New Ground State in ${}^{149}$La Removes Two-Neutron-Separation-Energy Anomaly in Lanthanum Isotopes

TL;DR

This study clarifies the ground state mass of ${}^{149}$La, resolving previous discrepancies and revealing a new nuclear shape transition indicated by a kink in two-neutron separation energies.

Contribution

The paper provides a precise measurement of ${}^{149}$La's ground state mass, resolving conflicting results and identifying a new nuclear shape transition around neutron number 91.

Findings

The ground state of ${}^{149}$La is lighter than previously reported by Jaries et al.

The two-neutron separation energy prominence disappears with the new mass measurement.

A new kink structure suggests a shape transition near N=91.

Abstract

Nuclear mass is a key indicator of how the nuclear shell structure evolves. The recent mass measurement study of neutron-rich lanthanum isotopes [A. Jaries, $et~al$., Phys. Rev. Lett. {\bf 134}, 042501(2025)] reveals the presence of a distinct prominence in their two-neutron separation energies. However, its presence has been called into question based on the results of another mass determination [B. Liu, Ph.D. thesis, University of Notre Dame (2025)]. In this letter, we report an effort to clarify these contradictory results through the use of the simultaneous mass-lifetime measurement of the neutron-rich lanthanum isotope ${}^{149}$La using a multi-reflection time-of-flight mass spectrograph combined with a $\beta$-TOF detector. The peak corresponding to a $\beta$-decaying state was observed in the time-of-flight spectra at a position of $221(6)~{\rm keV/c^2}$ lighter than the reported ${}^{149}$La mass in A. Jaries, $et~al$., but our measured result is in excellent agreement with the mass value reported in B. Liu. We have concluded that this peak is the ground state of ${}^{149}$La. With this, the previously reported distinct prominence in the two-neutron separation energies disappears, while a new kink structure, similar to that in the cerium isotopes, appears. Comparison with theoretical models suggests that a nuclear shape transition from octupole deformation to another type of deformation occurs around $N=91$ and is likely the cause of this kink structure.