New Ground State in ${}^{149}$La: Lanthanum Isotopes Lose Their Distinctive Feature in Two-Neutron Separation Energies

TL;DR

The study resolves conflicting mass measurements for 149La that impact the interpretation of the two-neutron separation energy $S_{2n}$ trends in neutron-rich lanthanum isotopes. By performing a simultaneous mass–lifetime measurement with a multi-reflection time-of-flight mass spectrograph (MRTOF-MS) and a beta–TOF detector, using spontaneous fission sources 252Cf and 248Cm, they identify the 149La ground state via a 221(6) keV/c^2 beta-decay signature lighter than the JYFL value. The ground-state mass leads to the disappearance of the distinct $S_{2n}$ prominence around $N \approx 93$ in La isotopes, instead yielding a kink near $N \approx 92$ that mirrors Ce isotopes and is well described by FRDM12; this points to a nuclear shape transition from octupole deformation to another deformation around $N = 91$. The analysis also clarifies the spin–parity issue by associating the observed beta-decaying state with a $7/2^-$ ground state, consistent with prior spectroscopy and resolving possible isomer interpretations. Overall, the work demonstrates the power of simultaneous mass–lifetime measurements to refine nuclear structure evolution in the mid-shell region and informs deformation-driven models of neutron-rich nuclei.

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. 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 study [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 $β$-TOF detector. The peak corresponding to a $β$-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. 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.

Figures (3)

• Figure 1: (a) TOF spectrum of the singly-charged $A=149$ isobar series produced form the ${}^{252}{\rm Cf}$ source. The red-colored lines indicate the fit results. The vertical dashed line represents the expected peak position of ${}^{149}{\rm La}$ based on the JYFL result. (b) the same as (a), but for doubly-charged ions with the ${}^{248}{\rm Cm}$ source. (c) Decay spectrum of the TOF-event-correlated $\beta$-decay events in (b). A comparison of the ${}^{149}{\rm La}$ half-life with the literature values is also presented. For all panels, the red-colored lines indicate the fit results.
• Figure 2: The measured mass excess values of ${}^{149}$La. Charge state and production method are labeled on the horizontal scale. The cyan- and orange-colored bands indicate the reported mass excess values by JYFL and CPT, respectively.
• Figure 3: (a) Two-neutron separation energies of the barium, lanthanum, and cerium isotope series. (b) Two-neutron shell gap of the lanthanum isotope series. For details regarding the theoretical predictions, see the main text. (c) The nuclear deformation parameters $\beta$ in the FRDM12. $\beta_2$ and $\beta_3$ correspond to the quadrupole and the octupole deformations, respectively.