Enhanced Core Polarization in ${}^{70}\mathrm{Ni}$ and ${}^{74}\mathrm{Zn}$

The reduced transition probabilities $B(E2;0^{+}\to 2_1^{+})$ of the neutron-rich ${}^{74}\mathrm{Zn}$ and ${}^{70}\mathrm{Ni}$ nuclei have been measured by Coulomb excitation in a ${}^{208}\mathrm{Pb}$ target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d’Ions Lourds via interactions of a $60A\mathrm{MeV}$ ${}^{76}\mathrm{Ge}$ beam with a Be target. The $B(E2)$ value for ${}^{70}\mathrm{Ni}_{42}$ is unexpectedly large, which indicates that neutrons added above $N=40$ strongly polarize the $Z=28$ proton core. In the Zn isotopic chain, the steep rise of $B(E2)$ values beyond $N=40$ continues up to ${}^{74}\mathrm{Zn}_{44}$. The enhanced proton core polarization in ${}^{70}\mathrm{Ni}$ is attributed to the monopole interaction between the neutron in the $g_{9/2}$ and protons in the $f_{7/2}$ and $f_{5/2}$ spin-orbit partner orbitals. This interaction could result in a weakening of magicity in ${}^{78}\mathrm{Ni}_{50}$.

Figure 1

Doppler-corrected $\gamma$-energy spectra obtained in the Ge clover detectors from the Coulomb excitation of the ${}^{76}\mathrm{Ge}$, ${}^{74}\mathrm{Zn}$, and ${}^{70}\mathrm{Ni}$ isotopes. Spectra of ${}^{74}\mathrm{Zn}$ (dashed line) and ${}^{70}\mathrm{Ni}$ (solid line) are superimposed in the right part, showing clearly the 1260 keV peak attributed to the $0^{+}\to 2^{+}$ excitation of ${}^{70}\mathrm{Ni}$.

Figure 2

Experimental $B(E2;0^{+}\to 2^{+})$ values in units of $e^2{\mathrm{fm}}^4$ in the Ni and Zn isotopic chains. Results on ${}^{70}\mathrm{Ni}$ and ${}^{74}\mathrm{Zn}$ nuclei are from the present study. Other values are taken from Refs. 20, 34. The number of neutrons in the $g_{9/2}$ orbital [written on top of each $B(E2)$ curve] and the $B(E2)$ values in Ni are calculated with the shell model of Ref. 11 (dashed line) or with the QRPA model of Ref. 13 (dotted line).