THE EGS5 CODE SYSTEM

1. Compute λ at the current location.
2. Let t 1 = λN λ .
3. Compute d, the distance to the nearest boundary along the photon’s direction.
4. Let t 2 equal the smaller of t 1 and d. Transport by distance t 2 .
5. Deduct t 2 /λ from N λ . If the result is zero (this happens when t 2 = t 1 ), then it is time to
interact—jump out of the loop.
6. This step is reached if t 2 = d. Thus, a boundary was reached. Do the necessary bookkeeping.
If the new region is a different material, go to Step 1. Otherwise, go to Step 2.
In regions where there is a vacuum, σ = 0 (λ = ∞) and special coding is used to account for this
situation.
Now let us consider charged particle transport. The relevant interactions considered in EGS5 are
elastic Coulomb scattering by nuclei, inelastic scattering by atomic electrons, positron annihilation,
and bremsstrahlung. Modeling of charged particle transport is particularly challenging because
analytical expressions for the cross sections for all of the above processes (with the exception of
annihilation) become infinite as the transferred energy approaches zero (the infrared catastrophe,
etc.). In actuality, these cross sections, when various corrections are taken into account (i.e.,
screening for nuclear scattering, electron binding for electron scattering, and Landau-Pomeranchuk-
Migdal effect corrections for bremsstrahlung), are not infinite, but they are very large and the exact
values for the total cross sections are not well known. Therefore, it is not practical to try to simulate
every interaction. Fortunately, the low momentum transfer events which give rise to the large total
cross section values do not result in large fluctuations in the shower behavior itself. For this reason,
large numbers of low momentum transfer collisions can be combined together and modeled as
continually-occurring processes. Cutoff energies are defined to demark the threshold between where
interactions are treated as discrete events (sometimes referred to as “hard collisions”) or aggregated
with other low transfer or “soft” interactions. The electron and photon threshold energiesused by
EGS5 (as set up in PEGS) are defined by the variables AE and AP, respectively, so that any electron
interaction which produces a delta-ray with total energy of at least AE or a photon with energy of at
least AP is considered to be a discrete event. All other interactions are treated as contributing to the
continuous processes of energy loss and deflection of the electron in between discrete interactions.
Continuous energy losses are due to soft interactions with the atomic electrons (excitation and
ionization loss) and to the emission of soft bremsstrahlung photons. Deflections are mostly due
to multiple Coulomb elastic scattering from the nucleus, with some contribution coming from soft
electron scattering.
Typically, Monte Carlo methods simulate charged particle transport as a series of “steps” of
distance t over which hundred or thousands (or more) of low momentum transfer elastic and inelastic
events may occur. It is usually assumed that the transport during each step can be modeled as
a single, straight-line, mono-energetic translation, at the end of which the aggregate effects of the
continuous energy loss and deflection incurred over the step t can be accounted for by sampling
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