Ponente
Descripción
In the neutron-rich regions around N≈60 and N≈90, well-known sudden shape transitions occur. These transitions are evidenced by many aspects. From the view of ground-state energy, a pronounced kink in the trend of two-neutron separation energies ($S_{2n}$) and negative values of the neutron shell gap energies ($Δ_{2n}$) at N≈60 and N≈90. From the perspective of excited-state spectroscopy, the excitation energy of the first 2+ state and the $B(E2; 2_1^+→0^+)$ value of even-even nuclei at N≈60 and N≈90 exhibit a sharp decrease and increase, respectively.
Recently, a high-precision mass measurements of $^{148-153}$La have revealed an unexpected sudden increase in $S_{2n}$ values from N=92 to N=94 for lanthanum isotopes [A. Jaries, et al., PRL 134(2025)042501]. None of the existing nuclear mass models, including phenomenological macroscopic-microscopic methods and self-consistent mean-field approaches, are capable of reproducing this observed bump. Instead, all the modes show a relatively smooth linear decrease in the $S_{2n}$ values of $^{150}$La and $^{151}$La. Currently, $^{149}$La is the most neutron-rich isotope for which excited-state spectroscopy data are available. This spectroscopy data proposed a shape transition from octupolar to prolate deformation at N≥92. The role of the 9/2$^+$[404] proton extruder orbital was adopted to explain the shift in the prominent bump observed in $S_{2n}$ values, moving from N=90 in heavier isotopic chains to N=92 in the La isotopic chain. However, more extensive spectroscopic studies on the more neutron-rich La isotopes are necessary to further understand this exotic shape-transition phenomenon.
In the last campaign of EURICA, the β-decay half-lives of $^{150,151}$Ba and $^{150,151}$La were measured [J. Wu, et al., PRL 118(2017)072701, PRC 101(2020)042801(R)]. Using the new γ-ray detection array with enhanced efficiency, along with increased $^{238}$U beam intensity, we propose to investigate the level structure of $^{150,151}$La through β-decay spectroscopy of $^{150,151}$Ba.
