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

Abstract

As one of the primordial radioactive isotopes, $$^{232}\mathrm{Th}$$232Th mainly undergoes $$\alpha $$α-decay with a half-life of $$1.402\cdot 10^{10}$$1.402·1010 years. However, it is also one of 35 double beta decay candidates in which the single $$\beta $$β-decay is forbidden or strongly suppressed. 181 mg of thorium contained in a gas mantle were measured in a HPGe well-detector at the Gran Sasso Underground Laboratory with a total exposure of 3.25 g$$\times $$×d. We obtain half-life limits on all double beta decay modes of $$^{232}\mathrm{Th}$$232Th to excited states of $$^{232}\mathrm{U}$$232U on the order of $$10^{11-15}$$1011-15 years. For the most likely transition into the 0$$^+_1$$1+ state we find a lower half-life limit of $$6.7\cdot 10^{14}$$6.7·1014 years (90% C.I.). These are the first constraints on double beta decay excited state transition in $$^{232}\mathrm{Th}$$232Th.