The evolution of nuclear structure: The NpNn scheme and related correlations

The evolution of collective nuclear structure is discussed from a horizontal perspective, that is, as a function of the number of valence protons and neutrons. Starting from an explicit recognition of the importance of the valence residual p-n interaction in the equilibrium shape and structure of nuclei, the phenomenological NpNn scheme is presented, and validated via an analysis of empirical p-n interaction strengths. Applications of the NpNn scheme are presented which exploit this paradigm of structural evolution. The physics underlying occasional deviations from the NpNn scheme is extracted and new signatures of structure and of nuclear shape components are developed which will be useful for the study of exotic nuclei with radioactive beams. Finally, the smooth correlations of many observables with NpNn leads to a study of the correlations between different collective observables themselves. The remarkable results stemming from this approach are summarized.