In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy particles electromagnetically interacting with it.
In materials of high atomic number (e.g. W, U, Pu) the electrons of energies >~10 MeV predominantly lose energy
by bremsstrahlung, and high-energy photons by
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 length scale for describing high-energy electromagnetic cascades.
The radiation length for a given material consisting of a single type of nucleus can be approximated by the following expression:
For , a good approximation is ,
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.
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