Evolution of nuclear structure in the neutron-rich ^96,97,99Nb 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 𝐽^𝜋 values of the hitherto unobserved states have been assigned through comparisons with 𝐽^𝜋 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.