The 1789 keV state in ${}^{30}\mathrm{Mg}$ was identified as the first excited $0^{+}$ state via its electric monopole ($E0$) transition to the ground state. The measured small value of ${\rho }^2(E0,0_2^{+}\to 0_1^{+})=(26.2\pm 7.5)\times {10}^{-3}$ 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 ${}^{30}\mathrm{Mg}$ 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 $\nu$ $1f_{7/2}$ 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.”

• Received 2 August 2008

DOI:https://doi.org/10.1103/PhysRevLett.103.012501