High-precisionB(E2)measurements of semi-magicNi58,60,62,64by Coulomb excitation

Abstract

High-precision reduced electric-quadrupole transition probabilities $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ have been measured from single-step Coulomb excitation of semi-magic $^{58,60,62,64}\mathrm{Ni}$ ($Z=28$) beams at 1.8 MeV per nucleon on a natural carbon target. The energy loss of the nickel beams through the carbon target were directly measured with a zero-degree Bragg detector and the absolute $B(E2)$ values were normalized by Rutherford scattering. The $B(E2)$ values disagree with recent lifetime studies that employed the Doppler-shift attenuation method. The present high-precision $B(E2)$ values reveal an asymmetry about $^{62}\mathrm{Ni}$, midshell between $N=28$ and 40, with larger values towards $^{56}\mathrm{Ni}$ ($Z=N=28$). The experimental $B(E2)$ values are compared with shell-model calculations in the full $pf$ model space and the results indicate a soft $^{56}\mathrm{Ni}$ core.