Rapid Analysis of ${}^{226}\mathrm{Ra}$ in Ultrapure Gadolinium Sulfate Octahydrate

Abstract

Numerous particle physics experiments utilize gadolinium (Gd), a rare earth element with the most significant neutron capture cross-section among all elements, to detect anti-neutrinos via inverse beta-decays or to remove neutron-induced background events. For example, to load Gd into water Cherenkov detectors, $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$ is dissolved and rare event search experiments are required to screen for radioactive impurities in $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$ before dissolution. This study developed a new method to rapidly measure the radium-226(${}^{226}\mathrm{Ra}$) concentration in $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$. This method requires only three days to measure a batch of samples, as opposed to the usual method using high-purity germanium detectors, which takes approximately 20 days after arrival. The detection limit for the measurement of ${}^{226}\mathrm{Ra}$ is 0.32 mBq/kg. This method has been already used for $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$ screening at the Super-Kamiokande Gd(SK-Gd) project, and it can be applied to future experiments.

10 pages, 3 figures