Core-Excitations in the Ground States of the Neutron-Rich Fluorine Isotopes.*

The breakdown of classical shell structure and the appearance of new magic numbers in nuclei with extreme N/Z ratios provide a valuable testing ground for shell-model and ab-initio descriptions.
In particular, there is substantial evidence for the appearance of N=16 as a new magic number in the oxygen isotopes as one approaches the dripline. Ab-initio methods including both NN and 3N interactions have been successful in describing the dripline at $^{24}$O, and have recently been extended into the Fluorine isotopes. They predict a fragmentation of the $\pi$1d5/2 strength as one moves towards stability due to core-coupling.

A recent experiment designed to study the evolution of core excitations in the ground states of the neutron-rich Fluorine isotopes was performed at the NSCL with GRETINA and the S800.
Using proton-knockout reactions from $^{21-25}$F secondary beams at 80-100 MeV/u, a consistent set of exclusive cross sections, momentum distributions, and derived spectroscopic factors can be obtained -- providing a stringent test of recent shell-model and ab-initio approaches.