Loss of collectivity in the transitional Er-156 nucleus at high spin

The Cd-114(Ca-48, 6n gamma) reaction at 215 MeV has been investigated using the Gammasphere spectrometer to study the high-spin structure of the nucleus Er-156(68)88. Many new transitions have been established along with 68 definitive spin-parity level assignments from a high-fold angular-distribution analysis. In addition, absolute B(M 1) and B(E 1) strengths have been inferred from measured gamma-ray branching ratios. Strong B(E 1) strength (10(-3) W.u.) is discussed in terms of possible octupole collectivity at low spin. At high spin, this nucleus undergoes a Coriolis-induced shape transition from a prolate state of collective rotation to a noncollective, triaxial-oblate configuration. The yrast positive-parity structure ultimately terminates in an energetically favored oblate state at I-pi = 42(+). Several weak high-energy gamma-ray transitions have been discovered that feed this favored state. State-of-the-art cranked Nilsson-Strutinsky calculations are used to interpret the high-spin behavior of Er-156 and comparisons are made with other N = 88 isotones. (Less)