The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound $^{ 28}\textrm{ F},$ determined to be a resonance in the $^{ 27}\textrm{ F} + n$ continuum at $2\underline{ 2}0 (\underline{ 5}0)$ keV. States in $^{ 28}\textrm{ F}$ were populated by the reactions of a 62 MeV/u $^{ 29}\textrm{ Ne}$ beam impinging on a 288 $\textrm{ mg/cm}^2$ beryllium target. The measured $^{ 28}\textrm{ F}$ ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that $pf$ shell intruder configurations play only a small role in the ground state structure of $^{ 28}\textrm{ F}$ and establishing a low-$Z$ boundary of the island of inversion for N=19 isotones.
Categories: nucl-ex