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

TL;DR

This paper reports the discovery of metastable isomeric states in $^{228}$Ac produced by $^{228}$Ra decay, which could influence background modeling in low-energy dark matter detection experiments.

Contribution

It identifies new isomeric states in $^{228}$Ac and discusses their potential impact on dark matter search backgrounds.

Findings

Identified metastable isomers at 6.28 keV, 6.67 keV, and 20.19 keV.

States have lifetimes around 100 ns.

Implications for low-background dark matter experiments.

Abstract

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 $\beta$-decay of $^{228}$Ra, a component of the $^{232}$Th decay chain, with $\beta$ Q-values of 39.52 keV, 39.13 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.