Insight Into the Measurement of Dissolved 227Ac In Seawater Using Radium Delayed Coincidence Counter

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Chemistry and Biochemistry


Mathematics and Natural Sciences


Due to the low abundance of 227Ac 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 227Ac 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 227Ac fixed on Mn-cartridges (backgrounds, detector efficiency, repeatability), and 2) the determination of the efficiency of seawater 227Ac 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 227Ac activities thus obtained are compared 1) to the 227Ac activities determined in several samples using Mn-fibers (assuming 100% yield of 227Ac extraction) and 2) to the 231Pa activities determined at the same stations during the GEOVIDE cruise (GEOTRACES GA01), 231Pa being the parent nuclide of 227Ac (Deng et al., 2018). Only few studies 227Ac and 231Pa have been published so far due to the difficulty to analyze these two nuclides. First, the 227Ac activities determined using Mn-cartridges agree well with the 227Ac activities determined using Mn-fibers. Second, at depths where 227Ac is usually found to be in secular equilibrium with 231Pa (0–2000 m), we found good agreement between 227Ac and 231Pa, which validates the method used to determine 227Ac activities, including the estimate of the Mn-cartridge extraction efficiency.

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Marine Chemistry



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