Observation of three-neutron sequential emission from O*25

Background: Measurements of neutron-unbound states can test nuclear models in very neutron-rich nuclei that, in some cases, cannot be probed with other methods.Purpose: Search for highly excited neutron-unbound states of $^{25}\mathrm{O}$ above the three-neutron separation energy.Method: The decay energy of $^{25}\mathrm{O}$ was reconstructed using the invariant mass spectroscopy method. A 101.3-MeV/u $^{27}\mathrm{Ne}$ beam collided with a liquid-deuterium target. Two-proton removal reactions populated excited $^{25}\mathrm{O}$ that decayed into three neutrons and an $^{22}\mathrm{O}$ fragment. The neutrons were detected by arrays of plastic scintillator bars, whereas a 4-Tm dipole magnet placed directly after the target redirected charged fragments to a series of charged-particle detectors. The data were compared with detailed Monte Carlo simulations of the reaction process and subsequent decay.Results: The data show evidence of neutron-unbound level(s) in $^{25}\mathrm{O}$ at an excitation energy of about 9 MeV which decay sequentially by the emission of three neutrons to $^{22}\mathrm{O}$.Conclusion: The observation of resonance strength in $^{25}\mathrm{O}$ at about 9 MeV is consistent with shell-model and eikonal calculations for the two-proton removal reaction from $^{27}\mathrm{Ne}$.