UC Berkeley

The high spin excited states of Lanthanum isotopes La140–143, above the N=82 closed shell, were populated in fission reactions. The prompt γ-ray transitions were measured using two complementary methods: (a) in coincidence with the isotopically identified fragments produced in the fission of the U238+Be9 system using the Variable Mode Spectrometer (VAMOS++) and the Advanced Gamma Tracking Array (AGATA) spectrometer, and (b) high statistics threefold γ-γ-γ and fourfold γ-γ-γ-γ coincidence data from the spontaneous fission of Cf252 using the Gammasphere. This work reports the first identification of a pair of parity doublet structures in La143 and the new high spin level structure in La140–142 from prompt γ-ray spectroscopy. The level structures are interpreted in terms of the systematics of neighboring odd-Z nuclei above the Z=50 shell closure and large-scale shell model calculations. The present results indicate the presence of stable octupole deformation in La143. The excitation energy pattern and their comparison with neighboring isotones, moving away from the N=82 closed shell, point towards a transition from single-particle structures to an alternating parity rotational band structure in the La isotopic chain.