Anomalous large angle alpha scattering

For α-nucleus potentials which contain a pocket, the scattering matrix may be split into 'barrier' and 'internal' components. Using simple parametrizations for these components, analytic expressions are derived for the corresponding 'barrier' and 'internal' cross sections. The oscillations present in these cross sections are interpreted as arising from interference between various terms in the Poisson summation formula for the scattering amplitudes. At backward angles the 'internal' cross section and the cross sections resulting from various Regge pole models are shown to have the same angular dependence. A method is given to locate roughly the position of the dominant Regge pole of the scattering matrix. At more foreward angles the 'internal' cross section, unlike those from the Regge pole models, describes the nuclear refraction undergone by α-particles that are transmitted through the target. Such refraction effects have previously been invoked to explain intermediate energy α-nucleus scattering. (orig.)