Attenuation length

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:


 * $$P(x) = e^{-x /\lambda} \!\,$$.

In general $λ$ is material- and energy-dependent.