The ground state of 28F has been observed as an unbound resonance
22​0 keV above the ground state of 27F. Comparison of this
result with USDA/USDB shell model predictions leads to the conclusion that the
28F ground state is primarily dominated by sd-shell configurations. Here
we present a detailed report on the experiment in which the ground state
resonance of 28F was first observed. Additionally, we report the first
observation of a neutron-unbound excited state in 27F at an excitation
energy of 250​0(22​0) keV.Comment: 10 pages, 11 figures, Accepted for publication in Phys. Rev.