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

摘要

Due to the low abundance of Ac-227 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 Pa-231 activities determined at the same stations during the GEOVIDE cruise (GEOTRACES GA01), Pa-231 being the parent nuclide of Ac-227 (Deng et al., 2018). Only few studies (227)AC and Pa-231 have been published so far due to the difficulty to analyze these two nuclides. First, the Ac-227 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 Pa-231 (0-2000 m), we found good agreement between Ac-227 and Pa-231, which validates the method used to determine (227)AC activities, including the estimate of the Mn-cartridge extraction efficiency.