# Attenuation length

In physics, the attenuation length or absorption length is the distance $\lambda$ 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.
$P(x) = e^{-x /\lambda} \!\,$.
In general $\lambda$ is material and energy dependent.