New β -decay spectroscopy of the Te137 nucleus

Abstract

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., $^{132}\mathrm{Sn}$, with only several valence particles. Such is the barely explored $^{137}\mathrm{I}$ nucleus, investigated here in detail.Purpose: To establish excited states in $^{137}\mathrm{I}$, $\ensuremath{\beta}$ decay of the $^{137}\mathrm{Te}$ ground state is studied. In addition, the unknown $\ensuremath{\beta}$-delayed neutron-emission channel of $^{137}\mathrm{Te}$ to $^{136}\mathrm{I}$ 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: $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectroscopy is employed to study excited states in $^{137}\mathrm{I}$. The nucleus is populated in the decay of a mass-separated beam of $^{137}\mathrm{Te}$, produced in neutron-induced fission of $^{235}\mathrm{U}$.Results: The new level scheme of $^{137}\mathrm{I}$ populated in $\ensuremath{\beta}$ decay is established. The half-life ${T}_{1/2}$ of $^{137}\mathrm{Te}$ is determined to be 2.46(5) s. The $\ensuremath{\beta}$-delayed neutron-emission probability ${P}_{n}$ value of $^{137}\mathrm{Te}$ 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 $Z=50$.