New limits on naturally occurring electron capture of 123Te

Electron capture of 123Te from the K shell has been investigated in a new underground search with an array of 340-g TeO2 thermal detectors. We find that some previous indication of this decay could be attributed to electron capture (EC) of 121Te resulting from neutron activation of natural tellurium. There is therefore so far no evidence for EC of 123Te from the K shell with a 90% C.L. lower limit t1/2k > 5 × 1019 years on the half lifetime. Taking into account the predicted K EC branching ratio, the corresponding lower limit on the 123Te EC half lifetime is t1/2 > 9.2 × 1016, which can be theoretically interpreted only on the basis of a strong suppression of the nuclear matrix elements. A complementary analysis based on the expected fraction of EC accompanied by internal bremsstrahlung is discussed.