PENELOPE 2003 - OECD Nuclear Energy Agency

5.5. The subroutine package pengeom 169
(if any) and so on. The process ends when we have determined the body IBODY that
contains the point, or as soon as we conclude that the point is outside the material
system (i.e. in a void region). Notice that, when we have found that a module KB does
contain the point, to do the next step we only need to consider the surfaces KS such
that FLAG(KB, KS) = 1, 2 or 4.
After the body IBODY that contains the initial position of the particle has been identified,
we can call subroutine STEP to move the particle a certain distance DS, dictated
by penelope, along the direction (U,V,W). We start by checking whether the track segment
crosses any of the surfaces that limit IBODY. If after travelling the distance DS the
particle remains within the same body, DSEF is set equal to DS and control is returned
to the main program. It is worth noting that the surfaces KS that define the initial body
are those with FLAG(IBODY,KS)=1 and 2 (proper limiting surfaces) or =3 (limiting surfaces
of limiting bodies). Although it may happen that a surface with FLAG=3 does not
directly limit the body, subroutine STEP cannot know this from the information at hand
and, consequently, all surfaces with FLAG=3 are analyzed after each move. It is clear
that, to reduce the number of surfaces to be considered, we should minimize the number
of bodies used to delimit other bodies.
When the particle leaves IBODY and enters a new material, STEP stops it just after
crossing the interface and determines the new body and material (in this case, the output
values of IBODY and MAT are different from the input ones). To do this, the limiting
surfaces of the parent module and of all the sisters of the initial body must be analyzed (if
they exist). If the new position is outside the parent module, we must analyze all surfaces
that limit the parent’s sisters and go downward in the genealogical tree to determine the
module that contains the point and, if necessary, go upwards again to find out what the
new body is. If the new material is the same as in the initial body, the particle is allowed
to move the remaining distance. Void regions (strict vacuum) are crossed freely (i.e. the
distance travelled within these regions is not counted). Furthermore, when the particle
starts from outside the enclosure, it is allowed to propagate freely until it reaches a
material body. The particle is stopped when it penetrates a different material or when
it leaves the system (i.e. when, after leaving a material body, its straight trajectory does
not intersect a non-void body; in this case, the value MAT=0 is returned). Evidently,
the speed of the geometry subroutines depends greatly on the structure of the modules’
genealogical tree. The responsibility of optimizing it rests with the user.
When STEP moves the particle across an interface, there is a risk that, owing to
numerical truncation errors, the particle is placed on the wrong side of the interface (i.e.
the track is stopped just before the interface). If this occurs, the program could go into
an endless loop in which STEP repeatedly tries to move the particle a very small distance
(of the order of 10 −15 cm) towards the interface but does not succeed, i.e. the particle
is trapped at the interface. To avoid this collapse of the trajectory, after each interface
crossing, STEP applies an additional small displacement (∼ 10 −8 cm) in the direction of
movement, which is physically irrelevant and sufficient to compensate for the effect of
truncation errors. The same strategy is used in subroutine LOCATE: when the particle
is too close to an interface, it is moved 10 −8 cm along the surface gradient direction