Identification of new isomers in Ac-228: impact on dark matter searches

Abstract

We report the identification of metastable isomeric states of Ac-228 at 6.28 keV, 6.67 keV and 20.19 keV, with lifetimes of an order of 100 ns. These states are produced by the beta-decay of Ra-228, a component of the Th-232 decay chain, with beta Q-values of 39.52 keV, 39.13 keV and 25.61 keV, respectively. Due to the low Q-value of Ra-228 as well as the relative abundance of Th-232 and their progeny in low background experiments, these observations potentially impact the low-energy background modeling of dark matter search experiments.