Physics Processes
Physics processes describe how particles interact with a
material. Seven major categories of processes are provided by
Geant4:
electromagnetic
,
hadronic
,
decay
,
photolepton-hadron
,
optical
,
parameterization
and
transportation
.
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
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.
Other base classes for processes
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:
G4VRestProcess
Processes using only the AtRestDoIt method.
example: neutron capture
G4VDiscreteProcess
Processes using only the PostStepDoIt method.
example: compton scattering, hadron inelastic interaction
The other four classes are provided for rather complex
processes:
G4VContinuousDiscreteProcess
Processes using both AlongStepDoIt and
PostStepDoIt methods.
example: transportation, ionisation(energy loss and delta ray)
G4VRestDiscreteProcess
Processes using both AtRestDoIt and
PostStepDoIt methods.
example: positron annihilation, decay (both in flight and at rest)
G4VRestContinuousProcess
Processes using both AtRestDoIt and
AlongStepDoIt methods.
G4VRestContinuousDiscreteProcess
Processes using AtRestDoIt,
AlongStepDoIt and PostStepDoIt methods.
Particle change
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.
Electromagnetic Interactions
This section summarizes the electromagnetic (EM) physics processes which
are provided with Geant4. Extended information are avalable at EM web
pages.
For details on the implementation of these
processes please refer to the
Physics Reference Manual.
"Standard" Electromagnetic Processes
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)
Multiple scattering (class name G4eMultipleScattering)
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
Bremsstrahlung (class name G4MuBremsstrahlung)
Ionisation and delta ray production (class name
G4MuIonisation)
Multiple scattering (class name G4MuMultipleScattering)
e+e- pair production (class name
G4MuPairProduction)
Hadron/ion processes
Bremsstrahlung (class name G4hBremsstrahlung)
Ionisation (class name G4hIonisation)
e+e- pair production (class name G4hPairProduction)
Ionisation for ions (class name G4ionIonisation)
Multiple scattering (class name G4hMultipleScattering)
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)
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)
It is recommended to use physics constructor classes provided
with rederence physics lists ($G4INSTALL/source/physics_lists/builders):
default EM physics (class name G4EmStandardPhysics)
optional EM physics providing similar performance as g4 7.1p01
(class name G4EmStandardPhysics_option1)
Experimental EM physics with enabled "ApplyCuts" option
(class name G4EmStandardPhysics_option2)
EM physics for simulation with high accuracy
(class name G4EmStandardPhysics_option3)
Examples of the registration of these physics constructor and
construction of alternative combinations of options are shown
in novice and extended examples ($G4INSTALL/examples/extended/electromagnetic).
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)
SetSpline(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.
Low Energy Electromagnetic Processes
A physical interaction is described by a process class which can handle
physics models, described by model classes. The following is a summary
of the Low Energy Electromagnetic physics models available in Geant4.
Further information is available in the web pages of the
Geant4 Low Energy Electromagnetic Physics Working Group, accessible
from the Geant4 web site, “who we are” section, then “working groups”.
The physics content of these models is documented in the Geant4
Physics Reference Manual.
They are based on the Livermore data library, on the
ICRU73 data tables or on the Penelope Monte Carlo code. They adopt the
same software design as the "standard" Geant4 electromagnetic models.
Examples of the registration of physics constructor with low-energy
electromagnetic models are shown in Geant4 extended examples
($G4INSTALL/examples/extended/electromagnetic).
Advanced examples ($G4INSTALL/examples/advanced)
illustrate alternative instantiation of these processes.
Both are available as part of the Geant4 release.
To use the low energy electromagnetic models, data files need to be
copied by the user to his/her code repository. These files are
distributed together with Geant4. The user should set the environment
variable G4LEDATA to the directory where he/she has copied the files.
Livermore based models
Photon models
Photo-electric effect (class G4LivermorePhotoElectricModel)
Polarized Photo-electric effect (class G4LivermorePolarizedPhotoElectricModel)
Compton scattering (class G4LivermoreComptonModel)
Polarized Compton scattering (class G4LivermorePolarizedComptonModel)
Rayleigh scattering (class G4LivermoreRayleighModel)
Polarized Rayleigh scattering (class G4LivermorePolarizedRayleighModel)
Gamma conversion (also called pair production, class G4LivermoreGammaConversionModel)
Polarized gamma conversion (class G4LivermorePolarizedGammaConversionModel)
Electron models
Bremsstrahlung (class G4LivermoreBremsstrahlungModel)
Ionisation and delta ray production (class G4LivermoreIonisationModel)
Options can be set in the G4LivermorePhotoElectricModel 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.
G4PhotoElectricAngularGeneratorSauterGavrilla is selected by default.
G4PhotoElectricAngularGeneratorSimple, G4PhotoElectricAngularGeneratorSauterGavrilla
and G4PhotoElectricAngularGeneratorPolarized can be set using respectively the
strings "default", "standard" and "polarized".
Options are available in the G4LivermoreBremsstrahlungModel 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 .
Other options G4LivermoreBremsstrahlungModel class are:
SetCutForLowEnSecPhotons(G4double)
Options are available in the G4LivermoreIonisationModel class:
ActivateAuger(G4bool)
SetCutForLowEnSecPhotons(G4double)
SetCutForLowEnSecElectrons(G4double)
ICRU73 based ion model
Ionisation and delta ray production (class G4IonParametrisedLossModel)
The ion model uses ICRU 73 stopping powers, if corresponding ion-material
combinations are covered by the ICRU 73 report (up to 1 GeV/nucleon), and
otherwise applies a Bethe-Bloch based formalism. For compounds, ICRU 73
stopping powers are employed if the material name coincides with the name
of Geant4 NIST materials (e.g. G4_WATER). Elemental materials are matched
to the corresponding ICRU 73 stopping powers by means of the atomic number
of the material. The material name may be arbitrary in this case. For a
list of applicable materials, the user is referred to the ICRU 73 report.
The model requires data files 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.
The model is dedicated to be used with the G4ionIonisation process and its
applicability is restricted to G4GenericIon particles. The ion model is
not used by default by this process and must be instantiated and registered
by the user:
G4ionIonisation* ionIoni = new G4ionIonisation();
ionIoni -> SetEmModel(new G4IonParametrisedLossModel());
Penelope based models
Photon models
Compton scattering (class G4PenelopeComptonModel)
Rayleigh scattering (class G4PenelopeRayleighModel)
Gamma conversion (also called pair production, class GPenelopeGammaConversionModel)
Photo-electric effect (class G4PenelopePhotoElectricModel)
Electron models
Bremsstrahlung (class G4PenelopeBremsstrahlungModel)
Ionisation and delta ray production (class G4PenelopeIonisationModel)
Positron models
Bremsstrahlung (class G4PenelopeBremsstrahlungModel)
Ionisation and delta ray production (class G4PenelopeIonisationModel)
Positron annihilation (class class G4PenelopeAnnihilationModel)
All Penelope models can be applied up to a maximum energy of 100 GeV,
although it is advisable not to use them above a few hundreds of MeV.
Options are available in the all Penelope Models, allowing to set
(and retrieve) the verbosity level of the model, namely the amount of
information which is printed on the screen.
SetVerbosityLevel(G4int)
GetVerbosityLevel()
The default verbosity level is 0 (namely, no textual output on the screen).
The default value should be used in general for normal runs. Higher
verbosity levels are suggested only for testing and debugging purposes.
The verbosity scale defined for all Penelope processes is the following:
0 = no printout on the screen (default)
1 = issue warnings only in the case of energy non-conservation in the final state (should never happen)
2 = reports full details on the energy budget in the final state
3 = writes also informations on cross section calculation, data file opening and sampling of atoms
4 = issues messages when entering in methods
Options are available in G4PenelopeComptonModel, G4PenelopePhotoElectricModel
and G4PenelopeIonisationModel to enable or disable the usage of atomic
de-excitation via the G4AtomicDeexcitation module.
SetDeexcitationFlag(G4bool)
DeexcitationFlag()
The default is “true”, namely vacancies in atomic shells produced by the
interaction are handled by the G4AtomicDeexcitation module, possibly with
the subsequent emission of fluorescence x-rays. If is set to “false”
by the user, the energy released in the re-arrangement of atomic vacancies
is treated in the model as a local energy deposit, without emission of
secondary particles. The methods are actually inherited from G4VEmModel,
so they work for all Penelope models; by the way, they have effect only
in G4PenelopeComptonModel, G4PenelopePhotoElectricModel and
G4PenelopeIonisationModel.
An option is also available in these models to enable the production of
Auger electrons by the G4AtomicDeexcitation module ActivateAuger(G4bool).
The default (coming from G4AtomicDeexcitation) is “false”, namely only
fluorescence x-rays are emitted but not Auger electrons. One should
notice that this option has effect only if the usage of the atomic
deexcitation is enabled. A warning message is printed if one tries to
enable the emission of the Auger electrons after having disabled the
atomic deexcitation via SetDeexcitationFlag(false).
Very Low energy Electromagnetic Processes (Geant4-DNA extension)
The Geant4 low energy electromagnetic Physics package has been extended
down to energies of a few electronVolts suitable for the simulation of
radiation effects in liquid water for applications in microdosimetry at
the cellular and sub-cellular level. These developments take place in
the framework of the on-going Geant4-DNA project (see the web pages
of the
Geant4 Low Energy Electromagnetic Physics Working Group).
The Geant4-DNA process and model classes apply to electrons, protons,
hydrogen, alpha particles and their charge states.
Electron processes and models
Elastic scattering :
process class is G4DNAElastic
two alternative model classes are : G4DNAScreenedRutherfordElasticModel
or G4DNAChampionElasticModel
Excitation
process class is G4DNAExcitation
model class is G4DNAEmfietzoglouExcitationModel
Ionisation
process class is G4DNAIonisation
model class is G4DNABornIonisationModel
Proton processes and models
Excitation
process class is G4DNAExcitation
two complementary model classes are G4DNAMillerGreenExcitationModel
(below 500 keV) and G4DNABornExcitationModel (above)
Ionisation
process class is G4DNAIonisation
two complementary model classes are G4DNARuddIonisationModel
(below 500 keV) and G4DNABornIonisationModel (above)
Charge decrease
process class is G4DNAChargeDecrease
model class is G4DNADingfelderChargeDecreaseModel
Hydrogen processes and models
Ionisation
process class is G4DNAIonisation
model class is G4DNARuddIonisationModel
Charge increase
process class is G4DNAChargeIncrease
model class is G4DNADingfelderChargeIncreaseModel
Helium (neutral) processes and models
Excitation
process class is G4DNAExcitation
model class is G4DNAMillerGreenExcitationModel
Ionisation
process class is G4DNAIonisation
model class is G4DNARuddIonisationModel
Charge increase
process class is G4DNAChargeIncrease
model class is G4DNADingfelderChargeIncreaseModel
Helium+ (ionized once) processes and models
Excitation
process class is G4DNAExcitation
model class is G4DNAMillerGreenExcitationModel
Ionisation
process class is G4DNAIonisation
model classes is G4DNARuddIonisationModel
Charge increase
process class is G4DNAChargeIncrease
model classes is G4DNADingfelderChargeIncreaseModel
Charge decrease
process class is G4DNAChargeDecrease
model classes is G4DNADingfelderChargeDecreaseModel
Helium++ (ionised twice) processes and models
Excitation
process class is G4DNAExcitation
model classes is G4DNAMillerGreenExcitationModel
Ionisation
process class is G4DNAIonisation
model classes is G4DNARuddIonisationModel
Charge decrease
process class is G4DNAChargeDecrease
model classes is G4DNADingfelderChargeDecreaseModel
An example of the registration of these processes in a physics list is
given here below and may be found in the microdosimetry advanced example.
#include "G4DNAElastic.hh"
#include "G4DNAChampionElasticModel.hh"
#include "G4DNAScreenedRutherfordElasticModel.hh"
#include "G4DNAExcitation.hh"
#include "G4DNAEmfietzoglouExcitationModel.hh"
#include "G4DNAMillerGreenExcitationModel.hh"
#include "G4DNABornExcitationModel.hh"
#include "G4DNAIonisation.hh"
#include "G4DNABornIonisationModel.hh"
#include "G4DNARuddIonisationModel.hh"
#include "G4DNAChargeDecrease.hh"
#include "G4DNADingfelderChargeDecreaseModel.hh"
#include "G4DNAChargeIncrease.hh"
#include "G4DNADingfelderChargeIncreaseModel.hh"
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
void PhysicsList::ConstructEM()
{
theParticleIterator->reset();
while( (*theParticleIterator)() )
{
G4ParticleDefinition* particle = theParticleIterator->value();
G4ProcessManager* pmanager = particle->GetProcessManager();
G4String particleName = particle->GetParticleName();
// DNA processes per particle type
if (particleName == "e-") {
G4DNAElastic* theDNAElasticProcess = new G4DNAElastic("e-_G4DNAElastic");
theDNAElasticProcess->SetModel(new G4DNAChampionElasticModel());
// or alternative model
// theDNAElasticProcess->SetModel(new G4DNAScreenedRutherfordElasticModel());
pmanager->AddDiscreteProcess(theDNAElasticProcess);
pmanager->AddDiscreteProcess(new G4DNAExcitation("e-_G4DNAExcitation"));
pmanager->AddDiscreteProcess(new G4DNAIonisation("e-_G4DNAIonisation"));
} else if ( particleName == "proton" ) {
pmanager->AddDiscreteProcess(new G4DNAExcitation("proton_G4DNAExcitation"));
pmanager->AddDiscreteProcess(new G4DNAIonisation("proton_G4DNAIonisation"));
pmanager->AddDiscreteProcess(new G4DNAChargeDecrease("proton_G4DNAChargeDecrease"));
} else if ( particleName == "hydrogen" ) {
pmanager->AddDiscreteProcess(new G4DNAIonisation("hydrogen_G4DNAIonisation"));
pmanager->AddDiscreteProcess(new G4DNAChargeIncrease("hydrogen_G4DNAChargeIncrease"));
} else if ( particleName == "alpha" ) {
pmanager->AddDiscreteProcess(new G4DNAExcitation("alpha_G4DNAExcitation"));
pmanager->AddDiscreteProcess(new G4DNAIonisation("alpha_G4DNAIonisation"));
pmanager->AddDiscreteProcess(new G4DNAChargeDecrease("alpha_G4DNAChargeDecrease"));
} else if ( particleName == "alpha+" ) {
pmanager->AddDiscreteProcess(new G4DNAExcitation("alpha+_G4DNAExcitation"));
pmanager->AddDiscreteProcess(new G4DNAIonisation("alpha+_G4DNAIonisation"));
pmanager->AddDiscreteProcess(new G4DNAChargeDecrease("alpha+_G4DNAChargeDecrease"));
pmanager->AddDiscreteProcess(new G4DNAChargeIncrease("alpha+_G4DNAChargeIncrease"));
} else if ( particleName == "helium" ) {
pmanager->AddDiscreteProcess(new G4DNAExcitation("helium_G4DNAExcitation"));
pmanager->AddDiscreteProcess(new G4DNAIonisation("helium_G4DNAIonisation"));
pmanager->AddDiscreteProcess(new G4DNAChargeIncrease("helium_G4DNAChargeIncrease"));
}
} // Loop on particles
}
//....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 PhysicsList::ConstructBaryons()
{
// construct baryons ---
// Geant4 DNA particles
G4GenericIon::GenericIonDefinition() ;
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.
Hadronic Interactions
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.
Treatment of Cross Sections
Cross section data sets
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 store
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.
Default cross sections
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.
Cross-sections for low energy neutron transport
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.
Hadrons at Rest
List of implemented "Hadron at Rest" processes
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.
Implementation Interface to Geant4
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.
Example of how to use a hadron at rest process
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 );
Hadrons in Flight
What processes do you need?
For hadrons in motion, there are four physics process classes.
shows each process and the
particles for which it is relevant.
How to register Models
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 , where
G4LEProtonInelastic.hh is the name of the include
file:
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 )
Which models are there, and what are the defaults
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.
Particle Decay Process
This section briefly introduces decay processes installed in
Geant4. For details of the implementation of particle decays,
please refer to the
Physics Reference Manual.
Particle Decay Class
Geant4 provides a G4Decay class for both ``at rest'' and
``in flight'' particle decays. G4Decay can be applied to all
particles except:
massless particles, i.e.,
G4ParticleDefinition::thePDGMass <= 0
particles with ``negative'' life time, i.e.,
G4ParticleDefinition::thePDGLifeTime < 0
shortlived particles, i.e.,
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 )
is shown in .
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);
}
}
}
Decay Table
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
.
Branching ratios and life time can be set in tracking time.
// set lifetime
G4Neutron::Neutron()->SetPDGLifeTime(885.7*second);
// allow neutron decay
G4Neutron::Neutron()->SetPDGStable(false);
Branching ratios and life time can be modified by using user commands, also.
Example: Set 100% br for dalitz decay of pi0
Idle> /particle/select pi0
Idle> /particle/property/decay/select 0
Idle> /particle/property/decay/br 0
Idle> /particle/property/decay/select 1
Idle> /particle/property/decay/br 1
Idle> /particle/property/decay/dump
G4DecayTable: pi0
0: BR: 0 [Phase Space] : gamma gamma
1: BR: 1 [Dalitz Decay] : gamma e- e+
Pre-assigned Decay Modes by Event Generators
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.
Photolepton-hadron Processes
To be delivered.
Optical Photon Processes
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 G4MaterialPropertyVectors 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 .
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);
Generation of Photons in
processes/electromagnetic/xrays - Cerenkov Effect
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
.
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);
}
}
}
Generation of Photons in
processes/electromagnetic/xrays - Scintillation
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
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
.
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.
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.
Generation of Photons in
processes/optical - Wavelength Shifting
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 .
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.
Tracking of Photons in processes/optical
Absorption
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.
Rayleigh Scattering
The differential cross section in Rayleigh scattering,
σ/ω, is proportional
to cos2(&thetas;),
where &thetas; 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.
Boundary Process
Reference: E. Hecht and A. Zajac, Optics
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.
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.].
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.
Martin Janecek and Bill Moses (Lawrence Berkeley National Laboratory)
built an instrument for measuring the angular reflectivity distribution
inside of BGO crystals with common surface treatments and reflectors
applied. These results have been incorporate into the Geant4 code. A
third class of reflection type besides dielectric_metal and
dielectric_dielectric is added: dielectric_LUT. The distributions have
been converted to 21 look-up-tables (LUT); so far for 1 scintillator
material (BGO) x 3 surface treatments x 7 reflector materials. The
modified code allows the user to specify the surface treatment
(rough-cut, chemically etched, or mechanically polished), the attached
reflector (Lumirror, Teflon, ESR film, Tyvek, or TiO2 paint), and the
bonding type (air-coupled or glued). The glue used is MeltMount, and the
ESR film used is VM2000. Each LUT consists of measured angular
distributions with 4º by 5º resolution in theta and phi, respectively,
for incidence angles from 0º to 90º degrees, in 1º-steps. The code might
in the future be updated by adding more LUTs, for instance, for other
scintillating materials (such as LSO or NaI). To use these LUT the user
has to download them from
Geant4 Software Download and set an environment variable,
G4REALSURFACEDATA, to the directory of
geant4/data/RealSurface1.0. For details see:
M. Janecek, W. Moses IEEE Transactions on Nuclear Science
.
The enumeration G4OpticalSurfaceFinish has been extended to include
(what follows should be a 2 column table):
polishedlumirrorair, // mechanically polished surface, with lumirror
polishedlumirrorglue, // mechanically polished surface, with lumirror & meltmount
polishedair, // mechanically polished surface
polishedteflonair, // mechanically polished surface, with teflon
polishedtioair, // mechanically polished surface, with tio paint
polishedtyvekair, // mechanically polished surface, with tyvek
polishedvm2000air, // mechanically polished surface, with esr film
polishedvm2000glue, // mechanically polished surface, with esr film & meltmount
etchedlumirrorair, // chemically etched surface, with lumirror
etchedlumirrorglue, // chemically etched surface, with lumirror & meltmount
etchedair, // chemically etched surface
etchedteflonair, // chemically etched surface, with teflon
etchedtioair, // chemically etched surface, with tio paint
etchedtyvekair, // chemically etched surface, with tyvek
etchedvm2000air, // chemically etched surface, with esr film
etchedvm2000glue, // chemically etched surface, with esr film & meltmount
groundlumirrorair, // rough-cut surface, with lumirror
groundlumirrorglue, // rough-cut surface, with lumirror & meltmount
groundair, // rough-cut surface
groundteflonair, // rough-cut surface, with teflon
groundtioair, // rough-cut surface, with tio paint
groundtyvekair, // rough-cut surface, with tyvek
groundvm2000air, // rough-cut surface, with esr film
groundvm2000glue // rough-cut surface, with esr film & meltmount
To use a look-up-table, all the user needs to specify for an
G4OpticalSurface is:
SetType(dielectric_LUT), SetModel(LUT) and for example,
SetFinish(polishedtyvekair).
Parameterization
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.
Generalities:
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 .
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.
Overview of Parameterisation Components
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 Abstract Class
Constructors:
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".
Virtual methods:
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.
The G4FastSimulationManager Class:
G4FastSimulationManager functionnalities regarding the use of ghost
volumes are explained in .
Constructor:
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.
G4VFastSimulationModel object management:
The following two methods provide the usual management
functions.
void AddFastSimulationModel(
G4VFastSimulationModel*)RemoveFastSimulationModel(
G4VFastSimulationModel*)
Interface with the G4FastSimulationManagerProcess:
This is described in the User's Guide for Toolkit Developers
(
section 3.9.6
)
The G4FastSimulationManagerProcess Class
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);
}
}
The G4GlobalFastSimulationManager Singleton Class
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.
Parameterisation Using 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.
Gflash Parameterization
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.
Using the Gflash Parameterisation
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.
Transportation Process
To be delivered by J. Apostolakis (John.Apostolakis@cern.ch).