Definition
High-energy electrons (>~10 MeV) 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 g·cm−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 for a given material consisting of a single type of nuclei can be approximated by the following expression:
,
where Z is the atomic number and A is mass number of the nucleus.
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
Read more about this topic: Radiation Length
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