Level structure of light neutron-rich La isotopes beyond the N=82 shell closure

TL;DR

This study investigates high spin states of neutron-rich La isotopes beyond N=82, revealing parity doublets and signs of octupole deformation, advancing understanding of nuclear structure near shell closures.

Contribution

First identification of parity doublet structures and new high spin level schemes in $^{140-143}$La, with insights into octupole deformation and structural evolution beyond N=82.

Findings

Identification of parity doublet structures in $^{143}$La

Observation of stable octupole deformation in $^{143}$La

Evidence of transition from single particle to rotational band structures

Abstract

The high spin excited states of Lanthanum isotopes $^{140-143}$La, above the $N=82$ closed shell, have been populated in fission reactions. The prompt $\gamma$-ray transitions were measured using two complementary methods; a) in coincidence with the isotopically identified fragments produced in the fission of the $^{238}$U+$^{9}$Be system using the VAMOS++ and the AGATA spectrometers and b) high statistics three-fold $\gamma-\gamma-\gamma$ and four-fold $\gamma-\gamma-\gamma-\gamma$ coincidence data from the spontaneous fission of $^{252}$Cf using the Gammasphere. This work reports the first identification of a pair of parity doublet structures in $^{143}$La and the new high spin level structure in $^{140-142}$La from prompt $\gamma$-ray spectroscopy. The level structures are interpreted in terms of the systematics of neighbouring odd-$Z$ nuclei above $Z=50$ shell closure and large-scale shell model calculations. The present results indicate the presence of stable octupole deformation, in $^{143}$La. The excitation energy pattern and their comparison with neighbouring 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.