First limits on double beta decays in 232Th.

As one of the primordial radioactive isotopes, 232 Th mainly undergoes α -decay with a half-life of 1.402 · 10 10 years. However, it is also one of 35 double beta decay candidates in which the single 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 × 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 years. For the most likely transition into the 0 1 + state we find a lower half-life limit of 6.7 · 10 14 years (90% C.I.). These are the first constraints on double beta decay excited state transition in 232 Th . [ABSTRACT FROM AUTHOR]