Tracking
Basic Concepts
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 G4EventManager class.
The tracking manager receives a track from the event manager and
takes the actions required to finish tracking it.
G4TrackingManager aggregates the pointers to
G4SteppingManager,G4Trajectory and
G4UserTrackingAction. Also there is a "use" relation to
G4Track and G4Step.
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 below.
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:
updating the kinetic energy of the current track particle (note
that 'sumEnergyChange' is the sum of the energy difference
before and after each process invocation)
updating 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.
Invoke the user intervention G4UserSteppingAction.
Handle hit information.
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 tracks via ParticleChange.
G4VProcess is a base class of all processes and
it has 3 kinds of DoIt and
GetPhysicalInteraction 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 G4Track keeps information at the beginning
of the step while the AlongStepDoIts are being
invoked for the step in progress.After finishing all
AlongStepDoIts, G4Track
is updated.
On the other hand, G4Track is
updated after each invocation of a PostStepDoIt.
What is a Step?
G4Step stores the transient information of a step. This
includes the two endpoints of the step, PreStepPoint and
PostStepPoint, which 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 using ParticleChange.
After each DoIt, ParticleChange
updates PostStepPoint based on proposed changes.
Then, G4Track is updated
after finishing all AlongStepDoIts and
after each PostStepDoIt.
Access to Track and Step Information
How to Get Track Information
Track information may be accessed by invoking various
Get methods provided in the G4Track
class. For details, see the
Software Reference Manual.
Typical information available includes:
(x,y,z)
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 )
Kinetic energy
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
Track status
(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
Pinter 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
G4Step/G4StepPoint
classes. For details, see the
Software Reference Manual.
Information in G4Step
includes:
Pointers to PreStep and
PostStepPoint
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 PreStepPoint
and PostStepPoint
Increment of momentum and energy between PreStepPoint
and 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.)
Pointer to G4Track
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
Information in G4StepPoint
(PreStepPoint and PostStepPoint)
includes:
(x, y, z, t)
(px, py, pz, Ek)
Momentum direction (unit vector)
Pointers to physical volumes
Safety
Beta, gamma
Polarization
Step status
Pointer to the physics process which defined the current step
and its DoIt type
Pointer to the physics process which defined the previous step
and its DoIt type
Total track length
Global time (time since the current event began)
Local time (time since the current track began)
Proper time
How to Get "particle change"
Particle change information can be accessed by invoking various
Get methods provided in the
G4ParticleChange class.
Typical information available includes (for details, see the
Software Reference Manual):
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 (G4TrackStatus)
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 G4Track)
Handling of Secondary Particles
Secondary particles are passed as G4Tracks from a physics
process to tracking.
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.
User Actions
There are two classes which allow the user to intervene in the
tracking. These are:
G4UserTrackingAction, and
G4UserSteppingAction.
Each provides methods which allow the user access to the Geant4
kernel at specific points in the tracking. For details, see the
Software Reference Manual.
Note-1:
Users SHOULD NOT (and CAN NOT) change G4Track
in UserSteppingAction.
Only the exception is the TrackStatus.
Note-2:
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.
Verbose Outputs
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
G4Trajectory and G4TrajectoryPoint
are default concrete classes provided by Geant4, which are derived from the
G4VTrajectory and G4VTrajectoryPoint
base classes, respectively.
A G4Trajectory class object is created by
G4TrackingManager when a G4Track
is passed from the G4EventManager.
G4Trajectory has the following data
members:
ID numbers of the track and the track's parent
particle name, charge, and PDG code
a collection of G4TrajectoryPoint pointers
G4TrajectoryPoint corresponds to a step point along
the path followed by the track. Its position is given by a
G4ThreeVector. A G4TrajectoryPoint
class object is created in the AppendStep() method of
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
G4UserTrackingAction::PreUserTrackingAction() method.
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 DrawTrajectory() or
ShowTrajectory() methods of 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:
negative: red
neutral: green
positive: blue
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
G4Track and G4Step are
transient classes; they are not available at the end of the event.
Thus, the concrete classes G4VTrajectory and
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. G4Trajectory and
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
AppendStep() method will be invoked by
G4TrackingManager.
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 to
sizeof(G4Trajectory). These two points do not
apply to G4VTrajectory because it has no
operator new or operator delete.