  The nuclear potential and resulting shell structure are well established for
the valley of stability, however, dramatic modifications to the familiar
ordering of single-particle orbitals in rare isotopes with a large imbalance of
proton and neutron numbers have been found: new shell gaps emerge and
conventional magic numbers are no longer valid. This article outlines some of
the recent in-beam gamma-ray spectroscopy measurements at NSCL aimed at
shedding light on the evolution of nuclear structure around neutron number N =
28 in neutron-rich Ar and S isotopes.
