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

Abstract

The level structure of the neutron-rich Cu77 nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of Ni77 are produced by in-flight 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 Cu77 are identified for the first time by correlating γ rays with the β decay of Ni77, 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 Ni78 and suggests a single-particle nature for both the 5/21- and 3/21- states in Cu77, leading to doubly magic Ni78.