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 ${}^{48}\mathrm{Ar}$ and ${}^{49}\mathrm{Ar}$ and find atomic mass excesses of $-22.28(31)\mathrm{MeV}$ and $-17.8(1.1)\mathrm{MeV}$, 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 shell-model calculations in the $sd$-$pf$ shell using SDPF-U and SDPF-MU Hamiltonians.

• Received 27 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.022501