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

TL;DR

This study probes the level structure of light neutron-rich La isotopes above the $N=82$ shell closure by combining prompt, isotope-tagged gamma spectroscopy from $^{238}$U+$^{9}$Be fission with high-fold gamma coincidences from $^{252}$Cf spontaneous fission. The authors establish new high-spin level schemes for $^{140-143}$La, identify a pair of parity-doublet bands in $^{143}$La, and provide evidence for stable octupole deformation at high spin, interpreted through systematics and large-scale shell-model calculations with the $V_{\text{MU}}+LS$ interaction. The results reveal a transition from single-particle to alternating-parity rotational structures in the La chain as neutrons are added beyond $N=82$, and they demonstrate the power of combining A,Z tagging with high-fold gamma spectroscopy to study challenging neutron-rich nuclei. The work thus advances understanding of octupole collectivity and shape evolution near the $Z=50$ shell closure and motivates further theoretical exploration with expanded model spaces and complementary experimental approaches.

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 $γ$-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 $γ-γ-γ$ and four-fold $γ-γ-γ-γ$ 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 $γ$-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.

Figures (21)

• Figure 1: (A,Z) gated Doppler corrected "singles" $\gamma$-ray spectra of $^{140-143}$La, detected in AGATA, from the fragment-$\gamma$ coincidences in $^{238}$U+$^{9}$Be-induced fission data. The insets show part of the spectra in an expanded scale.
• Figure 2: Level scheme of $^{140}$La, obtained from the present work. The $6^-$ level was reported in $\beta$-decay Meyer1990 and (n,$\gamma$) reactions Jurney1970. The new transitions from the present work are marked in red colour. The 211 and $1262$ keV $\gamma$ transitions are denoted as dashed lines to indicate the tentative nature of their placement (see text). Their assignment to $^{140}$La is confirmed from the (Z,A) gated spectrum.
• Figure 3: Coincidence spectrum corresponding to the $181$ keV (upper panel) and $1223$ keV (lower panel) $\gamma$-rays of $^{140}$La, obtained from the $^{238}$U+$^{9}$Be-induced fission data.
• Figure 4: Level scheme of $^{141}$La, obtained from the present work. The new transitions from the present work are marked in red colour.
• Figure 5: Coincidence spectrum corresponding (a) the 628 keV gate, (b) the added gates of 517 and 566 keV $\gamma$ rays, (c) the added gates of 590, 511, 416 and 957 keV $\gamma$ rays of $^{141}$La, obtained from $^{238}$U+$^{9}$Be-induced fission data.