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 25O above the three neutron separation energy.

Method: The decay energy of 25O was reconstructed using the invariant mass spectroscopy method. A 101.3 MeV/u 27Ne beam collided with a liquid deuterium target. Two-proton removal reactions populated excited 25O that decayed into three neutrons and an 22O fragment. The neutrons were detected by arrays of plastic scintillator bars, while 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 25O at an excitation energy of about 9 MeV which decay sequentially by the emission of three neutrons to 22O.

Conclusion: The observation of resonance strength in 25O at about 9 MeV is consistent with shell-model/eikonal calculations for the two-proton removal reaction from 27Ne.