Background: Neutron-rich nuclei around $Z\approx 40$ are well known for exhibiting multiple shape transitions. This region shows one of the sharpest shape transitions in the nuclear chart, evolving from a spherical vibrator at $N=58$ to a strongly deformed prolate shape at $N=60$. The largest deformations are observed for ${}_{38}\mathrm{Sr}$ and ${}_{40}\mathrm{Zr}$. This abrupt shape transition disappears at $Z=36$ and below, where a shape transition from spherical to oblate nuclei is predicted. On the other hand, for $Z\ge 42$ and $N\ge 60$, the shape is known to evolve from axial to triaxial. While the even-$Z$ nuclei in this region have already been extensively studied, new insights can be gained from the studies of odd-$Z$ isotopes for a better understanding of the underlying mechanisms driving these phenomena.

Purpose: The ${}_{41}\mathrm{Nb}$ nuclei lie at the boundary between axially deformed Zr and triaxially deformed Mo nuclei. This work investigates the nuclear structure of very neutron-rich Nb nuclei up to $N=68$. The goal is to understand how the nuclear shape evolves as a function of isospin in this isotopic chain and provide new insights into the emergence of triaxial deformation.

Methods: The structure of the neutron-rich Nb isotopes was investigated using state-of-the-art high-resolution $\gamma$-ray spectroscopy of fission fragments produced via two different fission reactions. The use of ${}^9\mathrm{Be}({}^{238}\mathrm{U},f)$ inverse kinematics, with a detection system comprising AGATA, EXOGAM, and $\mathrm{VAMOS}++$, enabled the measurement of prompt and delayed $\gamma$ rays from isotopically identified fission fragments, and $\gamma \text{−}\gamma \text{−}\gamma \text{−}\gamma$ highfold data were obtained from a spontaneous fission source of ${}^{252}\mathrm{Cf}$ using the Gammasphere array.

Results: The level scheme of ${}^{105}\mathrm{Nb}$ has been significantly extended, with the addition of two negative-parity bands observed for the first time. A new level scheme is proposed for ${}^{107}\mathrm{Nb}$, which is not in agreement with an earlier measurement, and new levels and transitions have been added to the very neutron-rich ${}^{109}\mathrm{Nb}$. The degree of triaxiality of the new bands is discussed on the basis of signature splitting analysis. The recently reported level scheme of ${}^{99}\mathrm{Nb}$ has been revised.

Conclusions: This systematic study on the Nb isotopic chain, compared to Zr and Mo, indicates that while the ground-state band exhibits a triaxial deformation, attributed to a proton hole coupled to a triaxially deformed Mo core, the negative-parity bands, based on isomeric bandheads, display an axially symmetric deformed structure, similar to that observed in the Zr isotopes, revealing the existence of a shape coexistence in the neutron-rich Nb nuclei.

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• Collective levels
• Electromagnetic transitions
• Energy levels
• Fission
• High spin states
• Nuclear shapes and moments
• Nuclear spin & parity
• Nuclear structure & decays
• Rare & new isotopes
• Gamma ray spectroscopy