THE EGS5 CODE SYSTEM

Figure 4.3: UCBEND simulation at 3.5 MeV (B=1.0 kG).
It becomes clear from pictures such as these that it is easier to focus the 8.5 MeV electrons through
the slit than the 3.5 MeV ones. A number of EGS5 “experiments” can be performed rather easily
now that the basic geometry and magnetic field transport is established. For example, one can
replace the air regions and/or the copper window with vacuum in order to determine what effect
they have. What was discovered by doing this is that the copper window has the greatest effect
on the transmission through the slit. Note that Figure 4.4, illustrates what happens when the
magnetic field is turned off. Users interested in charged particle transport in an electric field should
see Rawlinson, Bielajew, Munro and Galbraith [133].
4.3 Using Combinatorial Geometry with EGS5
All of the user codes presented thus far have treated problems with simple geometries involving
few regions and only basic shapes (parallel slabs and right circular cylinders). Construction of subroutine
HOWFAR for such geometries is rather straight-forward, as distances to boundaries are easily
determined for planar and cylindrical surfaces, and identifying new region numbers subsequent to
boundary crossings is almost trivial when there are few regions with limited intersections. However,
for simulations involving complex devices such as radiotherapy accelerator heads or high energy
physics detectors, subroutine HOWFAR can be quite cumbersome to construct. In such applications,
regions can have complex shapes for which distance to boundary computations are more involved
(e.g., in concave regions there can be multiple intersection points on the same surface), and/or have
surfaces which are bordered on one side by a large number of distinct volumes (e.g., at a collimators
interface), making region identification more difficult.
As the need to simulate complex geometry is a common problem in Monte Carlo modeling,
197