Background: The $N=80$ isotones offer an important opportunity for studying nuclear structure evolution and the interplay between single-particle motion and collective behavior near the $N=82$ shell closure. Although the nuclear shell model is a powerful and predictive framework, progress for the open-shell nuclei has been limited by the rapidly increasing configuration-space dimensions and the lack of a reliable effective interaction.

Purpose: To construct a feasible shell-model Hamiltonian capable of describing the $N=80$ even-even isotonic chain with $Z=50–74$, and to investigate their underlying low-lying configurations.

Methods: Full-configuration-interaction shell-model calculations are performed in a model space that includes the proton and neutron orbitals $0g_{7/2}, 1d_{5/2}, 1d_{3/2}, 2s_{1/2}$, and $0h_{11/2}$. The effective interaction is derived by optimizing the monopole and multipole components of the realistic interactions, reducing the number of independent parameters to eight via the principal component analysis approach. Further analysis is carried out using the nucleon-pair approximation of the shell model.

Results: The calculation reproduces excitation energies and electromagnetic properties for known $N=80$ isotones and provides predictions for ${}^{150}\mathrm{Yb}, {}^{152}\mathrm{Hf}$, and ${}^{154}\mathrm{W}$ near the proton drip line, including a level inversion between the yrast ${18}^{+}$ and ${20}^{+}$ states in ${}^{154}\mathrm{W}$. It further reveals a systematic evolution of the low-lying structures with increasing proton number.

Conclusions: The $N=80$ isotones exhibit coexistence of multiple configurations: the yrast $0^{+}–4^{+}$ states are (generalized-)seniority-dominated for lower-$Z$ nuclei, becoming increasingly vibrational-like with growing valence proton number, while the yrast $6^{+}–{10}^{+}$ states undergo transitions among neutron seniority-2, proton seniority-2, and proton-neutron coupled configurations along the isotonic chain, driven by proton-neutron interactions and the evolution of the proton Fermi surface. The analysis of the predicted level inversion in ${}^{154}\mathrm{W}$ highlights the essential role of the isoscalar spin-aligned proton-neutron correlation.

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• Collective levels
• Electromagnetic transitions
• Energy levels
• Isomer decays
• Lifetimes & widths
• Nuclear structure & decays
• Nucleon-nucleon interactions
• Spectroscopic factors & electromagnetic moments
• 150 ≤ A ≤ 189
• 90 ≤ A ≤ 149
• Shell model