Evolution of nuclear structure in the neutron-rich Nb^96,97,99 isotopes: Evidence for shape coexistence in N=58  ⁹⁹Nb

Excited states of the neutron-rich niobium isotopes 96,97,98,9941Nb have been populated in two experiments which used fusion-fission and multinucleon binary grazing reactions to populate high-spin yrast states. In the multinucleon-transfer experiment, a 530-MeV beam of 96Zr ions was incident on a thin 124Sn target; projectile-like ejectiles were detected and identified using the PRISMA magnetic spectrometer and the associated γ rays were detected using the CLARA array of Ge detectors. In the second experiment, the GASP array of escape-suppressed Ge detectors was used to detect γ rays from fusion-fission products formed following the interaction of a 230-MeV beam of 36S ions with a thick target of 176Yb. Level schemes of 96,97,99Nb were established up to excitation energies of 4545, 5409, and 3814 keV, respectively; states with proposed spin values up to about 15 ℏ were populated. Gamma-ray photopeaks corresponding to transitions in 98Nb were also observed in the PRISMA-CLARA experiment; however, it was not possible, in this case, to produce a level scheme based on γ-ray coincidence data from the GASP experiment. For 96Nb and 97Nb, the level schemes are in agreement with the results of earlier publications. Two new decay sequences have been populated in 99Nb; tentative Jπ values of the hitherto unobserved states have been assigned through comparisons with Jπ values of neighboring nuclei. In contrast with earlier published studies of the high-spin spectroscopy of 96Nb and 97Nb, the present work provides an unambiguous association of the observed γ rays with the A and Z of the excited nucleus. The structure of the yrast states of 96,97,99Nb is discussed within the context of shell-model calculations. The experimental results, supported by model calculations, indicate the first observation of shape coexistence at low spin and low excitation energy in the N=58 nucleus 99Nb. The results of TRS calculations indicate that the 9/2+ ground state is triaxial, tending to oblate shapes with a transition to a more deformed prolate shape beyond the 17/2+ member of the decay sequence; here the sequence has been observed to (29/2+). On the other hand, the previously unobserved decay sequence based on the 5/2− state at 631 keV exhibits the characteristics of a rotational sequence and has been assigned Nilsson quantum numbers 5/2−[303]. TRS calculations indicate that the 5/2−[303] band is gamma soft and this is consistent with the inability of the particle-rotor model to reproduce the observed behavior of the signature-splitting function.