Elastic recoil detection studies of near-surface hydrogen in cavity-grade niobium

Recent studies of the quality factor degradation mechanisms in superconducting RF niobium cavities at high surface magnetic fields revealed that RF performance may depend on the total hydrogen content in the 40 nm thick near-surface layer. Hydrogen distribution in niobium and its near-surface content variations after different chemical surface treatments has been addressed in previous studies. However, only chemical treatments were studied while heat treatments are equally important. In this work we use the elastic recoil detection (ERD) technique to systematically study the distribution of hydrogen in niobium sheet and cavity cutout samples subjected to chemical and heat treatments typically performed on niobium cavities. Our results indicate the near-surface segregation of hydrogen at the niobium oxide/niobium interface, and do not show any significant variation in hydrogen content after various heat and chemical treatments. We do not observe a direct correlation between total hydrogen content and the high field Q slope. Consequences of the observed hydrogen segregation are discussed in the framework of the NbH(x) phase diagram.