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

Area of Science:

• Nuclear Physics
• Atomic Physics

Background:

• Nuclear mass measurements are crucial for understanding nuclear shell structure evolution.
• Conflicting data exists regarding the two-neutron separation energies of neutron-rich lanthanum isotopes, specifically ^{149}La.

Purpose of the Study:

• To clarify contradictory mass measurements of ^{149}La.
• To investigate the nuclear shell structure of neutron-rich lanthanum isotopes.

Main Methods:

• Simultaneous mass-lifetime measurement of ^{149}La.
• Utilized a multireflection time-of-flight mass spectrograph and a beta-time-of-flight detector.

Main Results:

• The measured mass of ^{149}La agrees with Liu's thesis, contradicting Jaries et al.
• The distinct prominence in two-neutron separation energies disappears, replaced by a kink structure.
• A nuclear shape transition around N=91 is suggested as the cause of the kink.

Conclusions:

• The ground state mass of ^{149}La has been accurately determined, resolving experimental discrepancies.
• The observed kink structure indicates a nuclear shape transition, impacting our understanding of nuclear evolution.