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

Authors

Emilie Le Roy, Université de ToulouseFollow
Virginie Sanial, University of Southern Mississippi
Francois Lacan, Université de ToulouseFollow
Pieter Van Beek, Université de Toulouse
Marc Souhat, Université de Toulouse
Matthew A. Charette, Woods Hole Oceanographic InstitutionFollow
Paul B. Henderson, Woods Hole Oceanographic Institution

Document Type

Article

Publication Date

5-20-2019

Department

Chemistry and Biochemistry

School

Mathematics and Natural Sciences

Abstract

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.

Publication Title

Marine Chemistry

Volume

212

First Page

64

Last Page

73

Recommended Citation

Le Roy, E., Sanial, V., Lacan, F., Van Beek, P., Souhat, M., Charette, M. A., Henderson, P. B. (2019). Insight Into the Measurement of Dissolved ²²⁷Ac In Seawater Using Radium Delayed Coincidence Counter. Marine Chemistry, 212, 64-73.
Available at: https://aquila.usm.edu/fac_pubs/18039