Coulomb excitation at the border of the Island of inversion - the case of 31Mg

The ground state properties of 31Mg indicate a dramatic change of nuclear shape at N=19 with a highly deformed Jπ=1/2+ ground state, implying that 31Mg is part of the island of inversion. The unknown collective properties of excited states were subject of a Coulomb excitation experiment at REX-ISOLDE, CERN, employing a radioactive 31Mg beam. De-excitation γ-rays were detected by the MINIBALL γ-spectrometer in coincidence with scattered particles in a segmented Si-detector. The level scheme of 31Mg was extended. Spin and parity assignment of the 945 keV state yielded a (3/2+,5/2+) value. De-excitation of this state is dominated by a strong collective M1 transition to the (3/21+) state. The deduced B(E2) and B(M1)>0.145μN2 values are in good agreement with the properties of a rotational band built on the 1/2+ ground state.