Shape Coexistence in Light Se Isotopes: Evidence for Oblate Shapes

Lifetimes of states in the ground-state bands of ${}^{70}\mathrm{Se}$ and ${}^{72}\mathrm{Se}$ were measured using the recoil-distance Doppler shift method. The results deviate significantly from earlier measurements, requiring a revision of the conclusions drawn from a recent Coulomb excitation experiment concerning the shape of ${}^{70}\mathrm{Se}$. The new results lead to a coherent picture of shape coexistence in the neutron-deficient selenium and krypton isotopes. The coexistence and evolution of oblate and prolate shapes in this mass region is for the first time consistently described by new Hartree-Fock-Bogolyubov-based configuration-mixing calculations which were performed using the Gogny D1S interaction.

Figure 1

Spectra showing the stopped and Doppler-shifted components observed in the forward detectors at 36° for the transitions and distances indicated. The simulated line shape of the stopped component (for a lifetime of 3.3 ps) and the Gaussian fit of the shifted component are shown for the transition in ${}^{72}\mathrm{Se}$.

Figure 2

$B(E2)$ value as a function of the spectroscopic quadrupole moment $Q_s$ for the $2_1^{+}$ state in ${}^{70}\mathrm{Se}$. The sloping lines indicate the $1\sigma$ range of values compatible with the Coulomb excitation measurement [16]. The horizontal solid (red) and dashed (blue) lines represent the $1\sigma$ ranges from the present and the earlier [13] lifetime measurements, respectively. The asterisk indicates the theoretical value.

Figure 3

Spectroscopic quadrupole moments obtained from the configuration-mixing calculations as a function of spin for the low-lying states in ${}^{68}\mathrm{Se}$, ${}^{70}\mathrm{Se}$, and ${}^{72}\mathrm{Se}$.