CERN Program Library Long Writeup W5013 - CERNLIB ...

• GHELX3 for uniform fields along the z axis, IFIELD=3).
GRKUTA and GHELIX call the user subroutine GUFLD to obtain the components of the field in a given point.
GHELX3 takes the value of the field from the tracking medium parameter FIELDM.
5 The subroutine GUSTEP
The current track parameters are available in the common /GCTRAK/ together with all variables necessary
to the tracking routines for the control of the step size. In addition, a few flags and variables are stored in
the common block /GCTRAK/ to record the history of the current step:
• The flag INWVOL informs on the boundary crossing:
0 transport inside a volume;
1 entering a new volume or beginning of new track;
2 exiting a volume;
3 exiting the first mother;
• The flag ISTOP informs on the particle status:
0 normal transport;
1 particle has disappeared (decay, interaction...);
2 particle has crossed a threshold (time, energy...).
• The array LMEC informs on the mechanisms active in the current step. The mechanism names are
stored in ASCII equivalent in LMEC(1...NMEC).
• The total energy loss in the current step is stored in the variable DESTEP.
This information is necessary for the user to take the proper actions in the subroutine GUSTEP which is called
by GTRACK at the end of every step and when entering a new volume.
The variable NGKINE in common /GCKING/ contains the number of secondary particles generated at every
step, and which are stored in the same common. Depending on the application and on the particle type the
user may then take the appropriate action in GUSTEP:
• ignore the particle;
• store the secondary produced in the JSTAK stack for further tracking;
• store the secondary also in the JKINE/JVERTX structure where it will be kept till the end of the event.
6 Connection with the detector response package
The detector response package ([HITS]) allows to establish a correspondence between the volumes seen
by the particle and the active components of the detectors. When entering a new volume in GTRACK the
subroutine GFINDS is called. If the volume has been declared by the user as a sensitive detector through
appropriate calls to GSDET and if the corresponding tracking medium constant ISVOL is non zero, GFINDS
fills the common block /GCSETS/ with the information to identify uniquely the detector component. This
enables the user, in GUSTEP, to record the hits in the proper JHITS substructure [HITS].
7 Connection with the drawing package
The coordinates of the space points generated during the tracking are available at each step in the common
block/GCTRAK/. InGUSTEP the user can store them in the structure JXYZ with the help of the subroutine
GSXYZ. This information can be used later for debugging (subroutine GPJXYZ) or for the graphical representation
of the particle trajectories [DRAW].
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