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

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 ${\hat{I}}^3$-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 ${\hat{I}}^3-{\hat{I}}^3-{\hat{I}}^3$ and four-fold ${\hat{I}}^3-{\hat{I}}^3-{\hat{I}}^3-{\hat{I}}^3$ 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 ${\hat{I}}^3$-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.