Evolution of Nuclear Structure in the Neutron-Rich Nb96,97,99 Isotopes: Evidence for Shape Coexistence in N=58

Excited states of the neutron-rich niobium isotopes $_{\phantom{96,97,98,}41}^{96,97,98,99}\mathrm{Nb}$ 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 $^{96}\mathrm{Zr}$ ions was incident on a thin $^{124}\mathrm{Sn}$ target; projectile-like ejectiles were detected and identified using the PRISMA magnetic spectrometer and the associated $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$ rays from fusion-fission products formed following the interaction of a 230-MeV beam of $^{36}\mathrm{S}$ ions with a thick target of $^{176}\mathrm{Yb}$. Level schemes of $^{96,97,99}\mathrm{Nb}$ were established up to excitation energies of 4545, 5409, and 3814 keV, respectively; states with proposed spin values up to about 15 $\ensuremath{\hbar}$ were populated. Gamma-ray photopeaks corresponding to transitions in $^{98}\mathrm{Nb}$ were also observed in the PRISMA-CLARA experiment; however, it was not possible, in this case, to produce a level scheme based on $\ensuremath{\gamma}$-ray coincidence data from the GASP experiment. For $^{96}\mathrm{Nb}$ and $^{97}\mathrm{Nb}$, the level schemes are in agreement with the results of earlier publications. Two new decay sequences have been populated in $^{99}\mathrm{Nb}$; tentative ${J}^{\ensuremath{\pi}}$ values of the hitherto unobserved states have been assigned through comparisons with ${J}^{\ensuremath{\pi}}$ values of neighboring nuclei. In contrast with earlier published studies of the high-spin spectroscopy of $^{96}\mathrm{Nb}$ and $^{97}\mathrm{Nb}$, the present work provides an unambiguous association of the observed $\ensuremath{\gamma}$ rays with the $A$ and $Z$ of the excited nucleus. The structure of the yrast states of $^{96,97,99}\mathrm{Nb}$ 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 $^{99}\mathrm{Nb}$. 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 $5/{2}^{\ensuremath{-}}$ state at 631 keV exhibits the characteristics of a rotational sequence and has been assigned Nilsson quantum numbers $5/{2}^{\ensuremath{-}}[303]$. TRS calculations indicate that the $5/{2}^{\ensuremath{-}}[303]$ 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.