In materials of high atomic number (e.g. tungsten, uranium, plutonium) the electrons of energies >~10 MeV predominantly lose energy 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 X 0, usually measured in g·cm. 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.