THE EGS5 CODE SYSTEM

short steps accurate, but slow
step too long, inaccurate
Full CSDA Range
"optimal" step
Figure 2.15: Schematic illustrating the “broomstick” problem.
they scatter out of the cylinder is given by the single elastic scattering mean-free path, and any
Monte Carlo algorithm using a larger step-size would over-estimate the penetration (and hence
average total tracklength) in the cylinder. It should be clear then that for cylinders with arbitrary
radii, if we begin with results generated by using very small step-sizes and then gradually increase
the steps, we will eventually encounter divergence in the computed average electron track-length
inside the cylinder, as our model will at some point over-estimate the penetration prior to deflection
out toward the sides of the “broomstick.” This is illustrated in Figure 2.15.
Since spatial energy deposition profiles are essentially maps of region-dependent electron tracklengths,
and because most problem tallies will be correct if the spatial distribution of electron
tracklengths is correct, the largest value of scattering strength K 1 which produces converged results
for energy deposition for a given material in a reference geometric volume was initially chosen
to be the standard for the EGS5 step-size selection algorithm. Subsequent studies showed, however,
that many problems require a more stringent characterization of electron tracks than average
length, and so a standard based on the average position of track end points for electrons traversing
geometric volumes of given reference sizes was adopted as the standard. In this new adaptation,
instead of computing energy deposition in semi-infinite cylinders, we look at the average lateral
deflection 〈r〉 of electrons emerging from the far face of cylinders with lengths L equal to their diameters
D. Values of 〈r〉 over finite cylinders are clearly more sensitive to multiple scattering step-sizes
than values of 〈t〉, leading to more conservative estimates of the maximum permitted scattering
strength and thus better assuring accurate results for problems other than those involving energy
deposition.
Using this criteria for defining accuracy, tables of the material and energy dependent values
of the largest scattering strengths K 1 yielding, for volumes of a given size, values of 〈r〉 which
are within 1% of the converged results for small K 1 were compiled. Given this data, EGS5 is
able to provide a step-size control mechanism based on a single user input parameter in units of
length which characterizes the geometric granularity of the problem tallies. This value is called the
“characteristic dimension” for the problem, and is set in the user’s MAIN program by specifying a
non-zero value of the variable CHARD, which is material dependent. Given this parameter, EGS5
automatically selects the optimal energy dependent values of K 1 by interpolating the compiled data
tables of maximum scattering strengths in dimension, material and energy. The tables range in
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