Philosophy of Tracking
All Geant4 processes, including the transportation of particles, are
treated generically. In spite of the name “tracking”, particles are
not transported in the tracking category.
G4TrackingManager is an
interface class which brokers transactions between the event, track and
tracking categories. An instance of this class handles the message
passing between the upper hierarchical object, which is the event
manager, and lower hierarchical objects in the tracking category. The
event manager is a singleton instance of the
The tracking manager receives a track from the event manager and takes
the actions required to finish tracking it.
aggregates the pointers to
G4UserTrackingAction. Also there is a “use” relation to
G4SteppingManager plays an essential role in tracking the particle.
It takes care of all message passing between objects in the different
categories relevant to transporting a particle (for example, geometry
and interactions in matter). Its public method
Stepping() steers the
stepping of the particle. The algorithm to handle one step is given
If the particle stop (i.e. zero kinetic energy), each active AtRest process proposes a step length in time based on the interaction it describes. And the process proposing the smallest step length will be invoked.
Each active discrete or continuous process must propose a step length based on the interaction it describes. The smallest of these step lengths is taken.
The geometry navigator calculates “Safety”, the distance to the next volume boundary. If the minimum physical-step-length from the processes is shorter than “Safety”, the physical-step-length is selected as the next step length. In this case, no further geometrical calculations will be performed.
If the minimum physical-step-length from the processes is longer than “Safety”, the distance to the next boundary is re-calculated.
The smaller of the minimum physical-step-length and the geometric step length is taken.
All active continuous processes are invoked. Note that the particle’s kinetic energy will be updated only after all invoked processes have completed. The change in kinetic energy will be the sum of the contributions from these processes.
The current track properties are updated before discrete processes are invoked. In the same time, the secondary particles created by processes are stored in SecondaryList. The updated properties are:
the kinetic energy of the current track particle (note that ‘sumEnergyChange’ is the sum of the energy difference before and after each process invocation)
position and time
The kinetic energy of the particle is checked to see whether or not it has been terminated by a continuous process. If the kinetic energy goes down to zero, AtRest processes will be applied at the next step if applicable.
The discrete process is invoked. After the invocation,
the energy, position and time of the current track particle are updated, and
the secondaries are stored in SecondaryList.
The track is checked to see whether or not it has been terminated by the discrete process.
“Safety” is updated.
If the step was limited by the volume boundary, push the particle into the next volume.
Handle hit information.
Invoke the user intervention
Save data to Trajectory.
Update the mean free paths of the discrete processes.
If the parent particle is still alive, reset the maximum interaction length of the discrete process which has occurred.
One step completed.
What is a Process?
Only processes can change information of
G4Track and add secondary
G4VProcess is a base class of all
processes and it has 3 kinds of
methods in order to describe interactions generically. If a user want to
modify information of
G4Track, he (or she) SHOULD create a special
process for the purpose and register the process to the particle.
What is a Track?
G4Track keeps ‘current’ information of the particle. (i.e.
energy,momentum, position ,time and so on) and has ‘static’ information
(i.e. mass, charge, life and so on) also. Note that
information at the beginning of the step while the
are being invoked for the step in progress.After finishing all
G4Track is updated. On the other hand,
G4Track is updated after each invocation of a
What is a Step?
G4Step stores the transient information of a step. This includes the
two endpoints of the step,
contain the points’ coordinates and the volumes containing the points.
G4Step also stores the change in track properties between the two
points. These properties, such as energy and momentum, are updated as
the various active processes are invoked.
What is a ParticleChange?
Processes do NOT change any information of
G4Track directly in their
DoIt. Instead, they proposes changes as a result of interactions by
ParticleChange. After each
PostStepPoint based on proposed changes. Then,
is updated after finishing all
AlongStepDoIts and after each
Access to Track and Step Information¶
How to Get Track Information
Track information may be accessed by invoking various
provided in the
G4Track class. For details, see the
header file in
$G4INCLUDE. Typical information available includes:
Global time (time since the event was created)
Local time (time since the track was created)
Proper time (time in its rest frame since the track was created )
Momentum direction ( unit vector )
Accumulated geometrical track length
Accumulated true track length
Pointer to dynamic particle
Pointer to physical volume
Track ID number
Track ID number of the parent
Current step number
(x,y,z) at the start point (vertex position) of the track
Momentum direction at the start point (vertex position) of the track
Kinetic energy at the start point (vertex position) of the track
Pointer to the process which created the current track
How to Get Step Information
Step and step-point information can be retrieved by invoking various
Get methods provided in the
Information in G4Step includes:
Geometrical step length (step length before the correction of multiple scattering)
True step length (step length after the correction of multiple scattering)
Increment of position and time between
Increment of momentum and energy between
PostStepPoint. (Note: to get the energy deposited in the step, you cannot use this ‘Delta energy’. You have to use ‘Total energy deposit’ as below.)
Total energy deposited during the step - this is the sum of
the energy deposited by the energy loss process, and
the energy lost by secondaries which have NOT been generated because each of their energies was below the cut threshold
Energy deposited not by ionization during the step
Secondary tracks created during tracking for the current track.
NOTE: all secondaries are included. NOT only secondaries created in the CURRENT step.
(x, y, z, t)
(px, py, pz, Ek)
Momentum direction (unit vector)
Pointers to physical volumes
Pointer to the physics process which defined the current step and its
Pointer to the physics process which defined the previous step and its
Total track length
Global time (time since the current event began)
Local time (time since the current track began)
How to Get “particle change”
Particle change information can be accessed by invoking various
methods provided in the
class. Typical information available includes:
final momentum direction of the parent particle
final kinetic energy of the parent particle
final position of the parent particle
final global time of the parent particle
final proper time of the parent particle
final polarization of the parent particle
status of the parent particle (
true step length (this is used by multiple scattering to store the result of the transformation from the geometrical step length to the true step length)
local energy deposited - this consists of either
energy deposited by the energy loss process, or
the energy lost by secondaries which have NOT been generated because each of their energies was below the cut threshold.
number of secondaries particles
list of secondary particles (list of
Handling of Secondary Particles¶
Secondary particles are passed as
G4Tracks from a physics process
G4ParticleChange provides the following four methods
for a physics process:
AddSecondary( G4Track* aSecondary )
AddSecondary( G4DynamicParticle* aSecondary )
AddSecondary( G4DynamicParticle* aSecondary, G4ThreeVector position )
AddSecondary( G4DynamicParticle* aSecondary, G4double time)
In all but the first, the construction of
G4Track is done in the
methods using information given by the arguments.
There are two classes which allow the user to intervene in the tracking. These are:
Each provides methods which allow the user access to the Geant4 kernel at specific points in the tracking.
Users SHOULD NOT (and CAN NOT) change
UserSteppingAction. The only exception is the
Users have to be cautious to implement an unnatural/unphysical action in these user actions. See the section Killing Tracks in User Actions and Energy Conservation for more details.
The verbose information output flag can be turned on or off. The amount of information printed about the track/step, from brief to very detailed, can be controlled by the value of the verbose flag, for example,
G4UImanager* UI = G4UImanager::GetUIpointer(); UI->ApplyCommand("/tracking/verbose 1");
Trajectory and Trajectory Point¶
G4Trajectory and G4TrajectoryPoint¶
G4TrajectoryPoint are default concrete classes
provided by Geant4, which are derived from the
G4VTrajectoryPoint base classes, respectively. A
class object is created by
G4TrackingManager when a
passed from the
G4Trajectory has the following
ID numbers of the track and the track’s parent
particle name, charge, and PDG code
a collection of
G4TrajectoryPoint corresponds to a step point along the path
followed by the track. Its position is given by a
G4TrajectoryPoint class object is created in the
G4Trajectory and this method is invoked by
G4TrackingManager at the end of each step. The first point is
created when the
G4Trajectory is created, thus the first point is
the original vertex.
The creation of a trajectory can be controlled by invoking
G4TrackingManager::SetStoreTrajectory(G4bool). The UI command
/tracking/storeTrajectory _bool_ does the same. The user can set
this flag for each individual track from his/her
The user should not create trajectories for secondaries in a shower due to the large amount of memory consumed.
All the created trajectories in an event are stored in
G4TrajectoryContainer class object and this object will be kept by
G4Event. To draw or print trajectories generated in an event, the
user may invoke the
G4VTrajectory, respectively, from his/her
G4UserEventAction::EndOfEventAction(). The geometry must be drawn
before the trajectory drawing. The color of the drawn trajectory depends
on the particle charge:
Due to improvements in
G4Navigator, a track can execute more
than one turn of its spiral trajectory without being broken into
smaller steps as long as the trajectory does not cross a geometrical
boundary. Thus a drawn trajectory may not be circular.
Customizing trajectory and trajectory point¶
G4Step are transient classes; they are not available
at the end of the event. Thus, the concrete classes
G4VTrajectoryPoint are the only ones a user may employ for
end-of-event analysis or for persistency. As mentioned above, the
default classes which Geant4 provides, i.e.
G4TrajectoryPoint, have only very primitive quantities. The user can
customize his/her own trajectory and trajectory point classes by
deriving directly from the respective base classes.
To use the customized trajectory, the user must construct a concrete
trajectory class object in the
G4UserTrackingAction::PreUserTrackingAction() method and make its
pointer available to
G4TrackingManager by using the
SetTrajectory() method. The customized trajectory point class object
must be constructed in the
AppendStep() method of the user’s
implementation of the trajectory class. This
will be invoked by
To customize trajectory drawing, the user can override the
DrawTrajectory() method in his/her own trajectory class.
When a customized version of G4Trajectory declares any new class
variables, operator new and operator delete must be provided. It is
also useful to check that the allocation size in operator new is equal
sizeof(G4Trajectory). These two points do not apply to
G4VTrajectory because it has no operator new or operator delete.