Search for Nondirect Effects in theSi28(p,p0,1,2)Si28Excitation Function from 16-18.2 MeV

Excitation functions have been measured at laboratory angles of 60, 80, and 120° in 50-100-keV steps for proton scattering from the $0^{+}$, $2^{+}$, and $4^{+}$ members of the ground-state rotational band in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. Prominent structure of width 300 keV is seen in both the $2^{+}$ and $4^{+}$ cross sections at all three angles and throughout the entire energy range from 16 to 18.2 MeV. At 60°, there is a 30% change in the $2^{+}$ and $4^{+}$ cross sections over the resonance at 17.7 MeV. The presence of this structure makes very dubious the quadrupole and hexadecapole deformations extracted from direct-reaction analyses of proton scattering below 18 MeV.