[Ifeffit] reference

Hello All, Does anyone know of a reference that explicitly shows that sigma2 for a linear backscattering path is approximately twice a single scattering path? My example is for a uranyl moiety (UO2). The structure of this moiety is symmetric and linear with a dumbbell shape with O1=U=O2 all in a row. I need a reference that does the calculation to show that the 4 leg path (U-O1-U-O2-U) has a sigma2 value that is twice the single scattering path (U-O1-U) from one of the oxygen atoms. Thanks for your help Shelly