Publication Type
Journal
Journal Name
Physical Review Letters
Publication Date
Volume
114
Issue
2
Abstract
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 shell-model calculations in the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians.