2.4.  Tracking

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.

2.4.1.  Design Philosophy

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.

2.4.2.  Class Design

Figure 2.4 shows a general overview of the tracking design in Unified Modelling Language Notation.

Tracking design

Figure 2.4.  Tracking design


  • 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.

2.4.3.  Tracking Algorithm

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

  1. Actions before tracking the particle: Clear secondary particle vector
  2. Pre tracking user intervention process.
  3. Construct a trajectory if it is requested
  4. Give SteppingManager the pointer to the track which will be tracked
  5. Inform beginning of tracking to physics processes
  6. Track the particle Step-by-Step while it is alive
    • Call Stepping method of G4SteppingManager
    • Append a trajectory point to the trajectory object if it is requested
  7. Post tracking user intervention process.
  8. Destroy the trajectory if it was created

Stepping() in G4SteppingManager

  1. Initialize current step
  2. If particle is stopped, get the minimum life time from all the at rest processes and invoke InvokeAtRestDoItProcs for the selected AtRest processes
  3. If particle is not stopped:
    • Invoke DefinePhysicalStepLength, that finds the minimum step length demanded by the active processes
    • Invoke InvokeAlongStepDoItProcs
    • Update current track properties by taking into account all changes by AlongStepDoIt
    • Update the safety
    • Invoke PostStepDoIt of the active discrete process.
    • Update the track length
    • Send G4Step information to Hit/Dig if the volume is sensitive
    • Invoke the user intervention process.
    • Return the value of the StepStatus.

2.4.4.  Interaction with Physics Processes

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:

    • Forced : Corresponding PostStepDoIt is forced.
    • NotForced : Corresponding PostStepDoIt is not forced unless this process limits the step.
    • Conditionally : Only when AlongStepDoIt limits the step, corresponding PoststepDoIt is invoked.
    • ExclusivelyForced : Corresponding PostStepDoIt is exclusively forced.

    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:

    • CandidateForSelection: this process can be the winner. If its step length is the smallest, it will be the process defining the step (the process
    • NotCandidateForSelection: this process cannot be the winner. Even if its step length is taken as the smallest, it will not be the process defining the step

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:

    • Update PostStepPoint of Step according to ParticleChange
    • Update G4Track according to ParticleChange after each PostStepDoIt
    • Update safety after each invocation of PostStepDoIts
    • 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 status is set according to what the process defined

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

2.4.5.  Ordering of Methods of Physics Processes

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:

  1. 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.

  2. 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?