Structure of ¹³Be probed via secondary-beam reactions

The low-lying level structure of the unbound neutron-rich nucleus ¹³Be has been investigated via breakup on a carbon target of secondary beams of ^14,15B at 35 MeV/nucleon. The coincident detection of the beam velocity ¹²Be fragments and neutrons permitted the invariant mass of the ¹²Be+n and ¹²Be+n+n systems to be reconstructed. In the case of the breakup of ¹⁵B, a very narrow structure at threshold was observed in the ¹²Be+n channel. Analysis of the ¹²Be+n+n events demonstrated that this resulted from the sequential decay of the unbound ¹⁴Be(2⁺) state rather than a strongly interacting s-wave virtual state in ¹³Be, as had been surmised in stable beam fragmentation studies. Single-proton removal from ¹⁴B was found to populate a broad low-lying structure some 0.7 MeV above the neutron-decay threshold, in addition to a less prominent feature at around 2.4 MeV. Based on the selectivity of the reaction and a comparison with (0-3)ℏω shell-model calculations, the low-lying structure is concluded to arise from closely spaced J^π=1/2⁺ and 5/2⁺ resonances (E_r=0.40±0.03 and 0.85^+0.15_-0.11 MeV), while the broad higher-lying feature is a second 5/2⁺ level (E_r=2.35±0.14 MeV). Taken in conjunction with earlier studies, the results suggest that the lowest 1/2⁺ and 1/2^- levels lie relatively close together below 1 MeV.