First limits on double beta decays in $^{\bf 232}$Th

Abstract

As one of the primordial radioactive isotopes, $^{232}$Th mainly undergoes $\alpha$-decay with a half-life of $1.402 \times 10^{10}$ yr. However, it is also one of 35 double beta decay candidates in which the single $\beta$-decay is forbidden or strongly suppressed. 181 mg of thorium contained in a gas mantle was measured in an HPGe well-detector at the Gran Sasso Underground Laboratory (LNGS) with a total exposure of 3.25 g $\times$ d. We obtain half-life limits on all double beta decay modes of $^{232}$Th to excited states of $^{232}$U on the order of $10^{11-15}$ yr. For the most likely transition into the $0_1^+$ state we find a lower half-life limit of $6.3 \times 10^{14}$ yr (90% credibility). These are the first constraints on double beta decay excited state transition in $^{232}$Th.