Symmetry and the Origin of Nuclear Deformation

Evidence is presented that the systematic features of nuclear deformation are determined primarily by filling of the normal-parity shell model orbitals of the valence shells, and that abnormal-parity orbitals play important but sec ondary roles in the microscopic origin of deformation. This contradicts many common ideas concerning deformation, but is in accord with the point of view underlying the Fermion Dynamical Symmetry Model. We argue that the deformation systematics of the FDSM and the Nilsson model are mutually consistent, and that in the FDSM the n−p quadrupole residual interaction is responsible for deformation. The same principles applied to superdeformation indicate that abnormal parity orbitals have a much more direct influence on superdeformation than on normal deformation.