Uranium isotope-amount ratios in certified reference material 116-A - Uranium (enriched) metal assay and isotopic standard

Certified reference material (CRM) 116-A, uranium (enriched) metal assay and isotopic standard, was analyzed using TRITON and MAT261 thermal ionization mass spectrometer (TIMS) instruments to characterize the uranium isotope-amount ratios. The certified n(²³⁸U)/n(²³⁵U) "major" ratio in CRM 116-A was determined using a combination of two analytical techniques: total evaporation (TE) and modified total evaporation (MTE). The "minor" isotope-amount ratios n(²³⁴U)/n(²³⁵U) and n(²³⁶U)/n(²³⁵U) in CRM 116-A were characterized using a combination of MTE and conventional analysis techniques. For the n( ²³⁴U)/n(²³⁵U) and n(²³⁶U)/n(²³⁵U) ratios, both the MTE and conventional analysis routines incorporate an internal mass bias correction using the measured n(²³⁸U)/n(²³⁵U) ratio as well as corrections for peak tailing from ²³⁵U to ²³⁸U. The abundance of ²³³U, present in CRM 116-A at trace levels, was characterized using a conventional analysis technique that incorporates a secondary electron multiplier (SEM) equipped with an energy filter. CRM 116-A isotope-amount ratios are traceable to the national measurement base and to the International System of Units (S.I.). The measurements leading to the certification of the uranium isotopic abundances in CRM 116-A are discussed.