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Adding a new process

Adding a new process. CERN User’s Workshop 13 November 2002 V.Ivanchenko, CERN, Budker Institute for Nuclear Physics Based on presentation of M.Verderi at SLAC. Introduction. Geant4 provides a variety of physics processes – electromagnetic, hadronic, optical….

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Adding a new process

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  1. Adding a new process CERN User’s Workshop 13 November 2002 V.Ivanchenko, CERN, Budker Institute for Nuclear Physics Based on presentation of M.Verderi at SLAC

  2. Introduction • Geant4 provides a variety of physics processes – electromagnetic, hadronic, optical…. • Geant4 is a free toolkit - users can implement any new process • Processes are designed in a generic/general way • Process interface is very stable • Simple processes are easy to realize ! • Sometimes details of process interface are not so obvious and should be discussed V.Ivanchenko CERN 13.11.02

  3. Outline • Overview of the process interface • Mandatory and optional methods • Details of method signatures • Processes types • Examples of electromagnetic processes • Hadron processes V.Ivanchenko CERN 13.11.02

  4. G4 processes • Physics is described via abstract interface called process associated with particles • Distinction between process and model – one process may includes many models • Generation of final state is independent from the access and use of cross sections and from tracking • Transparent access to cross sections and secondary generators V.Ivanchenko CERN 13.11.02

  5. All processes inherit from the base class G4VProcess Standard interface: a process provides Interaction Lenths, StepLimits, and DoIt methods Processes active AlongStep, PostStep, AtRest There are 7 types of processes with predefined interfaces: ContinuousProcess ContinuouesDiscreteProcess DiscreteProcess RestContinuouesDiscreteProcess RestContinuouesProcess RestDiscreteProcess RestProcess G4VProcess interface overview V.Ivanchenko CERN 13.11.02

  6. The mandatory methods • G4VProcess defines 6 pure virtual methods: • AtRestGetPhysicalInteractionLength, • AtRestDoIt; • AlongStepGetPhysicalInteractionLength, • AlongStepDoIt; • PostStepGetPhysicalInteractionLength, • PostStepDoIt. • You need to override to implement a new process • Either directly, or by using some intermediate class, like G4VDiscreteProcess • The signatures of those methods are described later • G4VProcess defined insource/processes/management; V.Ivanchenko CERN 13.11.02

  7. The optional methods • G4VProcess defines also several virtual (but non pure-virtual) methods. For example: • void BuildPhysicsTable(const G4ParticleDefinition&); • Invoked by the run manager, after cuts are (re)set; • G4bool IsApplicable(const G4ParticleDefinition&); • Process is assign only for true particles • void StartTracking(): void EndTracking(); • Invoked before and after the tracking of the track has finished; • But several other virtual methods exist. Please review G4VProcess.hh for details; V.Ivanchenko CERN 13.11.02

  8. « Life cycle » of a process • Implementation mandatory • Implementation optional • Construction; • BuildPhysicsTable(); • StartTracking(); • If particle at rest: • AtRestGetPhysicalInteractionLength(); • AtRestDoIt(); • Else: • PostStepGetPhysicalInteractionLength(); • AlongStepGetPhysicalInteractionLength(); • AlongStepDoIt(); • PostStepDoIt(); • EndTracking(); • Destruction; Tracking loop Stepping loop V.Ivanchenko CERN 13.11.02

  9. Number of interaction length  • G4VProcess and derived classes implement a scheme based on the « number of interaction length » to define the time/space point of the interaction and to choose the process of interaction • assumed for the processes dealing with the exponential law • At the beginning of the tracking the process is given a sampled value « number of interaction lenght » Nint • At the beginning of each step • The (concrete) process evaluates the current « mean free path » lfree, given the current material; • The « true path length » the process allows to the particle before the interaction occurs is then: Nintlfree ; • This value is returned by GetPhysicalInteractionLength; V.Ivanchenko CERN 13.11.02

  10. Number of interaction length • Then the step occurs with the actual step length Lstep value; • At the beginning of the new step: • If the process has limited the previous step (ie its interaction occured), it gets a new Nint value; • Otherwise, the process converts backLstep into a number of « consumed » interaction length, which is substracted to its Nint amount; • Please review for example G4VDiscreteProcess; • Note that all related methods are virtual, allowing to redefine them, if needed. V.Ivanchenko CERN 13.11.02

  11. Some advices • Do not overwrite GPIL virtual functions if it is possible! • Try to use one of 7 predefined interfaces • In that case the accurate control on interaction length is provided for you V.Ivanchenko CERN 13.11.02

  12. Example: G4VDiscreteProcess inline G4doubleG4VDiscreteProcess::PostStepGetPhysicalInteractionLength(const G4Track&track,G4doublepreviousStepSize, G4ForceCondition* condition) { if ( (previousStepSize <=0.0) || (theNumberOfInteractionLengthLeft<=0.0)) {// beginning of tracking (or just after DoIt of this process)ResetNumberOfInteractionLengthLeft(); } else {//subtract NumberOfInteractionLengthLeftSubtractNumberOfInteractionLengthLeft(previousStepSize); if(theNumberOfInteractionLengthLeft<perMillion)theNumberOfInteractionLengthLeft=0.; } //… //get mean free pathcurrentInteractionLength = GetMeanFreePath(track, previousStepSize, condition);G4doublevalue = theNumberOfInteractionLengthLeft*currentInteractionLength;returnvalue; } V.Ivanchenko CERN 13.11.02

  13. Interfaces to be used • protected: • // For all processes • virtual G4double GetMeanFreePath( const G4Track& track,G4double previousStepSize, G4ForceCondition* condition) = 0 • // For continuous processes • virtual G4doubleGetContinuousStepLimit( const G4Track& track,G4double previousStepSize, G4double currentMinimumStep, G4ForceCondition* condition) = 0; V.Ivanchenko CERN 13.11.02

  14. When process is active? • All Continuous processes are invocated at each step of the particle • If Discrete or Rest process limits the step then it is invocated • To help to activate Rest or Discrete process at each step one should use G4ForceCondition V.Ivanchenko CERN 13.11.02

  15. G4ForceConditions • GetPhysicalInteractionLength methods involve G4ForceCondition & G4GPILSelection; • These are two enumerations: • They define signals, that processes send to the stepping, to require the treatment they wish from the stepping; • Involve « delicate » aspects; • Defined in source/track; V.Ivanchenko CERN 13.11.02

  16. G4ForceConditions • G4ForceCondition (AtRest and PostStep) defines requests for treatment of the DoIt methods. • It can take the values: • NotForced: Usual case : the DoIt method is invoked if the related GetPhysicalInteractionLength has limited the step; • Forced: The related DoIt is applied anyway; • Conditionally: The PostStepDoIt is applied if the AlongStep has limited the step; • ExclusivelyForced: Only the PostStepDoIt of the process is applied: all other AlongStep and PostStep are ignored; V.Ivanchenko CERN 13.11.02

  17. G4GPILSelection • More delicate… • G4GPILSelection (AlongStep) defines requests for the treatment of the GetPhysicalInteractionLength methods. • It can takes the values: • CandidateForSelection: usual case : the process will be « declared » to have limited the step if it returns the smallest length; • NotCandidateForSelection: the process will not be « declared » to have limited the step, even if it returns the smallest lenght; • In practice, only the multiple-scattering makes use of the « NotCandidateForSelection » signal; V.Ivanchenko CERN 13.11.02

  18. Examples of processes • G4hIonisation – notForced ContinuousDiscrete • G4Decay – notForced RestDiscrete • G4Cherenkov – Continuous • G4Scintillation – Forced RestDiscrete • G4MuonMinusCaptureAtRest – Rest • G4ProtonInelasticProcess – notForced Discrete V.Ivanchenko CERN 13.11.02

  19. DoIt signature • virtualG4VParticleChange*AtRestDoIt(const G4Track& track,const G4Step& step ) = 0; • virtualG4VParticleChange*AlongStepDoIt(const G4Track& track,const G4Step& step ) = 0; • virtualG4VParticleChange*PostStepDoIt(const G4Track& track,const G4Step& step ) = 0; V.Ivanchenko CERN 13.11.02

  20. DoIt signature • All DoIt methods have the same signature: • They receive constG4Track and G4Step • It is not allowed to change directly the track, nor the step • They return a G4VParticleChange: • This G4VParticleChangereturns the changes of the track to the stepping • Not the « delta » • It is assumed to create of secondary G4Track • Need to be familiar with, to implement a process ; V.Ivanchenko CERN 13.11.02

  21. G4VParticleChange • G4VParticleChange is defined in source/track • It defines the virtual methods: • virtual G4Step*UpdateStepForAtRest(G4Step*); • virtual G4Step*UpdateStepForAlongStep(G4Step*); • virtual G4Step*UpdateStepForPostStep(G4Step*); • Which are used to communicate the changes to be applied on the primary; • They return the G4Step after having updated it; • Each concrete G4VParticleChange should modify only the necessary members of the G4Step; • Can be relevant if your G4VParticleChange is often used; V.Ivanchenko CERN 13.11.02

  22. G4VParticleChange • To create secondaries by the process, the following methods have to be used: • void SetNumberOfSecondaries(G4int); • To declare the maximum number of secondaries which will be created by the process; • void AddSecondary(G4Track* aSecondary); • Which has to be called for each secondary created; • G4VParticleChange has a method Initialize(const G4Track&) which is used to initialize the members which will be changed by the process V.Ivanchenko CERN 13.11.02

  23. G4TrackStatus • G4TrackStatus defines the possible status a track can undertake; • It is needed when writing a process: fAlive, // Continue the tracking fStopButAlive, // Invoke active rest physics processes and // and kill the current track afterward fStopAndKill, // Kill the current track fKillTrackAndSecondaries, // Kill the current track and also // associated secondaries. fSuspend, // Suspend the current track fPostponeToNextEvent // Postpones the tracking of thecurrent // track to the next event. V.Ivanchenko CERN 13.11.02

  24. Example with G4GammaConversion (1) • Example with G4GammaConversion, which uses a particle change defined in the base class G4VDiscreteProcess; G4VParticleChange*G4GammaConversion::PostStepDoIt( const G4Track&aTrack,const G4Step&aStep){ aParticleChange.Initialize(aTrack); //Does the physics… aParticleChange.SetNumberOfSecondaries(2) ; //…G4doublelocalEnergyDeposit = 0.; if (ElectKineEnergy > fminimalEnergy) { //… // create G4DynamicParticle object for the particle1G4DynamicParticle*aParticle1= newG4DynamicParticle(G4Electron::Electron(), ElectDirection, ElectKineEnergy);aParticleChange.AddSecondary(aParticle1); } else { localEnergyDeposit += ElectKineEnergy;} V.Ivanchenko CERN 13.11.02

  25. Example with G4GammaConversion (2) // the e+ is always created (even with Ekine=0) for further annihilation. //… if (PositKineEnergy < fminimalEnergy) { localEnergyDeposit += PositKineEnergy; PositKineEnergy = 0.;} //… // create G4DynamicParticle object for the particle2 G4DynamicParticle*aParticle2= newG4DynamicParticle(G4Positron::Positron(), PositDirection, PositKineEnergy);aParticleChange.AddSecondary(aParticle2); aParticleChange.SetLocalEnergyDeposit(localEnergyDeposit); // // Kill the incident photon //aParticleChange.SetEnergyChange( 0. ); aParticleChange.SetStatusChange( fStopAndKill );returnG4VDiscreteProcess::PostStepDoIt( aTrack, aStep );} V.Ivanchenko CERN 13.11.02

  26. Some remarks • It is not necessary to know Geant4 kernel in order to implement a new process • One have to follow the described interfaces • Having several implementation for given process is normal for Geant4 V.Ivanchenko CERN 13.11.02

  27. Some limits and constrains • Signature should be adequate to physics • Energy loss processes are coupled in Geant4 via dE/dx, Range, and InverseRange tables • Hadronic physics utilizes model approach, so cross sections and secondary generators are separate • One can implement also a model for hadronic interaction or specific hadronic cross section V.Ivanchenko CERN 13.11.02

  28. Conclusion remarks • User can substitute any of existing processes • It is assumed that Geant4 user is at the same time a developer • Geant4 team will appreciate an efforts of any user to implement his/her own process, if the is correct from physics point of view • New process can be easily utilized in Geant4 thanks to OO technology V.Ivanchenko CERN 13.11.02

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