Accelerator Mass Spectrometry of 237Np, 239Pu and 240Pu for environmental studies at the Centro Nacional de Aceleradores

Abstract

Low-energy Accelerator Mass Spectrometry (AMS) systems (i.e. working with 1 MV terminal voltages and below) have demonstrated in the past their capability for the ultra-trace analyses of U and Pu in different field studies. The situation for Np has been different, mostly due to the constraints imposed by the lack of a Np isotopic tracer. Here, it is presented a comprehensive study on the performance of the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) for: i) the determination of Np using Pu as the AMS normalisation isotope and, ii) the efficient analysis of Pu, Pu and Np when they coexist in the same cathode with traces of natural uranium. Working procedures to analyse Np and Pu on one hand, and Pu on the other, are developed in order to meet competitive U suppression factors and counting efficiencies for every radionuclide. It is demonstrated that the sputtering-process-based temporal trend in the Np/Pu AMS ratios show a coherent behaviour between the different studied matrixes, demonstrating the reliability of the necessary standard correction procedure in AMS. It is proved that Np and Pu concentrations at the 1 fg level and Pu concentrations at the 0.1 fg level can be quantified in samples containing U traces. The technique is applied to the Pu and Np analysis of a seawater reference sample supplied by the International Atomic Energy Agency (IAEA): IAEA-443.