Editing Cross section (physics) (section)

== Attenuation of a beam of particles ==
{{See also|Attenuation}}
If a beam of particles enters a thin layer of material of thickness {{math|d''z''}}, the [[flux]] {{math|Φ}} of the beam will decrease by {{math|dΦ}} according to
: <math>\frac{\mathrm d \Phi}{\mathrm d z} = -n \sigma \Phi,</math>
where {{math|''σ''}} is the total cross section of ''all'' events, including [[scattering]], [[absorption (electromagnetic radiation)|absorption]], or transformation to another species. The volumetric number density of scattering centers is designated by {{math|''n''}}. Solving this equation exhibits the exponential attenuation of the beam intensity:
: <math>\Phi = \Phi_0 e^{-n \sigma z},</math>
where {{math|Φ<sub>0</sub>}} is the initial flux, and {{math|''z''}} is the total thickness of the material. For light, this is called the [[Beer–Lambert law]].