Effective Potential and Scattering Length Density

According to Eq. (3.3), a neutron incident on a solid or liquid would see an array of 5-function potentials 

It is important to realize that the wave incident on each nucleus is not the same as the incident wave on the entire sample. A good discussion on treating the problem of multiple scattering has been given by Sears [15] but falls beyond the scope of this introduction to neutron scattering theory. It can be shown [16] that the effective potential for a neutron scattered by a collection of nuclei is simply the volume average of the Fermi potenhals 

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