Elastic scattering of 104 MeV alpha particles

Abstract

Differential cross sections were measured for elastic scattering of 104 MeV alpha particles on 6Li, 9Be, 12C, 14N, 16O, 20Ne, 40Ar, 64Ni, 90Zr, 124Sn, 208Pb and 209Bi. All angular distributions show a pronounced diffraction pattern. For the lightest nuclei measured ( 6Li to 16O) the cross section exceeds the Rutherford cross section by a factor of up to 15 at angles around 45°. The cross section data were analysed in terms of parametrized phase-shift functions using 9 free parameters and in terms of the spinless optical model using up to 7 free parameters. Good fits were obtained. A strong absorption was found for the heavy nuclei, A ≧ 124. For these nuclei, only the outermost region of the nuclear surface contributes to the scattering. A potential radius cannot be extracted regardless of the pronounced diffraction pattern. The absorption is not so strong for the medium weight nuclei, A = 20–90. Information about the nuclear surface can be extracted from the differential cross section. The well known phase ambiguity in the optical model was found for these nuclei. For the light nuclei ( A ≦ 16) the inner region of the interaction potential contributes to the scattering. A repulsive core for small interaction distances explains the observed cross sections which indicates the nonlocal character of the true interaction. The phase shift analysis leads to good fits, but the phase shifts obtained differ significantly from those of the optical-model analysis. This indicates that the phase-shift analysis is ambiguous also.