Shell structure of S 43 and collapse of the N=28 shell closure

Abstract

The single-particle structure of the N=27 isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly magic Ca48 toward the drip line. S43 was studied employing the one-neutron knockout reaction from a radioactive S44 beam. Using a combination of prompt and delayed γ-ray spectroscopy the level structure of S43 was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the S44 ground-state configuration has a strong intruder component. The results were confronted with shell-model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the 2p3/2 orbital in the experiment indicates that the breakdown of the N=28 magic number is more rapid than the theoretical calculations suggest.