Attenuation length

In physics, the attenuation length is the distance <math>\tau<math> into a material when the probability has dropped to <math>1/e<math> that a particle has not been absorbed. Alternatively, if there is a beam of particles incident on the material, the attentuation length is the distance where the intensity of the beam has dropped to <math>1/e<math>, or about 63% of the particles have been stopped.

Mathematically, the probability of finding a particle at depth x into the material is

<math>P(x) = e^{-x /\tau} \!\,<math>.

In general <math>\tau<math> is material dependant.