The structural and chemical composition of the surface layer (100–140 nm) of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so-called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable-energy synchrotron x-ray photoelectron spectroscopy (XPS), to study the niobium surface subjected to the following treatments: vacuum annealing at 800 °C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy-distribution curves revealed a slightly increased thickness of the native oxide Nb₂O₅ for the infused samples (∼3.8 nm) as compared to the annealed one (∼3.5 nm) which indicates insignificant oxygen incorporation into niobium during 120 °C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in-situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high-temperature stage is the prerequisite for success of any treatment recipe.

中文翻译：

---

采用 X 射线光电子能谱对超导射频腔进行铌热处理的系统研究

在低温下工作的铌射频腔体表层（100-140 nm）的结构和化学成分对其超导特性有巨大影响。在过去几年中，采用新的热处理配方（即所谓的氮气灌注）处理的型腔已显示出更高的效率和更高的加速梯度。氮气的作用和重要性一直是许多争论的话题。在目前的工作中，我们采用可变能量同步加速器 X 射线光电子能谱 (XPS)，研究经过以下处理的铌表面：800 °C 真空退火、氮气灌注和真空热处理，但灌注过程没有氮气供应。与退火样品 (~3.5 nm) 相比，注入样品 (~3.8 nm) 的 2 O 5 表明在 120 °C 烘烤过程中，铌中的氧含量微不足道，氮对氧化物或其他铌相的形成_没有影响_。通过进行额外的就地退火实验和失败注入过程后分析铌，我们得出结论，真空炉卫生，特别是在高温阶段是任何处理配方成功的先决条件。