Nuclear collective quadrupole-octupole excitations in the U(16) spdf interacting boson model

The algebraic description of nuclei in which quadrupole and octupole collective degrees of freedom are simultaneously present is studied within the framework of the U(16) spdf interacting boson model. These degrees of freedom seem to be important in the collective structure of the rare-earth and actinide nuclei. In Part I, we introduce and discuss the algebraic structure of the U(16) spdf interacting boson model. Dynamical symmetry breaking schemes are explicitly constructed, primarily using a simple ansatz. The generators, commutation relations, Killing forms, linear and quadratic Casimir operators and the expectation values of the Casimir operators are all listed for each subalgebra. In Part II, we study some related non-dynamical symmetry phenomenology in nuclei. In particular we apply some of the algebraic results of Part I to explain the suggested experimental appearance of octupole deformation in the even-even rare-earth nuclei near 146Ba. We show that the U(13) sdf model does not provide any reasonable explanation of the experimental E1 transition data, even with the introduction of strong ground state octupole deformation. We then present a simple description of the energy and electromagnetic transition data within the framework of the U(16) spdf model, utilizing a mass dependent E1 operator, and show that even 1-pf boson provides a reasonable description of all the data. We next study the energies and electromagnetic transitions of collective positive and negative parity states in the even-even 56Ba, 58Ce, 60Nd and 62Sm isotopes, with 82 < N < 94, using a U(16) hamiltonian that describes the transition from a vibrational structure [U(7) x U(5) x U(3) x U(1)] to rotational structure [SU(3) x SU(3)]