Enhanced core polarization in 70Ni and 74Zn

The reduced transition probabilities B(E2;0(+)-> 2(1)(+)) of the neutron-rich Zn-74 and Ni-70 nuclei have been measured by Coulomb excitation in a Pb-208 target at intermediate energy. These nuclei have been produced at Grand Accelerateur National d'Ions Lourds via interactions of a 60A MeV Ge-76 beam with a Be target. The B(E2) value for Ni-70(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 Zn-74(44). The enhanced proton core polarization in Ni-70 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 Ni-78(50).