An inelastic proton scattering experiment was performed at the National Super-conducting Cyclotron Laboratory to study quadrupole collectivity in the even-even silicon and sulfur isotopes near N = 28. Experiments on neutron-rich sulfur isotopes have found significant collectivity and have been interpreted as pointing to the collapse of the N = 28 shell gap. Narrowing of a proton subshell gap in the sulfur isotopes may, however, be responsible for the increased collectivity. This experiment gives a quantitative measurement of the decrease in collectivity between 42S and 44S showing that the N = 28 shell gap does not vanish at Z = 16. In the silicon isotopes, the large, stable Z = 14 subshell gap directly ties collective trends to the strength of the N = 28 shell closure. Quadrupole collectivity and 2+1 excitation energies in the isotopes 36,38,40Si give clear evidence for the narrowing of the N = 28 shell gap in the absence of strong proton collectivity.
展开▼
