As one of the primordial radioactive isotopes, 232Th mainly undergoes
α-decay with a half-life of 1.402×1010 yr. 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 was measured in an HPGe well-detector at the Gran Sasso Underground
Laboratory (LNGS) with a total exposure of 3.25 g × d. We obtain
half-life limits on all double beta decay modes of 232Th to excited states
of 232U on the order of 1011−15 yr. For the most likely transition
into the 01+​ state we find a lower half-life limit of 6.3×1014
yr (90% credibility). These are the first constraints on double beta decay
excited state transition in 232Th