THE EGS5 CODE SYSTEM

x
Final
Direction
φ
Θ
∆x
t
Initial
Direction
z
∆y
y
s
Figure 2.8: Schematic of electron transport mechanics model.
along its initial direction and then determine its new direction by sampling the scattering angle Θ
from a multiple scattering p.d.f. dependent upon the material, the total distance traveled, t, and
the particle energy.
Ignoring lateral deflections introduces significant errors, however, unless restrictions (often quite
severe) are placed on the maximum sizes of the electron transport tracklengths t. These restrictions
were greatly eased by the introduction of the PRESTA algorithm, which treats ∆x and ∆y
explicitly during the transport simulation and which also includes a more accurate prescription for
relating the straight line transport distance s to the actual pathlength t. It should be noted here
that because of the random nature of the particle trajectory, s, ∆x, and ∆y are actually random
variables, dependent upon the scattering angle Θ and the tracklength t. In PRESTA, ∆x and ∆y
range between 0 and t/2 and s is given by some fraction of t. There are two major drawbacks
to the PRESTA formalism, however. First, in situations where an electron is traveling close to a
region boundary, translating it lateral distances ∆x and ∆y perpendicular to its initial direction
can sometimes result in moving it across the boundary and into a region with different material
properties. Thus PRESTA required computationally expensive interrogation of the problem geometry
and sometimes resulted in very small steps when particles were traveling roughly parallel
to nearby region boundaries. Second, PRESTA is not adept at modeling backscattering. Electron
backscattering in general results from a single, very large angle collision and not as the aggregate
effect of a large number of small-angle collisions. It is clear from Figure 2.8 that physically, if
an electron were to experience a 180 degree collision immediately at t = 0, it could potentially
travel a distance s = −t in the backward direction. Thus the set of all possible final positions
for an electron traveling a pathlength t is a sphere of radius t (this is sometimes referred to as
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