Scattering amplitude
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The scattering amplitude describes the amplitude of an outgoing, elemetary, spherical wave relative to a plane, incoming wave scattered on a point size particle. Since the spherical wave amplitude varies with 1/R (note, the intensity is the |square| of the amplitude), R being the distance to the point scatterer, the unit of the scattering amplitude includes the unit of a length with respect to the units of the incoming wave. Therefore, the scattering amplitude is often also called scattering length.
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[edit] cross section
The scattering amplitude f relates to the scattering cross section s = 4*Pi * f^2.
[edit] X-rays
The scattering length for X-rays is the Thompson scattering length or classical electron radius r0.
[edit] neutrons
The nuclear neutron scattering process involves the coherent neutron scattering length, often described by b.
[edit] quantum mechanical formalism
A quantum mechanical approach is given by the S matrix formalism.
