Implementation of Guide to the expression of Uncertainty in Measurement (GUM) to multi-collector TIMS uranium isotope ratio metrology

The application of the GUM (Guide to the expression of Uncertainty in Measurement) to calculate standard uncertainties for routine uranium isotope mass spectrometry measurements for nuclear safeguards and nuclear metrology is introduced. The benefit of this approach is an improved coherency and transparency of the uncertainty calculation, which should include contributions from all potentially significant sources of uncertainty to the mass spectrometric measurement result. The GUM approach puts the responsibility for quantifying the uncertainty on the analyst who makes the measurements and not with the user of the data. The uncertainty budget also serves to provide a feedback to the analyst. It identifies the dominant components of the measurement uncertainty and allows for better understanding, management, and improvement of the measurement process. Detailed examples of uncertainty calculations are presented for the most common types of uranium isotope measurements by multi-collector thermal ionization mass spectrometry (TIMS), e.g., total evaporation, conventional Faraday cup measurements using internal normalization, and combined measurements using a secondary electron multiplier and Faraday cups. Various sources of uncertainty common to multi-collector TIMS, such as baseline noise, peak-tailing effects, peak flatness, detector inter-calibration, and detector linearity response are discussed with respect to the determination of their uncertainty contribution and their influence on the results. Different approaches are explained with their advantages and disadvantages.