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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Evidence for the ground state of the neutron-unbound nucleus 26 O was observed for the first time in the single proton-knockout reaction from a 82 MeV/u 27 F beam. Neutrons were measured in coincidence with 24 O fragments. 26 O was determined to be unbound by 150 +50 −150 keV from the observation of low-energy neutrons. This result agrees with recent shell model calculations based on microscopic two-and three-nucleon forces.A major challenge in nuclear physics remains the description of nuclei based on fundamental interactions. "Ab-initio" approaches have been developed to calculate nuclear properties based on nucleon-nucleon scattering data up to A ∼ 12 [1]. Recent advances in nuclear theory made it possible to describe some fundamental properties of light nuclei up to oxygen based on two-and three-nucleon interactions [2][3][4][5][6]. On the way to heavier nuclides it will be critical for these models to describe the dramatic change in the location of the neutron dripline from oxygen (N = 16) to fluorine (N ≥ 22) which was first pointed out by Sakurai et al. [7]. The addition of one proton binds at least six additional neutrons. The two-neutron separation energy of 26 O serves as an important benchmark for these calculations. The majority of the current nuclear structure models predict 26 O to be bound [8][9][10][11][12][13] respect to two-neutron emission. 26 O is thus also an excellent candidate for di-neutron emission. Furthermore calculations by Grigorenko et al. predict that the emission of a pair of correlated neutrons might be hindered so that for very low decay energies lifetimes on the order of pico-to nanoseconds could be possible [27].We searched for unbound states in 26 O using oneproton knockout reactions from 27 F and by measuring neutrons in coincidence with 24 O fragments. Figure 1 shows a schematic level scheme of the possible decay paths for predicted states of 26 O. In this letter we present the first evidence for the observation of the unbound ground state of 26 O.