An improved self-checking weighing practice and a method for contamination detection for use in IDMS nuclear material assay

In Nuclear Safeguards applications and applications related to Nuclear Material Accountancy the need for accurate nuclear material assay is self-evident. Isotope Dilution Mass Spectrometry (IDMS) has the potential to meet the demands for accuracy and is generally regarded as a primary reference method. In Safeguards Analytical Laboratories, IDMS is also used either to calibrate or to perform the Quality Assurance of other nuclear material assay methods, such as titration, calorimetry and Hybrid K-edge, and in many laboratories IDMS has become the main point of reference for many laboratory activities. IDMS is based on the availability of accurate reference materials and makes use of the high accuracy with which good mass spectrometry can determine isotope ratios. Both aspects are an essential ingredient of accurate IDMS, but an assay by IDMS involves additional steps — such as accurate aliquoting / weighing and contamination-free handling — which often receive less attention than they deserve. This short paper discusses phenomena, often encountered during weighing, which have the potential of causing significant bias in the nuclear material assay. With these phenomena understood we will present, and prove the validity of, an alternative weighing practice. This alternative practice has a superior performance over traditional simple weighing practices and also offers a built-in consistency check, so that bad weighings can be detected long before laboratory resources, such as separation chemistry, alpha counting and mass spectrometry, are wasted. Accidental contamination of any IDMS-related material also has the potential of invalidating the IDMS results, as has poor and unstable mass spectrometry. We will therefore also explore to what extent the routine IDMS data can be inspected for internal consistency, so that contamination or a failure in mass spectrometry can be detected.