Search for unbound 15Be states in the 3n+12Be channel

Background: Be-15 is expected to have low-lying 3/2(+) and 5/2(+) states. A first search did not find the 3/2(+) [A. Spyrou et al., Phys. Rev. C 84, 044309 (2011)]; however, a resonance in Be-15 was populated in a second attempt and determined to be unbound with respect to Be-14 by 1.8(1) MeV with a tentative spin-parity assignment of 5/2(+) [J. Snyder et al., Phys. Rev. C 88, 031303(R) (2013)]. Purpose: Search for the predicted Be-15 3/2(+) state in the three-neutron decay channel. Method: A two-proton removal reaction from a 55 MeV/u C-17 beam was used to populate neutron-unbound states in Be-15. The two-, three-, and four-body decay energies of the Be-12+ neutron(s) detected in coincidence were reconstructed using invariant mass spectroscopy. Monte Carlo simulations were performed to extract the resonance and decay properties from the observed spectra. Results: The low-energy regions of the decay energy spectra can be described with the first excited unbound state of Be-14 (E-x = 1.54 MeV, E-r = 0.28 MeV). Including a state in Be-15 that decays through the first excited Be-14 state slightly improves the fit at higher energies though the cross section is small. Conclusions: A Be-15 component is not needed to describe the data. If the 3/2(+) state in Be-15 is populated, the decay by three-neutron emission through Be-14 is weak, <= 11% up to 4 MeV. In the best fit, Be-15 is unbound with respect to Be-12 by 1.4 MeV (unbound with respect to Be-14 by 2.66 MeV) with a strength of 7%.