Shell Structure At N=28 Near The Dripline: Spectroscopy Of Si-42, P-43, And S-44

Measurements of the N=28 isotones Si-42, P-43, and S-44 using one- and two-proton knockout reactions from the radioactive beam nuclei S-44 and Ar-46 are reported. The knockout reaction cross sections for populating Si-42 and P-43 and a 184 keV gamma-ray observed in P-43 establish that the d(3/2) and s(1/2) proton orbits are nearly degenerate in these nuclei and that there is a substantial Z=14 subshell closure separating these two orbits from the d(5/2) orbit. The increase in the inclusive two-proton knockout cross section from Si-42 to S-44 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 d(5/2) proton strength in P-43 via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure.