Competing single-particle and collective behavior in 71Se

The high-spin decay of Se-71 was studied using the Fe-54(Na-23,alpha pn) reaction at 80 MeV and the Florida State University Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. Based on prompt gamma-gamma coincidences measured in the experiment, the known level scheme was enhanced and extended to higher spin with 19 new transitions. A band that was previously suggested to have positive parity was reassigned as the "missing" signature partner of an existing negative-parity band. Spins were assigned based on directional correlation of oriented nuclei ratios. Lifetimes of 17 excited states were measured using the Doppler-shift attenuation method. Experimental Q(t) values imply an intermediate degree of collective behavior for 71Se at high spin. Theoretical Q(t) values determined from cranked Woods-Saxon (CWS) calculations show better agreement with the experimental ones for the positive-parity states than the negative-parity states. Shape competition and gamma softness characterize the low-spin states of the lowest positive- and negative-parity bands based on the CWS calculations. At high spin, triaxial shapes with gamma > 0 degrees are predicted.