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

We report the identification of metastable isomeric states of ^228 Ac 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 β -decay of ^228 Ra, a component of the ^232 Th decay chain, with β Q-values of 39.52 keV, 39.13 keV and 25.61 keV, respectively. Due to the low Q-value of ^228 Ra as well as the relative abundance of ^232 Th and their progeny in low background experiments, these observations potentially impact the low-energy background modeling of dark matter search experiments.