Reflections Along the N=Z Line: The Neutron–Proton Exchange Symmetry

Abstract

The study of nuclear states on, and around, the line of N = Z is one of the cornerstones of contemporary nuclear structure physics. In the lightest systems it can allow us to test the rapidly developing ab-initio model methodology and, as we progress toward the sd-shell, key states of importance in understanding astrophysical nucleosynthesis pathways can be studied. Moving higher in mass, beyond A = 40, such pursuits become experimentally challenging as the lines of stability and of N = Z separate forever. Further up, the arrival of the heaviest well-studied N = Z doubly magic nucleus, 56Ni, signals a region where the study of competing discrete gamma- and charged-particle emission from excited states probes multi-dimensional quantum tunnelling processes in a unique way [1]. The line of N = Z around A ∼ 70 passes through a region where the overlap of the neutron and proton wave functions, coupled to large shell-gaps (for both types of nucleon) for a variety of nuclear shapes, yields a region of rich shape-coexistence yielding the most stringent tests of nuclear models [2]. As the N = Z line approaches the proton dripline between A ∼ 70 and A = 100, short-lived proton emitting ground- and excited states are expected—some of which again are important for nucleosynthesis [3]. One emerging area, as heavier N ∼ Z nuclei become accessible, is the degree to which isospin (or mirror) symmetry is distorted as the effects of weak binding take effect. Finally, the expected spherical 100Sn shell closure is approached: this last bound doubly magic N=Z nucleus lies on the proton drip‐line.