EGS4

The EGS (Electron Gamma Shower) code system consists of a set of routines sufficient to simulate (using Monte Carlo methods) the coupled transport of electrons and photons in any element, compound or mixture, and in a geometry specified by the user (via the routine HOWFAR), with the results being output by the user specified routine AUSGAB. The code consists of two user callable routines, HATCH and SHOWER, together with a number of routines specifying the physics of the transport of an electromagnetic shower. The authors claim that the current version of EGS (EGS4, which was developed with medical physics applications in mind) can follow electrons down to an energy of 10 keV, and photons down to 1 keV in energy, well below the energies necessary for simulations of the SNO detector.

An adjunct to the code system is the PEGS program (Preprocessor for EGS) which prepares the cross sectional data for the media specified by the user. This data is subsequently read into the code by the HATCH routine.

The following processes are included in the EGS code:-

1. Bhabha (e$^{+}$e$^{-}$) and Møller (e$^{-}$e$^{-}$) scattering, using exact formulae.

2. Bremsstrahlung production is catered for, but excludes the Elwert correction factor necessary to calculate accurately the bremsstrahlung cross-section for electrons with a kinetic energy of approximately 1 MeV or less. This means that EGS incorrectly estimates the fraction of the total energy loss due to the production of photons, but, as the stopping powers are thought to be correct, this is not considered to be a major problem.

3. Coherent Rayleigh scattering is an option, but is not currently implemented.

4. Compton Scattering.

5. Continuous energy loss is applied to charged particle tracks between discrete interactions. The stopping power consists of soft Bremsstrahlung production and collision loss terms, the latter being determined by the restricted Bethe-Block formula, with the Sternheimer treatment of the density effect. The boundary between discrete interactions and continuous energy loss is defined by a pair energies specified by the user for photons and electrons respectively. Any particle that could be created by a discrete interaction, but is below the appropriate energy is instead absorbed into the continuous energy loss, and is not subsequently followed by the code.

6. Multiple scattering of electrons is handled using the theory of Molière, as formulated by Bethe.

7. Pair production.

8. The photoelectric effect. However, only the primary electron is tracked and neither fluorescent photons nor Auger electrons are considered.

9. Positron annihilation in flight and at rest.

The initial particle, as well as the products of any interactions are followed until they drop below preset cut-off energies.

A full write up for the code system, and the physics contained therein may be found in the EGS4 manual [7].