Characterization and first application of a new ²³⁶Np spike for (A)MS measurements of ²³⁷Np

The anthropogenic radionuclide ²³⁷Np has excellent potential as an environmental tracer, complementing more established anthropogenic actinide tracers such as ²³⁶U/²³⁸U or ^239,240Pu. A major limitation in quantitatively measuring ²³⁷Np concentrations by Accelerator Mass Spectrometry (AMS) has been the unavailability of sufficiently pure isotopic spike material for normalisation of ²³⁷Np counting results. A new ²³⁶Np spike material has been produced via the ²³²Th(⁷Li,3n)²³⁶Np reaction, and its isotopic and isobaric composition has been characterised at the Vienna Environmental Research Accelerator (VERA). (1.64 ± 0.14) × 10⁹ atoms ²³⁶Np with a²³⁶Np/²³⁷Np ratio of 6.93 ± 0.27 have been produced. The ²³⁶Np normalisation has been validated using the IAEA-381 and IAEA-385 environmental reference materials, as well as a²³⁷Np dilution series in deep seawater. The ²³⁶Np-normalised data reproduced the nominal values for the ²³⁷Np concentrations within 1 σ for the dilution series, whereas the complementary ²⁴²Pu normalisation was significantly below the nominal values (2-17 σ) for all but one sample in the dilution series. First results from the analysis of environmental samples using the ²³⁶Np spike indicate a different vertical distribution of Pu and Np in peat, while for seawater samples collected near the Fukushima Daiichi Nuclear Power Plant no influence of the ALPS-treated water releases on ²³⁷Np concentrations could be observed. For the new Zurich Seawater intercomparison sample (ZSW), a²³⁷Np concentration of (9.50 ± 1.2) × 10⁶ at/kg can be reported. The new ²³⁶Np spike material has significantly outperformed the non-isotopic normalisation. Remaining challenges due to limited availability and a considerable ²³⁷Np co-production need to be addressed by scaling up production.