N = 8 Shell Breaking in ¹²Be from a Single-Particle Perspective

Abstract

Experimental observations of the low-lying states in ¹²Be and their accurate modelling play an essential role in understanding the disappearance of the N = 8 magic number. Long-standing experimental ambiguities have been clarified using a one-neutron-adding (d,p) reaction on ¹¹Be using the ISOLDE Solenoidal Spectrometer at CERN’s HIE-ISOLDE facility. The single-particle energies of 1s_1/2, 0d_5/2, and 0p_1/2 orbitals in ¹²Be have been determined from the extracted spectroscopic factors. A significant reduction between the separation of 1s_1/2 and 0p_1/2 orbitals is found in comparison with the carbon isotones, highlighting the breakdown of the N = 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 N = 8 shell breaking and the exotic near-threshold phenomena observed in ¹²Be, including the narrow unnatural-parity resonance 0^–₁ and the possible halolike nature of the 0⁺₂ isomer.