Shape evolution in heavy sulfur isotopes and erosion of the N=28 shell closure

The structure of neutron-rich ${}^{40,42,44}\mathrm{S}$ nuclei has been investigated through in-beam $\ensuremath{\gamma}$-ray spectroscopy using the fragmentation reaction of a 60 MeV A ${}^{48}\mathrm{Ca}$ beam on a thin Be target. ${E}_{\ensuremath{\gamma}},$ ${I}_{\ensuremath{\gamma}},$ $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence, and $\ensuremath{\gamma}$-ray angular distributions were measured for each produced fragment. The level schemes previously containing only a single $\ensuremath{\gamma}$ transition were extended, and spin values were proposed for the new states. The experimental results were interpreted by use of microscopic collective-model and large-scale shell-model calculations. The results of the model calculations are consistent with each other, and give a reasonable description of the experimental results. Both models predict an erosion of the $N=28$ shell closure at $Z=16$ and suggest a deformed ground state for ${}^{40,42}\mathrm{S}$ and a spherical-deformed mixed configuration for ${}^{44}\mathrm{S}.$