Valencep-ninteractions and the development of collectivity in heavy nuclei

A universal, physically intuitive description of collectivity in heavy nuclei is obtained in terms of the parameter P=Nₚ{N}ₙ$/(${N}ₚ$+${N}ₙ$) which gives the average number of interactions of each valence nucleon with those of the other type. Regardless of mass region, deformation sets in when P=${P}_{crit{{}}4--}$5. This, in turn, gives a rule that medium-mass and heavy nuclei with effective ${N}ₚ$ or ${N}ₙ${≤}4 cannot be deformed. An interpretation in terms of the monopole and quadrupole p-n interactions is discussed.