Backward Angle Anomaly in Alpha-Nucleus Scattering and Parity-Dependent Optical Model

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Abstract
An extended optical potential model is proposed, which has a parity-dependent part in the nuclear surface region. Backward angle anomaly in the differential cross section appearing in the α-particle scattering on the nuclei in the calcium region is analyzed systematically by this model. It is shown that the scattering amplitudes of sufrace-grazing partial waves superposed by the parity-dependent potential are summed up coherently at backward angles to give the backward rising of the differential cross section. The α-Ca and -Ar scattering with an incident energy near 20 ∼30 MeV are analyzed and a fairly good fit is obtained. The gross behavior of a broad peak in the excitation function at an extreme back angle is reproduced with an energy dependence of the parity-dependent part and it is shown that this broad peak is brought about by a coherent contribution of the additional amplitudes in some surface partial waves.