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, Ba148,150, has been studied via β decay at the ISOLDE Decay Station. This paper reports on the first measurement of the positive- and negative-parity low-spin excited states of Ba150 and presents an extension of the β-decay scheme of Cs148. Employing the fast timing technique, half-lives for the 21+ level in both nuclei have been determined, resulting in T1/2=1.51(1) ns for Ba148 and T1/2=3.4(2) ns for Ba150. The systematics of low-spin states, together with the experimental determination of the B(E2:2+→0+) transition probabilities, indicate an increasing collectivity in Ba148-150, 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.