Correlations that favor pairs of like nucleons coupled to zero total angular momentum in the simplest spherical picture. They help explain why an unpaired nucleon often dominates an odd nucleus's spin, and why neighboring nuclear separation energies show an odd-even staggering. Nuclear pairing belongs to the residual-interaction physics beyond merely filling independent energy levels. Mixing different occupations and deformation can also be necessary; the closed-core picture is most useful near suitable nuclear shell closures.
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