Shape evolution in neutron-rich odd-even $^{105-109}$Nb isotopes

TL;DR

This study maps the shape evolution and onset of triaxiality in neutron-rich odd-$Z$ Nb isotopes ($^{99-109}$Nb) by combining inverse-kinematics fission data with AGATA/VAMOS++ and high-fold Gammasphere measurements. The authors extend and revise level schemes, notably reassigning the $^{99}$Nb negative-parity band to the $\,\pi 1/2^-[301]\,$ configuration and establishing new bands in $^{105-107}$Nb, including a long-lived isomer in $^{105}$Nb. Systematics across $N=60-68$ reveal two coexisting structures: a triaxial ground-state band based on $\,\pi 5/2^+[422]$ and an axially deformed excited band linked to $\,\pi 5/2^-[303]$ or $\,\pi 3/2^-[301]$, with signature splitting indicating increasing triaxiality for the ground-state band and axial character for the negative-parity bands. The results support shape coexistence in this region and challenge some model predictions, highlighting the need for advanced theories to describe odd-$A$ nuclei through rapid shape transitions. The work demonstrates the power of combining isotopically identified prompt-delayed gamma spectroscopy with high-statistics coincidence data to resolve complex level schemes in neutron-rich nuclei.

Abstract

Neutron-rich nuclei around $Z\sim40$ exhibit multiple shape transitions. This region shows one of the sharpest transitions in the nuclear chart, from a spherical vibrator at $N=58$ to a strongly deformed prolate shape at $N=60$, with largest deformations seen for $_{38}$Sr and $_{40}$Zr. Below $Z=36$, a spherical-to-oblate transition is predicted, while above $Z=42$ and $N\ge60$, the shape evolves from axial to triaxial. Even-$Z$ nuclei have been well studied, but odd-$Z$ isotopes such as Nb offer additional insights into these mechanisms. The Nb isotopes lie at the boundary between axially deformed Zr and triaxially deformed Mo nuclei. This work explores the structure of neutron-rich Nb nuclei up to $N=68$, aiming to understand shape evolution with isospin and the onset of triaxiality. Two complementary fission experiments were used: (i) $^{238}$U+$^9$Be at GANIL in inverse kinematics with AGATA, EXOGAM, and VAMOS++, allowing prompt and delayed $γ$-ray spectroscopy with isotopic identification; (ii) spontaneous fission of $^{252}$Cf with the Gammasphere array providing high-fold $γ$-coincidence data. The level scheme of $^{105}$Nb was significantly extended with two new negative-parity bands. A revised scheme is proposed for $^{107}$Nb, differing from previous results, and new structures are reported in $^{109}$Nb. The signature splitting analysis indicates triaxial deformation for positive-parity bands, while negative-parity bands show axial symmetry, similar to Zr. This reveals a shape coexistence in neutron-rich Nb nuclei.

Figures (19)

• Figure 1: VAMOS++ identification spectra for fission fragments produced in the E680 experiment. (a) Two-dimensional spectrum of energy loss ($\Delta$E) as a function of total energy (E) measured in ionization chambers of VAMOS++. The elements Kr, Nb and Pd are labeled. (b) A distribution for $Z=41$ isotopes identified in VAMOS++ measured in coincidence with $\gamma$ rays in AGATA.
• Figure 2: Doppler-corrected $\gamma$-ray spectra measured in AGATA of $^{99,101,103,105,107,109}$Nb isotopically identified with VAMOS++. The labeled transitions are new. For $\gamma$-ray energies above 300 keV, the Y-axis is displayed using a different scale when necessary, associated with the right Y-axis (in Counts/keV).
• Figure 3: Revised level scheme of $^{99}$Nb. New transitions and levels are marked in red, while those in blue correspond to those recently reported Kumar2023. The width of the arrows represents the observed intensities relative to the strongest transition.
• Figure 4: Doppler-corrected $\gamma$-ray coincidence spectra for selected gates in the positive-parity band of $^{99}$Nb shown in Fig. \ref{['fig:99Nb_LS']}.
• Figure 5: Level schemes of $^{105, 107, 109}$Nb obtained from the present work. New $\gamma$-ray transitions and levels are marked in red. The label “X” indicates unknown band-head energy.