Shape Coexistence Near Neutron Number N=20: First Identification of the E0 Decay from the Deformed First Excited J^π=0⁺ State in ³⁰Mg

The 1789 keV state in ³⁰Mg was identified as the first excited 0⁺ state via its electric monopole (E0) transition to the ground state. The measured small value of ρ²(E0,0₂⁺→0₁⁺)=(26.2±7.5)×10^-3 implies within a two-level model a small mixing of competing configurations with largely different intrinsic quadrupole deformation near the neutron shell closure at N=20. Axially symmetric configuration mixing calculations identify the ground state of ³⁰Mg to be based on neutron configurations below the N=20 shell closure, while the excited 0⁺ state mainly consists of two neutrons excited into the ν 1f_7/2 orbital. The experimental result represents the first case where an E0 back decay from a strongly deformed second to the normal deformed first nuclear potential minimum well has been unambiguously identified, thus directly proving shape coexistence at the borderline of the much-debated “island of inversion.”