Abstract
As one of the primordial radioactive isotopes, ^{232}mathrm{Th} mainly undergoes alpha -decay with a half-life of 1.402cdot 10^{10} years. 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 were measured in a HPGe well-detector at the Gran Sasso Underground Laboratory with a total exposure of 3.25 gtimes d.We obtain half-life limits on all double beta decay modes of ^{232}mathrm{Th} to excited states of ^{232}mathrm{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 6.7cdot 10^{14} years (90% C.I.). These are the first constraints on double beta decay excited state transition in ^{232}mathrm{Th}.
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