Shape Coexistence Near Neutron NumberN=20: First Identification of theE0Decay from the Deformed First ExcitedJπ=0+State inMg30

The 1789 keV state in {sup 30}Mg was identified as the first excited 0{sup +} state via its electric monopole (E0) transition to the ground state. The measured small value of rho{sup 2}(E0,0{sub 2}{sup +}->0{sub 1}{sup +})=(26.2+-7.5)x10{sup -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 {sup 30}Mg to be based on neutron configurations below the N=20 shell closure, while the excited 0{sup +} state mainly consists of two neutrons excited into the nu 1f{sub 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.'