Physics processes describe how particles interact with a material. Seven major categories of processes are provided by Geant4:
hadronic ,
decay ,
optical ,
parameterization and
The generalization and abstraction of physics processes is a key issue in the design of Geant4. All physics processes are treated in the same manner from the tracking point of view. The Geant4 approach enables anyone to create a process and assign it to a particle type. This openness should allow the creation of processes for novel, domain-specific or customised purposes by individuals or groups of users.
Each process has two groups of methods which play an important
role in tracking, GetPhysicalInteractionLength
(GPIL) and
DoIt
. The GPIL method gives the step length from the
current space-time point to the next space-time point. It does this
by calculating the probability of interaction based on the
process's cross section information. At the end of this step the
DoIt
method should be invoked. The DoIt
method
implements the details of the interaction, changing the particle's
energy, momentum, direction and position, and producing secondary
tracks if required. These changes are recorded as
G4VParticleChange objects(see
Particle Change).
G4VProcess is the base class for all physics processes.
Each physics process must implement virtual methods of
G4VProcess which describe the interaction (DoIt) and
determine when an interaction should occur (GPIL). In order to
accommodate various types of interactions G4VProcess
provides three DoIt
methods:
G4VParticleChange* AlongStepDoIt( const G4Track& track,
const G4Step& stepData )
This method is invoked while G4SteppingManager is
transporting a particle through one step. The corresponding
AlongStepDoIt
for each defined process is applied for
every step regardless of which process produces the minimum step
length. Each resulting change to the track information is recorded
and accumulated in G4Step. After all processes have been
invoked, changes due to AlongStepDoIt
are applied to
G4Track, including the particle relocation and the safety
update. Note that after the invocation of AlongStepDoIt
,
the endpoint of the G4Track object is in a new volume if the
step was limited by a geometric boundary. In order to obtain
information about the old volume, G4Step must be accessed,
since it contains information about both endpoints of a step.
G4VParticleChange* PostStepDoIt( const G4Track& track,
const G4Step& stepData )
This method is invoked at the end point of a step, only if its
process has produced the minimum step length, or if the process is
forced to occur. G4Track will be updated after each
invocation of PostStepDoIt
, in contrast to the
AlongStepDoIt
method.
G4VParticleChange* AtRestDoIt( const G4Track& track,
const G4Step& stepData )
This method is invoked only for stopped particles, and only if its process produced the minimum step length or the process is forced to occur.
For each of the above DoIt
methods G4VProcess
provides a corresponding pure virtual GPIL method:
G4double PostStepGetPhysicalInteractionLength( const
G4Track& track, G4double previousStepSize, G4ForceCondition*
condition )
This method generates the step length allowed by its process. It also provides a flag to force the interaction to occur regardless of its step length.
G4double AlongStepGetPhysicalInteractionLength( const
G4Track& track, G4double previousStepSize, G4double
currentMinimumStep, G4double& proposedSafety, G4GPILSelection*
selection )
This method generates the step length allowed by its process.
G4double AtRestGetPhysicalInteractionLength( const
G4Track& track, G4ForceCondition* condition )
This method generates the step length in time allowed by its process. It also provides a flag to force the interaction to occur regardless of its step length.
Other pure virtual methods in G4VProcess follow:
virtual G4bool IsApplicable(const
G4ParticleDefinition&)
returns true if this process object is applicable to the particle type.
virtual void PreparePhysicsTable(const
G4ParticleDefinition&)
and
virtual void BuildPhysicsTable(const
G4ParticleDefinition&)
is messaged by the process manager, whenever cross section tables should be prepared and rebuilt due to changing cut-off values. It is not mandatory if the process is not affected by cut-off values.
virtual void StartTracking()
and
virtual void EndTracking()
are messaged by the tracking manager at the beginning and end of tracking the current track.
Specialized processes may be derived from seven additional virtual base classes which are themselves derived from G4VProcess. Three of these classes are used for simple processes:
Processes using only the AtRestDoIt
method.
example: neutron capture
Processes using only the PostStepDoIt
method.
example: compton scattering, hadron inelastic interaction
The other four classes are provided for rather complex processes:
Processes using both AlongStepDoIt
and
PostStepDoIt
methods.
example: transportation, ionisation(energy loss and delta ray)
Processes using both AtRestDoIt
and
PostStepDoIt
methods.
example: positron annihilation, decay (both in flight and at rest)
Processes using both AtRestDoIt
and
AlongStepDoIt
methods.
Processes using AtRestDoIt
,
AlongStepDoIt and
PostStepDoIt methods.
G4VParticleChange and its descendants are used to store
the final state information of the track, including secondary
tracks, which has been generated by the DoIt
methods. The
instance of G4VParticleChange is the only object whose
information is updated by the physics processes, hence it is
responsible for updating the step. The stepping manager collects
secondary tracks and only sends requests via particle change to
update G4Step.
G4VParticleChange is introduced as an abstract class. It has a minimal set of methods for updating G4Step and handling secondaries. A physics process can therefore define its own particle change derived from G4VParticleChange. Three pure virtual methods are provided,
virtual G4Step* UpdateStepForAtRest( G4Step* step)
,
virtual G4Step* UpdateStepForAlongStep( G4Step* step )
and
virtual G4Step* UpdateStepForPostStep( G4Step* step)
,
which correspond to the three DoIt
methods of
G4VProcess. Each derived class should implement these
methods.
This section summarizes the electromagnetic physics processes which are installed in Geant4. For details on the implementation of these processes please refer to the Physics Reference Manual.
The following is a summary of the standard electromagnetic processes available in Geant4.
Photon processes
Compton scattering (class name G4ComptonScattering)
Gamma conversion (also called pair production, class name G4GammaConversion)
Photo-electric effect (class name G4PhotoElectricEffect)
Muon pair production (class name G4GammaConversionToMuons)
Electron/positron processes
Ionisation and delta ray production (class name G4eIonisation)
Bremsstrahlung (class name G4eBremsstrahlung)
Positron annihilation into two gammas (class name G4eplusAnnihilation)
Positron annihilation into two muons (class name G4AnnihiToMuPair)
Positron annihilation into hadrons (class name G4eeToHadrons)
Muon processes
Ionisation and delta ray production (class name G4MuIonisation)
Bremsstrahlung (class name G4MuBremsstrahlung)
e+e- pair production (class name G4MuPairProduction)
Hadron/ion processes
Ionisation (class name G4hIonisation)
Ionisation for ions (class name G4ionIonisation)
Ionisation for ions in low-density media (class name G4ionGasIonisation)
Ionisation for heavy exotic particles (class name G4hhIonisation)
Ionisation for classical magnetic monopole (class name G4mplIonisation)
Coulomb scattering processes
A general process in the sense that the same process/class is used to simulate the multiple scattering of the all charged particles (class name G4MultipleScattering)
Specialised process for more fast simulation the multiple scattering of muons and hadrons (class name G4hMultipleScattering)
Alternative process (beta-version) for the multiple scattering of muons (class name G4MuMultipleScattering)
Alternative process for simulation of single Coulomb scattering of all charged particles (class name G4CoulombScattering)
Alternative process for simulation of single Coulomb scattering of ions (class name G4ScreenedNuclearRecoil)
Processes for simulation of polarized electron and gamma beams
Compton scattering of circularly polarized gamma beam on polarized target (class name G4PolarizedCompton)
Pair production induced by circularly polarized gamma beam (class name G4PolarizedGammaConversion)
Photo-electric effect induced by circularly polarized gamma beam (class name G4PolarizedPhotoElectricEffect)
Bremsstrahlung of polarized electrons and positrons (class name G4ePolarizedBremsstrahlung)
Ionisation of polarized electron and positron beam (class name G4ePolarizedIonisation)
Annihilation of polarized positrons (class name G4eplusPolarizedAnnihilation)
Processes for simulation of X-rays and optical protons production by charged particles
Synchrotron radiation (class name G4SynchrotronRadiation)
Transition radiation (class name G4TransitionRadiation)
Cerenkov radiation (class name G4Cerenkov)
Scintillations (class name G4Scintillation)
The processes described above use physics model classes, which may be combined according to particle energy. It is possible to change the energy range over which different models are valid, and to apply other models specific to particle type, energy range, and G4Region. The following alternative models are available:
Ionisation in thin absorbers (class name G4PAIModel)
An example of the registration of these processes in a physics list is given in Example 5.1, similar method is used in EM-builders of reference physics lists ($G4INSTALL/source/physics_lists/builders) and in EM examples ($G4INSTALL/examples/extended/electromagnetic).
Example 5.1.
Registration of standard electromagnetic processes
void PhysicsList::ConstructEM() { theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (particleName == "gamma") { pmanager->AddDiscreteProcess(new G4PhotoElectricEffect); pmanager->AddDiscreteProcess(new G4ComptonScattering); pmanager->AddDiscreteProcess(new G4GammaConversion); } else if (particleName == "e-") { pmanager->AddProcess(new G4MultipleScattering, -1, 1, 1); pmanager->AddProcess(new G4eIonisation, -1, 2, 2); pmanager->AddProcess(new G4eBremsstrahlung, -1, 3, 3); } else if (particleName == "e+") { pmanager->AddProcess(new G4MultipleScattering, -1, 1, 1); pmanager->AddProcess(new G4eIonisation, -1, 2, 2); pmanager->AddProcess(new G4eBremsstrahlung, -1, 3, 3); pmanager->AddProcess(new G4eplusAnnihilation, 0,-1, 4); } else if( particleName == "mu+" || particleName == "mu-" ) { pmanager->AddProcess(new G4hMultipleScattering,-1, 1, 1); pmanager->AddProcess(new G4MuIonisation, -1, 2, 2); pmanager->AddProcess(new G4MuBremsstrahlung, -1, 3, 3); pmanager->AddProcess(new G4MuPairProduction, -1, 4, 4); } else if (particleName == "alpha" || particleName == "He3" || particleName == "GenericIon") { // ions with charge >= +2 pmanager->AddProcess(new G4hMultipleScattering,-1, 1, 1); pmanager->AddProcess(new G4ionIonisation, -1, 2, 2); } else if ((!particle->IsShortLived()) && (particle->GetPDGCharge() != 0.0) && (particle->GetParticleName() != "chargedgeantino")) { //all others charged particles except geantino and short-lived pmanager->AddProcess(new G4hMultipleScattering,-1, 1, 1); pmanager->AddProcess(new G4hIonisation, -1, 2, 2); } } }
Novice and extended electromagnetic examples illustrating the use of electromagnetic processes are available as part of the Geant4 release.
Options are available for steering the standard electromagnetic processes. These options may be invoked either by UI commands or by the interface class G4EmProcessOptions. This class has the following public methods:
SetLossFluctuations(G4bool)
SetSubCutoff(G4bool, const G4Region* r=0)
SetIntegral(G4bool)
SetMinSubRange(G4double)
SetMinEnergy(G4double)
SetMaxEnergy(G4double)
SetMaxEnergyForCSDARange(G4double)
SetMaxEnergyForMuons(G4double)
SetDEDXBinning(G4int)
SetDEDXBinningForCSDARange(G4int)
SetLambdaBinning(G4int)
SetStepFunction(G4double, G4double)
SetRandomStep(G4bool)
SetApplyCuts(G4bool)
SetBuildCSDARange(G4bool)
SetVerbose(G4int, const G4String name= "all")
SetLambdaFactor(G4double)
SetLinearLossLimit(G4double)
ActivateDeexcitation(G4bool val, const G4Region* r = 0)
SetMscStepLimitation(G4MscStepLimitType val)
SetMscLateralDisplacement(G4bool val)
SetSkin(G4double)
SetMscRangeFactor(G4double)
SetMscGeomFactor(G4double)
SetLPMFlag(G4bool)
SetBremsstrahlungTh(G4double)
The corresponding UI command can be accessed in the UI subdirectory "/process/eLoss". The following types of step limitation by multiple scattering are available:
fSimple - step limitation used in g4 7.1 version (used in QGSP_EMV Physics List)
fUseSafety - default
fUseDistanceToBoundary - advance method of step limitation used in EM examples, required parameter skin > 0, should be used for setup without magnetic field
G4EmCalculator is a class which provides access to cross sections and stopping powers. This class can be used anywhere in the user code provided the physics list has already been initialised (G4State_Idle). G4EmCalculator has "Get" methods which can be applied to materials for which physics tables are already built, and "Compute" methods which can be applied to any material defined in the application or existing in the Geant4 internal database. The public methods of this class are:
GetDEDX(kinEnergy,particle,material,G4Region region=0)
GetRangeFromRestrictedDEDX(kinEnergy,particle,material,G4Region* region=0)
GetCSDARange(kinEnergy,particle,material,G4Region* region=0)
GetRange(kinEnergy,particle,material,G4Region* region=0)
GetKinEnergy(range,particle,material,G4Region* region=0)
GetCrosSectionPerVolume(kinEnergy,particle,material,G4Region* region=0)
GetMeanFreePath(kinEnergy,particle,material,G4Region* region=0)
PrintDEDXTable(particle)
PrintRangeTable(particle)
PrintInverseRangeTable(particle)
ComputeDEDX(kinEnergy,particle,process,material,cut=DBL_MAX)
ComputeElectronicDEDX(kinEnergy,particle,material,cut=DBL_MAX)
ComputeNuclearDEDX(kinEnergy,particle,material,cut=DBL_MAX)
ComputeTotalDEDX(kinEnergy,particle,material,cut=DBL_MAX)
ComputeCrosSectionPerVolume(kinEnergy,particle,process,material,cut=0)
ComputeCrosSectionPerAtom(kinEnergy,particle,process,Z,A,cut=0)
ComputeMeanFreePath(kinEnergy,particle,process,material,cut=0)
ComputeEnergyCutFromRangeCut(range,particle,material)
FindParticle(const G4String&)
FindMaterial(const G4String&)
FindRegion(const G4String&)
FindCouple(const G4Material*, const G4Region* region=0)
SetVerbose(G4int)
For these interfaces, particles, materials, or processes may be pointers or strings with names.
The following is a summary of the Low Energy Electromagnetic processes available in Geant4. Further information is available in the homepage of the Geant4 Low Energy Electromagnetic Physics Working Group. The physics content of these processes is documented in Geant4 Physics Reference Manual and in other papers.
Photon processes
Compton scattering (class G4LowEnergyCompton)
Polarized Compton scattering (class G4LowEnergyPolarizedCompton)
Rayleigh scattering (class G4LowEnergyRayleigh)
Gamma conversion (also called pair production, class G4LowEnergyGammaConversion)
Photo-electric effect (classG4LowEnergyPhotoElectric)
Electron processes
Bremsstrahlung (class G4LowEnergyBremsstrahlung)
Ionisation and delta ray production (class G4LowEnergyIonisation)
Hadron and ion processes
Ionisation and delta ray production (class G4hLowEnergyIonisation)
An example of the registration of these processes in a physics list is given in Example 5.2.
Example 5.2. Registration of electromagnetic low energy electron/photon processes.
void LowEnPhysicsList::ConstructEM() { theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (particleName == "gamma") { theLEPhotoElectric = new G4LowEnergyPhotoElectric(); theLECompton = new G4LowEnergyCompton(); theLEGammaConversion = new G4LowEnergyGammaConversion(); theLERayleigh = new G4LowEnergyRayleigh(); pmanager->AddDiscreteProcess(theLEPhotoElectric); pmanager->AddDiscreteProcess(theLECompton); pmanager->AddDiscreteProcess(theLERayleigh); pmanager->AddDiscreteProcess(theLEGammaConversion); } else if (particleName == "e-") { theLEIonisation = new G4LowEnergyIonisation(); theLEBremsstrahlung = new G4LowEnergyBremsstrahlung(); theeminusMultipleScattering = new G4MultipleScattering(); pmanager->AddProcess(theeminusMultipleScattering,-1,1,1); pmanager->AddProcess(theLEIonisation,-1,2,2); pmanager->AddProcess(theLEBremsstrahlung,-1,-1,3); } else if (particleName == "e+") { theeplusMultipleScattering = new G4MultipleScattering(); theeplusIonisation = new G4eIonisation(); theeplusBremsstrahlung = new G4eBremsstrahlung(); theeplusAnnihilation = new G4eplusAnnihilation(); pmanager->AddProcess(theeplusMultipleScattering,-1,1,1); pmanager->AddProcess(theeplusIonisation,-1,2,2); pmanager->AddProcess(theeplusBremsstrahlung,-1,-1,3); pmanager->AddProcess(theeplusAnnihilation,0,-1,4); } } }
Advanced examples illustrating the use of Low Energy Electromagnetic processes are available as part of the Geant4 release and are further documented here.
To run the Low Energy code for photon and electron electromagnetic processes, data files need to be copied by the user to his/her code repository. These files are distributed together with Geant4 release.
The user should set the environment variable G4LEDATA to the directory where he/she has copied the files.
Options are available for low energy electromagnetic processes for hadrons and ions in terms of public member functions of the G4hLowEnergyIonisation class:
SetHighEnergyForProtonParametrisation(G4double)
SetLowEnergyForProtonParametrisation(G4double)
SetHighEnergyForAntiProtonParametrisation(G4double)
SetLowEnergyForAntiProtonParametrisation(G4double)
SetElectronicStoppingPowerModel(const G4ParticleDefinition*,const G4String& )
SetNuclearStoppingPowerModel(const G4String&)
SetNuclearStoppingOn()
SetNuclearStoppingOff()
SetBarkasOn()
SetBarkasOff()
SetFluorescence(const G4bool)
ActivateAugerElectronProduction(G4bool)
SetCutForSecondaryPhotons(G4double)
SetCutForSecondaryElectrons(G4double)
The available models for ElectronicStoppingPower and NuclearStoppingPower are documented in the class diagrams.
Options are available for low energy electromagnetic processes for electrons in the G4LowEnergyIonisation class:
ActivateAuger(G4bool)
SetCutForLowEnSecPhotons(G4double)
SetCutForLowEnSecElectrons(G4double)
Options are available for low energy electromagnetic processes for electrons/positrons in the G4LowEnergyBremsstrahlung class, that allow the use of alternative bremsstrahlung angular generators:
SetAngularGenerator(G4VBremAngularDistribution* distribution);
SetAngularGenerator(const G4String& name);
Currently three angular generators are available: G4ModifiedTsai, 2BNGenerator and 2BSGenerator. G4ModifiedTsai is set by default, but it can be forced using the string "tsai". 2BNGenerator and 2BSGenerator can be set using the strings "2bs" and "2bn". Information regarding conditions of use, performance and energy limits of different models are available in the Physics Reference Manual and in the Geant4 Low Energy Electromagnetic Physics Working Group homepage.
Other options G4LowEnergyBremsstrahlung class are:
SetCutForLowEnSecPhotons(G4double)
Options can also be set in the G4LowEnergyPhotoElectric class, that allow the use of alternative photoelectron angular generators:
SetAngularGenerator(G4VPhotoElectricAngularDistribution* distribution);
SetAngularGenerator(const G4String& name);
Currently three angular generators are available: G4PhotoElectricAngularGeneratorSimple, G4PhotoElectricAngularGeneratorSauterGavrilla and G4PhotoElectricAngularGeneratorPolarized. G4PhotoElectricAngularGeneratorSimple is set by default, but it can be forced using the string "default". G4PhotoElectricAngularGeneratorSauterGavrilla and G4PhotoElectricAngularGeneratorPolarized can be set using the strings "standard" and "polarized". Information regarding conditions of use, performance and energy limits of different models are available in the Physics Reference Manual and in the Geant4 Low Energy Electromagnetic Physics Working Group homepage.
Geant4 low energy electromagnetic Physics processes have been extended down to energies of a few electronVolts suitable for the simulation of radiation effects in liquid water for applications at the cellular and sub-cellular level. These developments take place in the framework of the Geant4 DNA project [ http://www.ge.infn.it/geant4/dna ] and are fully described in the paper [ Chauvie2007 ].
Their implementation in Geant4 is based on the usage of innovative techniques first introduced in Monte Carlo simulation (policy-based class design), to ensure openness to future extension and evolution as well as flexibility of configuration in user applications. In this new design, a generic Geant4-DNA physics process is configured by template specialization in order to acquire physical properties (cross section, final state), using policy classes : a Cross Section policy class and a Final State policy class.
These processes apply to electrons, protons, hydrogen, alpha particles and their charge states.
Elastic scattering (two complementary models available depending on energy range)
Cross section policy class name, common to both models : G4CrossSectionElasticScreenedRutherford
Final state policy class names : G4FinalStateElasticScreenedRutherford or G4FinalStateElasticBrennerZaider
Excitation (one model)
Cross section policy class name : G4CrossSectionExcitationEmfietzoglou
Final state policy class name : G4FinalStateExcitationEmfietzoglou
Ionisation (one model)
Cross section policy class name : G4CrossSectionIonisationBorn
Final state policy class names : G4FinalStateIonisationBorn
Excitation (two complementary models available depending on energy range)
Cross section policy class name : G4CrossSectionExcitationMillerGreen
Final state policy class name : G4FinalStateExcitationMillerGreen
Cross section policy class name : G4CrossSectionExcitationBorn
Final state policy class name : G4FinalStateExcitationBorn
Ionisation (two complementary models available depending on energy range)
Cross section policy class name : G4CrossSectionIonisationRudd
Final state policy class name : G4FinalStateIonisationRudd
Cross section policy class name : G4CrossSectionIonisationBorn
Final state policy class name : G4FinalStateIonisationBorn
Charge decrease (one model)
Cross section policy class name : G4CrossSectionChargeDecrease
Final state policy class name : G4FinalStateChargeDecrease
Ionisation (one model)
Cross section policy class name : G4CrossSectionIonisationRudd
Final state policy class name : G4FinalStateIonisationRudd
Charge increase (one model)
Cross section policy class name : G4CrossSectionChargeIncrease
Final state policy class name : G4FinalStateChargeIncrease
Excitation (one model)
Cross section policy class name : G4CrossSectionExcitationMillerGreen
Final state policy class name : G4FinalStateExcitationMillerGreen
Ionisation (one model)
Cross section policy class name : G4CrossSectionIonisationRudd
Final state policy class name : G4FinalStateIonisationRudd
Charge increase (one model)
Cross section policy class name : G4CrossSectionChargeIncrease
Final state policy class name : G4FinalStateChargeIncrease
Excitation (one model)
Cross section policy class name : G4CrossSectionExcitationMillerGreen
Final state policy class name : G4FinalStateExcitationMillerGreen
Ionisation (one model)
Cross section policy class name : G4CrossSectionIonisationRudd
Final state policy class name : G4FinalStateIonisationRudd
Charge increase (one model)
Cross section policy class name : G4CrossSectionChargeIncrease
Final state policy class name : G4FinalStateChargeIncrease
Charge decrease (one model)
Cross section policy class name : G4CrossSectionChargeDecrease
Final state policy class name : G4FinalStateChargeDecrease
Excitation (one model)
Cross section policy class name : G4CrossSectionExcitationMillerGreen
Final state policy class name : G4FinalStateExcitationMillerGreen
Ionisation (one model)
Cross section policy class name : G4CrossSectionIonisationRudd
Final state policy class name : G4FinalStateIonisationRudd
Charge decrease (one model)
Cross section policy class name : G4CrossSectionChargeDecrease
Final state policy class name : G4FinalStateChargeDecrease
An example of the registration of these processes in a physics list is given here below :
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo...... // Geant4 DNA header files #include "G4DNAGenericIonsManager.hh" #include "G4FinalStateProduct.hh" #include "G4DNAProcess.hh" #include "G4CrossSectionExcitationEmfietzoglou.hh" #include "G4FinalStateExcitationEmfietzoglou.hh" #include "G4CrossSectionElasticScreenedRutherford.hh" #include "G4FinalStateElasticScreenedRutherford.hh" #include "G4FinalStateElasticBrennerZaider.hh" #include "G4CrossSectionExcitationBorn.hh" #include "G4FinalStateExcitationBorn.hh" #include "G4CrossSectionIonisationBorn.hh" #include "G4FinalStateIonisationBorn.hh" #include "G4CrossSectionIonisationRudd.hh" #include "G4FinalStateIonisationRudd.hh" #include "G4CrossSectionExcitationMillerGreen.hh" #include "G4FinalStateExcitationMillerGreen.hh" #include "G4CrossSectionChargeDecrease.hh" #include "G4FinalStateChargeDecrease.hh" #include "G4CrossSectionChargeIncrease.hh" #include "G4FinalStateChargeIncrease.hh" // Processes definition typedef G4DNAProcess<G4CrossSectionElasticScreenedRutherford,G4FinalStateElasticScreenedRutherford> ElasticScreenedRutherford; typedef G4DNAProcess<G4CrossSectionElasticScreenedRutherford,G4FinalStateElasticBrennerZaider> ElasticBrennerZaider; typedef G4DNAProcess<G4CrossSectionExcitationEmfietzoglou,G4FinalStateExcitationEmfietzoglou> ExcitationEmfietzoglou; typedef G4DNAProcess<G4CrossSectionExcitationBorn,G4FinalStateExcitationBorn> ExcitationBorn; typedef G4DNAProcess<G4CrossSectionIonisationBorn,G4FinalStateIonisationBorn> IonisationBorn; typedef G4DNAProcess<G4CrossSectionIonisationRudd,G4FinalStateIonisationRudd> IonisationRudd; typedef G4DNAProcess<G4CrossSectionExcitationMillerGreen,G4FinalStateExcitationMillerGreen> ExcitationMillerGreen; typedef G4DNAProcess<G4CrossSectionChargeDecrease,G4FinalStateChargeDecrease> ChargeDecrease; typedef G4DNAProcess<G4CrossSectionChargeIncrease,G4FinalStateChargeIncrease> ChargeIncrease; // Processes registration void MicrodosimetryPhysicsList::ConstructEM() { theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* processManager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (particleName == "e-") { processManager->AddDiscreteProcess(new ExcitationEmfietzoglou); processManager->AddDiscreteProcess(new ElasticScreenedRutherford); processManager->AddDiscreteProcess(new ElasticBrennerZaider); processManager->AddDiscreteProcess(new IonisationBorn); } else if ( particleName == "proton" ) { processManager->AddDiscreteProcess(new ExcitationMillerGreen); processManager->AddDiscreteProcess(new ExcitationBorn); processManager->AddDiscreteProcess(new IonisationRudd); processManager->AddDiscreteProcess(new IonisationBorn); processManager->AddDiscreteProcess(new ChargeDecrease); } else if ( particleName == "hydrogen" ) { processManager->AddDiscreteProcess(new IonisationRudd); processManager->AddDiscreteProcess(new ChargeIncrease); } else if ( particleName == "alpha" ) { processManager->AddDiscreteProcess(new ExcitationMillerGreen); processManager->AddDiscreteProcess(new IonisationRudd); processManager->AddDiscreteProcess(new ChargeDecrease); } else if ( particleName == "alpha+" ) { processManager->AddDiscreteProcess(new ExcitationMillerGreen); processManager->AddDiscreteProcess(new IonisationRudd); processManager->AddDiscreteProcess(new ChargeDecrease); processManager->AddDiscreteProcess(new ChargeIncrease); } else if ( particleName == "helium" ) { processManager->AddDiscreteProcess(new ExcitationMillerGreen); processManager->AddDiscreteProcess(new IonisationRudd); processManager->AddDiscreteProcess(new ChargeIncrease); } } } //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
Note that in the above example, "alpha" particles are helium atoms ionised twice and "helium" particles are neutral helium atoms. The definition of particles in the physics list may be for example implemented as follows :
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo...... #include "G4DNAGenericIonsManager.hh" void MicrodosimetryPhysicsList::ConstructBaryons() { // construct baryons --- // Geant4 DNA particles G4DNAGenericIonsManager * genericIonsManager; genericIonsManager=G4DNAGenericIonsManager::Instance(); genericIonsManager->GetIon("alpha++"); genericIonsManager->GetIon("alpha+"); genericIonsManager->GetIon("helium"); genericIonsManager->GetIon("hydrogen"); } //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
To run the Geant4 DNA extension, data files need to be copied by the user to his/her code repository. These files are distributed together with the Geant4 release.
The user should set the environment variable G4LEDATA to the directory where he/she has copied the files.
This section briefly introduces the hadronic physics processes installed in Geant4. For details of the implementation of hadronic interactions available in Geant4, please refer to the Physics Reference Manual.
Each hadronic process object (derived from G4HadronicProcess) may have one or more cross section data sets associated with it. The term "data set" is meant, in a broad sense, to be an object that encapsulates methods and data for calculating total cross sections for a given process. The methods and data may take many forms, from a simple equation using a few hard-wired numbers to a sophisticated parameterisation using large data tables. Cross section data sets are derived from the abstract class G4VCrossSectionDataSet, and are required to implement the following methods:
G4bool IsApplicable( const G4DynamicParticle*, const G4Element* )
This method must return True
if the data set is able to
calculate a total cross section for the given particle and
material, and False
otherwise.
G4double GetCrossSection( const G4DynamicParticle*, const G4Element* )
This method, which will be invoked only if True
was
returned by IsApplicable
, must return a cross section, in
Geant4 default units, for the given particle and material.
void BuildPhysicsTable( const G4ParticleDefinition& )
This method may be invoked to request the data set to recalculate its internal database or otherwise reset its state after a change in the cuts or other parameters of the given particle type.
void DumpPhysicsTable( const G4ParticleDefinition& ) = 0
This method may be invoked to request the data set to print its internal database and/or other state information, for the given particle type, to the standard output stream.
Cross section data sets are used by the process for the calculation of the physical interaction length. A given cross section data set may only apply to a certain energy range, or may only be able to calculate cross sections for a particular type of particle. The class G4CrossSectionDataStore has been provided to allow the user to specify, if desired, a series of data sets for a process, and to arrange the priority of data sets so that the appropriate one is used for a given energy range, particle, and material. It implements the following public methods:
G4CrossSectionDataStore() ~G4CrossSectionDataStore()
and
G4double GetCrossSection( const G4DynamicParticle*, const G4Element* )
For a given particle and material, this method returns a cross
section value provided by one of the collection of cross section
data sets listed in the data store object. If there are no known
data sets, a G4Exception
is thrown and DBL_MIN
is
returned. Otherwise, each data set in the list is queried, in
reverse list order, by invoking its IsApplicable
method
for the given particle and material. The first data set object that
responds positively will then be asked to return a cross section
value via its GetCrossSection
method. If no data set
responds positively, a G4Exception
is thrown and
DBL_MIN
is returned.
void AddDataSet( G4VCrossSectionDataSet* aDataSet )
This method adds the given cross section data set to the end of the
list of data sets in the data store. For the evaluation of cross
sections, the list has a LIFO (Last In First Out) priority, meaning
that data sets added later to the list will have priority over
those added earlier to the list. Another way of saying this, is
that the data store, when given a GetCrossSection
request,
does the IsApplicable
queries in the reverse list order,
starting with the last data set in the list and proceeding to the
first, and the first data set that responds positively is used to
calculate the cross section.
void BuildPhysicsTable( const G4ParticleDefinition& aParticleType )
This method may be invoked to indicate to the data store that there
has been a change in the cuts or other parameters of the given
particle type. In response, the data store will invoke the
BuildPhysicsTable
of each of its data sets.
void DumpPhysicsTable( const G4ParticleDefinition& )
This method may be used to request the data store to invoke the
DumpPhysicsTable
method of each of its data sets.
The defaults for total cross section data and calculations have been encapsulated in the singleton class G4HadronCrossSections. Each hadronic process: G4HadronInelasticProcess, G4HadronElasticProcess, G4HadronFissionProcess, and G4HadronCaptureProcess, comes already equipped with a cross section data store and a default cross section data set. The data set objects are really just shells that invoke the singleton G4HadronCrossSections to do the real work of calculating cross sections.
The default cross sections can be overridden in whole or in part by the user. To this end, the base class G4HadronicProcess has a ``get'' method:
G4CrossSectionDataStore* GetCrossSectionDataStore()
which gives public access to the data store for each process. The user's cross section data sets can be added to the data store according to the following framework:
G4Hadron...Process aProcess(...) MyCrossSectionDataSet myDataSet(...) aProcess.GetCrossSectionDataStore()->AddDataSet( &MyDataSet )
The added data set will override the default cross section data
whenever so indicated by its IsApplicable
method.
In addition to the ``get'' method, G4HadronicProcess also has the method
void SetCrossSectionDataStore( G4CrossSectionDataStore* )
which allows the user to completely replace the default data store with a new data store.
It should be noted that a process does not send any information about itself to its associated data store (and hence data set) objects. Thus, each data set is assumed to be formulated to calculate cross sections for one and only one type of process. Of course, this does not prevent different data sets from sharing common data and/or calculation methods, as in the case of the G4HadronCrossSections class mentioned above. Indeed, G4VCrossSectionDataSet specifies only the abstract interface between physics processes and their data sets, and leaves the user free to implement whatever sort of underlying structure is appropriate.
The current implementation of the data set G4HadronCrossSections reuses the total cross-sections for inelastic and elastic scattering, radiative capture and fission as used with GHEISHA to provide cross-sections for calculation of the respective mean free paths of a given particle in a given material.
The cross section data for low energy neutron transport are
organized in a set of files that are read in by the corresponding
data set classes at time zero. Hereby the file system is used, in
order to allow highly granular access to the data. The ``root''
directory of the cross-section directory structure is accessed
through an environment variable, NeutronHPCrossSections
,
which is to be set by the user. The classes accessing the total
cross-sections of the individual processes, i.e., the cross-section
data set classes for low energy neutron transport, are
G4NeutronHPElasticData,
G4NeutronHPCaptureData,
G4NeutronHPFissionData,
and G4NeutronHPInelasticData.
For detailed descriptions of the low energy neutron total cross-sections, they may be registered by the user as described above with the data stores of the corresponding processes for neutron interactions.
It should be noted that using these total cross section classes does not require that the neutron_hp models also be used. It is up to the user to decide whethee this is desirable or not for his particular problem.
The following process classes have been implemented:
pi- absorption (class name G4PionMinusAbsorptionAtRest or G4PiMinusAbsorptionAtRest)
kaon- absorption (class name G4KaonMinusAbsorptionAtRest or G4KaonMinusAbsorption)
neutron capture (class name G4NeutronCaptureAtRest)
anti-proton annihilation (class name G4AntiProtonAnnihilationAtRest)
anti-neutron annihilation (class name G4AntiNeutronAnnihilationAtRest)
mu- capture (class name G4MuonMinusCaptureAtRest)
alternative CHIPS model for any negativly charged particle (class name G4QCaptureAtRest)
Obviously the last process does not, strictly speaking, deal with a ``hadron at rest''. It does, nonetheless, share common features with the others in the above list because of the implementation model chosen. The differences between the alternative implementation for kaon and pion absorption concern the fast part of the emitted particle spectrum. G4PiMinusAbsorptionAtRest, and G4KaonMinusAbsorptionAtRest focus especially on a good description of this part of the spectrum.
All of these classes are derived from the abstract class G4VRestProcess. In addition to the constructor and destructor methods, the following public methods of the abstract class have been implemented for each of the above six processes:
AtRestGetPhysicalInteractionLength( const G4Track&,
G4ForceCondition* )
This method returns the time taken before the interaction actually occurs. In all processes listed above, except for muon capture, a value of zero is returned. For the muon capture process the muon capture lifetime is returned.
AtRestDoIt( const G4Track&, const G4Step&)
This method generates the secondary particles produced by the process.
IsApplicable( const G4ParticleDefinition& )
This method returns the result of a check to see if the process is possible for a given particle.
Including a ``hadron at rest'' process for a particle, a pi- for example, into the Geant4 system is straightforward and can be done in the following way:
create a process:
theProcess = new G4PionMinusAbsorptionAtRest();
register the process with the particle's process manager:
theParticleDef = G4PionMinus::PionMinus(); G4ProcessManager* pman = theParticleDef->GetProcessManager(); pman->AddRestProcess( theProcess );
For hadrons in motion, there are four physics process classes. Table 5.1 shows each process and the particles for which it is relevant.
G4HadronElasticProcess | pi+, pi-, K+, K0S, K0L, K-, p, p-bar, n, n-bar, lambda, lambda-bar, Sigma+, Sigma-, Sigma+-bar, Sigma--bar, Xi0, Xi-, Xi0-bar, Xi--bar |
G4HadronInelasticProcess | pi+, pi-, K+, K0S, K0L, K-, p, p-bar, n, n-bar, lambda, lambda-bar, Sigma+, Sigma-, Sigma+-bar, Sigma--bar, Xi0, Xi-, Xi0-bar, Xi--bar |
G4HadronFissionProcess | all |
G4CaptureProcess | n, n-bar |
Table 5.1. Hadronic processes and relevant particles.
To register an inelastic process model for a particle, a proton for example, first get the pointer to the particle's process manager:
G4ParticleDefinition *theProton = G4Proton::ProtonDefinition(); G4ProcessManager *theProtonProcMan = theProton->GetProcessManager();
Create an instance of the particle's inelastic process:
G4ProtonInelasticProcess *theProtonIEProc = new G4ProtonInelasticProcess();
Create an instance of the model which determines the secondaries produced in the interaction, and calculates the momenta of the particles:
G4LEProtonInelastic *theProtonIE = new G4LEProtonInelastic();
Register the model with the particle's inelastic process:
theProtonIEProc->RegisterMe( theProtonIE );
Finally, add the particle's inelastic process to the list of discrete processes:
theProtonProcMan->AddDiscreteProcess( theProtonIEProc );
The particle's inelastic process class,
G4ProtonInelasticProcess in the example above, derives from
the G4HadronicInelasticProcess class, and simply defines the
process name and calls the G4HadronicInelasticProcess
constructor. All of the specific particle inelastic processes
derive from the G4HadronicInelasticProcess class, which
calls the PostStepDoIt
function, which returns the
particle change object from the G4HadronicProcess function
GeneralPostStepDoIt
. This class also gets the mean free
path, builds the physics table, and gets the microscopic cross
section. The G4HadronicInelasticProcess class derives from
the G4HadronicProcess class, which is the top level hadronic
process class. The G4HadronicProcess class derives from the
G4VDiscreteProcess class. The inelastic, elastic, capture,
and fission processes derive from the G4HadronicProcess
class. This pure virtual class also provides the energy range
manager object and the RegisterMe
access function.
A sample case for the proton's inelastic interaction model class
is shown in Example 5.3, where
G4LEProtonInelastic.hh
is the name of the include
file:
Example 5.3. An example of a proton inelastic interaction model class.
----------------------------- include file ------------------------------------------ #include "G4InelasticInteraction.hh" class G4LEProtonInelastic : public G4InelasticInteraction { public: G4LEProtonInelastic() : G4InelasticInteraction() { SetMinEnergy( 0.0 ); SetMaxEnergy( 25.*GeV ); } ~G4LEProtonInelastic() { } G4ParticleChange *ApplyYourself( const G4Track &aTrack, G4Nucleus &targetNucleus ); private: void CascadeAndCalculateMomenta( required arguments ); }; ----------------------------- source file ------------------------------------------ #include "G4LEProtonInelastic.hh" G4ParticleChange * G4LEProton Inelastic::ApplyYourself( const G4Track &aTrack, G4Nucleus &targetNucleus ) { theParticleChange.Initialize( aTrack ); const G4DynamicParticle *incidentParticle = aTrack.GetDynamicParticle(); // create the target particle G4DynamicParticle *targetParticle = targetNucleus.ReturnTargetParticle(); CascadeAndCalculateMomenta( required arguments ) { ... } return &theParticleChange; }
The CascadeAndCalculateMomenta
function is the bulk of
the model and is to be provided by the model's creator. It should
determine what secondary particles are produced in the interaction,
calculate the momenta for all the particles, and put this
information into the ParticleChange object which is
returned.
The G4LEProtonInelastic class derives from the
G4InelasticInteraction class, which is an abstract base
class since the pure virtual function ApplyYourself
is not
defined there. G4InelasticInteraction itself derives from
the G4HadronicInteraction abstract base class. This class is
the base class for all the model classes. It sorts out the energy
range for the models and provides class utilities. The
G4HadronicInteraction class provides the
Set/GetMinEnergy
and the Set/GetMaxEnergy
functions which determine the minimum and maximum energy range for
the model. An energy range can be set for a specific element, a
specific material, or for general applicability:
void SetMinEnergy( G4double anEnergy, G4Element *anElement ) void SetMinEnergy( G4double anEnergy, G4Material *aMaterial ) void SetMinEnergy( const G4double anEnergy ) void SetMaxEnergy( G4double anEnergy, G4Element *anElement ) void SetMaxEnergy( G4double anEnergy, G4Material *aMaterial ) void SetMaxEnergy( const G4double anEnergy )
In Geant4, any model can be run together with any other model without the need for the implementation of a special interface, or batch suite, and the ranges of applicability for the different models can be steered at initialisation time. This way, highly specialised models (valid only for one material and particle, and applicable only in a very restricted energy range) can be used in the same application, together with more general code, in a coherent fashion.
Each model has an intrinsic range of applicability, and the model chosen for a simulation depends very much on the use-case. Consequently, there are no ``defaults''. However, physics lists are provided which specify sets of models for various purposes.
Three types of hadronic shower models have been implemented: parametrisation driven models, data driven models, and theory driven models.
Parametrisation driven models are used for all processes
pertaining to particles coming to rest, and interacting with the
nucleus. For particles in flight, two sets of models exist for
inelastic scattering; low energy, and high energy models. Both sets
are based originally on the GHEISHA
package of Geant3.21,
and the original approaches to primary interaction, nuclear
excitation, intra-nuclear cascade and evaporation is kept. The
models are located in the sub-directories
hadronics/models/low_energy
and
hadronics/models/high_energy
. The low energy models are
targeted towards energies below 20 GeV; the high energy models
cover the energy range from 20 GeV to O(TeV). Fission, capture and
coherent elastic scattering are also modeled through parametrised
models.
Data driven models are available for the transport of low
energy neutrons in matter in sub-directory
hadronics/models/neutron_hp
. The modeling is based
on the data formats of ENDF/B-VI,
and all distributions of this standard data format are implemented.
The data sets used are selected from data libraries that conform to
these standard formats. The file system is used in order to allow granular
access to, and flexibility in, the use of the cross sections for different
isotopes, and channels. The energy coverage of these models is from
thermal energies to 20 MeV.
Theory driven models are available for inelastic scattering in
a first implementation, covering the full energy range of LHC
experiments. They are located in sub-directory
hadronics/models/generator
. The current philosophy
implies the usage of parton string models at high energies, of
intra-nuclear transport models at intermediate energies, and of
statistical break-up models for de-excitation.
This section briefly introduces decay processes installed in Geant4. For details of the implementation of particle decays, please refer to the Physics Reference Manual.
Geant4 provides a G4Decay class for both ``at rest'' and ``in flight'' particle decays. G4Decay can be applied to all particles except:
G4ParticleDefinition::thePDGMass <= 0
G4ParticleDefinition::thePDGLifeTime < 0
G4ParticleDefinition::fShortLivedFlag = True
Decay for some particles may be switched on or off by using
G4ParticleDefinition::SetPDGStable()
as well as
ActivateProcess()
and InActivateProcess()
methods of G4ProcessManager.
G4Decay proposes the step length (or step time for
AtRest
) according to the lifetime of the particle unless
PreAssignedDecayProperTime
is defined in
G4DynamicParticle.
The G4Decay class itself does not define decay modes of the particle. Geant4 provides two ways of doing this:
using G4DecayChannel in G4DecayTable, and
using thePreAssignedDecayProducts
of
G4DynamicParticle
The G4Decay class calculates the
PhysicalInteractionLength
and boosts decay products
created by G4VDecayChannel or event generators. See below
for information on the determination of the decay modes.
An object of G4Decay can be shared by particles.
Registration of the decay process to particles in the
ConstructPhysics
method of PhysicsList
(see Section 2.5.3)
is shown in Example 5.4.
Example 5.4.
Registration of the decay process to particles in the
ConstructPhysics
method of PhysicsList.
#include "G4Decay.hh" void ExN02PhysicsList::ConstructGeneral() { // Add Decay Process G4Decay* theDecayProcess = new G4Decay(); theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); if (theDecayProcess->IsApplicable(*particle)) { pmanager ->AddProcess(theDecayProcess); // set ordering for PostStepDoIt and AtRestDoIt pmanager ->SetProcessOrdering(theDecayProcess, idxPostStep); pmanager ->SetProcessOrdering(theDecayProcess, idxAtRest); } } }
Each particle has its G4DecayTable, which stores information on the decay modes of the particle. Each decay mode, with its branching ratio, corresponds to an object of various ``decay channel'' classes derived from G4VDecayChannel. Default decay modes are created in the constructors of particle classes. For example, the decay table of the neutral pion has G4PhaseSpaceDecayChannel and G4DalitzDecayChannel as follows:
// create a decay channel G4VDecayChannel* mode; // pi0 -> gamma + gamma mode = new G4PhaseSpaceDecayChannel("pi0",0.988,2,"gamma","gamma"); table->Insert(mode); // pi0 -> gamma + e+ + e- mode = new G4DalitzDecayChannel("pi0",0.012,"e-","e+"); table->Insert(mode);
Decay modes and branching ratios defined in Geant4 are listed in Section 5.3.2.
Decays of heavy flavor particles such as B mesons are very complex,
with many varieties of decay modes and decay mechanisms. There are
many models for heavy particle decay provided by various event
generators and it is impossible to define all the decay modes of
heavy particles by using G4VDecayChannel. In other words,
decays of heavy particles cannot be defined by the Geant4 decay
process, but should be defined by event generators or other
external packages. Geant4 provides two ways to do this:
pre-assigned decay mode
and external decayer
.
In the latter approach, the class G4VExtDecayer is used for the interface to an external package which defines decay modes for a particle. If an instance of G4VExtDecayer is attached to G4Decay, daughter particles will be generated by the external decay handler.
In the former case, decays of heavy particles are simulated by an event generator and the primary event contains the decay information. G4VPrimaryGenerator automatically attaches any daughter particles to the parent particle as the PreAssignedDecayProducts member of G4DynamicParticle. G4Decay adopts these pre-assigned daughter particles instead of asking G4VDecayChannel to generate decay products.
In addition, the user may assign a pre-assigned
decay
time for a specific track in its rest frame (i.e. decay time is
defined in the proper time) by using the
G4PrimaryParticle::SetProperTime() method.
G4VPrimaryGenerator sets the PreAssignedDecayProperTime
member of G4DynamicParticle. G4Decay
uses this decay time instead of the life time of the particle type.
A photon is considered to be optical when its wavelength is much greater than the typical atomic spacing. In GEANT4 optical photons are treated as a class of particle distinct from their higher energy gamma cousins. This implementation allows the wave-like properties of electromagnetic radiation to be incorporated into the optical photon process. Because this theoretical description breaks down at higher energies, there is no smooth transition as a function of energy between the optical photon and gamma particle classes.
For the simulation of optical photons to work correctly in GEANT4, they must be imputed a linear polarization. This is unlike most other particles in GEANT4 but is automatically and correctly done for optical photons that are generated as secondaries by existing processes in GEANT4. Not so, if the user wishes to start optical photons as primary particles. In this case, the user must set the linear polarization using particle gun methods, the General Particle Source, or his/her PrimaryGeneratorAction. For an unpolarized source, the linear polarization should be sampled randomly for each new primary photon.
The GEANT4 catalogue of processes at optical wavelengths includes refraction and reflection at medium boundaries, bulk absorption and Rayleigh scattering. Processes which produce optical photons include the Cerenkov effect, transition radiation and scintillation. Optical photons are generated in GEANT4 without energy conservation and their energy must therefore not be tallied as part of the energy balance of an event.
The optical properties of the medium which are key to the
implementation of these types of processes are stored as entries in
a G4MaterialPropertiesTable
which is linked to the
G4Material
in question. These properties may be constants
or they may be expressed as a function of the photon's wavelength.
This table is a private data member of the G4Material
class. The G4MaterialPropertiesTable
is implemented as a
hash directory, in which each entry consists of a value and
a key. The key is used to quickly and efficiently retrieve
the corresponding value. All values in the dictionary are either
instantiations of G4double
or the class
G4MaterialPropertyVector
, and all keys are of type
G4String
.
A G4MaterialPropertyVector
is composed of
instantiations of the class G4MPVEntry
. The
G4MPVEntry
is a pair of numbers, which in the case of an
optical property, are the photon momentum and corresponding
property value. The G4MaterialPropertyVector
is
implemented as a G4std::vector
, with the sorting operation
defined as
MPVEntry1 < MPVEntry2 ==
photon_momentum1 < photon_momentum2.
This results in all G4MaterialPropertyVector
s being sorted in
ascending order of photon momenta. It is possible for the user to
add as many material (optical) properties to the material as he
wishes using the methods supplied by the
G4MaterialPropertiesTable
class. An example of this is
shown in Example 5.5.
Example 5.5.
Optical properties added to a G4MaterialPropertiesTable
and linked to a G4Material
const G4int NUMENTRIES = 32; G4double ppckov[NUMENTRIES] = {2.034*eV, ......, 4.136*eV}; G4double rindex[NUMENTRIES] = {1.3435, ......, 1.3608}; G4double absorption[NUMENTRIES] = {344.8*cm, ......, 1450.0*cm]; G4MaterialPropertiesTable *MPT = new G4MaterialPropertiesTable(); MPT -> AddConstProperty("SCINTILLATIONYIELD",100./MeV); MPT -> AddProperty("RINDEX",ppckov,rindex,NUMENTRIES}; MPT -> AddProperty("ABSLENGTH",ppckov,absorption,NUMENTRIES}; scintillator -> SetMaterialPropertiesTable(MPT);
The radiation of Cerenkov light occurs when a charged particle moves through a dispersive medium faster than the group velocity of light in that medium. Photons are emitted on the surface of a cone, whose opening angle with respect to the particle's instantaneous direction decreases as the particle slows down. At the same time, the frequency of the photons emitted increases, and the number produced decreases. When the particle velocity drops below the local speed of light, the radiation ceases and the emission cone angle collapses to zero. The photons produced by this process have an inherent polarization perpendicular to the cone's surface at production.
The flux, spectrum, polarization and emission of Cerenkov
radiation in the AlongStepDoIt
method of the class
G4Cerenkov
follow well-known formulae, with two inherent
computational limitations. The first arises from step-wise
simulation, and the second comes from the requirement that
numerical integration calculate the average number of Cerenkov
photons per step. The process makes use of a
G4PhysicsTable
which contains incremental integrals to
expedite this calculation.
The time and position of Cerenkov photon emission are calculated
from quantities known at the beginning of a charged particle's
step. The step is assumed to be rectilinear even in the presence of
a magnetic field. The user may limit the step size by specifying a
maximum (average) number of Cerenkov photons created during the
step, using the SetMaxNumPhotonsPerStep(const G4int
NumPhotons)
method. The actual number generated will
necessarily be different due to the Poissonian nature of the
production. In the present implementation, the production density
of photons is distributed evenly along the particle's track
segment, even if the particle has slowed significantly during the
step.
The frequently very large number of secondaries produced in a
single step (about 300/cm in water), compelled the idea in
GEANT3.21 of suspending the primary particle until all its progeny
have been tracked. Despite the fact that GEANT4 employs dynamic
memory allocation and thus does not suffer from the limitations of
GEANT3.21 with its fixed large initial ZEBRA store, GEANT4
nevertheless provides for an analogous functionality with the
public method SetTrackSecondariesFirst
. An example of the
registration of the Cerenkov process is given in
Example 5.6.
Example 5.6.
Registration of the Cerenkov process in PhysicsList
.
#include "G4Cerenkov.hh" void ExptPhysicsList::ConstructOp(){ G4Cerenkov* theCerenkovProcess = new G4Cerenkov("Cerenkov"); G4int MaxNumPhotons = 300; theCerenkovProcess->SetTrackSecondariesFirst(true); theCerenkovProcess->SetMaxNumPhotonsPerStep(MaxNumPhotons); theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (theCerenkovProcess->IsApplicable(*particle)) { pmanager->AddContinuousProcess(theCerenkovProcess); } } }
Every scintillating material has a characteristic light yield,
SCINTILLATIONYIELD
, and an intrinsic resolution,
RESOLUTIONSCALE
, which generally broadens the statistical
distribution of generated photons. A wider intrinsic resolution is
due to impurities which are typical for doped crystals like NaI(Tl)
and CsI(Tl). On the other hand, the intrinsic resolution can also
be narrower when the Fano factor plays a role. The actual number of
emitted photons during a step fluctuates around the mean number of
photons with a width given by
ResolutionScale*sqrt(MeanNumberOfPhotons)
. The average
light yield, MeanNumberOfPhotons
, has a linear dependence
on the local energy deposition, but it may be different for minimum
ionizing and non-minimum ionizing particles.
A scintillator is also characterized by its photon emission
spectrum and by the exponential decay of its time spectrum. In
GEANT4 the scintillator can have a fast and a slow component. The
relative strength of the fast component as a fraction of total
scintillation yield is given by the YIELDRATIO
.
Scintillation may be simulated by specifying these empirical
parameters for each material. It is sufficient to specify in the
user's DetectorConstruction
class a relative spectral
distribution as a function of photon energy for the scintillating
material. An example of this is shown in
Example 5.7
Example 5.7.
Specification of scintillation properties in
DetectorConstruction
.
const G4int NUMENTRIES = 9; G4double Scnt_PP[NUMENTRIES] = { 6.6*eV, 6.7*eV, 6.8*eV, 6.9*eV, 7.0*eV, 7.1*eV, 7.2*eV, 7.3*eV, 7.4*eV }; G4double Scnt_FAST[NUMENTRIES] = { 0.000134, 0.004432, 0.053991, 0.241971, 0.398942, 0.000134, 0.004432, 0.053991, 0.241971 }; G4double Scnt_SLOW[NUMENTRIES] = { 0.000010, 0.000020, 0.000030, 0.004000, 0.008000, 0.005000, 0.020000, 0.001000, 0.000010 }; G4Material* Scnt; G4MaterialPropertiesTable* Scnt_MPT = new G4MaterialPropertiesTable(); Scnt_MPT->AddProperty("FASTCOMPONENT", Scnt_PP, Scnt_FAST, NUMENTRIES); Scnt_MPT->AddProperty("SLOWCOMPONENT", Scnt_PP, Scnt_SLOW, NUMENTRIES); Scnt_MPT->AddConstProperty("SCINTILLATIONYIELD", 5000./MeV); Scnt_MPT->AddConstProperty("RESOLUTIONSCALE", 2.0); Scnt_MPT->AddConstProperty("FASTTIMECONSTANT", 1.*ns); Scnt_MPT->AddConstProperty("SLOWTIMECONSTANT", 10.*ns); Scnt_MPT->AddConstProperty("YIELDRATIO", 0.8); Scnt->SetMaterialPropertiesTable(Scnt_MPT);
In cases where the scintillation yield of a scintillator depends
on the particle type, different scintillation processes may be
defined for them. How this yield scales to the one specified for
the material is expressed with the
ScintillationYieldFactor
in the user's
PhysicsList
as shown in
Example 5.8.
In those cases where the fast to slow excitation ratio changes with particle
type, the method SetScintillationExcitationRatio
can be
called for each scintillation process (see the advanced
underground_physics example). This overwrites the
YieldRatio
obtained from the
G4MaterialPropertiesTable
.
Example 5.8.
Implementation of the scintillation process in
PhysicsList
.
G4Scintillation* theMuonScintProcess = new G4Scintillation("Scintillation"); theMuonScintProcess->SetTrackSecondariesFirst(true); theMuonScintProcess->SetScintillationYieldFactor(0.8); theParticleIterator->reset(); while( (*theParticleIterator)() ){ G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (theMuonScintProcess->IsApplicable(*particle)) { if (particleName == "mu+") { pmanager->AddProcess(theMuonScintProcess); pmanager->SetProcessOrderingToLast(theMuonScintProcess, idxAtRest); pmanager->SetProcessOrderingToLast(theMuonScintProcess, idxPostStep); } } }
A Gaussian-distributed number of photons is generated according
to the energy lost during the step. A resolution scale of 1.0
produces a statistical fluctuation around the average yield set
with AddConstProperty("SCINTILLATIONYIELD")
, while values
> 1 broaden the fluctuation. A value of zero produces no
fluctuation. Each photon's frequency is sampled from the empirical
spectrum. The photons originate evenly along the track segment and
are emitted uniformly into 4π with a random linear polarization
and at times characteristic for the scintillation component.
Wavelength Shifting (WLS) fibers are used in many high-energy particle physics experiments. They absorb light at one wavelength and re-emit light at a different wavelength and are used for several reasons. For one, they tend to decrease the self-absorption of the detector so that as much light reaches the PMTs as possible. WLS fibers are also used to match the emission spectrum of the detector with the input spectrum of the PMT.
A WLS material is characterized by its photon absorption and
photon emission spectrum and by a possible time delay between the
absorption and re-emission of the photon. Wavelength Shifting may
be simulated by specifying these empirical parameters for each WLS
material in the simulation. It is sufficient to specify in the
user's DetectorConstruction
class a relative spectral
distribution as a function of photon energy for the WLS material.
WLSABSLENGTH is the absorption length of the material as a function
of the photon's momentum. WLSCOMPONENT is the relative emission
spectrum of the material as a function of the photon's momentum,
and WLSTIMECONSTANT accounts for any time delay which may occur
between absorption and re-emission of the photon. An example is
shown in Example 5.9.
Example 5.9.
Specification of WLS properties in DetectorConstruction
.
const G4int nEntries = 9; G4double PhotonEnergy[nEntries] = { 6.6*eV, 6.7*eV, 6.8*eV, 6.9*eV, 7.0*eV, 7.1*eV, 7.2*eV, 7.3*eV, 7.4*eV }; G4double RIndexFiber[nEntries] = { 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60 }; G4double AbsFiber[nEntries] = {0.1*mm,0.2*mm,0.3*mm,0.4*cm,1.0*cm,10*cm,1.0*m,10.0*m,10.0*m}; G4double EmissionFiber[nEntries] = {0.0, 0.0, 0.0, 0.1, 0.5, 1.0, 5.0, 10.0, 10.0 }; G4Material* WLSFiber; G4MaterialPropertiesTable* MPTFiber = new G4MaterialPropertiesTable(); MPTFiber->AddProperty("RINDEX",PhotonEnergy,RIndexFiber,nEntries); MPTFiber->AddProperty("WLSABSLENGTH",PhotonEnergy,AbsFiber,nEntries); MPTFiber->AddProperty("WLSCOMPONENT",PhotonEnergy,EmissionFiber,nEntries); MPTFiber->AddConstProperty("WLSTIMECONSTANT", 0.5*ns); WLSFiber->SetMaterialPropertiesTable(MPTFiber);
The process is defined in the PhysicsList in the usual way. The process class name is G4OpWLS. It should be instantiated with theWLSProcess = new G4OpWLS("OpWLS") and attached to the process manager of the optical photon as a DiscreteProcess. The way the WLSTIMECONSTANT is used depends on the time profile method chosen by the user. If in the PhysicsList theWLSProcess->UseTimeGenerator("exponential") option is set, the time delay between absorption and re-emission of the photon is sampled from an exponential distribution, with the decay term equal to WLSTIMECONSTANT. If, on the other hand, theWLSProcess->UseTimeGenerator("delta") is chosen, the time delay is a delta function and equal to WLSTIMECONSTANT. The default is "delta" in case the G4OpWLS::UseTimeGenerator(const G4String name) method is not used.
The implementation of optical photon bulk absorption,
G4OpAbsorption
, is trivial in that the process merely
kills the particle. The procedure requires the user to fill the
relevant G4MaterialPropertiesTable
with empirical data for
the absorption length, using ABSLENGTH
as the property key
in the public method AddProperty
. The absorption length is
the average distance traveled by a photon before being absorpted by
the medium; i.e. it is the mean free path returned by the
GetMeanFreePath
method.
The differential cross section in Rayleigh scattering,
σ/ω, is proportional
to cos2(θ),
where θ is the polar of the new polarization vector with
respect to the old polarization vector. The G4OpRayleigh
scattering process samples this angle accordingly and then
calculates the scattered photon's new direction by requiring that
it be perpendicular to the photon's new polarization in such a way
that the final direction, initial and final polarizations are all
in one plane. This process thus depends on the particle's
polarization (spin). The photon's polarization is a data member of
the G4DynamicParticle
class.
A photon which is not assigned a polarization at production,
either via the SetPolarization
method of the
G4PrimaryParticle
class, or indirectly with the
SetParticlePolarization
method of the
G4ParticleGun
class, may not be Rayleigh scattered.
Optical photons produced by the G4Cerenkov
process have
inherently a polarization perpendicular to the cone's surface at
production. Scintillation photons have a random linear polarization
perpendicular to their direction.
The process requires a G4MaterialPropertiesTable
to be
filled by the user with Rayleigh scattering length data. The
Rayleigh scattering attenuation length is the average distance
traveled by a photon before it is Rayleigh scattered in the medium
and it is the distance returned by the GetMeanFreePath
method. The G4OpRayleigh
class provides a
RayleighAttenuationLengthGenerator
method which calculates
the attenuation coefficient of a medium following the
Einstein-Smoluchowski formula whose derivation requires the use of
statistical mechanics, includes temperature, and depends on the
isothermal compressibility of the medium. This generator is
convenient when the Rayleigh attenuation length is not known from
measurement but may be calculated from first principles using the
above material constants. For a medium named Water and no
Rayleigh scattering attenutation length specified by the user, the
program automatically calls the
RayleighAttenuationLengthGenerator
which calculates it for 10 degrees Celsius liquid water.
Reference: E. Hecht and A. Zajac, Optics [ Hecht1974 ]
For the simple case of a perfectly smooth interface between two
dielectric materials, all the user needs to provide are the
refractive indices of the two materials stored in their respective
G4MaterialPropertiesTable
. In all other cases, the optical
boundary process design relies on the concept of surfaces.
The information is split into two classes. One class in the
material category keeps information about the physical properties
of the surface itself, and a second class in the geometry category
holds pointers to the relevant physical and logical volumes
involved and has an association to the physical class. Surface
objects of the second type are stored in a related table and can be
retrieved by either specifying the two ordered pairs of physical
volumes touching at the surface, or by the logical volume entirely
surrounded by this surface. The former is called a border
surface while the latter is referred to as the skin
surface. This second type of surface is useful in situations
where a volume is coded with a reflector and is placed into many
different mother volumes. A limitation is that the skin surface can
only have one and the same optical property for all of the enclosed
volume's sides. The border surface is an ordered pair of physical
volumes, so in principle, the user can choose different optical
properties for photons arriving from the reverse side of the same
interface. For the optical boundary process to use a border
surface, the two volumes must have been positioned with
G4PVPlacement
. The ordered combination can exist at many
places in the simulation. When the surface concept is not needed,
and a perfectly smooth surface exists beteen two dielectic
materials, the only relevant property is the index of refraction, a
quantity stored with the material, and no restriction exists on how
the volumes were positioned.
The physical surface object also specifies which model the
boundary process should use to simulate interactions with that
surface. In addition, the physical surface can have a material
property table all its own. The usage of this table allows all
specular constants to be wavelength dependent. In case the surface
is painted or wrapped (but not a cladding), the table may include
the thin layer's index of refraction. This allows the simulation of
boundary effects at the intersection between the medium and the
surface layer, as well as the Lambertian reflection at the far side
of the thin layer. This occurs within the process itself and does
not invoke the G4Navigator
. Combinations of surface finish
properties, such as polished or
ground and front
painted or back painted, enumerate the different
situations which can be simulated.
When a photon arrives at a medium boundary its behavior depends on the nature of the two materials that join at that boundary. Medium boundaries may be formed between two dielectric materials or a dielectric and a metal. In the case of two dielectric materials, the photon can undergo total internal reflection, refraction or reflection, depending on the photon's wavelength, angle of incidence, and the refractive indices on both sides of the boundary. Furthermore, reflection and transmission probabilites are sensitive to the state of linear polarization. In the case of an interface between a dielectric and a metal, the photon can be absorbed by the metal or reflected back into the dielectric. If the photon is absorbed it can be detected according to the photoelectron efficiency of the metal.
As expressed in Maxwell's equations, Fresnel reflection and refraction are intertwined through their relative probabilities of occurrence. Therefore neither of these processes, nor total internal reflection, are viewed as individual processes deserving separate class implementation. Nonetheless, an attempt was made to adhere to the abstraction of having independent processes by splitting the code into different methods where practicable.
One implementation of the G4OpBoundaryProcess
class
employs the
UNIFIED model
[A. Levin and C. Moisan, A More Physical Approach
to Model the Surface Treatment of Scintillation Counters and its
Implementation into DETECT, TRIUMF Preprint TRI-PP-96-64, Oct.
1996] of the DETECT program [G.F. Knoll, T.F. Knoll and T.M.
Henderson, Light Collection Scintillation Detector Composites for
Neutron Detection, IEEE Trans. Nucl. Sci., 35 (1988) 872.]. It
applies to dielectric-dielectric interfaces and tries to provide a
realistic simulation, which deals with all aspects of surface
finish and reflector coating. The surface may be assumed as smooth
and covered with a metallized coating representing a specular
reflector with given reflection coefficient, or painted with a
diffuse reflecting material where Lambertian reflection occurs. The
surfaces may or may not be in optical contact with another
component and most importantly, one may consider a surface to be
made up of micro-facets with normal vectors that follow given
distributions around the nominal normal for the volume at the
impact point. For very rough surfaces, it is possible for the
photon to inversely aim at the same surface again after reflection
of refraction and so multiple interactions with the boundary are
possible within the process itself and without the need for
relocation by G4Navigator
.
The UNIFIED model provides for a range of different reflection mechanisms. The specular lobe constant represents the reflection probability about the normal of a micro facet. The specular spike constant, in turn, illustrates the probability of reflection about the average surface normal. The diffuse lobe constant is for the probability of internal Lambertian reflection, and finally the back-scatter spike constant is for the case of several reflections within a deep groove with the ultimate result of exact back-scattering. The four probabilities must add up to one, with the diffuse lobe constant being implicit. The reader may consult the reference for a thorough description of the model.
Example 5.10. Dielectric-dielectric surface properties defined via the G4OpticalSurface.
G4VPhysicalVolume* volume1; G4VPhysicalVolume* volume2; G4OpticalSurface* OpSurface = new G4OpticalSurface("name"); G4LogicalBorderSurface* Surface = new G4LogicalBorderSurface("name",volume1,volume2,OpSurface); G4double sigma_alpha = 0.1; OpSurface -> SetType(dielectric_dielectric); OpSurface -> SetModel(unified); OpSurface -> SetFinish(groundbackpainted); OpSurface -> SetSigmaAlpha(sigma_alpha); const G4int NUM = 2; G4double pp[NUM] = {2.038*eV, 4.144*eV}; G4double specularlobe[NUM] = {0.3, 0.3}; G4double specularspike[NUM] = {0.2, 0.2}; G4double backscatter[NUM] = {0.1, 0.1}; G4double rindex[NUM] = {1.35, 1.40}; G4double reflectivity[NUM] = {0.3, 0.5}; G4double efficiency[NUM] = {0.8, 0.1}; G4MaterialPropertiesTable* SMPT = new G4MaterialPropertiesTable(); SMPT -> AddProperty("RINDEX",pp,rindex,NUM); SMPT -> AddProperty("SPECULARLOBECONSTANT",pp,specularlobe,NUM); SMPT -> AddProperty("SPECULARSPIKECONSTANT",pp,specularspike,NUM); SMPT -> AddProperty("BACKSCATTERCONSTANT",pp,backscatter,NUM); SMPT -> AddProperty("REFLECTIVITY",pp,reflectivity,NUM); SMPT -> AddProperty("EFFICIENCY",pp,efficiency,NUM); OpSurface -> SetMaterialPropertiesTable(SMPT);
The original GEANT3.21 implementation of this process is also available via the GLISUR methods flag. [GEANT Detector Description and Simulation Tool, Application Software Group, Computing and Networks Division, CERN, PHYS260-6 tp 260-7.].
Example 5.11. Dielectric metal surface properties defined via the G4OpticalSurface.
G4LogicalVolume* volume_log; G4OpticalSurface* OpSurface = new G4OpticalSurface("name"); G4LogicalSkinSurface* Surface = new G4LogicalSkinSurface("name",volume_log,OpSurface); OpSurface -> SetType(dielectric_metal); OpSurface -> SetFinish(ground); OpSurface -> SetModel(glisur); G4double polish = 0.8; G4MaterialPropertiesTable *OpSurfaceProperty = new G4MaterialPropertiesTable(); OpSurfaceProperty -> AddProperty("REFLECTIVITY",pp,reflectivity,NUM); OpSurfaceProperty -> AddProperty("EFFICIENCY",pp,efficiency,NUM); OpSurface -> SetMaterialPropertiesTable(OpSurfaceProperty);
The reflectivity off a metal surface can also be calculated by way of a complex index of refraction. Instead of storing the REFLECTIVITY directly, the user stores the real part (REALRINDEX) and the imaginary part (IMAGINARYRINDEX) as a function of photon energy separately in the G4MaterialPropertyTable. Geant4 then calculates the reflectivity depending on the incident angle, photon energy, degree of TE and TM polarization, and this complex refractive index.
The program defaults to the GLISUR model and polished
surface finish when no specific model and surface finish is
specified by the user. In the case of a dielectric-metal interface,
or when the GLISUR model is specified, the only surface finish
options available are polished or ground. For
dielectric-metal surfaces, the G4OpBoundaryProcess
also
defaults to unit reflectivity and zero detection efficiency. In
cases where the user specifies the UNIFIED model, but does not
otherwise specify the model reflection probability constants, the
default becomes Lambertian reflection.
In this section we describe how to use the parameterization or "fast simulation" facilities of GEANT4. Examples are provided in the examples/novice/N05 directory.
The Geant4 parameterization facilities allow you to shortcut the detailed tracking in a given volume and for given particle types in order for you to provide your own implementation of the physics and of the detector response.
Parameterisations are bound to a
G4Region
object, which, in the case of fast simulation is also called an
envelope. Prior to release 8.0,
parameterisations were bound
to a G4LogicalVolume
, the root of a volume hierarchy.
These root volumes are now attributes of the G4Region
.
Envelopes often correspond to the volumes of sub-detectors:
electromagnetic calorimeters, tracking chambers, etc. With GEANT4
it is also possible to define envelopes by overlaying a parallel or
"ghost" geometry as discussed in Section 5.2.6.7.
In GEANT4, parameterisations have three main features. You must specify:
the particle types for which your parameterisation is valid;
the dynamics conditions for which your parameterisation is valid and must be triggered;
the parameterisation itself: where the primary will be killed or moved, whether or not to create it or create secondaries, etc., and where the detector response will be computed.
GEANT4 will message your parameterisation code for each step
starting in any root G4LogicalVolume (including daughters.
sub-daughters, etc. of this volume) of the G4Region
.
It will proceed by first asking the available parameterisations for
the current particle type if one of them (and only one) wants to
issue a trigger. If so it will invoke its parameterisation. In this
case, the tracking
will not apply physics
to the particle in the step. Instead, the UserSteppingAction will be
invoked.
Parameterisations look like a "user stepping action" but are more advanced because:
parameterisation code is messaged only in the
G4Region
to which it is bound;
parameterisation code is messaged anywhere in the
G4Region
, that is, any volume in which the track is
located;
GEANT4 will provide information to your parameterisation code
about the current root volume of the G4Region
in which the track is travelling.
The GEANT4 components which allow the implementation and control of parameterisations are:
G4VFastSimulationModel
This is the abstract class for the implementation of parameterisations. You must inherit from it to implement your concrete parameterisation model.
G4FastSimulationManager
The G4VFastSimulationModel objects are attached to the
G4Region
through a G4FastSimulationManager.
This object will manage the list of models and will message them at
tracking time.
G4Region/Envelope
As mentioned before, an envelope in GEANT4 is a
G4Region
.
The parameterisation is bound to the G4Region
by
setting a G4FastSimulationManager
pointer to it.
The figure below shows how the G4VFastSimulationModel
and G4FastSimulationManager
objects are bound to the
G4Region
. Then for all root G4LogicalVolume's held by
the G4Region, the fast simulation code is active.
G4FastSimulationManagerProcess
This is a G4VProcess
. It provides the interface
between the tracking and the parameterisation. It must be set in the
process list of the particles you want to parameterise.
G4GlobalFastSimulationManager
This a singleton class which provides the management of the
G4FastSimulationManager
objects and some ghost
facilities.
The G4VFastSimulationModel
class has two constructors.
The second one allows you to get started quickly:
G4VFastSimulationModel(
const G4String& aName)
:
Here aName
identifies the parameterisation model.
G4VFastSimulationModel(const G4String&
aName, G4Region*, G4bool IsUnique=false):
In addition to the model name, this constructor accepts a G4Region pointer. The needed G4FastSimulationManager object is constructed if necessary, passing to it the G4Region pointer and the boolean value. If it already exists, the model is simply added to this manager. Note that the G4VFastSimulationModel object will not keep track of the G4Region passed in the constructor. The boolean argument is there for optimization purposes: if you know that the G4Region has a unique root G4LogicalVolume, uniquely placed, you can set the boolean value to "true".
The G4VFastSimulationModel has three pure virtual methods which must be overriden in your concrete class:
G4VFastSimulationModel(
const G4String& aName):
Here aName identifies the parameterisation model.
G4bool ModelTrigger(
const G4FastTrack&):
You must return "true" when the dynamic conditions to trigger your parameterisation are fulfilled. G4FastTrack provides access to the current G4Track, gives simple access to the current root G4LogicalVolume related features (its G4VSolid, and G4AffineTransform references between the global and the root G4LogicalVolume local coordinates systems) and simple access to the position and momentum expressed in the root G4LogicalVolume coordinate system. Using these quantities and the G4VSolid methods, you can for example easily check how far you are from the root G4LogicalVolume boundary.
G4bool IsApplicable(
const G4ParticleDefinition&):
In your implementation, you must return "true" when your model is applicable to the G4ParticleDefinition passed to this method. The G4ParticleDefinition provides all intrinsic particle information (mass, charge, spin, name ...).
If you want to implement a model which is valid only for certain particle types, it is recommended for efficiency that you use the static pointer of the corresponding particle classes.
As an example, in a model valid for gammas only, the IsApplicable() method should take the form:
#include "G4Gamma.hh" G4bool MyGammaModel::IsApplicable(const G4ParticleDefinition& partDef) { return &partDef == G4Gamma::GammaDefinition(); }
G4bool ModelTrigger(
const G4FastTrack&):
You must return "true" when the dynamic conditions to trigger your parameterisation are fulfilled. The G4FastTrack provides access to the current G4Track, gives simple access to envelope related features (G4LogicalVolume, G4VSolid, and G4AffineTransform references between the global and the envelope local coordinates systems) and simple access to the position and momentum expressed in the envelope coordinate system. Using these quantities and the G4VSolid methods, you can for example easily check how far you are from the envelope boundary.
void DoIt(
const G4FastTrack&, G4FastStep&):
The details of your parameterisation will be implemented in this method. The G4FastTrack reference provides the input information, and the final state of the particles after parameterisation must be returned through the G4FastStep reference. Tracking for the final state particles is requested after your parameterisation has been invoked.
G4FastSimulationManager functionnalities regarding the use of ghost volumes are explained in Section 5.2.6.7.
G4FastSimulationManager(
G4Region *anEnvelope, G4bool IsUnique=false):
This is the only constructor. You specify the G4Region by providing its pointer. The G4FastSimulationManager object will bind itself to this G4Region. If you know that this G4Region has a single root G4LogicalVolume, placed only once, you can set the IsUnique boolean to "true" to allow some optimization.
Note that if you choose to use the G4VFastSimulationModel(const G4String&, G4Region*, G4bool) constructor for your model, the G4FastSimulationManager will be constructed using the given G4Region* and G4bool values of the model constructor.
The following two methods provide the usual management functions.
void AddFastSimulationModel(
G4VFastSimulationModel*)
RemoveFastSimulationModel(
G4VFastSimulationModel*)
This is described in the User's Guide for Toolkit Developers ( section 3.9.6 )
This G4VProcess serves as an interface between the tracking and the parameterisation. At tracking time, it collaborates with the G4FastSimulationManager of the current volume, if any, to allow the models to trigger. If no manager exists or if no model issues a trigger, the tracking goes on normally.
In the present implementation, you must set this process in the G4ProcessManager of the particles you parameterise to enable your parameterisation.
The processes ordering is:
[n-3] ... [n-2] Multiple Scattering [n-1] G4FastSimulationManagerProcess [ n ] G4Transportation
This ordering is important if you use ghost geometries, since the G4FastSimulationManagerProcess will provide navigation in the ghost world to limit the step on ghost boundaries.
The G4FastSimulationManager must be added to the process list of a particle as a continuous and discrete process if you use ghost geometries for this particle. You can add it as a discrete process if you don't use ghosts.
The following code registers the G4FastSimulationManagerProcess with all the particles as a discrete and continuous process:
void MyPhysicsList::addParameterisation() { G4FastSimulationManagerProcess* theFastSimulationManagerProcess = new G4FastSimulationManagerProcess(); theParticleIterator->reset(); while( (*theParticleIterator)() ) { G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); pmanager->AddProcess(theFastSimulationManagerProcess, -1, 0, 0); } }
This class is a singleton which can be accessed as follows:
#include "G4GlobalFastSimulationManager.hh" ... ... G4GlobalFastSimulationManager* globalFSM; globalFSM = G4GlobalFastSimulationManager::getGlobalFastSimulationManager(); ... ...
Presently, you will mainly need to use the GlobalFastSimulationManager if you use ghost geometries.
In some cases, volumes of the tracking geometry do not allow envelopes to be defined. This may be the case with a geometry coming from a CAD system. Since such a geometry is flat, a parallel geometry must be used to define the envelopes.
Another interesting case involves defining an envelope which groups the electromagnetic and hadronic calorimeters of a detector into one volume. This may be useful when parameterizing the interaction of charged pions. You will very likely not want electrons to see this envelope, which means that ghost geometries have to be organized by particle flavours.
Using ghost geometries implies some more overhead in the parameterisation mechanism for the particles sensitive to ghosts, since navigation is provided in the ghost geometry by the G4FastSimulationManagerProcess. Usually, however, only a few volumes will be placed in this ghost world, so that the geometry computations will remain rather cheap.
In the existing implementation (temporary implementation with G4Region but before parallel geometry implementation), you may only consider ghost G4Regions with just one root G4LogicalVolume. The G4GlobalFastSimulationManager provides the construction of the ghost geometry by making first an empty "clone" of the world for tracking provided by the construct() method of your G4VUserDetectorConstruction concrete class. You provide the placement of the G4Region root G4LogicalVolume relative to the ghost world coordinates in the G4FastSimulationManager objects. A ghost G4Region is recognized by the fact that its associated G4FastSimulationManager retains a non-empty list of placements.
The G4GlobalFastSimulationManager will then use both those placements and the IsApplicable() methods of the models attached to the G4FastSimulationManager objects to build the flavour-dependant ghost geometries.
Then at the beginning of the tracking of a particle, the appropriate ghost world, if any, will be selected.
The steps required to build one ghost G4Region are:
built the ghost G4Region : myGhostRegion;
build the root G4LogicalVolume: myGhostLogical, set it to myGhostRegion;
build a G4FastSimulationManager object, myGhostFSManager, giving myGhostRegion as argument of the constructor;
give to the G4FastSimulationManager the placement of the myGhostLogical, by invoking for the G4FastSimulationManager method:
AddGhostPlacement(G4RotationMatrix*, const G4ThreeVector&);
or:
AddGhostPlacement(G4Transform3D*);
where the rotation matrix and translation vector of the 3-D transformation describe the placement relative to the ghost world coordinates.
build your G4VFastSimulationModel objects and add them to the myGhostFSManager. The IsApplicable() methods of your models will be used by the G4GlobalFastSimulationManager to build the ghost geometries corresponding to a given particle type.
Invoke the G4GlobalFastSimulationManager method:
G4GlobalFastSimulationManager::getGlobalFastSimulationManager()-> CloseFastSimulation();
This last call will cause the G4GlobalFastSimulationManager to build the flavour-dependent ghost geometries. This call must be done before the RunManager closes the geometry. (It is foreseen that the run manager in the future will invoke the CloseFastSimulation() to synchronize properly with the closing of the geometry).
Visualization facilities are provided for ghosts geometries. After the CloseFastSimulation() invocation, it is possible to ask for the drawing of ghosts in an interactive session. The basic commands are:
/vis/draw/Ghosts particle_name
which makes the drawing of the ghost geometry associated with the particle specified by name in the command line.
/vis/draw/Ghosts
which draws all the ghost geometries.
This section describes how to use the Gflash library. Gflash is a concrete parameterization which is based on the equations and parameters of the original Gflash package from H1(hep-ex/0001020, Grindhammer & Peters, see physics manual) and uses the "fast simulation" facilities of GEANT4 described above. Briefly, whenever a e-/e+ particle enters the calorimeter, it is parameterized if it has a minimum energy and the shower is expected to be contained in the calorimeter (or " parameterization envelope"). If this is fulfilled the particle is killed, as well as all secondaries, and the energy is deposited according to the Gflash equations. An example, provided in examples/extended/parametrisation/gflash/, shows how to interface Gflash to your application. The simulation time is measured, so the user can immediately see the speed increase resulting from the use of Gflash.
To use Gflash "out of the box" the following steps are necessary:
The user must add the fast simulation process to his process manager:
void MyPhysicsList::addParameterisation() { G4FastSimulationManagerProcess* theFastSimulationManagerProcess = new G4FastSimulationManagerProcess(); theParticleIterator->reset(); while( (*theParticleIterator)() ) { G4ParticleDefinition* particle = theParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); pmanager->AddProcess(theFastSimulationManagerProcess, -1, 0, 0); } }
The envelope in which the parameterization should be performed must be specified (below: G4Region m_calo_region) and the GFlashShowerModel must be assigned to this region. Furthermore, the classes GFlashParticleBounds (which provides thresholds for the parameterization like minimal energy etc.), GflashHitMaker(a helper class to generate hits in the sensitive detector) and GFlashHomoShowerParamterisation (which does the computations) must be constructed (by the user at the moment) and assigned to the GFlashShowerModel. Please note that at the moment only homogeneous calorimeters are supported.
m_theFastShowerModel = new GFlashShowerModel("fastShowerModel",m_calo_region); m_theParametrisation = new GFlashHomoShowerParamterisation(matManager->getMaterial(mat)); m_theParticleBounds = new GFlashParticleBounds(); m_theHMaker = new GFlashHitMaker(); m_theFastShowerModel->SetParametrisation(*m_theParametrisation); m_theFastShowerModel->SetParticleBounds(*m_theParticleBounds) ; m_theFastShowerModel->SetHitMaker(*m_theHMaker);
The user must also set the material of the calorimeter, since the computation depends on the material.
It is mandatory to use G4VGFlashSensitiveDetector as (additional) base class for the sensitive detector.
class ExGflashSensitiveDetector: public G4VSensitiveDetector ,public G4VGFlashSensitiveDetector
Here it is necessary to implement a separate interface, where the GFlash spots are processed.
(ProcessHits(G4GFlashSpot*aSpot ,G4TouchableHistory* ROhist))
A separate interface is used, because the Gflash spots naturally contain less information than the full simulation.
Since the parameters in the Gflash package are taken from fits to full simulations with Geant3, some retuning might be necessary for good agreement with Geant4 showers. For experiment-specific geometries some retuning might be necessary anyway. The tuning is quite complicated since there are many parameters (some correlated) and cannot be described here (see again hep-ex/0001020). For brave users the Gflash framework already forsees the possibility of passing a class with the (users) parameters,GVFlashHomoShowerTuning, to the GFlashHomoShowerParamterisation constructor. The default parameters are the original Gflash parameters:
GFlashHomoShowerParameterisation(G4Material * aMat, GVFlashHomoShowerTuning * aPar = 0);
Now there is also a preliminary implemenation of a parameterization for sampling calorimeters.
The user must specify the active and passive material, as well as the thickness of the active and passive layer.
The sampling structure of the calorimeter is taken into account by using an "effective medium" to compute the shower shape.
All material properties needed are calculated automatically. If tuning is required, the user can pass his own parameter set in the class GFlashSamplingShowerTuning. Here the user can also set his calorimeter resolution.
All in all the constructor looks the following:
GFlashSamplingShowerParamterisation(G4Material * Mat1, G4Material * Mat2,G4double d1,G4double d2, GVFlashSamplingShowerTuning * aPar = 0);
An implementation of some tools that should help the user to tune the parameterization is forseen.
To be delivered by J. Apostolakis (<John.Apostolakis@cern.ch>
).