Mapping of the onset of a new region of deformation: the masses of 31Mg and 32Mg

Qsub(β)-values are measured by the shape-fitting method, applied to selected γ-coincident β energy spectra of 31Mg and 32Na. The deduced mass excesses of 31Mg and 32Mg are 5.0+-0.6 and -1.9+-1.4 MeV, respectively. The corresponding two-neutron binding energies Ssub(2n) are in marked excess over the values extrapolated from the Ssub(2n) variation with A established for less neutron rich nuclei. This is taken as an indication for the onset of deformation at N=19 for Mg isotopes