PENELOPE 2003 - OECD Nuclear Energy Agency

188 Chapter 6. Structure and operation of the code system
(iii) CALL START to initiate the simulation of the track.
(iv) CALL JUMP(DSMAX,DS) to determine the length DS of the next track segment (for
electrons and positrons, DS will never exceed the input value DSMAX).
(v) Compute the position of the following event:
• If the track has crossed an interface, stop the particle at the position where
the track intersects the interface, and shorten the step length DS accordingly.
Change to the new material (the one behind the interface) by redefining the
variables IBODY and MAT.
When the particle escapes from the system, the simulation of the track has
been finished; increment counters and go to step (vii).
Go to step (iii).
(vi) CALL KNOCK(DE,ICOL) to simulate the following event.
• If the energy is less than EABS(KPAR,MAT), end the track, increment counters
and go to step (vii).
• Go to step (iv).
(vii) CALL SECPAR(LEFT) to start the track of a particle in the secondary stack (this
particle is then automatically removed from the stack).
• If LEFT>0, go to step (iii). The initial state of a secondary particle has already
been set.
• If LEFT=0, the simulation of the shower produced by the primary particle has
been completed. Go to step (i) to generate a new primary particle (or leave
the simulation loop after simulating a sufficiently large number of showers).
Notice that subroutines JUMP and KNOCK keep the position coordinates unaltered;
the positions of successive events have to be followed by the MAIN program (simply
by performing a displacement of length DS along the direction of movement after each
call to JUMP). The energy of the particle is automatically reduced by subroutine KNOCK,
after generating the energy loss from the relevant probability distribution. KNOCK also
modifies the direction of movement according to the scattering angles of the simulated
event. Thus, at the output of KNOCK, the values of the energy E, the position r =(X,Y,Z)
and the direction of movement ˆd =(U,V,W) define the particle state immediately after
the interaction event.
In order to avoid problems related with possible overflows of the secondary stack,
when a secondary particle is produced its energy is temporarily assumed as locally deposited.
Hence, the energy E of a secondary must be subtracted from the corresponding
dose counter when the secondary track is started. Occasional overflows of the secondary
stack are remedied by eliminating the less energetic secondary electron or photon in the
stack (positrons are not eliminated since they will eventually produce quite energetic
annihilation radiation). As the main effect of secondary particles is to spread out the
energy deposited by the primary one, the elimination of the less energetic secondary
electrons and photons should not invalidate local dose calculations.