# 11.2: Some terminology for general scattering

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The incident flux (I) of particles with momentum $${\bf p}= \hbar {\bf k}$$ is the number of incident particles crossing unit area perpendicular to the beam direction per unit time.

The scattered flux (S) of particles with momentum $${\bf p'} = \hbar {\bf k'}$$, is the number of scattered particles scattered into the element of solid angle $$d\Omega$$ about the direction $$\theta$$, $$\phi$$ per unit time per unit solid angle.

The differential cross section is the ratio of the scattered flux in direction $$\theta$$, $$\phi$$ to the incident flux.

$\frac{d\sigma}{d\Omega} = S/I \nonumber$

The total cross section is the ratio of the scattered flux in any direction to the incident flux.

$\sigma_T = \int \int \frac{d\sigma}{d\Omega} \sin \theta d\theta d\phi \nonumber$

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