First β -delayed γ -ray spectroscopy of ¹⁰⁹ Nb:  Single-quasiparticle states with prolate shape in ¹⁰⁹Mo

The β-delayed γ-ray spectroscopy of ¹⁰⁹Nb was performed to investigate the structure of ¹⁰⁹Mo for the first time at the RIBF, Japan. Over one hundred transitions were assigned to ¹⁰⁹Mo, of these two were from the known isomeric state, and a new 33.1 keV excited state below the isomeric state was identified. The multipolarities of the transitions depopulating the isomeric state were restricted considering the internal conversion coefficients and empirically recommended upper limits of transition strengths. The intrinsic gyromagnetic ratio of the isomeric-state band was extracted, and the rotational band of the ground state was identified. The configurations of the ground and isomeric states in  ¹⁰⁹Mo were assigned to the ν1/2⁺[411] and ν5/2⁺[413] orbitals, respectively, with prolate shape. From the logft analysis, the ground state in ¹⁰⁹Nb and a new excited state in ¹⁰⁹Mo with a small logft value were assigned to the π5/2⁺[422] and ν5/2⁺[402] orbitals, respectively. The assignments were supported by potential energy surface calculations using the configuration-constrained method with a triaxially deformed Woods-Saxon potential. Although the ν1/2⁺[411] orbital was successfully predicted as the ground state, the order of quasiparticle configurations for excited states was not reproduced. The neutron emission probability in ¹⁰⁹Nb β-decay was evaluated to be 27(11)% utilizing the β-delayed γ rays of  ¹⁰⁹Tc and ¹⁰⁸Mo.