Evolution of the nuclear structure in neutron-rich Cu isotopes: first spectroscopy of odd-odd 74 Cu and 76 Cu

Excited states in the odd-odd isotopes 74Cu and 76Cu were observed for the first time in a spectroscopic study following the β decay of 74Ni and 76Ni. The experiment was performed at the Radioactive Isotope Beam Factory at RIKEN Nishina Center using the EURICA setup and the BigRIPS separator. Level schemes were constructed based on the analysis of γ-γ coincidences. The time correlations between β and γ decay showed no evidence for population of isomeric states in either nucleus. The experimental results are compared to large-scale Monte Carlo shell-model calculations with the A3DA-m interaction. The low-lying states in 74Cu and 76Cu are interpreted as spin multiplets arising from the coupling of an odd proton in the 2p3/2 or 1f5/2 orbital with an odd neutron in the 1g9/2 or 2p1/2 orbital. The results provide information on the evolution of single-particle orbitals along the chain of neutron-rich Cu isotopes and on the proton-neutron interaction in the vicinity of 78Ni, which serve as important benchmarks for shell-model calculations. The non-observation of the low-energy isomeric state at 65 keV in 76Cu, combined with the observed decay pattern from higher-lying states, strongly supports a spin-parity assignment of 3(-) for the ground state and (6-) for the isomeric state, resolving a long-standing ambiguity discussed in previous studies.