Low-lying level structure of the neutron-rich nucleus 109Nb: A possible oblate-shape isomer

The neutron-rich nuclei 109Nb and 109Zr have been populated using in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. A T1/2=150(30)ns isomer at 313 keV has been identified in 109Nb for the first time. The low-lying levels in 109Nb have been also populated following the β-decay of 109Zr. Based on the difference in feeding pattern between the isomeric and β decays, the decay scheme from the isomeric state in 109Nb was established. The observed hindrances of the electromagnetic transitions deexciting the isomeric state are discussed in terms of possible shape coexistence. Potential energy surface calculations for single-proton configurations predict the presence of low-lying oblate-deformed states in 109Nb.