New beta-decay spectroscopy of the Te-137 nucleus

Background: Nuclear spectroscopy of neutron-rich isotopes provides important information on their nuclear structure and has a valuable impact on the modeling of the r-process path. Particularly interesting are nuclei close to doubly-magic species, e.g., Sn-132, with only several valence particles. Such is the barely explored I-137 nucleus, investigated here in detail. Purpose: To establish excited states in I-137, ss decay of the Te-137 ground state is studied. In addition, the unknown ss-delayed neutron-emission channel of Te-137 to I-136 is inspected. Search for levels and for candidates for Gamow-Teller and first-forbidden transitions between the mother nucleus and excited states in the daughter nucleus is conducted within the experimental observations. Methods: ss-delayed gamma-ray spectroscopy is employed to study excited states in I-137. The nucleus is populated in the decay of a mass-separated beam of Te-137, produced in neutron-induced fission of U-235. Results: The new level scheme of I-137 populated in ss decay is established. The half-life T-1/2 of Te-137 is determined to be 2.46(5) s. The ss-delayed neutron-emission probability P-n value of Te-137 is deduced as a lower limit to be 2.63(85)%. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.