Evolution of nuclear shapes in 157-161Yb as a function of spin and neutron number

The population distributions of the entry states, the entry lines versus multiplicity, the energy spectra and the angular distributions of the continuum #betta#-rays as a function of multiplicity in 157-161Yb from the reactions of 136 MeV and 149 MeV 20Ne with 144Nd and 146Nd have been investigated with a 4π multidetector system gated with a Ge detector. The observed entry lines and #betta#-decay modes indicate changes in the nuclear structure with increasing spin. This systematic change for N = 87 - 91 Yb isotopes suggests an evolution of nuclear shapes from prolate at low spins to particle aligned oblate structure for 157Yb and 158Yb followed by the onset of high-K bands built on largely deformed oblate states at increasingly higher spin between 38 and 50 (h/2π) with increasing neutron number for 157-161Yb. Furthermore evidence for an evolution to triaxial shape at I approx.= 50 for 158Yb was found. (orig.)