Elastic and inelastic scattering of 16 O and 18 O on 28 Si at 56 MeV incident energy

Elastic and inelastic scatterings of /sup 16/O and /sup 18/O projectiles have been studied on /sup 28/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 appeared in the angular distribution for the /sup 16/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 /sup 16/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 /sup 18/O scattering angular distributions is just a normal strong absorptive potential. The absence of any surface transparency in the case of /sup 18/O scattering can be attributed to the fact that many direct reaction channels are well Q matched.