A study of transitional collective behavior in heavy nuclei.

摘要

Throughout the nuclear landscape, regions of structural transition have provided a sensitive means of testing our understanding of nuclear structure. In this work, two aspects of transitional behavior in nuclear structure will be examined, using both theoretical and experimental methods.;In the weakly vibrational nucleus 140Nd, mixed symmetry states -- collective excitations in which the protons and neutrons behave collectively, but out of phase -- are sought via beta decay experiments at the Wright Nuclear Structure Laboratory at Yale University. Angular correlations techniques are used to identify 2+ states near 2 MeV that decay to the 2+1 state via a predominant M1 transition, and two low-lying mixed symmetry candidates are identified. The relationship between underlying shell structure and the fragmentation of mixed symmetry states in the N = 80 isotones is discussed.;To study of transitions between spherical and deformed collective structures, internal conversion electron measurements of the deformed nucleus 158Dy have been carried out at the Australian National University. Measurements in the deformed region, where data are scarce, may hold the key to understanding the physics behind observed E0 transition strengths in the transitional region. To that end, the E0 transition between the 0+2 and 0+1 states in 158Dy has been measured, and an X(E0/E2) value for this transition is presented.;Finally, the Interacting Boson Model-1 is used to study the evolution of quantum (shape) phase transitional behavior in collective nuclei. Energies, transition strengths, and shape invariants in yrast states are studied over a large range of system sizes. The roles that system size, angular momentum, excitation energy, and even choice of observable play in our discussion of such systems are explored through a variety of methods. Angular momentum, in particular, is found to play a significant role in the evolution of ground state band energies in finite nuclear systems, but does not have an effect on the evolution of any observables in the large boson limit.