Backscattering of slow electrons by positive ions

The scattering cross section of slow electrons by positive ions at scattering angle 1800 is considered as a function of energy. At this angle there will be no oscillations due to the factor exp[i(z/k) ln sin(theta/2)] in the Coulomb part of the amplitude. The shape of the cross section vs. energy curve depends on the non-Coulomb part of the interaction. The possibility of a deep minimum on this curve is indicated and the conditions of its existence are examined