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

Abstract

Recent studies of the quality factor degradation mechanisms in superconducting RFniobium cavities at high surface magnetic fields revealed that RF performance may dependon the total hydrogen content in the 40 nm thick near-surface layer. Hydrogen distributionin niobium and its near-surface content variations after different chemical surfacetreatments has been addressed in previous studies. However, only chemical treatments werestudied while heat treatments are equally important. In this work we use the elastic recoildetection (ERD) technique to systematically study the distribution of hydrogen in niobiumsheet and cavity cutout samples subjected to chemical and heat treatments typicallyperformed on niobium cavities. Our results indicate the near-surface segregation ofhydrogen at the niobium oxide/niobium interface, and do not show any significantvariation in hydrogen content after various heat and chemical treatments. We do notobserve a direct correlation between total hydrogen content and the high fieldQ slope. Consequences of the observed hydrogen segregation are discussed in the framework of theNbHx phase diagram.