Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles with it. 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 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.