Shell structure at N = 28 near the dripline : spectroscopy of 42 Si , 43 P and 44

Measurements of the N = 28 isotones 42Si, 43P and 44S using oneand two-proton knockout reactions from the radioactive beam nuclei 44S and 46Ar are reported. The knockout reaction cross sections for populating 42Si and 43P and a 184 keV γ-ray observed in 43P establish that the d3/2 and s1/2 proton orbits are nearly degenerate in these nuclei and that there is a substantial Z = 14 subshell closure separating these two orbits from the d5/2 orbit. The increase in the inclusive twoproton knockout cross section from 42Si to 44S demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the d5/2 proton strength in 43P via a higher statistics one-proton knockout experiment could help determine the size of the Z = 14 closure. PACS numbers: 21.10.Pc,21.60.Cs,25.70.Hi,27.40.+z