THE EGS5 CODE SYSTEM

tally, as in EGS4. Depositing energy continually in this manner, rather than only in discrete,
large, chunks at the hinge points, obviates requiring very small values of DEINITIAL to attain
small statistical uncertainty in most energy deposition problems. It does introduce some artifacts,
however, in that while translating a particle to an energy hinge point, energy is being deposited
into the medium without being decremented from the particle. Similarly, along the track segment
between the hinge and the end of the energy transport step (the part described by DERESID), the
full transport step energy loss will have been decremented from the electron, but not all of that will
have been deposited via continuous stopping loss until the end of the transport step. Thus energy
is not strictly conserved, in that a particle’s total energy plus the energy of its daughter particles
plus the energy that it has deposited do not sum to the particle’s initial energy except at the exact
transitions between hinge steps (which are not stopping points on the translation steps). This has
some interesting consequences. For instance, if an electron escapes the problem geometry before
reaching its next energy hinge, a hinge must be imposed at the boundary to and the amount of
energy deposited prior to the escape of the particle much be decremented from the particle before
it is tallied as an escaped particle. Similarly, if an electron escapes during the residual part of a
previous hinge (i.e., before the full DERESID has been deposited), its kinetic energy must adjusted
upward to account for the fact that the full hinge energy has already been decremented from the
particle, but a portion of it has yet to be deposited, because the end of the hinge step had yet to
be reached. See Figure 2.14 for a schematic illustrating this problem.
Likewise, when a hard collision occurs, a particle will be somewhere in the midst of some
transport step, on either the residual side of the previous energy hinge or the initial side of the
upcoming hinge, and so the kinetic energy available for the collision must be adjusted to account
for the energy deposited during the translation step. If the electron has yet to reach the end of
the previous hinge still (i.e., it’s still on the residual side), we must adjust the particle kinetic
energy by increasing it to account for energy decremented but not deposited before proceeding the
collision analysis. And, as with the case of escaping particles, if the electron has passed the end of
the previous hinge step and is on the initial part of the next hinge step, we must impose a hinge
and decrement the already deposited energy from the electron. These adjustments are necessary
not only to preserve energy deposition, but also to determine the kinetic energy available for the
hard collision. In terminating hinges before hard collisions, we adjust the electron energy exactly
as was done for escaping electrons, by setting E = E − DEINITIAL + DENSTEP.
A final interesting problem occurs at final electron energy hinge. Because the electron energy
is set to be equivalent to ECUT at the final hinge point, which is reached before the electron has
been translated through the DERESID portion of its transport step, some provision must be made
for tracking electrons during the residual parts of their final hinge steps. This requirement is
complicated somewhat in that such electrons will have total energy which is exactly equal to the
problem cutoff energy. In the present EGS5 algorithm, when the last energy hinge point is reached,
the electron energy is set to ECUT, DENSTEP is set to DERESID, and DEINITIAL is set to zero (DERESID
is then set to zero as well). The multiple scattering hinge step at this point is set to infinity, as
is the distance to the next hard collision. The particle is thus transported linearly through the
distance corresponding to DERESID, depositing the appropriate CSDA energy along the way.
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