Investigating nuclear shell structure in the vicinity of Ni 78: Low-lying excited states in the neutron-rich isotopes Zn 80,82

The low-lying level structures of nuclei in the vicinity of Ni78 were investigated using in-beam γ-ray spectroscopy to clarify the nature of the nuclear magic numbers Z=28 and N=50 in systems close to the neutron drip line. Nucleon knockout reactions were employed to populate excited states in Zn80 and Zn82. A candidate for the 41+ level in Zn80 was identified at 1979(30) keV, and the lifetime of this state was estimated to be 136-67+92 ps from a line-shape analysis. Moreover, the energy of the 21+ state in Zn82 is reported to lie at 621(11) keV. The large drop in the 21+ energy at Zn82 indicates the presence of a significant peak in the E(21+) systematics at N=50. Furthermore, the E(41+)/E(21+) and B(E2;41+→21+)/B(E2;21+→0g.s.+) ratios in Zn80 were deduced to be 1.32(3) and 1.12-60+80, respectively. These results imply that Zn80 can be described in terms of two-proton configurations with a Ni78 core and are consistent with a robust N=50 magic number along the Zn isotopic chain. These observations, therefore, indicate a persistent N=50 shell closure in nuclei far from the line of β stability, which in turn suggests a doubly magic structure for Ni78.