Isomer-delayed gamma-ray spectroscopy of neutron-rich Tb-166

This short paper presents the identification of a metastable, isomeric-state decay in the neutron-rich odd-odd, prolate-deformed nucleus Tb-166. The nucleus of interest was formed using the in-flight fission of a 345 MeV per nucleon U-238 primary beam at the RIBF facility, RIKEN, Japan. Gamma-ray transitions decaying from the observed isomeric states in Tb-166 were identified using the EURICA gamma-ray spectrometer, positioned at the final focus of the BigRIPS fragments separator. The current work identifies a single discrete gamma-ray transition of energy 119 keV which de-excites an isomeric state in Tb-166 with a measured half-life of 3.5(4) mu s. The multipolarity assignment for this transition is an electric dipole and is made on the basis internal conversion and decay lifetime arguments. Possible two quasi-particle Nilsson configurations for the initial and final states which are linked by this transition in Tb-166 are made on the basis of comparison with Blocked BCS Nilsson calculations, with the predicted ground state configuration for this nucleus arising from the coupling of the. nu 1/2(-) [521] and pi 3/2(+) [411] Nilsson orbitals.