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

Abstract

The low-lying level structure of the unbound neutron-rich nucleus ${}^{13}$Be has been investigated via breakup on a carbon target of secondary beams of ${}^{14,15}$B at 35 MeV/nucleon. The coincident detection of the beam velocity ${}^{12}$Be fragments and neutrons permitted the invariant mass of the ${}^{12}$Be+$n$ and ${}^{12}$Be+$n$+$n$ systems to be reconstructed. In the case of the breakup of ${}^{15}$B, a very narrow structure at threshold was observed in the ${}^{12}$Be+$n$ channel. Contrary to earlier stable beam fragmentation studies which identified this as a strongly interacting $s$-wave virtual state in ${}^{13}$Be, analysis here of the ${}^{12}$Be+$n$+$n$ events demonstrated that this was an artifact resulting from the sequential-decay of the ${}^{14}$Be(2${}^{+}$) state. Single-proton removal from ${}^{14}$B was found to populate a broad low-lying structure some 0.70 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)$\hslash \ddot{I}‰$ shell-model calculations, the low-lying structure is concluded to most probably arise from closely spaced J${}^{\ddot{I}}€$=1/2${}^{+}$ and 5/2${}^{+}$ resonances (E${}_r$=0.40$\pm$0.03 and 0.85${}_{-0.11}^{+0.15}$ MeV), whilst the broad higher-lying feature is a second 5/2${}^{+}$ level (E${}_r$=2.35$\pm$0.14 MeV). Taken in conjunction with earlier studies, it would appear that the lowest 1/2${}^{+}$ and 1/2${}^{-}$ levels lie relatively close together below 1 MeV.

14 pages, 13 figures, 2 tables. Accepted for publication in Physical Review C