ABSTRACT
This chapter points out that the shell and independent-particle models explains many nuclear properties, but fail to account for the large nuclear quadrupole moments and spheroidal shapes which many nuclei have. It examines the level structures of many of these so-called vibrational nuclei up to an excitation of about 5 MeV; well-defined rotation-like bands appear, just as in permanently deformed nuclei. The chapter outlines the theory of K. Kumar and M. Baranger, which treats spherical, deformed, and transitional nuclei within a single framework and considers the Y20-type of deformation for axially symmetric well-deformed nuclei. The nuclear motion is collective, or cooperative, if this pattern changes very slowly, or, in the extreme case, is permanent. In the early development of a collective model, all nuclei were broadly classified into two categories: spherical and permanently deformed. The chapter discusses the simplest vibrational and rotational models for stiff nuclei.
