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. 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 e+e− pair production. The characteristicamount 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-energyelectron loses all but ​1⁄e of its energy by bremsstrahlung, and ​7⁄9 of the mean freepath 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: X 0 = 716.4 g c m − 2 A Z ( Z + 1 ) ln ⁡ 287 Z = 1433 g c m − 2 A Z ( Z + 1 ) ( 11.319 − ln ⁡ Z ) {displaystyle X_{0}=716.4;mathrm {g} ,mathrm {cm} ^{-2}{frac {A}{Z(Z+1)ln {frac {287}{sqrt {Z}}}}}=1433;mathrm {g} ,mathrm {cm} ^{-2}{frac {A}{Z(Z+1)(11.319-ln {Z})}}} , where Z is the atomic number and A is mass number of the nucleus. For Z > 4 {displaystyle Z>4} , a good approximation is 1 X 0 = 4 ( ℏ m e c ) 2 Z ( Z + 1 ) α 3 n a log ⁡ ( 183 Z 1 / 3 ) {displaystyle {frac {1}{X_{0}}}=4left({frac {hbar }{m_{mathrm {e} }c}} ight)^{2}Z(Z+1)alpha ^{3}n_{mathrm {a} }log left({frac {183}{Z^{1/3}}} ight)} , where n a {displaystyle n_{mathrm {a} }} is the number density of the nucleus, ℏ {displaystyle hbar } denotes the reduced Planck constant, m e {displaystyle m_{mathrm {e} }} the electron rest mass, c {displaystyle c} the speed of light, and α {displaystyle alpha } 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.