Lifetime Measurements in $^{71}$Se

In the light selenium isotopes, $^{71}$Se appears to be a transitional nucleus, showing signs of competing single particle and collective structures, but its level structure is not well known. The present work measured lifetimes in $^{71}$Se in order to quantify the degree of collectivity as a function of spin as the configuration of the unpaired neutron changes. $^{71}$Se nuclei were produced at high spin by a $^{54}$Fe($^{23}$Na,$\alpha$pn) fusion reaction at 80 MeV conducted at Florida State University. Fifteen lifetimes were measured from the resulting gamma-ray coincidence data using the Doppler-shift attenuation method. Experimental transition quadrupole moments $Q_t$ were inferred from the lifetimes and found to be in rough agreement with the predictions of cranked Woods-Saxon calculations. Comparisons with neighboring odd-mass nuclei confirmed that $^{71}$Se exhibits moderate collectivity. Based on coincidence relations and systematic arguments, the level scheme was enhanced and extended to higher spin. A band that was previously assigned positive parity was reassigned as the ``missing'' signature partner of an existing negative-parity band.