Electron capture of ${}^{123}\mathrm{Te}$ from the K shell has been investigated in a new underground search with an array of 340-g ${\mathrm{TeO}}_2$ thermal detectors. We find that some previous indication of this decay could be attributed to electron capture (EC) of ${}^{121}\mathrm{Te}$ resulting from neutron activation of natural tellurium. There is therefore so far no evidence for EC of ${}^{123}\mathrm{Te}$ from the K shell with a 90% C.L. lower limit $t_{1/2}^K>5\mathrm{}\times {10}^{19}$ years on the half lifetime. Taking into account the predicted K EC branching ratio, the corresponding lower limit on the ${}^{123}\mathrm{Te}$ EC half lifetime is $t_{1/2}>9.2\mathrm{}\times {10}^{16},$ 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.

• Received 28 November 2001

DOI:https://doi.org/10.1103/PhysRevC.67.014323