$B(E2)\uparrow$ Measurements for Radioactive Neutron-Rich Ge Isotopes: Reaching the $N=50$ Closed Shell

The $B(E2;0_1^{+}\to 2_1^{+})$ values for the radioactive neutron-rich germanium isotopes ${}^{78,80}\mathrm{Ge}$ and the closed neutron shell nucleus ${}^{82}\mathrm{Ge}$ were measured at the HRIBF using Coulomb excitation in inverse kinematics. These data allow a study of the systematic trend between the subshell closures at $N=40$ and $50$. The $B(E2)$ behavior approaching $N=50$ is similar to the trend observed for heavier isotopic chains. A comparison of the experimental results with a shell model calculation demonstrates persistence of the $N=50$ shell gap and a strong sensitivity of the $B(E2)$ values to the effective interaction.

Figure 1

Coincidence $\gamma$-ray spectrum for the $A=78$ RIB experiment after the event-by-event Doppler correction. A clear separation between the $2_1^{+}\to 0_1^{+}$ transitions of ${}^{78}\mathrm{Ge}$ and ${}^{78}\mathrm{Se}$, differing by only $5\mathrm{keV}$, was achieved. The fitted peaks and the resulting difference are shown at the bottom.

Figure 2

Overlay of the $\gamma$-ray coincidence spectra for the $A=82$ RIB experiment (solid line) with the one obtained with a pure ${}^{82}\mathrm{Se}$ beam (dotted line), normalized to the Se peak.

Figure 3

Experimental $B(E2)$ values obtained in the present work (circles) for Se (top panel) and Ge (bottom panel) isotopes. A comparison with adopted $B(E2)$ values (squares) is shown for the stable nuclei. The solid circles correspond to the radioactive nuclei, which are compared with shell model calculations (triangles).

Figure 4

Trend of the experimental $B(E2)\uparrow$ values. In the top panel the behavior of the Ge chain as a function of $N$ is compared with neighboring isotopic chains. The bottom panel shows the behavior of the $N=50$ isotones as a function of $Z$ compared with the shell model predictions (dotted line). Solid circles indicate RIB measurements obtained in this work.