EVIDENCE FOR NATURALLY-OCCURRING ELECTRON-CAPTURE OF TE-123

Evidence for electron capture of Te-123 from the K shell has been obta ined 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 t he 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 s ingle beta process and lower by six orders of magnitude with respect t o the previously reported one for this decay. An analysis based on the expected fraction of electron captures accompanied by internal bremss trahlung allows to set a stringent limit, independent on the decay cha nnel, for the lifetime of Te-123.