Elastic and inelastic scatterings of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ projectiles have been studied on ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ nucleus at 56 MeV incident energy. Optical model and coupled-channel analyses have been performed in order to define the nature of the optical potential. It has turned out that the enhanced backward angle oscillations that appeared in the angular distribution for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ scattering are mainly due to refractive phenomena namely orbiting rather than diffractive phenomena. The potential necessary to reproduce the experimental data has to be surface transparent for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ projectile, with a pocket in the effective potential in the grazing wave region and with a sharp diffusivity in the imaginary part in order to produce a mirror effect due to strong and abrupt volume absorption. On the other hand the potential necessary to fit the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ scattering angular distributions is just a normal strong absorptive potential. The absence of any surface transparency in the case of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ scattering can be attributed to the fact that many direct reaction channels are well $Q$ matched.

NUCLEAR REACTIONS ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$) and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$), $E_{{}_{}{}^{16}{}_{}{}^{}\mathrm{O}}$, $E_{{}_{}{}^{18}{}_{}{}^{}\mathrm{O}}=56\mathrm{}$ MeV; elastic and inelastic angular distributions measured between 5° and 71° (lab). Optical model and coupled-channel analysis.

• Received 31 August 1978

DOI:https://doi.org/10.1103/PhysRevC.19.794