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Insight into the measurement of dissolved 227Ac in seawater using radium delayed coincidence counter

Abstract : Due to the low abundance of 227 Ac in seawater, the analysis of this radionuclide requires the use of high-sensitivity, low-background instruments and the collection of large volume samples. A promising technique relies on the pre-concentration of 227 Ac in seawater using cartridges impregnated with manganese oxide (Mn-cartridges) that are mounted on in situ pumps, and its measurement on a Radium Delayed Coincidence Counter (RaDeCC), usually used to analyze short-lived radium isotopes. In this work, we present an evaluation of this technique, including 1) the study of the performance of the RaDeCC measurements for 227 Ac fixed on Mn-cartridges (backgrounds, detector efficiency, repeatability), and 2) the determination of the efficiency of seawater 227 Ac extraction of the Mn-cartridges and its reproducibility for the first time, by using Mn-cartridges placed in series. Overall, we found a Mn-cartridge extraction efficiency of 47 ± 12% (1 SD). Repeatability experiments allowed us to estimate the uncertainties of the entire measurement of 19% (1 SD). Finally, in the aim to validate the method, the 227 Ac activities thus obtained are compared 1) to the 227 Ac activities determined in several samples using Mn-fibers (assuming 100% yield of 227 Ac extraction) and 2) to the 231 Pa activities determined at the same stations during the GEOVIDE cruise (GEOTRACES GA01), 231 Pa being the parent nuclide of 227 Ac (Deng et al., 2018). Only few studies 227 Ac and 231 Pa have been published so far due to the difficulty to analyze these two nuclides. First, the 227 Ac activities determined using Mn-cartridges agree well with the 227 Ac activities determined using Mn-fibers. Second, at depths where 227 Ac is usually found to be in secular equilibrium with 231 Pa (0-2000 m), we found good agreement between 227 Ac and 231 Pa, which validates the method used to determine 227 Ac activities, including the estimate of the Mn-cartridge extraction efficiency.
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Emilie Le Roy, Virginie Sanial, Francois Lacan, Pieter van Beek, Marc Souhaut, et al.. Insight into the measurement of dissolved 227Ac in seawater using radium delayed coincidence counter. Marine Chemistry, Elsevier, 2019, 212, pp.64-73. ⟨10.1016/j.marchem.2019.04.002⟩. ⟨hal-02511001⟩

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