Isomer spectroscopy of neutron-rich Tb-168(103)

Abstract

In-flight fission of a 345 MeV per nucleon U-238 primary beam on a 2 mm thick Be-9 target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus Dy-170 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 beta-decay. Discrete gamma-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 Tb-168, which corresponds to a single proton-neutron hole configuration in the valence maximum nucleus Dy-170. 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 Tb-168 ground state the observed isomeric state by comparison with Blocked BCS-Nilsson calculations.