Systematic study of the nuclear potential through high precision back-angle quasi-elastic scattering measurements of¹⁶O and³²S on various targets

Elastic and inelastic scattering have proven an excellent method to probe the nuclear potential in the surface region [1]. A surface diffuseness parameter of around 0.63 fm is accepted as describing elastic and inelastic scattering data. A new method to determine the diffuseness was proposed [2] using high precision quasi-elastic excitation functions at sub-barrier energies, and at backward angles. In response to this proposal, we have made measurements for 32S + Er, 186W, 197Au, 208Pb [3], and more recently for the reactions ^l6O + ^144,154Sm, ^l86W, ^l97Au, ²⁰⁸Pb [4]. Both single channel and coupled-channels calculations have been performed to extract the diffuseness parameter of the nuclear potential, assuming a Woods-Saxon shape. For the reactions involving near-spherical targets, both theoretical analyses give the same diffuseness parameter, as had been hoped, showing that the effect of couplings to intrinsic excited states in the target and projectile is negligible. On the other hand, for deformed systems, coupling effects are surprisingly important even at deep sub-barrier energies, reducing the flux at backward angles. Energy spectra recorded with the back-angle Si detector show a large number of distinct peaks as well as a broad structure at even higher excitation energies. A comparison of the back-scattered energy spectra with coupled-channels calculations demonstrates that including couplings only to low lying collective states in coupled-channels calculations is inadequate in describing the reaction mechanism even at energies close to the barrier.