Differential cross sections (DCS) for positron collisions with atoms require, in principal, the scattering contributions from an infinite number of partial waves. The traditional way to accomplish this is to make use of a closure scheme that involves the First Born Approximation (FBA). Done properly, this is an excellent technique that assures a converged result. However, over time, "Born closure" has come to imply an additional approximation that involves replacing the actual interaction potential with only the long-range analytic form when calculating the FBA terms. To read more about this subject, download the short paper in one of the formats listed below.
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