Shape coexistence and signature splitting in 77Br

High angular momentum states in 77Br were populated via the 65Cu(18O,α2n)77Br reaction at 65 MeV using the Florida State University (FSU) FN Tandem-LINAC accelerator. The Pitt-FSU array with 9 Compton-suppressed high purity Ge detectors was used to measure prompt γ-γ coincidences. Analysis of the coincidences led to an extension of the positive- and negative-parity bands to spins of (41/2+) and (43/2-). Directional correlation of oriented nuclei ratios have been deduced where possible, and spin assignments made accordingly. A cranked-shell-model analysis shows that in the positive-parity band the νg9/2 alignment occurs at the same frequency as in the neighboring isotopes, 75,79Br, but with a larger interaction strength. In the negative-parity bands the lower πg9/2 crossing also occurs at about the same frequency as in 75Br, but with a smaller interaction. The higher frequency νg9/2 crossing is delayed in 77Br. Hartree-Fock-Bogolyubov cranking calculations predict considerable γ softness in 77Br, and particle-rotor model calculations support the interpretation of γ softness or variations in triaxiality within the bands