Flux, Scattering Cross Section, and Intensity

In the case of a spherical wave, that is, radiation emitted or scattered by a point source, it is more convenient to express the flux J by the amount of energy transmitted per second through a unit solid angle rather than a unit area. In this way the measure of the flux becomes independent of the distance R from the source to the point of observation. In terms of the particle stream, the flux is then given as the number of photons or neutrons transmitted per solid angle per second. It is also understood that the amplitude A of the spherical wave is so defined that its square still gives the flux J independent of R. 

In terms of the particle language, the differential scattering cross section da/dQ is the probability that a photon or a neutron impinging on the sample is scattered into a unit solid angle in the given direction, or 

Integrating the differential scattering cross section throughout the solid angle Q gives the total scattering cross section 

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