NUCLEAR SHELL STRUCTURE EVOLUTION IN NEUTRON-RICH ISOTOPES BEYOND THE N=82 MAGIC NUMBER: A THEORETICAL INVESTIGATION

Abstract

The development of nucleus shell configuration in a neutron-enriched nucleus is still a major problem in atomic physics due to its importance in terms of both nuclear structure and quick neutron-capture (r-) process nucleosynthesis. This review synthesizes current theoretical and experimental advances concerning shell evolution beyond the N = 82 magic number, with particular emphasis on the region approaching the closure of the N = 126 shell that governs the third r-process abundance peak’s formation. Recent breakthroughs in large-scale shell-model computations, chiral effective field theory, ab initio many-body techniques, energy density functional approaches, and relativistic mean-field models are critically evaluated for their descriptions of shell-gap evolution, effective single-particle energies, and proton–neutron interactions in neutron-rich nuclei. The review highlights the roles of tensor-force effects, monopole interactions, three-nucleon forces, and pairing correlations in modifying shell structure under extreme isospin conditions while summarizing current experimental evidence supporting these theoretical predictions. Although the N = 82 shell gap exhibits moderate evolution with decreasing proton number, the N = 126 shell closure remains comparatively robust despite model-dependent uncertainties in its predicted behaviour. The implications of shell evolution for neutron separation energies, β-decay properties, and r-process abundance calculations are also discussed. Overall, this review provides an integrated perspective on the microscopic mechanisms governing shell evolution beyond N = 82 and identifies key challenges and future directions for improving theoretical predictions and experimental constraints.