Nuclear structure of 76Se from inelastic neutron scattering measurements*

The low-lying, low-spin levels of $^{76}$Se were studied with the (n,n$'\gamma$) reaction. Gamma-ray excitation function measurements were performed at incident neutron energies from 2.0 to 3.5 MeV, and $\gamma$-ray angular distributions were measured at neutron energies of 2.4, 3.0 and 3.7 MeV. From these measurements, level spins, level lifetimes, $\gamma$-ray intensities, and multipole mixing ratios were determined. Interpreting the nuclear structure of the stable Se nuclei is challenging, with shape transitions, shape coexistence, and triaxiality in evidence. The low-lying structure of $^{76}$Se appears to be the most vibrational of the Se isotopes, with a two-phonon (0$^+$, 2$^+$, 4$^+$) triplet of collective states. In addition to these clearly collective excitations, we have identified and characterized a $4^+\rightarrow2^+\rightarrow 0^+$ cascade of two $E2$ transitions built on the first excited 0$^+$ state at 1122 keV. The picture for $^{76}$Se thus differs from $^{72}$Se and $^{74}$Se, and indicates that the configuration mixing of this coexisting band is less than exhibited in the other Se nuclei. Comparison of the low-lying level schemes of $^{76}$Ge and $^{76}$Se, the double-beta decay daughter, shows a marked difference.