This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. This work was partially supported by KAKENHI (Grants No. 25247045, No. 23.01752, and No. 25800130). The authors acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. This work was partially supported by the Norwegian Research Council under project Contracts No. 240104, 262952, 263030, and 288061. The Monte Carlo shell-model calculations were performed on the K computer at RIKEN AICS (hp160211, hp170230, hp180179, hp190160). This work was supported in part by MEXT as “Priority Issue on PostK Computer” (Elucidation of the Fundamental Laws and Evolution of the Universe) and as “Program for Promoting Researches on the Supercomputer Fugaku” (Simulation for basic science: from fundamental laws of particles to creation of nuclei) (hp200130) and JICFuS. Support from German BMBF Grant No. 05P19RDFN1 and No. 05P21RDFN1, from US DOE Grant No. DE-FG02-91ER-40609, from the National Research, Development and Innovation Fund of Hungary via Project No. K128947 and TKP2021-NKTA42 is acknowledged. University of Brighton authors were supported by STFC Grant No. ST/J000132/1. This work was partially supported by Helmholtz Forschungsakademie Hessen für FAIR (HFHF), GSI Helmholtzzentrum für Schwerionenforschung, Campus Darmstadt, 64289 Darmstadt, Germany, and by Generalitat Valenciana, Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital (CISEJI/2022/25).

Nuclei in the vicinity of Ni78 are important benchmarks for nuclear structure, which can reveal changes in the shell structure far from stability. Spectroscopy of the odd-odd isotope Cu78 was performed for the first time in an experiment with the EURICA setup at the Radioactive Isotope Beam Factory at RIKEN Nishina Center. Excited states in the neutron-rich isotope were populated following the β decay of Ni78 produced by in-flight fission and separated by the BigRIPS separator. A level scheme based on the analysis of γ-γ coincidences is presented. Tentative spin and parity assignments were made when possible based on the β-decay feeding intensities and γ-decay properties of the excited states. Time correlations between β and γ decay show clear indications of an isomeric state with a half-life of 3.8(4) ms. Large-scale Monte Carlo shell-model calculations were performed using the A3DA-m interaction and a valence space comprising the full fp shell and the 1g9/2 and 2d5/2 orbitals for both protons and neutrons. The comparison of the experimental results with the shell-model calculations allows interpreting the excited states in terms of spin multiplets arising from the proton-neutron interaction. The results provide further insight into the evolution of the proton single-particle orbitals as a function of neutron number, and quantitative information about the proton-neutron interaction outside the doubly magic Ni78 core.

10 pags., 5 figs., 2 tabs.

Peer reviewed