Isomer spectroscopy of neutron-rich ¹⁶⁸Tb₁₀₃

In-flight fission of a 345 MeV per nucleon ²³⁸U primary beam on a 2 mm thick ⁹Be target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus ¹⁷⁰Dy at the RIBF Facility, RIKEN, Japan. The produced secondary fragments of interest were identified event-by-event using the BigRIPS separator. The fragments were implanted into the WAS3ABI position sensitive silicon active stopper which allowed pixelated correlations between implants and their subsequent β-decay. Discrete γ-ray transitions emitted following decays from either metastable states or excited states populated following beta decay were identified using the 84 coaxial high-purity germanium (HPGe) detectors of the EURICA spectrometer, which was complemented by 18 additional cerium-doped lanthanum bromide (LaBr₃) fast-timing scintillation detectors from the FATIMA collaboration. This paper presents the internal decay of a metastable isomeric excited state in the odd-odd nucleus ¹⁶⁸Tb, which corresponds to a single proton-neutron hole configuration in the valence maximum nucleus ¹⁷⁰Dy. These data represent the first information on excited states in this nucleus, which is the most neutron-rich odd-odd isotope of terbium (Z=65) studied to date. Nilsson configurations associated with an axially symmetric, prolate-deformed nucleus are proposed for the ¹⁶⁸Tb ground state the observed isomeric state by comparison with Blocked BCS-Nilsson calculations.