PENELOPE 2003 - OECD Nuclear Energy Agency

168 Chapter 5. Constructive quadric geometry
STEP displaces the particle and stops it at the end of the step, or just after entering
a new material. The output value DSEF is the distance travelled within the initial
material. If the particle enters a void region, STEP continues the particle track, as
a straight segment, until it penetrates a material body or leaves the system (the
path length through inner void regions is not included in DSEF). When the particle
arrives from a void region (MAT = 0), it is stopped after entering the first material
body. The output value MAT = 0 indicates that the particle has escaped from the
system.
◦ Input-output values (through COMMON/TRACK/):
X, Y, Z: Input: coordinates of the initial position.
Output: coordinates of the final position.
U, V, W: Direction cosines of the displacement. They are kept unaltered.
IBODY Input: initial body, i.e. the one that contains the initial position.
Output: final body.
MAT: Material in body IBODY (automatically changed when the particle crosses an
interface).
◦ Input argument:
DS: Distance to travel (unaltered).
◦ Output arguments:
DSEF: Travelled path length before leaving the initial material or completing the
jump (less than DS if the track crosses an interface).
NCROSS: Number of interface crossings (=0 if the particle does not leave the initial
material, greater than 0 if the particle enters a new material).
For the handling and storage of geometric information we take advantage of the
structure of the genealogical tree. It is assumed that an enclosure has been defined so
that it is the only common ancestor for all bodies and modules. To understand the
operation of the geometry routines, it is convenient to define a matrix FLAG(KB,KS) as
follows (the indices KS and KB indicate the label of a surface and a body or module,
respectively),
FLAG(KB,KS) = 1, if KS is a limiting surface of KB and KB is inside KS (i.e. side
pointer = −1).
= 2, if KS is a limiting surface of KB and KB is outside KS (i.e. side
pointer = +1).
= 3, if KB is a body and KS does not directly limit KB, but appears
in the definition of a body that limits KB.
= 4, if KB is a module and KS limits one of its daughters (bodies and
submodules), but does not appear in the definition of KB.
= 5, otherwise.
To locate a point we call subroutine LOCATE, where we proceed upwards in the
genealogical tree of modules. If the point is outside the enclosure, we set MAT = 0 and
return to the main program. Otherwise, we look for a module or body of the second
generation that contains the point. If it exists, we continue analyzing its descendants