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In physics, the attenuation length or absorption length is the distance λ into a material when the probability has dropped to 1/e that a particle has not been absorbed. Alternatively, if there is a beam of particles incident on the material, the attenuation length is the distance where the intensity of the beam has dropped to 1/e, or about 63% of the particles have been stopped.
Mathematically, the probability of finding a particle at depth x into the material is calculated by the Beer–Lambert law:
- .
In general λ is material- and energy-dependent.