Evolution of deformation in neutron-rich Ba isotopes up to $A=150$

The occurrence of octupolar shapes in the Ba isotopic chain was recently established experimentally up to $N=90$. To further extend the systematics, the evolution of shapes in the most neutron-rich members of the $Z=56$ isotopic chain accessible at present, $^{148,150}$Ba, has been studied via $\beta$ decay at the ISOLDE Decay Station. This paper reports on the first measurement of the positive- and negative-parity low-spin excited states of $^{150}$Ba and presents an extension of the $\beta$-decay scheme of $^{148}$Cs. Employing the fast timing technique, half-lives for the $2_1^+$ level in both nuclei have been determined, resulting in $T_{1/2}=1.51(1)$ ns for $^{148}$Ba and $T_{1/2}=3.4(2)$ ns for $^{150}$Ba. The systematics of low-spin states, together with the experimental determination of the $B(E2:2^+ \rightarrow 0^+$) transition probabilities, indicate an increasing collectivity in $^{148–150}$Ba, towards prolate deformed shapes. The experimental data are compared to symmetry conserving configuration mixing (SCCM) calculations, confirming an evolution of increasingly quadrupole deformed shapes with a definite octupolar character.