Shell Evolution towards 78Ni: Low-Lying States in 77Cu

The level structure of the neutron-rich Cu-77 nucleus is investigated through beta-delayed gamma-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of Ni-77 are produced by inflight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in Cu-77 are identified for the first time by correlating gamma rays with the beta decay of Ni-77, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the single-particle structure near Ni-78 and suggests a single-particle nature for both the 5/2(1)(-) and 3/2(1)(-) states in Cu-77, leading to doubly magic Ni-78.