Is nuclear structure really so simple? (The P factor revisited)

The parameter, P=NpNn/(Np+Nn), where Np(Nn) is the number of valence protons (neutrons) relative to the nearest shell or subshell closure, provides a simple and physically intuitive framework for understanding the evolution of collectivity in medium and heavy nuclei in terms of the competition between the spherical-driving, like-particle pairing interaction and the deformation-driving, proton-neutron interaction. In the past several years the P formalism has proven to be useful for correlating a variety of properties of nuclear ground states and structures at low excitation energy. Two of these, the systematics of ground state masses and of E(41+)/E(21+) ratios, are reviewed. Recently, efforts have been made to characterize structures appearing at higher excitation energies in terms of P. A discussion of rotational bands, with special emphasis on the phenomenon of identical bands in heavy nuclei, is presented in this context. The P systematics of intruder states is also presented