Spectroscopic Quadrupole Moments in ${}^{96,98}\mathrm{Sr}$: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at $N=60$

Neutron-rich ${}^{96,98}\mathrm{Sr}$ isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at $N=60$.

Figure 1

Total $\gamma$-ray energy spectrum following Coulomb excitation of the ${}^{98}\mathrm{Sr}$ beam impinging on a ${}^{208}\mathrm{Pb}$ target, Doppler corrected for the projectile. Contaminating transitions, attributed to ${}^{98}\mathrm{Rb}$, are marked with asterisks.

Figure 2

Comparison between the theoretical and experimental level schemes of ${}^{96}\mathrm{Sr}$ (left) and ${}^{98}\mathrm{Sr}$ (right). The excitation energies (in keV) are given in brackets. The widths and labels of the arrows represent the measured and calculated $B(E2)$ in $e^2{b}^2$. The measured and calculated spectroscopic quadrupole moments are given in ${\mathrm{efm}}^2$ next to each state.