In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles with it.

[edit] Definition

High-energy electrons predominantly lose energy in matter by bremsstrahlung, and high-energy photons by e⁺e⁻ pair production. The characteristic amount of matter traversed for these related interactions is called the radiation length X₀, usually measured in gcm^-2. It is both the mean distance over which a high-energy electron loses all but 1/e of its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. It is also the appropriate scale length for describing high-energy electromagnetic cascades.

The radiation length is given, to good approximation, by the expression

. ^[1]

where Z is the atomic number and A is the mass number.

For electrons at lower energies (below few tens of MeVs), the energy loss by ionization is predominant.

While this definition may also be used for other electromagnetic interacting particles beyond leptons and photons, the presence of the stronger hadronic and nuclear interaction makes it a far less interesting characterisation of the material; the nuclear collision length and nuclear interaction length are more relevant.

Comprehensive tables for radiation lengths and other properties of materials are available from http://pdg.lbl.gov/AtomicNuclearProperties

[edit] See also

• Mean free path
• Attenuation length
• Attenuation coefficient
• Attenuation
• Range (particle radiation)
• Stopping power (particle radiation)
• Electron energy loss spectroscopy

[edit] References

• S. Eidelman et al. Particle Data Group, Review of particle physics, Phys. Lett. B 592 (2004) (http://pdg.lbl.gov/)

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