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.

This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (January 2010)

Mathematically, the probability of finding a particle at depth x into the material is calculated by the Beer–Lambert law:

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

In general λ is material- and energy-dependent.

• Beer's Law
• Mean free path
• Attenuation coefficient
• Attenuation (electromagnetic radiation)
• Radiation length

• S. Eidelman; et al. (2004). Particle Data Group (ed.). "Review of particle physics". Phys. Lett. B. 592 (1–4): 1–5. arXiv:astro-ph/0406663. doi:10.1016/j.physletb.2004.06.001. PMID 10020536. S2CID 118588567.
• https://web.archive.org/web/20050215215652/http://www.ct.infn.it/~rivel/Glossario/node2.doc

• http://henke.lbl.gov/optical_constants/atten2.doc

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