Rotational bands in77Rb: Spectroscopy near theZ=38deformed shell gap

Abstract

The $\ensuremath{\gamma}$ decays of high spin states in ${}^{77}$Rb populated in the reaction ${}^{40}$Ca${(}^{40}$Ca,$3p)$${}^{77}$Rb were studied with the EUROGAM I $\ensuremath{\gamma}$-ray spectrometer at Daresbury Laboratory, in conjunction with the recoil mass separator. The data were used to extend or establish three positive parity and five negative parity bands in ${}^{77}$Rb. Level lifetimes and sidefeeding times were then measured with the NORDBALL $\ensuremath{\gamma}$-ray spectrometer, using the Doppler shift attenuation method. A very detailed level scheme including many weakly populated states was established and the bands were observed over large spin ranges. The deduced lifetimes demonstrate the high collectivity of the excited states and are consistent with their having large prolate deformations. The structures of the bands and their crossings are discussed in relation to the large shell gap at prolate deformation ${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.4$.