Neutron-unbound states in 27F and 28F have been measured using the technique of invariant mass spectroscopy, with the unbound states populated from nucleon knock out reactions, 29Ne (9Be, X). Neutrons resulting from the decay of unbound states were detected in the Modular Neutron Array (MoNA), which recorded their positions and times of flight. Residual charged fragments were deflected by the dipole Sweeper magnet and passed through a series of charged particle detectors that provided position, energy loss, and total kinetic energy measurements. The charged particle measurements were sufficient for isotope separation and identification as well as reconstruction of momentum vectors at the target. In addition to the neutron and charged particle detectors, a CsI(Na) array (CAESAR) surrounded the target, allowing a unique determination of the decay path of the unbound states.In 27F, a resonance was observed to decay to the ground state of 26F with 380 ± 60 keV relative energy, corresponding to an excited level in 27F at 2500 ± 220 keV. The 28F relative energy spectrum indicates the presence of multiple, unresolved resonances; however, it was possible to determine the location of the ground state resonance as 210+50-60 keV above the ground state of 27F. This translates to a 28F binding energy of 186.47 ± 0.20 MeV.Comparison of the 28F binding energy to USDA/USDB shell model predictions provides insight into the role of intruder configurations in the ground state structure of 28F and the low-Z limit of the &ldquo island of inversion &rdquo around N = 20. The USDA/USDB calculations are in good agreement with the present measurement, in sharp contrast to other neutron rich, N = 19 nuclei (29Ne, 30Na, and 31Mg). This proves that configurations lying outside of the sd model space are not necessary to obtain a good description of the ground state binding energy of 28F and suggests that 28F does not exhibit inverted single particle structure in its ground state.
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