Observation of new neutron-rich Mn, Fe, Co, Ni, and Cu isotopes in the vicinity of 78Ni

Neutron-rich nuclei in the vicinity of Ni-78 were produced using a U-238 beam at the RIKEN Radioactive Isotope Beam Factory. The particle-identification plot for the in-flight fission fragments highlights the first observation of eight new isotopes: Mn-73, Fe-76, Co-77,Co-78, Ni-80,Ni-81,Ni-82, and Cu-83. Although the beta-decay half-lives of Co-77 and Ni-80 were recently reported by Xu et al. [Phys. Rev. Lett. 113, 032505 ( 2014)] using data from the same experiment, the current work provides the first direct, quantitative evidence for the existence of these isotopes. The experimental production cross sections are reproduced in a satisfactory manner by theoretical predictions. An odd-even staggering of the cross sections was observed, and the effect appears to become more pronounced for the most exotic nuclei that were investigated. The staggering effect was interpreted as an increase of the neutron-evaporation probability for odd-N isotopes, owing to the decrease of the neutron-separation energy, S-n. The predicted cross section for Ni-80 is significantly overestimated, which may be related to a weak binding of the neutron pair above the N = 50 shell closure.