Progress on the Proton Decays of A ∼ 20 Nuclei Around the Proton Drip Line

Experimental studies on the spontaneous nucleon emission in nuclei around the drip line enable us to explore new isotopes or resonant states, and to reveal exotic structures and decay properties of nuclei located far from the β stability line; consequently, they are of critical importance for probing limits of nuclear stability and understanding nucleon–nucleon interactions under extreme conditions of isospin asymmetry. With the radioactive isotope beam 20Mg provided by the National Superconducting Cyclotron Laboratory at Michigan State University, we studied the proton decays of nuclei around the proton drip line at A∼20 mass region. Complete-kinematics measurements were performed for proton decays of one-proton resonant states in 18Na, two-proton resonant states in 20Mg, three-proton resonant states in 21Al, and four-proton resonant states in 18Mg, yielding decay energy spectra for all four nuclei. With the invariant mass method, the ground state of 18Na was firmly identified, clarifying previous ambiguities of its position. The isotope 18Mg, which is located two neutrons beyond the proton drip line, was experimentally observed for the first time. Multi-body correlation analysis of emitted protons from 20Mg, 21Al, and 18Mg, combined with Monte Carlo simulations, reveals that the identified resonant states in 20Mg and 21Al predominantly decay via two and three sequential steps of 1p emission, respectively, whereas the 18Mg ground state decays mainly through a two-step cascade of prompt 2p emission.