Background: ${}^{15}\mathrm{Be}$ 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 ${}^{15}\mathrm{Be}$ was populated in a second attempt and determined to be unbound with respect to ${}^{14}\mathrm{Be}$ 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 ${}^{15}\mathrm{Be}3/2^{+}$ state in the three-neutron decay channel.

Method: A two-proton removal reaction from a 55 MeV/u ${}^{17}\mathrm{C}$ beam was used to populate neutron-unbound states in ${}^{15}\mathrm{Be}$. The two-, three-, and four-body decay energies of the ${}^{12}\mathrm{Be}+$ 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 ${}^{14}\mathrm{Be}$ ($E_x=1.54$ MeV, $E_r=0.28$ MeV). Including a state in ${}^{15}\mathrm{Be}$ that decays through the first excited ${}^{14}\mathrm{Be}$ state slightly improves the fit at higher energies though the cross section is small.

Conclusions: A ${}^{15}\mathrm{Be}$ component is not needed to describe the data. If the $3/2^{+}$ state in ${}^{15}\mathrm{Be}$ is populated, the decay by three-neutron emission through ${}^{14}\mathrm{Be}$ is weak, $\le 11\%$ up to 4 MeV. In the best fit, ${}^{15}\mathrm{Be}$ is unbound with respect to ${}^{12}\mathrm{Be}$ by 1.4 MeV (unbound with respect to ${}^{14}\mathrm{Be}$ by 2.66 MeV) with a strength of $7\%$.

• Received 9 December 2014
• Revised 30 December 2014

DOI:https://doi.org/10.1103/PhysRevC.91.017304