Band structures of 76Se and 78Se

Band structures of 76Se and 78Se have been studied with the sup(74,76)Ge(α, 2nγ)sup(76,78)Se reactions by using a variety of in-beam γ-techniques: γ-ray singles spectra, γ-ray excitation functions, γ-γ-t coincidences, γ-ray angular distributions and γ-ray linear polarizations. Spins and parities have been assigned uniquely for many new levels in 76Se and 78Se and four bands have been identified in both nuclei: (I) the ground-state band, (II) a positive-parity ΔJ = 1 band built on the second 2+ state (γ-vibrational band), (III) a negative-parity ΔJ = 2 band built on the 3- state (octupole band) and (IV) a ΔJ = 2 band built on the high-lying J = 4 state. In addition, the second 8+ and 10+ states, which are possibly the lowest members of a band, have been found in both nuclei. Systematics of the band structures obtained are discussed. Level energies of the band members and B(E2) ratios for some of the inter-band transitions between γ- and ground-state bands have been calculated with the proton-neutron interacting boson model IBM-2 and a reasonable agreement with the present data has been obtained. (orig.)