Evidence for naturally occurring electron capture of Te-123

Evidence for electron capture of Te-123 from the K shell has been obtained in an experiment performed underground with an array of four 340 g TeO2 thermal detectors. This is the first real proof of the decay of this isotope. In our thermal approach, unlike in previous experiments, the TeO2 bolometer acts at the same time as source and detector of the Te-123 decay and its resolution allows to discriminate between the lines produced by background x ray excitation and electron capture. In addition anticoincidence with the nearby detectors reduces the signal of x rays produced by excitation of Tellurium. The partial width for K electron capture of Te-123 is the smallest one ever measured for a single beta process and lower by six orders of magnitude with respect to the previously reported one for this decay. An analysis based on the expected fraction of electron captures accompanied by internal bremsstrahlung allows to set a stringent limit, independent on the decay channel, for the lifetime of Te-123.