THE EGS5 CODE SYSTEM

Chapter 2RADIATION TRANSPORT INEGS52.1 Description of Radiation Transport-Shower ProcessElectrons 1 , as they traverse matter, lose energy by two basic processes: collision and radiation.The collision process is one whereby either the atom is left in an excited state or it is ionized. Mostof the time the ejected electron, in the case of ionization, has a small amount of energy that isdeposited locally. On occasion, however, an orbital electron is given a significant amount of kineticenergy such that it is regarded as a secondary particle called a delta-ray.Energy loss by radiation (bremsstrahlung) is farily uniformly distributed among secondaryphotons of all energies from zero up to the kinetic energy of the primary particle itself. At lowelectron energies the collision loss mechanism dominates the electron stopping process, while athigh energies bremsstrahlung events are more important, implying that there must exist an energyat which the rate of energy loss from the two mechanisms are equivalent. This energy coincidesapproximately with the critical energy of the material, a parameter that is used in shower theory forscaling purposes[141]. Note that the energetic photons produced in bremsstrahlung collisions maythemselves interact in the medium through one of three photon-processes, in relative probabilitiesdepending on the energy of the photon and the nature of the medium. At high energies, themost likely photon interaction is materialization into an electron-positron pair, while at somewhatlower energies (in the MeV range), the most prevalent process is Compton scattering from atomicelectrons. Both processes result in a return of energy to the system in the form of electrons whichcan generate additional bremsstrahlung photons, resulting in a multiplicative cycle known as anelectromagnetic cascade shower. The third photon interactive process, the photoelectric effect, aswell as multiple Coulomb scattering of the electrons by atoms, perturbs the shower to some degree1 In this report, we often refer to both positrons and electrons as simply electrons. Distinguishing features will benoted in context.20