The low-lying level structures of nuclei in the vicinity of ${}^{78}\mathrm{Ni}$ were investigated using in-beam $\gamma$-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 ${}^{80}\mathrm{Zn}$ and ${}^{82}\mathrm{Zn}$. A candidate for the $4_1^{+}$ level in ${}^{80}\mathrm{Zn}$ 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 $2_1^{+}$ state in ${}^{82}\mathrm{Zn}$ is reported to lie at 621(11) keV. The large drop in the $2_1^{+}$ energy at ${}^{82}\mathrm{Zn}$ indicates the presence of a significant peak in the $E\left(2_1^{+}\right)$ systematics at $N=50$. Furthermore, the $E\left(4_1^{+}\right)/E\left(2_1^{+}\right)$ and $B(E2;4_1^{+}\to 2_1^{+})/B(E2;2_1^{+}\to 0_{\mathrm{g}.\mathrm{s}.}^{+})$ ratios in ${}^{80}\mathrm{Zn}$ were deduced to be $1.32\left(3\right)$ and $1.{12}_{-60}^{+80}$, respectively. These results imply that ${}^{80}\mathrm{Zn}$ can be described in terms of two-proton configurations with a ${}^{78}\mathrm{Ni}$ 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 $\beta$ stability, which in turn suggests a doubly magic structure for ${}^{78}\mathrm{Ni}$.

• Received 20 November 2015

DOI:https://doi.org/10.1103/PhysRevC.93.024320