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

Measurements of the N=28 isotones ⁴²Si, ⁴³P, and ⁴⁴S using one- and 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 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 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 d_5/2 proton strength in ⁴³P via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure.