In the Geant simulation code, the user is expected to provide the information regarding the geometry of the set-up and its position in the master reference frame where the tracking is performed. In the master reference frame, the user also supplies the kinematical information about the reaction under study. In addition, the user must make the necessary arrangement for the read out of the desired quantities. The information needed to run the simulation code is supplied via subroutines where the user is free to take any action.
In the case
of the pion beta calorimeter, the nine different shapes were defined and
positioned as displayed in figure 
. The pentagonal shape,
being the only regular geometry, was simple to define.
Figure: The pion beta apparatus as defined and positioned
in Geant for the simulations of the reactions relevant to the experiment and the
design and understanding of the triggers. Shown here from the center, are the active
target, the plastic veto detector and the calorimeter. The pentagons, the
hexagons and the vetoes were each defined as two entities which are patched
together to get the correct geometries: this process was necessary due to the
difficulty in defining irregular objects in Geant. This segmentation in
Geant reveals one property of the geodesic breakdown: from a pentagon center
to another pentagon center, one can see portions of geodesics as explained in
section 6.1
The other shapes, especially the
hexagons have been defined as two trapezoids which were patched together to
reproduce the original hexagons. The half hexagon D's were defined as
trapezoids. The shower vetoes, due to their peculiar geometry, were also
defined as two trapezoids connected together. The pentagons, although they can
be single entities, were defined as two geometries patched together as shown in
figure .
Once the shapes were defined, the next step in
constructing the simulation code was the rotations and translations necessary
for positioning each module in order to generate the calorimeter. This was a
straightforward task since the rotation angles and translation vectors follow
as the result of the geodesic breakdown described in section 6.2.
The kinematics parameters for the pion beta process or other reactions of
interest were set-up in the appropriate user subroutine and the energy
deposited within each of the modules was recorded at the end of each track
or whenever necessary during the tracking process via another user subroutine.
The tracking medium parameters---their optimization is very important to
interpret accurately the simulation results---were adjusted to reflect fine
tracking step size and low tracking cut-offs for all shower particles. The
light collection non-uniformity and the photoelectron statistics which have
been measured during the pion beta test runs, were taken into account for
more realistic simulations. A complete simulation code is described in
appendix C and one simulated pion beta decay event is shown in
figure .
Figure: A cut view of the calorimeter and the
simulation of one pion beta decay event in Geant. The two photons from 
decay
are the blue dashed lines emerging from the target. The photons reach the calorimeter
where they initiate electromagnetic showers all of which are contained within the
modules. The 
(the red line in the target) from pion beta decay
annihilates in the target with an atomic electron. The two gamma rays resulting
from the annihilation process are also shown as blue lines coming from the
target. The red lines are charged particles.