Anomalous Diffraction (AD) method has proved to be an adequate approximation of the exact method (GMM code computed by Xu) to obtain the scattering cross section for aggregate whose the primary particle size parameter is upper than 2. Indeed, the error of this approximated method is less than approximately 10% when the primary particles are in SiO2. However this method is strongly related to the aggregate morphology since it includes in its formulation the concept of chord length. We initially studied the chord distribution for various aggregates, and then we reformulated the anomalous diffraction method, to approximate the scattering cross section, now completely analytically. This new expression is entitled ADr with the r for rapid because this one is at least a hundred times faster than the standard AD method.