Large proton contribution to the 2+ excitation in Mg20 studied by intermediate energy inelastic scattering

Coulomb excitation of the proton-rich nucleus Mg20 was studied using a radioactive Mg20 beam at 58A MeV impinging on a lead target. The reduced transition probability B(E2;0g.s.+→21+) was extracted to be 177(32)e2 fm4, which agrees with the theoretical predictions by a cluster model assuming 16O+2p+2p structure, a mean-field approach based on the angular momentum projected generator coordinate method, and the USD shell model. The ratio of the neutron-to-proton multipole matrix elements Mn/Mp in the mirror nucleus O20 was deduced to be 2.51(25) with the Mn value evaluated from the measured B(E2) value for Mg20 with the help of isospin symmetry. The results confirm the large Mn/Mp value previously reported in O20, leading to the dominant role of the four valence nucleons in the 21+ excitation and persistence of the O16 core in O20 and Mg20.