The Onset of Collectivity in Neutron-Deficient POLONIUM-200,198,196

This dissertation concerns the study of discrete, low spin and high spin states in the three even-mass polonium (Z = 84) isotopes; ^{200}Po, ^{198}Po and ^{196}Po. The reactions used to study the nuclei were: ^{172} Yb(^{28}Si,4n) ^{196}Po at beam energies of 141 and 145 MeV; ^{174}Yb( ^{29}Si,5n)^ {198}Po at beam energies of 141 and 146 MeV; ^{176}Yb( ^{29}Si,5n)^{200 }Po at a beam energy of 146 MeV. The experiment studying ^{196}Po was performed at Argonne National Laboratory using the Argonne -Notre Dame array, which consists of 12 Compton-suppressed Ge detectors and a 50 element BGO inner ball of detectors. The other two nuclei were studied at Lawrence Berkeley Laboratory using the High Energy Resolution Array, which consisted of 20 Compton suppressed Ge detectors and a 40 element BGO inner ball. Decay schemes of yrast and near-yrast states, including probable spin and parity assignments, were built in spite of sizable backgrounds from fission, Coulomb excitation, and transfer reactions. The purpose of the present work was to study the transition from single-particle to collective behavior in the low-lying states. The systematic behavior of the low-lying states, including ratios of the yrast state energies and branching ratios of transitions depopulating the 2 _sp{2}{+} and 4_sp{2}{+} states, were studied. The transition was described using a composite of the shell model and the vibrational limit of the liquid drop model. It was determined that the yrast 2 ^+ and 4^+ states are collective for ^{208-196} Po. However, the yrast 6^+ state remains predominantly a non-collective excitation until ^{196}Po, where it becomes an excellent example of vibrational behavior. This can be attributed to the opening of the neutron i _{13/2} orbital near ^ {198}Po (N = 114), which causes larger overlap between the valence protons and neutrons. In addition to the transitional behavior of the low-lying states, high spin level schemes were established for all three nuclei. Tentative assignments were made for their configurations. In particular, three cascades in ^{198}Po, and two in ^{200}Po show signs of being 2 neutron quasi-particle states coupled to the ground state vibrational band.