Structure of $^{13}$Be probed via secondary beam reactions

The low-lying level structure of the unbound neutron-rich nucleus Be-13 has been investigated via breakup on a carbon target of secondary beams of B-14,B-15 at 35 MeV/nucleon. The coincident detection of the beam velocity Be-12 fragments and neutrons permitted the invariant mass of the Be-12 + n and Be-12 + n + n systems to be reconstructed. In the case of the breakup of B-15, a very narrow structure at threshold was observed in the Be-12 + n channel. Analysis of the Be-12 + n + n events demonstrated that this resulted from the sequential decay of the unbound Be-14(2(+)) state rather than a strongly interacting s-wave virtual state in Be-13, as had been surmised in stable beam fragmentation studies. Single-proton removal from B-14 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)(h) over bar omega shell-model calculations, the low-lying structure is concluded to arise from closely spaced J(pi) = 1/2(+) and 5/2(+) resonances (E-r = 0.40 +/- 0.03 and 0.85(-0.11)(+0.15) 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.