Abstract
This short paper presents the identification of a metastable, isomeric-state decay in the neutron-rich odd-odd, prolate-deformed nucleus 166 Tb. The nucleus of interest was formed using the in-flight fission of a 345 MeV per nucleon 238 U primary beam at the RIBF facility, RIKEN, Japan. Gamma-ray transitions decaying from the observed isomeric states in 166 Tb 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 166 Tb with a measured half-life of 3.5(4) μs. 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 166 Tb 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 v (1 - /2)[521] and π (3 + /2) Nilsson orbitals.
Highlights
The valence maximum nucleus 170Dy104 lies in the centre of the deformed region of prolate nuclear rotors [1]
Neutron-rich nuclei in the vicinity 170Dy were produced following the projectile fission of a 345 MeV per nucleon 238U primary beam on a 2 mm thick berylium production target at the Radioactive Isotope Beam Factory (RIBF) [3], RIKEN, Japan
Isomer-delayed spectroscopy has been performed on the neutron-rich, odd-odd prolate deformed nucleus 166Tb at the RIBF facility, RIKEN following the production of this isotope via high-energy projectile fission
Summary
The valence maximum nucleus 170Dy104 lies in the centre of the deformed region of prolate nuclear rotors [1]. Note that both hydrogen-like (Z = Q-1) and fully-stripped (Z = Q) ions of 166Tb are transmitted in this particular magnetic rigidity setting. We note that the identified gamma-ray transition at 119 keV in the present work was previously noted in the study of the β− decay of 166Dy by Ichikawa et al, [2]. The 119 keV transition was reported as being in coincidence with a 40 keV gamma ray which fed the proposed ground state of 166Tb. While the 40 keV transition is not clearly separated in the current work, there are counts to the left of the Kα X-rays identified in Fig. 2 which are consistent with a transition at that energy. If the K X-rays arise from the competing internal conversion branch of the 119 keV transition from the isomer, this suggests an electric dipole multipolarity for the 119 keV gamma ray, since other likely multipolarities (M1, E2 and M2) all have much higher internal conversion coefficients and would
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