Quadrupole Transition Strength in the ${}^{74}\mathrm{Ni}$ Nucleus and Core Polarization Effects in the Neutron-Rich Ni Isotopes

The reduced transition probability $B(E2;0^{+}\to 2^{+})$ has been measured for the neutron-rich nucleus ${}^{74}\mathrm{Ni}$ in an intermediate energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The obtained $B(E2;0^{+}\to 2^{+})=642_{-226}^{+216}{e}^2{\mathrm{fm}}^4$ value defines a trend which is unexpectedly small if referred to ${}^{70}\mathrm{Ni}$ and to a previous indirect determination of the transition strength in ${}^{74}\mathrm{Ni}$. This indicates a reduced polarization of the $Z=28$ core by the valence neutrons. Calculations in the $pfgd$ model space reproduce well the experimental result indicating that the $B(E2)$ strength predominantly corresponds to neutron excitations. The ratio of the neutron and proton multipole matrix elements supports such an interpretation.

Figure 1

Doppler-corrected $\gamma$-ray spectrum following Coulomb excitation obtained from the CAESAR detectors when incoming and outgoing particle selections are applied and with the “safe” Coulex impact parameter condition (see text for details). The peak at 1.024 MeV corresponding to the deexcitation of the $2_1^{+}$ state is clearly visible. The spectrum is fitted with the result of a geant4 simulation of the response function of the apparatus (dot-dashed blue line) plus two exponential curves (dashed red and green lines) chosen in comparison to background spectra obtained from ${}^{76}\mathrm{Cu}$ and ${}^{77}\mathrm{Zn}$ present in the cocktail beam. In the inset, the particle identification plot in the S800 focal plane is reported and the charge state of each isotope is indicated. It shows the energy loss in the focal plane detector versus the time of flight. ${}^{74}\mathrm{Ni}$, clearly separated in the plot, was produced with an average rate of 0.7 pps and a purity of 1.5% out of the total.

Figure 2

Experimental systematic of the $B(E2;0^{+}\to 2^{+})$ in the Ni isotopes [14]. The experimental point for ${}^{74}\mathrm{Ni}$ corresponds to the result of the present Coulomb excitation experiment. From a proton inelastic scattering experiment [17], a model dependent $B(E2;0^{+}\to 2^{+})$ value of $1270\pm 380e^2{\mathrm{fm}}^4$ has also been deduced for ${}^{74}\mathrm{Ni}$ [14]. The results are compared with the predictions of large-scale shell-model calculations in the $fpgd$ model space and using empirical or derived residual interactions (see text).