First observation of new isomers in $^{228}$Ac : Impact on dark matter searches.

We report the discovery of two metastable isomeric states of $^{228}$Ac at 6.28\,keV and 20.19\,keV, with lifetimes of 299$\pm$11\,ns and 115$\pm$25\,ns, respectively. These states are produced by the $\beta$-decay of $^{228}$Ra, a component of the $^{232}$Th decay chain, with $\beta$ Q-values of 39.52\,keV and 25.61\,keV, respectively. Due to its low Q-value 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.