THE EGS5 CODE SYSTEM

2.15.5 EGS5 Transport Mechanics Algorithm
The transport between hard collisions (bremsstrahlung or delta-ray collisions) is superimposed on
the decoupled hinge mechanics as an independent, third possible transport process. To retain the
decoupling of geometry from all physics processes, for hard collisions EGS5 holds fixed over all
boundary crossing an initially sampled number of mean free paths before the next hard collision,
updating the corresponding distance (computed from the new total cross section), when entering
a new region. Again, while the random energy hinge preserves the average distance between hard
collisions, it does not preserve the exact distribution of collision distances if the hard collision cross
section exhibits an energy dependence between the energy hinges. In practice, however, this leads
to only small errors in cases where the energy hinge steps are very large and the hard collision
mean free path is sharply varying with energy.
Thus the dual random hinge transport mechanics can be described as follows: at the beginning
of the particle simulation, four possible events are considered: an energy loss hinge (determined
by a hinge on specified energy loss ∆E); a multiple scattering hinge (determined by a hinge on
a specified scattering strength K 1 ); a hard collision (specified by a randomly sampled number of
mean free paths); and boundary crossing (specified by the problem geometry). The distances to
each of these possible events is computed using the appropriate stopping power, scattering power,
total cross section or region geometry, and the particle is transported linearly through the shortest
of those 4 distances. The appropriate process is applied, values of the stopping power, scattering
power, cross section and boundary condition are updated if need be, and new values of the distances
to the varies events are computed to reflect any changes. Transport along all hinges then continues
through to the next event. Effectively, there are four transport processes occurring simultaneously
at each single translation of the electron.
The details of the implementation of the dual random hinge, because it is such a radical departure
from other transport mechanics models, can sometimes lead to some confusion (and, in any
case, the energy hinge definitely leads to important implications for many common tallies), and so
we present an expanded explication here. Ignoring hard collisions and boundary crossings for the
moment and generalizing here for the sake of brevity, we note that a step t involves transport over
the initial step prior to the hinge a distance t init given by (ζt) followed by transport through the
residual step distance t resid by ((1 − ζ)t). In practice, once a particle reaches the hinge point at
t init , we do not simply transport the particle through t resid to the end of the current step, because
nothing actually occurs there, as the physics was applied at the hinge point. So instead, immediately
after each hinge the distance to the next hinge point is determined and total step that the
particle must be transported before it reaches its next hinge is given by t step = t 1 resid + t2 init , where
the superscripts refer to the 1 st and 2 nd hinges steps. We thus have the somewhat counter-intuitive
situation in that when a particle is translated between two hinge points, it is actually being moved
a distance which corresponds to the residual part of one transport step plus the initial part of the
second transport step. Thus we distinguish between translation hinge steps, over which particles
are moved from one hinge point to the next, linearly and with constant energy, and transport steps,
which refer to the conventional condensed history Monte Carlo distances over which energy losses
and multiple scattering angles are computed and applied. Translation hinge steps and transport
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