The EGS computer code system is a general purpose package for the Monte Carlo

Monte Carlo method

Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...

 simulation of the coupled transport of electrons

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 and photons

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 in an arbitrary geometry for particles with energies from a few keV up to several TeV. It was developed at SLAC.

The SLAC version of EGS is no longer actively developed. There are two forks which are being actively maintained:

• EGSnrc, maintained by the Ionizing Radiation Standards Group, Institute for National Measurement Standards, National Research Council of Canada
  National Research Council of Canada
  The National Research Council is an agency of the Government of Canada which conducts scientific research and development.- History :...
  
   http://irs.inms.nrc.ca/software/egsnrc/
• EGS, maintained by KEK
  KEK
  , known as KEK, is a national organization whose purpose is to operate the largest particle physics laboratory in Japan, which is situated in Tsukuba of Ibaraki prefecture. Established in 1997. The term "KEK" is also used to refer to the laboratory itself, which employs approximately 900 employees...
  
  , the Japanese particle physics research facility http://rcwww.kek.jp/research/egs/kek/