Radiation length
In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy particles electromagnetically interacting with it. Definition. 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 , usually measured in g·cm. It is both the mean distance over which a high-energy electron loses all but of its energy by bremsstrahlung, and 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: formula_1, where is the atomic number and is mass number of the nucleus. For formula_2, a good approximation is formula_3, where formula_4 is the number density of the nucleus, formula_5 denotes the reduced Planck constant, formula_6 the electron rest mass, formula_7 the speed of light, and formula_8 the fine structure constant. For electrons at lower energies (below  few tens of MeV), 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 the Particle Data Group.