Orateur
Description
The ¹²Be nucleus lies far from the valley of stability in the neutron-rich region of the beryllium isotopes. As a light exotic nucleus exhibiting a breakdown of the classical magic number N = 8, ¹²Be provides a valuable testing ground for exploring the interplay between shell evolution, clustering, and deformation.
The structure of ¹²Be was investigated through the resonant elastic scattering reaction ⁴He(⁸He,⁸He)⁴He, measured with the ACTAR TPC active target at GANIL in March 2024. The detector, filled with helium gas, enabled a continuous scan of the excitation-energy region between 11.5 and 13 MeV. The achieved energy resolution of 83 keV (FWHM), a significant improvement over previous measurements (~800 keV), allows precise reconstruction of excitation functions and angular distributions, leading to reliable spin–parity assignments.
Two resonant states were identified:
• A previously observed resonance is reassigned a spin–parity of 4⁺. In agreement with GCM calculations, this new spin assignment impacts the previous interpretation of the rotational band of ¹²Be. This state exhibits a strong ⁴He + ⁸He cluster structure, with some mixing of a ⁶He + ⁶He configuration.
• A new 3⁻ state is observed just above the ⁵He + ⁷He threshold. Unlike the 4⁺ state, it does not show strong ⁶He + ⁶He or ⁴He + ⁸He clustering. Instead, it is a candidate for a possible ⁵He + ⁷He molecular configuration.
These results highlight the structural complexity of ¹²Be, where multiple cluster configurations coexist and mix.