Onset of band structure in ⁷⁰Ga

Excited states in odd-odd ⁷⁰Ga were studied using the ⁶²Ni(¹⁴C,αpn) fusion-evaporation reaction performed at the John D. Fox Superconducting Accelerator Facility at Florida State University with a beam energy of 50 MeV. The depopulating γ decays were measured in coincidence using a Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. An investigation of these coincidences resulted in the addition of 16 new transitions in the ⁷⁰Ga level scheme, including some which belong to the onset of a new positive-parity band structure likely based on the π g_9/2 νg_9/2 configuration. Spins and parities were assigned based on directional correlation of oriented nuclei ratios and linear polarization measurements. The excitation energies predicted by shell-model calculations using the JUN45 effective interaction compare favorably with the experimental ones for the positive-parity states, but are generally about 400-500 keV too low for the negative-parity states. Total Routhian surface calculations for the lowest positive-parity configuration with signature α = 0 predict significant triaxiality (γ ≈ -20°) with competing non-collective excitations developing at a spin (J = 8) that corresponds to the onset of the positive-parity band observed experimentally. The calculations for the lowest negative-parity states with α = 0 yielded surfaces that were qualitatively similar.