New level schemes and octupole correlations of light neutron-rich lanthanum isotopes 143,144La

The yrast and near-yrast level scheme of light neutron-rich 143La (Z=57, N=86) is reinvestigated and expanded and that of 144La (N=87) is proposed for the first time by measuring prompt γ rays from the spontaneous fission of 252Cf at Gammasphere. Spins/parities are assigned to the lowest-lying levels of 143,144La based on the early studies of β− decay, and the assignments for high-spin levels of 143,144La are made by measuring internal conversion coefficients and following the level systematics of the neighboring heavier La isotopes and even–even Ba and Ce nuclei. The B(E1)/B(E2) ratios, energy displacements δE(I) and rotational frequency ratios ω−(I)/ω+(I) of the new parity-doublets of 143,144La indicate that octupole deformations/correlations also develop in these nuclei. The band-crossings observed in a rotational frequency range of 0.31 to 0.34 MeV for the two even-parity bands in 143La but being absent in 144La are interpreted as due to alignment of a pair of i13/2 neutrons in 143La.