  We present results from recent time-of-flight nuclear mass measurements at
the National Superconducting Cyclotron Laboratory at Michigan State University.
We report the first mass measurements of 48Ar and 49Ar and find atomic mass
excesses of -22.28(31) MeV and -17.8(1.1) MeV, respectively. These masses
provide strong evidence for the closed shell nature of neutron number N=28 in
argon, which is therefore the lowest even-Z element exhibiting the N=28 closed
shell. The resulting trend in binding-energy differences, which probes the
strength of the N=28 shell, compares favorably with shellmodel calculations in
the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians.
