π = 8 Shell Breaking in 12 Be from a Single-Particle Perspective
Experimental observations of the low-lying states in 12 Be and their accurate modeling play an essential role in understanding the disappearance of the π = 8 magic number. Long-standing experimental ambiguities have been clarified using an one-neutron adding (π, π) reaction on 11 Be using the ISOLDE Solenoidal Spectrometer at CERNβs HIE-ISOLDE facility. The single-particle energies of 1β’π 1/2 , 0β’π 5/2 , and 0β’π 1/2 orbitals in 12 Be have been determined from the extracted spectroscopic factors. A significant reduction between the separation of 1β’π 1/2 and 0β’π 1/2 orbitals is found in comparison with the carbon isotones, highlighting the breakdown of the π = 8 shell. These observations serve as an important test of different effects incorporated in theoretical models. It is found that two synergistic mechanisms, core deformation and weak binding, are responsible for the π = 8 shell breaking and the exotic near-threshold phenomena observed in 12 Be, including the narrow unnatural-parity resonance $0^{-}_1$ and the possible halolike nature of the $0^+_2$ isomer.