The form factor of the real part of the α-nucleus potential studied over a wide energy range

Differential cross sections for elastic scattering of α-particles from 90Zr and 92Zr were measured over a wide angular range at a number of energies between 40 and 120 MeV. Excitation functions for scattering from 90Zr were measured at three angles over the range 28-54 MeV. The data were analyzed in terms of the optical model. Apart from the standard Woods-Saxon form factor, a number of other parameterizations of the real potential form factor were used. The very best fits to the data were obtained in a model-independent analysis in which the real potential was represented by a spline function. Folding model potentials were also employed. The data for Esub(α)>=80 MeV can be very well accounted for by an optical potential with constant form factors and real and imaginary well depths which vary smoothly with energy. At lower energies, however, the data can only be fitted in detail if for the real potential a form factor is used which is different from the one used at high energies. Attempts to explain the energy dependence of the form factor, e.g. in terms of effects which are neglected in the regular optical model description, are discussed. No satisfactory explanation has been obtained. The investigation of the uniqueness of the determination of the real potential revealed that at high energies as well as at low energies the elastic scattering is sensitive to the shape of the real potential over the range 4 to 9 fm, and very insensitive to the deep interior part and the tail of the potential. (Auth.)