Backangle anomaly in scattering of α -particles from ²⁸ Si at low energies

In order to resolve the differences in the literature on the existence of quasi-molecular states in the α -²⁸ Si system, excitation functions were measured for the scattering of α -particles from ²⁸ Si in the incident energy range E_lab=3 -7.8 MeV. An angular distribution measurement was carried out in the angular range θ_lab=30° -174.5 ° for every potential resonance observed in the excitation functions. Data was analysed using a Regge-pole formalism by coherently adding specific resonances to an underlying diffraction term calculated by a strong absorption model. Furthermore, the usual compound elastic contribution was incoherently added to the direct interaction part of the cross section. The 6.8 MeV resonance was confirmed with J=3 and some evidence was observed for a J=1 resonance around 6.0 MeV.