Collective behavior in ⁷⁸Rb

Mean lifetimes of 10 states in ⁷⁸Rb have been measured using the recoil-distance method. The γ-ray spectra of the recoils produced following the ⁵⁸Ni(²³Na,2pn)⁷⁸Rb reaction at 70 MeV were observed in prompt coincidence using the FSU plunger in the Pitt-FSU detector array. Different decay curves in different coincidence gates provide additional confirmation for the 153 keV doublet in the level scheme. The alternating pattern of B(M1) strengths observed previously in the yrast band was seen to continue down to lower spins but with an attenuated magnitude, in agreement with neighboring odd-odd nuclei ⁷⁴Br and ⁸²Y. A slight reduction in collectivity with decreasing spin was inferred in the lowest positive- and negative-parity bands based on the gradual decrease in the B(E2) strengths. Particle-rotor model calculations predict strong alternations in both the signature splitting and B(M1) strength pattern in the positive-parity yrast band, in qualitative agreement with the experimental results.