THE EGS5 CODE SYSTEM

despite being able to take much longer steps at high energies because of the random hinge transport
mechanics. To circumvent this problem, EGS5 reverts to specifying the multiple scattering step-size
based on a fractional energy loss over the step, which is now called EFRACH, to distinguish it from
ESTEPE in EGS4. EFRACH is material dependent and specified in the PEGS input NAMELIST.
One consequence of the work done to assure that EGS5 expends most of its computational
effort where it was most important (i.e., at higher energies), was the recognition that additional
efficiency might be achieved by allowing the fractional energy loss of the multiple scattering steps
to vary over the energy range of the problem. Thus, in addition to EFRACH, which is now defined to
be the fractional energy loss over a multiple scattering step at the highest problem energy, a second
parameter EFRACL, corresponding to the fractional energy loss over a multiple scattering step at
the lowest problem energy, is used. Like EFRACH, EFRACL is also material dependent and is specified
in the PEGS input NAMELIST, and the fractional energy loss permitting for multiple-scattering
steps swings logarithmically over the energy range of the problem.
Since the computational effort required to solve electron transport problems by Monte Carlo is
directly related to the number (and therefore the size) of the multiple scattering steps, an “optimal”
step-size in a simulation would therefore be the longest one for which the transport mechanics error
is less than the desired accuracy. Users thus need to judiciously select EFRACH and EFRACL to glean
maximum peformance.
In practice, however, optimal selection of step-sizes is difficult because the intricate interplay
between electron step-size and accuracy almost always depends on not only the particular quantity
of interest being computed, but also on the fineness or granularity of desired output tallies. For
example, a simulation of the spatial energy deposition distribution in a voxelized geometry is a
much more fine grain simulation than a computation of bulk energy deposited in a large detector
module. Even given that optimization of step-size selection necessarily requires some input from
the user because of it is inherently problem-dependent, it is clear that even expert users would be
unduly burdened by the need to optimally select both EFRACH and EFRACL in order to maximize the
performance of EGS5. To address this problem, a method has been devised which will automatically
select step-sizes based on a single user input parameter based on the problem geometry, as described
below.
2.15.9 Multiple Scattering Step-Size Optimization Using Media “Characteristic
Dimensions”
As optimization always involves a speed/accuracy trade-off, development of an automated method
for optimization of step-size selection first requires the adoption of a practical standard defining
“accuracy.” Perhaps the most severe test of a Monte Carlo program’s electron transport algorithm
is the “broomstick” problem, in which the tracks of electrons normally incident on the planar faces
of semi-infinite right-circular cylinders of progressive smaller radii (to isolate the effects of the transport
mechanics, hard-collisions are usually ignored in this problem) are simulated. For cylinders of
radii approaching infinitesimal thinness, the average total tracklength of incident electrons before
112