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

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.