THE EGS5 CODE SYSTEM

2.3 Simulating the Physical Processes—An Overview
In some approaches, the Boltzmann transport equation is written for a system and from it a Monte
Carlo simulation of the system is derived. This method gives correct average quantities, such as
fluences, but may not correctly represent fluctuations in the real situation when variance reduction
techniques are employed. The reader is referred to Chapter 3 in the book by Carter and Cashwell[41]
for details of this particular method.
In all versions of EGS we have taken a different and more simple-minded approach in that we
attempt to simulate the actual physical processes as closely as possible. We have not introduced
any inherent variance reduction techniques, so that fluctuations in the Monte Carlo results should
truly be representative of real-life fluctuations. For the design of high energy particle detectors,
this is an important consideration. On the other hand, fluctuations are not usually of interest
in radiation shielding-type problems, and so EGS5 includes several variance reduction techniques
which may be optionally invoked to make certain classes of calculations run more efficiently. None
of the variance reduction techniques are invoked by default, however, and so the method of EGS5
can generally be described as analog 3 Monte Carlo.
The simulation of an electromagnetic cascade shower can be decomposed into a simulation of the
transport and interactions of single high energy particle, along with some necessary bookkeeping.
A last-in-first-out (LIFO) stack is used to store the properties of particles which have yet to be
simulated. Initially, only the incident particle is on the stack (more correctly, the properties of the
incident particle are stored in the first position of corresponding arrays). The basic strategy is to
transport the top particle in the stack either until an interaction takes place, until its energy drops
below a predetermined cutoff energy, or until it enters a particular region of space. In the latter
two cases, the particle is taken off the stack and the simulation resumes with the new top particle.
If an interaction takes place, and if there is more than one secondary product particle, the particle
with the lowest available energy is put on the top of the stack. By “available energy” we mean
the maximum energy which can be imparted by a given particle to new secondary particles in a
collision: E for photons; E − m for electrons; and E + m for positrons, where E is the particle’s
total energy and m is the electron rest mass energy. By always tracking the lowest energy particle
first, we ensure that the depth of the stack will never exceed log 2 (E max /E cut ), where E max is the
largest incident energy to be simulated and E cut is the lowest cutoff energy. When the final particle
is removed from the stack and none remain, the simulation of the shower event is ended. The
complete simulation of each individual shower event is commonly called a Monte Carlo “history”.
3 The electron transport model in EGS5 is not strictly “analog” in that all scattering collisions are not treated on
an individual basis, but it is “analog” in the sense that the models of the aggregate effects of the large numbers of
collisions which are grouped together are analytic and can in most circumstances preserve the random nature of the
fluctuations in showers.
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