Mark Caprio

The collective structure of atomic nuclei intermediate between spherical and quadrupole deformed structure presents challenges to theoretical understanding. However, models have recently been proposed in terms of potentials which are largely independent of the quadrupole deformation beta [beta]. These models, E(5) and X(5), describe a transitional nucleus either free to undergo deviations from axial symmetry (gamma[gamma]-soft) or confined to axial symmetry, respectively. To test these models, information is needed on low-spin states of transitional nuclei. The present work involves measurement of electromagnetic decay properties of low-spin states for nuclei in the A ≈ 100 (gamma-soft) and N ≈ 90 (axially symmetric) transition regions. Population in beta-decay and thermal neutron capture are used, and measurements are carried out using gamma-ray coincidence spectroscopy, fast electronic scintillation timing, and gamma-ray induced Doppler broadening techniques. Results are obtained for 102 Pd, 152Sm, 154Dy, 156Dy, 162Yb, and 162Er, from experiments at the Yale University Wright Nuclear Structure Laboratory, the TRIUMF ISAC radioactive beam facility (Vancouver), and the Institut Laue-Langevin high-flux reactor (Grenoble). The present data allow detailed comparison of transitional nuclei to model predictions. Many characteristics of the N = 90 transitional nuclei (Nd, Sm, Gd, Dy) are found to be reproduced by X(5) and similar beta-soft axially symmetric descriptions. The nucleus 162Er, bordering the N ≈ 90 region, appears to support a low-lying beta-vibrational excitation, indicating comparatively beta-soft structure. And 102Pd is found to match descriptions involving a beta-soft, nearly gamma-independent, potential. To facilitate interpretation of these nuclei, a new approach is developed that simplifies the application of the geometric collective model (GCM) by use of scaling properties. Solutions are also obtained for the E(5) Hamiltonian for finite well depths. These results demonstrate the relevance of beta-soft potentials, and in particular the new E(5) and X(5) models, to transitional nuclei. They suggest that such nuclei, historically among the most difficult to describe theoretically, are amenable to descriptions of comparable simplicity to those used for spherical and well-deformed nuclei.