THE EGS5 CODE SYSTEM

1
0.1
0.01
Electrons on Ti
20MeV
10MeV
7MeV
5MeV
3MeV
2MeV
1MeV
700keV
500keV
300keV
K1
0.001
0.0001
1e-05
1e-05 0.0001 0.001 0.01 0.1 1 10
Characteristic Dimension (cm)
Figure 2.19: Optimal initial scattering strength K 1 vs. broomstick diameter (equivalent to the
characteristic dimension) in titanium at various energies.
The sample problem tutor4 described in the next chapter investigates and illustrates the effectiveness
of the automated step-size selection method based on characteristic dimension.
2.15.10 Treatment of Initial Steps of Primary Electrons
It must be noted that not all classes of problems are guaranteed to be modeled accurately whenever
the average tracklength or average lateral deflection is modeled correctly in given region volumes. In
particular, problems using tallies which have a spatial or directional dependence on secondary particle
production occurring prior to the first multiple scattering hinge point (such as deep penetration
shower simulations) can exhibit step-size artifacts not present in EGS4, since high energy bremsstrahlung
directions can be correlated with electron directions. In such cases, while EGS4 always
imposes multiple scattering prior to secondary particle production, the random hinge methodology
of EGS5 does not always assure at least some deflection prior to secondary particle generating,
sometimes leading to over-estimation of particles in the forward direction.
To counter this problem and still permit EGS5 to take very very long steps as often as possible,
for all problems in which secondary particles are being produced and the primary source particles are
electrons, a mechanism has been introduced in EGS5 to force very small initial multiple scattering
hinge steps. If and only if an electron is determined by EGS subroutine SHOWER to be a primary
118