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

Abstract

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 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 150 Ba and presents an extension of the β -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 ( E 2 : 2 + → 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.