Prominent bump in the two-neutron separation energies of neutron-rich lanthanum isotopes revealed by high-precision mass spectrometry

TL;DR

This study uses high-precision mass spectrometry to reveal a significant bump in the two-neutron separation energies of neutron-rich lanthanum isotopes, indicating notable nuclear structure effects away from traditional shell closures.

Contribution

First experimental determination of masses for $^{152,153}$La and detailed analysis of two-neutron separation energies in neutron-rich lanthanum isotopes.

Findings

Identification of a prominent bump in two-neutron separation energies at N=93-94 in lanthanum.

Confirmation of a sharp kink at N=93 in cerium isotopes.

Discovery of a 0.4 MeV energy increase in lanthanum isotopes, one of the strongest observed away from shell closures.

Abstract

We report on high-precision atomic mass measurements of $^{148\text{-}153}$La and $^{151}$Ce performed with the JYFLTRAP double Penning trap using the Phase-Imaging Ion-Cyclotron-Resonance technique. The masses of $^{152,153}$La were experimentally determined for the first time. We confirm the sharp kink in the two-neutron separation energies at the neutron number ${N=93}$ in the cerium (${Z=58}$) isotopic chain. Our precision mass measurements of the most exotic neutron-rich lanthanum (${Z=57}$) isotopes reveal a sudden increase in two-neutron separation energies from ${N=92}$ to ${N=93}$. Unlike in the cerium isotopic chain, the kink is not sharp but extends to ${N=94}$ forming a prominent bump. The gain in energy is about 0.4 MeV, making it one of the strongest changes in two-neutron separation energies over the whole chart of nuclides, away from nuclear shell closures. The results call for further studies to elucidate the structure of neutron-rich lanthanum isotopes.