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 La-143 (Z = 57, N = 86) is reinvestigated and expanded and that of La-144 (N = 87) is proposed for the first time by measuring prompt gamma rays from the spontaneous fission of Cf-252 at Gammasphere. Spins/parities are assigned to the lowest-lying levels of La-143,La-144 based on the early studies of beta(-) decay, and the assignments for high-spin levels of La-143,La-144 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 delta E(1) and rotational frequency ratios omega(-)(I)/omega(+)(I) of the new parity-doublets of La-143,La-144 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 La-143 but being absent in 144La are interpreted as due to alignment of a pair of i(13/2) neutrons in La-143. (C) 2008 Elsevier B.V. All rights reserved.