Nuclear barriers in 16O scattering

The method of determining the barrier height for 16O with respect to several medium weight nuclei from 40Sa to 120Sm based on the optical model analysis of elastic scattering is discussed. The Florida State University Super FN Tandem Van de Graaf Accelerator was used to produce a beam of 30 - 60 MeV16O ions of charge state 5 or 6. Fifteen point angular distributions were measured in 2 MeV steps using 15 Si surface barries detectors mounted in a ring at 10deg intervals from 30deg to 170deg in the lab. The overall energy resolution was about 300-500 keV. The angular distribution measurements were not carried out to the large angles at high energies in order to be able to use the extensions of the optical model. Extensive three and four parameter searches were carried out for real and imaginary well depths and the geometry parameters rsub(o)and a where r = rsub(o)(A1sup(1/3)+A2sup(1/3) and a is the diffuseness. A compound nuclear contribution to the data was estimated from the statistical model with simplications where the sum over all exit channels into which the compound nucleus may decay is replaced by an explicit expression obtained from the Fermi gas model. The disagreement between experimental and computed cross sections at back angles is observed which suggests that some extension of the optical model is necessary for heavy ion scattering. (S.B.)