Description of backward maxima in the elastic scattering of α particles by 12C and 40Ca nuclei

The backward maxima of the cross section are described quantitatively by representing the elastic scattering amplitude as the sum of an optical (potential) amplitude with terms added for exchange processes. The correctness of the use of a simplified variant of the distorted-wave method (the eikonal approximation with a cutoff in the integration over the interior of the nucleus) to calculate the matrix elements of exchange processes is justified numerically. The microscopic approach makes it possible not only to describe the backward maxima of the cross sections for the two nuclei but also to significantly improve the agreement with experiment in a wide range of angles and to obtain the correct general behavior of the excitation functions at very large angles. Comparison of the microscopic and phenomenological reflection coefficients shows that eta/sub L/ cannot be described in terms of a single phenomenological model