Differential cross sections were measured in small angular steps at forward angles for the elastic scattering of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ from ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$ at $E_{\mathrm{lab}}=60\mathrm{}$ MeV and from ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at six other incident energies between 45 and 63 MeV. The angular position of a peak in the diffraction pattern at ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}\cong 75°$ was measured as a function of the incident energy between 55 and 63 MeV in 0.5 MeV steps. Close fits to these angular distributions and those of other authors at energies between 41 and 81 MeV, some spanning the whole angular range up to 180°, and the excitation functions at 90° and 180°, were obtained in an optical model analysis. A consistent description of the data was found by the use of a surface-transparent and parity-dependent potential with a real part able to generate a pocket in the total potential. The real and imaginary strengths depend quite strongly and smoothly on the incident energy. This potential shows a transition from surface transparency to strong absorption as $E_{\mathrm{lab}}$ approaches 81 MeV. The ambiguities in the strengths of the potential are discussed. The broad dispersive potential resonances which are present in several partial waves at every energy are also discussed and their relative importance is examined.

NUCLEAR REACTIONS ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$, $E=45-63\mathrm{}$ MeV, measured $\sigma (E; \theta )$. Optical potential fit, $E=41-81\mathrm{}$ MeV. Parameter ambiguities, parity dependence, potential resonances.

• Received 18 November 1982

DOI:https://doi.org/10.1103/PhysRevC.28.717