The tracking category manages the contribution of the processes to the evolution of a track's state and provides information in sensitive volumes for hits and digitization.
It is well known that the overall performance of a detector simulation depends critically on the CPU time spent propagating the particle through one step. The most important consideration in the object design of the tracking category is maintaining high execution speed in the Geant4 simulation while utilizing the power of the object-oriented approach.
An extreme approach to the particle tracking design would be to integrate all functionalities required for the propagation of a particle into a single class. This design approach looks object-oriented because a particle in the real world propagates by itself while interacting with the material surrounding it. However, in terms of data hiding, which is one of the most important ingredients in the object-oriented approach, the design can be improved.
Combining all the necessary functionalities into a single class exposes all the data attributes to a large number of methods in the class. This is basically equivalent to using a common block in Fortran.
Instead of the 'big-class' approach, a hierarchical design was employed by Geant4. The hierarchical approach, which includes inheritance and aggregation, enables large, complex software systems to be designed in a structured way. The simulation of a particle passing through matter is a complex task involving particles, detector geometry, physics interactions and hits in the detector. It is well-suited to the hierarchical approach. The hierarchical design manages the complexity of the tracking category by separating the system into layers. Each layer may then be designed independently of the others.
In order to maintain high-performance tracking, use of the
inheritance ('is-a' relation) hierarchy in the tracking category
was avoided as much as possible. For example, track
and particle
classes might have been designed so that a
track
'is a' particle
. In this
scheme, however, whenever a track
object is used,
time is spent copying the data from the particle
object
into the track
object. Adopting the aggregation ('has-a' relation) hierarchy
requires only pointers to be copied, thus providing a
performance advantage.
Figure 2.4 shows a general overview of the tracking design in Unified Modelling Language Notation.
G4TrackingManager is an interface
between the event and track categories and the tracking category.
It handles the message passing between the upper hierarchical object,
which is the event manager (G4EventManagerz
),
and lower hierarchical objects in the tracking category.
G4TrackingManager
is responsible for processing
one track which it receives from the event manager.
G4TrackingManager
aggregates the pointers to
G4SteppingManager
, G4Trajectory
and G4UserTrackingAction
. It also has a 'use'
relation to G4Track
.
G4SteppingManager plays an essential
role in particle tracking. It performs message passing to objects
in all categories related to particle transport, such as
geometry and physics processes. Its public method
Stepping()
steers the stepping of the particle.
The algorithm employed in this method is basically the same as that
in Geant3. The Geant4 implementation, however, relies on the inheritance
hierarchy of the physics interactions. The hierarchical
design of the physics interactions enables the stepping
manager to handle them as abstract objects. Hence, the manager
is not concerned with concrete interaction objects such as
bremsstrahlung or pair creation. The actual invocations of
various interactions during the stepping are done through a
dynamic binding mechanism. This mechanism shields the
tracking category from any change in the design of the physics
process classes, including the addition or subtraction of new
processes.
G4SteppingManager
also aggregates
the pointers to G4Navigator
from the geometry
category, to the current G4Track
, and
the list of secondaries from the current track
(through a G4TrackVector
) to
G4UserSteppingAction
and to
G4VSteppingVerbose
.
It also has a 'use' relation to G4ProcessManager
and G4ParticleChange
in the physics processes
class category.
G4Track - the class
G4Track
represents a particle
which is pushed by G4SteppingManager
. It holds
information required for stepping a particle, for example, the
current position, the time since the start of stepping, the
identification of the geometrical volume which contains the particle,
etc. Dynamic information, such as particle momentum and energy,
is held in the class through a pointer to the
G4DynamicParticle
class. Static
information, such as the particle mass and charge is stored in the
G4DynamicParticle
class through the pointer to the
G4ParticleDefinition
class. Here the aggregation
hierarchical design is extensively employed to maintain high tracking
performance.
G4TrajectoryPoint and G4Trajectory -
the class G4TrajectoryPoint
holds the state of
the particle after propagating one step. Among other things, it
includes information on space-time, energy-momentum and geometrical
volumes.
G4Trajectory
aggregates all
G4TrajectoryPoint
objects which belong to the
particle being propagated.
G4TrackingManager
takes care of adding the
G4TrajectoryPoint
to a G4Trajectory
object if the user requested it (see
Geant4 User's Guide - For Application Developers.
The life of a G4Trajectory
object spans an event, contrary to
G4Track
objects, which are deleted from memory after being
processed.
G4UserTrackingAction and G4UserSteppingAction -
G4UserTrackingAction
is a base class from which user actions
at the beginning or end of tracking may be derived. Similarly,
G4UserSteppingAction
is a base class from which user actions
at the beginning or end of each step may be derived.
The key classes for tracking in Geant4 are
G4TrackingManager
and G4SteppingManager
.
The singleton object "TrackingManager" from G4TrackingManager
keeps all information related to a particular track, and it
also manages all actions necessary to complete the tracking. The
tracking proceeds by pushing a particle by a step, the length
of which is defined by one of the active processes.
The "TrackingManager" object delegates management of each of the
steps to the "SteppingManager" object. This object keeps all
information related to a particular step.
The public method ProcessOneTrack()
in
G4TrackingManager
is the key to managing the tracking, while the public method
Stepping()
is the key to managing one step. The algorithms
used in these methods are explained below.
ProcessOneTrack() in G4TrackingManager
Stepping() in G4SteppingManager
The interaction of the tracking category with the physics processes
is done in two ways. First each process can limit the step length
through one of its three GetPhysicalInteractionLength()
methods, AtRest, AlongStep, or PostStep. Second, for the selected processes
the DoIt (AtRest, AlongStep or PostStep) methods are invoked.
All this interaction is managed by the Stepping method of
G4SteppingManager
. To calculate the step length, the
DefinePhysicalStepLength()
method is called. The flow of
this method is the following:
Obtain maximum allowed Step in the volume define by the user through G4UserLimits.
The PostStepGetPhysicalInteractionLength of all active processes is called. Each process returns a step length and the minimum one is chosen. This method also returns a G4ForceCondition flag, to indicate if the process is forced or not:
All other DoIt including AlongStepDoIts are ignored.
The AlongStepGetPhysicalInteractionLength method of all active processes is called. Each process returns a step length and the minimum of these is chosen. This method also returns a fGPILSelection flag, to indicate if the process is the selected one can be is forced or not:
The method G4SteppingManager::InvokeAlongStepDoIts()
is in charge of calling the AlongStepDoIt methods of the different
processes:
If the current step is defined by a 'ExclusivelyForced' PostStepGetPhysicalInteractionLength, no AlongStepDoIt method will be invoked
Else, all the active continuous processes will be invoked, and they return the ParticleChange. After it for each process the following is executed:
Update the G4Step information by using final state information of the track given by a physics process. This is done through the UpdateStepForAlongStep method of the ParticleChange
Then for each secondary:
It is checked if its kinetic energy is smaller than the energy threshold for the material. In this case the particle is assigned a 0. kinetic energy and its energy is added as deposited energy of the parent track. This check is only done if the flag ApplyCutFlag is set for the particle (by default it is set to 'false' for all particles, user may change it in its G4VUserPhysicsList). If the track has the flag IsGoodForTracking 'true' this check will have no effect (used mainly to track particles below threshold)
The parentID and the process pointer which created this track are set
The secondary track is added to the list of secondaries. If it has 0. kinetic energy, it is only added if it it invokes a rest process at the beginning of the tracking
The track status is set according to what the process defined
The method G4SteppingManager::InvokePostStepDoIts
is on
charge of calling the PostStepDoIt methods of the different processes.
Invoke the PostStepDoIt methods of the specified discrete process (the one selected by the PostStepGetPhysicalInteractionLength, and they return the ParticleChange. The order of invocation of processes is inverse to the order used for the GPIL methods. After it for each process the following is executed:
It is checked if its kinetic energy is smaller than the energy threshold for the material. In this case the particle is assigned a 0. kinetic energy and its energy is added as deposited energy of the parent track. This check is only done if the flag ApplyCutFlag is set for the particle (by default it is set to 'false' for all particles, user may change it in its G4VUserPhysicsList). If the track has the flag IsGoodForTracking 'true' this check will have no effect (used mainly to track particles below threshold)
The parentID and the process pointer which created this track are set
The secondary track is added to the list of secondaries. If it has 0. kinetic energy, it is only added if it it invokes a rest process at the beginning of the tracking
The method G4SteppingManager::InvokeAtRestDoIts
is called
instead of the three above methods in case the track status is
fStopAndALive. It is on charge of selecting
the rest process which has the shortest time before and then invoke it:
To select the process with shortest tiem, the AtRestGPIL method of all active processes is called. Each process returns an lifetime and the minimum one is chosen. This method returm also a G4ForceCondition flag, to indicate if the process is forced or not: = Forced : Corresponding AtRestDoIt is forced. = NotForced : Corresponding AtRestDoIt is not forced unless this process limits the step.
Set the step length of current track and step to 0.
Invoke the AtRestDoIt methods of the specified at rest process, and they return the ParticleChange. The order of invocation of processes is inverse to the order used for the GPIL methods.
After it for each process the following is executed:
Set the current process as a process which defined this Step length.
Update the G4Step information by using final state information of the track given by a physics process. This is done through the UpdateStepForAtRest method of the ParticleChange.
The secondaries from ParticleChange are stored to SecondaryList
Then for each secondary:
It is checked if its kinetic energy is smaller than the energy threshold for the material. In this case the particle is assigned a 0. kinetic energy and its energy is added as deposited energy of the parent track. This check is only done if the flag ApplyCutFlag is set for the particle (by default it is set to 'false' for all particles, user may change it in its G4VUserPhysicsList). If the track has the flag IsGoodForTracking 'true' this check will have no effect (used mainly to track particles below threshold)
The parentID and the process pointer which created this track are set
The secondary track is added to the list of secondaries. If it has 0. kinetic energy, it is only added if it it invokes a rest process at the beginning of the tracking
The track is updated and its status is set according to what the process defined
The ProcessManager of a particle is responsible for providing the
correct ordering of process invocations. G4SteppingManager
invokes the processes at each phase just following the order given
by the ProcessManager of the corresponding particle.
For some processes the order is important. Geant4 provides by default the right ordering. It is always possible for the user to choose the order of process invocations at the initial set up phase of Geant4. This default ordering is the following:
Ordering of GetPhysicalInteractionLength
In the loop of GetPhysicalInteractionLength of AlongStepDoIt, the Transportation process has to be invoked at the end.
In the loop of GetPhysicalInteractionLength of AlongStepDoIt, the Multiple Scattering process has to be invoked just before the Transportation process.
Ordering of DoIts
There is only some special cases. For example, the Cherenkov process needs the energy loss information of the current step for its DoIt invocation. Therefore, the EnergyLoss process has to be invoked before the Cherenkov process. This ordering is provided by the process manager. Energy loss information necessary for the Cherenkov process is passed using G4Step (or the static dE/dX table is used together with the step length information in G4Step to obtain the energy loss information). Any other?