated Nb has lower levels of surface H than non heat treated Nb and subsequent removal of surface oxide by etching causes intake of H in a heat treated Nb sample. This result helps confirm the role of surface oxide as a hydrogen barrier. To further understand the oxide, Nb samples were anodized to obtain a thicker surface oxide and H and D were implanted into this oxide to check for the appearance of implant peaks. SIMS depth profile analyses were carried out and confirmed the presence of the implant shape for these elements in the oxide. Relative Sensitivity Factor (RSFs) could then be calculated for quantification of H in the oxide. Since the Nb matrix signal showed little change from the oxide to the substrate, the same RSF was used to estimate the H concentration in the Nb at 2×1022 atoms/cm3 (approximately 40% mole fraction H).