  The region near Z=28, N=40 is a subject of great interest for nuclear
structure studies due to spectroscopic signatures in $^{68}$Ni suggesting a
subshell closure at N=40. Trends in nuclear masses and their derivatives
provide a complementary approach to shell structure investigations via
separation energies. Penning trap mass spectrometry has provided precise
measurements for a number of nuclei in this region, however a complete picture
of the mass surfaces has so far been limited by the large uncertainty remaining
for nuclei with N > 40 along the iron and cobalt chains. Here we present the
first Penning trap measurements of $^{68,69}$Co, performed at the Low-Energy
Beam and Ion Trap facility at the National Superconducting Cyclotron
Laboratory. In addition, we perform ab initio calculations of ground state and
two-neutron separation energies of cobalt isotopes with the valence-space
in-medium similarity renormalization group approach based on a particular set
of two- and three-nucleon forces which predict saturation in infinite matter.
We discuss the importance of these measurements and calculations for
understanding the evolution of nuclear structure near $^{68}$Ni.

Nuclear Experiment