  Accurate SIMS measurement of nitrogen in niobium relies on the use of closely
equivalent standards, made by ion implantation, to convert nitrogen signal
intensity to nitrogen content by determination of relative sensitivity factors
(RSF). Accurate RSF values for ppm-range nitrogen contents are increasingly
critical, as more precision is sought in processes for next-generation
superconducting radio-frequency (SRF) accelerator cavities. Factors influencing
RSF value measurements were investigated with the aim of reliably attaining
better than 10% accuracy in N concentrations at various depths into the bulk.
This has been accomplished for materials typical of SRF cavities at the cost of
great attention to all aspects.
