Evolution of structure and shapes in 158Er to ultrahigh spin

The level structure of 158Er has been studied using the Gammasphere spectrometer via the 114Cd(48Ca, 4n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level scheme has been con-siderably extended with more than 200 new transitions and six new rotational structures, including two strongly coupled high -K bands. Configuration assignments for the new structures are based on their observed alignments, B(M1)/B(E2) ratios of reduced transition probabilities, excitation energies, and comparisons with neighboring nuclei and theoretical calculations. With increasing angular momentum, this nucleus exhibits Coriolis-induced alignments of both neutrons and protons before it then undergoes a rotation-induced transition from near-prolate collective rotation to a noncollective oblate configuration. This transition occurs via the mechanism of band termination around spin 45 h over bar in three rotational structures. Two distinct lifetime branches, consistent with the crossing of a collective "fast" rotational structure by an energetically favored "slow" terminating sequence, are confirmed for the positive-parity states, and similar behavior is established in the negative-parity states. Weak-intensity, high-energy transitions are observed to feed into the terminating states. At the highest spins,