Evidence for shape coexistence in Br-77

The nucleus Br-77 has been studied in the reaction Cu-65(O-16,2p2n) at E = 75 MeV. Lifetimes have been measured for several states using the Doppler-s hift attenuation technique. The B(E2) values for the inband transitions, de duced from the lifetime results, reflect strong collectivity for both the p ositive-parity yrast band (band 1) and the ground-state negative-parity ban d (band 2). States with spin above 17/2(+) in band 1 are found to have a la rge average quadrupole deformation of beta (2) = 0.35. The experimental res ults for band 2 suggest that the 25/2(-) and 29/2(-) states are somewhat mo re strongly deformed than the lower-spin states. A comparison of the experi mental results with particle-rotor-model calculations provides evidence for shape coexistence in 77Br, favouring a prolate shape for the g(9/2) yrast band and an oblate shape for the lowest negative-parity band. (C) 2001 Else vier Science B.V. All rights reserved.