Low-energy Coulomb excitation of the neutron-rich Mg istotopes 30 Mg and 32 Mg

Abstract

The first safe Coulomb excitation experiments with beam energies below the Coulomb barrier have been performed at the newly commissioned radioactive beam facility REX-ISOLDE at CERN in conjunction with the modern HPGe gamma-spectrometer MINIBALL. REX-ISOLDE and MINIBALL offer unique possibilities to study collective and single-particle properties of nuclei far from stability by standard techniques such as safe Coulomb excitation and transfer reactions in inverse kinematics. From the Coulomb excitation experiments with beams of 30Mg and 32Mg, the reduced transition probabilities B(E2; 0_gs^+ -> 2_1^+) of these isotopes could be extracted in a model-independent way to be 253(21) e^2 fm^4 and 434(52) e^2 fm^4, respectively. While the B(E2) value of 30Mg is that expected for a pure sd-shell nucleus, the unusually large collective value observed for the semi-magic N=20 nucleus 32Mg reflects the vanishing neutron shell gap between sd- and the f_7/2-shell configurations which has been conjectured for the neutron-rich isotopes of Ne, Na and Mg. The large B(E2) value of 32Mg can indeed be well described by a pure intruder configuration. The present result thus manifests a surprisingly abrupt transition to the so-called island of inversion between 30Mg and 32Mg.