Year
2021
Abstract
The International Atomic Energy Agency’s (IAEA) environmental sampling (ES) helps to provide assurance to the international community regarding the absence of undeclared nuclear material and activities. In order to detect Pu at the femtogram (10-15) level in environmental swipe samples, highly sensitive analytical techniques are needed. The most sensitive technique is the so-called bulk analysis, providing both average U and Pu isotopic composition and amount information. Isotope dilution mass spectrometry (IDMS) provides the most accurate assay and isotope ratio results if both the spike and the measurement method fulfill certain criteria. 244Pu is considered the ideal spike reference material for low-level Pu assay and isotope ratio analysis because 1) it is the least abundant isotope in the sample, and 2) it is relatively stable. Until 2018, its use for ES analysis was, however, not feasible due to the lack of isotopic purity (244Pu isotopic abundance < 98 %) in commercially available reference materials. Highest accuracy and lowest detection limits can only be achieved by using a high-purity rare spike material, in which the other Pu isotopes are “quasi” absent. In 1991, the IAEA initiated the production of a high-purity 244Pu spike reference material. The final 244Pu spike product - having a 244Pu purity of 99.983 ± 0.001 at.% - was produced by Lawrence Livermore National Lab (US Department of Energy), and delivered to the IAEA in 2018. The availability of this superior spike material led to a modification of the multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) collector block arrangement so that all Pu isotopes can be directly normalized to 244Pu or 239Pu, resulting in improved accuracy for both assay and isotope ratio measurements. Improved accuracy is further achieved by on-line monitoring of Pb interferences that occur in the presence of elevated Pb concentrations in the samples. The availability of this high-purity spike together with the modification to the MC-ICP-MS collector block have resulted in improved accuracy, timeliness and detection capability for low-level Pu assay and isotope ratio measurements for ES analysis, the analytical performance of which will be demonstrated in this paper.