Enhanced Core Polarization in 70Ni and 74Zn

GANIL Expérience; The reduced transition probabilities B(E2;0+\rightarrow 2+_1) of the neutron-rich 74Zn and 70Ni nuclei have been measured by Coulomb excitation in a 208Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d'Ions Lourds via interactions of a 60A MeV 76Ge beam with a Be target. The B(E2) value for 70Ni_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 74Zn 44. The enhanced proton core polarization in 70Ni 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 78Ni_50.