Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter 20Al

The previously unknown nucleus 20 Al has been observed for the first time by detecting its in-flight decays. Tracking trajectories of all decay products with silicon microstrip detectors allowed for a conclusion that 20 Al is unbound with respect to three-proton (3⁢𝑝) emission. The 3⁢𝑝-decay energy of the 20 Al ground state has been determined to be 1.93(+0.12−0.10)  MeV through a detailed study of angular correlations of its decay products, 17 Ne +𝑝+𝑝+𝑝. This value is significantly smaller than the predictions inferred from the isospin symmetry by using the known neutron separation energy of its mirror nucleus 20 N, which indicates a possible isospin symmetry breaking in the mirror nuclei 20 Al and 20 N. This observed isospin symmetry breaking is supported by the calculations of the continuum embedded theoretical frameworks, describing the observed 20 Al ground state as a 1⁢𝑝 𝑠-wave state with a spin-parity of 1−, which differs from the spin-parity (2−) of the 20 N
ground state. The 20 Al ground state decays by sequential 1⁢𝑝−2⁢𝑝 emission via the intermediate ground state of 19 Mg, which is the first observed case of “daughter” 2⁢𝑝 radioactivity following 1⁢𝑝 decay of the parent state.