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 t^K_1/2>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 t_1/2>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.