The neutron unbound ground state of (25)O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a (26)F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_+20(-10) keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the nu1s(1/2) and nu0d(3/2) orbitals. The neutron separation energies for (25)O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an (26)O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of (26)O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

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A two-neutron unbound excited state of 24 O was populated through a (d,d') reaction at 83.4 MeV/nucleon. A state at E = 715 ± 110 (stat) ±45 (sys) keV with a width of Γ < 2 MeV was observed above the two-neutron separation energy placing it at 7.65 ± 0.2 MeV with respect to the ground state. Three-body correlations for the decay of 24 O → 22 O + 2n show clear evidence for a sequential decay through an intermediate state in23 O. Neither a di-neutron nor phase-space model for the three-body breakup were able to describe these correlations.

A search for the neutron-unbound nucleus 21 C was performed via the single proton removal reaction from a beam of 22 N at 68 MeV/u. Neutrons were detected with the Modular Neutron Array (MoNA) in coincidence with 20 C fragments. No evidence for a low-lying state was found, and the reconstructed 20 C+n decay energy spectrum could be described with an s-wave line shape with a scattering length limit of |a s | < 2.8 fm, consistent with shell model predictions. A comparison with a renormalized zero-range three-body model suggests that 22 C is bound by less than 70 keV.