Low-lying neutron unbound states in 12Be

The neutron decay of an unbound resonance in Be-12 has been measured at 1243 +/- 21 keV decay energy with a width of 634 +/- 60 keV. This state was populated with a one-proton removal reaction from a 71 MeV/u B-13 beam incident upon a beryllium target. The invariant mass reconstruction of the resonance was achieved by measuring the daughter fragment in coincidence with neutrons. Despite being above the 2n separation energy, the state decays predominantly by the emission of one neutron to Be-11, setting an upper limit on the branching ratio for the two-neutron decay channel to Be-10 of less than 5%. From the characteristics of the population and decay of the resonance, it is concluded that this state cannot correspond to the previously observed state at 4580 +/- 5 keV.