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 236U and 239,240Pu in different field studies. The situation for 237Np 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 237Np using 242Pu as the AMS normalisation isotope and, ii) the efficient analysis of 239Pu, 240Pu and 237Np when they coexist in the same cathode with traces of natural uranium. Working procedures to analyse 237Np and 239Pu on one hand, and 240Pu on the other, are developed in order to meet competitive 238U suppression factors and counting efficiencies for every radionuclide. It is demonstrated that the sputtering-process-based temporal trend in the 237Np/242Pu 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 237Np and 239Pu concentrations at the 1 fg level and 240Pu concentrations at the 0.1 fg level can be quantified in samples containing 238U traces. The technique is applied to the 239,240Pu and 237Np analysis of a seawater reference sample supplied by the International Atomic Energy Agency (IAEA): IAEA-443.