THE EGS5 CODE SYSTEM

Chapter 2
RADIATION TRANSPORT IN
EGS5
2.1 Description of Radiation Transport-Shower Process
Electrons 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. Most
of the time the ejected electron, in the case of ionization, has a small amount of energy that is
deposited locally. On occasion, however, an orbital electron is given a significant amount of kinetic
energy such that it is regarded as a secondary particle called a delta-ray.
Energy loss by radiation (bremsstrahlung) is farily uniformly distributed among secondary
photons of all energies from zero up to the kinetic energy of the primary particle itself. At low
electron energies the collision loss mechanism dominates the electron stopping process, while at
high energies bremsstrahlung events are more important, implying that there must exist an energy
at which the rate of energy loss from the two mechanisms are equivalent. This energy coincides
approximately with the critical energy of the material, a parameter that is used in shower theory for
scaling purposes[141]. Note that the energetic photons produced in bremsstrahlung collisions may
themselves interact in the medium through one of three photon-processes, in relative probabilities
depending on the energy of the photon and the nature of the medium. At high energies, the
most likely photon interaction is materialization into an electron-positron pair, while at somewhat
lower energies (in the MeV range), the most prevalent process is Compton scattering from atomic
electrons. Both processes result in a return of energy to the system in the form of electrons which
can generate additional bremsstrahlung photons, resulting in a multiplicative cycle known as an
electromagnetic cascade shower. The third photon interactive process, the photoelectric effect, as
well as multiple Coulomb scattering of the electrons by atoms, perturbs the shower to some degree
1 In this report, we often refer to both positrons and electrons as simply electrons. Distinguishing features will be
noted in context.
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