The following is a summary of the electromagnetic processes available in Geant4.

It is recommended to use physics constructor classes provided with rederence physics lists ($G4INSTALL/source/physics_lists/builders):

Examples of the registration of these physics constructor and construction of alternative combinations of options are shown in basic, extended and advanced examples ($G4INSTALL/examples/basic, $G4INSTALL/examples/extended/electromagnetic and $G4INSTALL/examples/advanced). Examples illustrating the use of electromagnetic processes are available as part of the Geant4 release.

Options are available for steering of electromagnetic processes. These options may be invoked either by UI commands or by the interface class G4EmProcessOptions. This class has the following public methods:

The corresponding UI command can be accessed in the UI subdirectories "/process/eLoss", "/process/em", and "/process/msc". The following types of step limitation by multiple scattering are available:

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:

For these interfaces, particles, materials, or processes may be pointers or strings with names.

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 Penelope2008 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.

Remember that production cuts for secondaries can be specified as range cuts, which are converted at initialisation time into energy thresholds for secondary gamma, electron, positron and proton production.

A range cut value is set by default to 0.7 mm in Geant4 reference physics lists. This value can be specified in the optional SetCuts() method of the user Physics list or via UI commands. For eg. to set a range cut of 10 micrometers, one can use:

    /run/setCut  0.01 mm
  

or, for a given particle type (for eg. electron),

    /run/setCutForAGivenParticle e- 0.01 mm   
  

If a range cut equivalent to an energy lower than 990 eV is specified, the energy cut is still set to 990 eV. In order to decrease this value (for eg. down to 250 eV, in order to simulate low energy emission lines of the fluorescence spectrum), one may use the following UI command before the "/run/initialize" command:

   /cuts/setLowEdge 250 eV
 

or alternatively directly in the user Physics list, in the optional SetCuts() method, using:

   G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(250*eV, 1*GeV);   
 

For part of EM processes it is possible to factorise out sampling of secondary energy and direction. Using an interface G4VEmModel base class SetAngularDistribution(G4VEmAngularDistribution*) it is possible to substitute default angular generator of a model. Angular generators in standard and lowenergy sub-packages follow the same abstract interface.

For photoelectric models several angular generators are available:

For bremsstrahlung models following angular generators are available:

It may be useful to create more than one secondary at an interaction. For example, electrons incident on a target in a medical linac produce photons through bremsstrahlung. The variance reduction technique of bremsstrahlung splitting involves choosing N photons from the expected distribution, and assigning each a weight of 1/N.

Similarly, if the secondaries are not important, one can kill them with a survival probability of 1/N. The weight of the survivors is increased by a factor N. This is known as Russian roulette.

Neither biasing technique is applied if the resulting daughter particles would have a weight below 1/N, in the case of brem splitting, or above 1, in the case of Russian roulette.

These techniques can be enabled in Geant4 electromagnetics with the macro commands

/process/em/setSecBiasing processName Region factor energyLimit energyUnit

where: processName is the name of the process to apply the biasing to; Region is the region in which to apply biasing; factor is the inverse of the brem splitting or Russian roulette factor (1/N); energyLimit energyUnit is the high energy limit. If the first secondary has energy above this limit, no biasing is applied.

For example,

/process/em/setSecBiasing eBrem target 10 100 MeV

will result in electrons undergoing bremsstrahlung in the target region being split 10 times (if the first photon sampled has an energy less than 100 MeV).

Note that the biasing needs to be specified for each process individually. To apply Russian Roulette to daughter electrons from interactions of photons, issue the macro command for the processes phot, compt, conv.

Reference: BEAMnrc Users Manual, D.W.O Rogers, B. Walters, I. Kawrakow. NRCC Report PIRS-0509(A)revL, available at http://www.irs.inms.nrc.ca/inms/irs/BEAM/beamhome.html

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());

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.

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:

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 more in the Geant4-DNA web pages or in 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

Proton processes and models

Hydrogen processes and models

Helium (neutral) processes and models

Helium+ (ionized once) processes and models

Helium++ (ionised twice) processes and models

C, N, O, Fe processes and models

An example of the registration of these processes in a physics list is given in the G4EmDNAPhysics constructor (in $G4INSTALL/source/physics_lists/builders). An example of the usage of this constructor in a physics list is given in the "dnaphysics" advanced example, which explains how to extract basic information from Geant4-DNA Physics processes.

The "microdosimetry" advanced example illustrates how to combine Geant4-DNA processes with Standard electromagnetic processes (combination of discrete and condensed history Geant4 electromagnetic processes at different scales).

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.

A full list of publications regarding Geant4-DNA is directly available from the Geant4-DNA website or from the Geant4@IN2P3 web site).

A unique interface named G4VAtomicDeexcitation is available in Geant4 for the simulation of atomic deexcitation using Standard, Low Energy and Very Low Energy electromagnetic processes. Atomic deexcitation includes fluorescence and Auger electron emission induced by photons, electrons and ions (PIXE); see more details in:

It can be activated for processes producing vacancies in atomic shells. Currently these processes are the photoelectric effect, ionization and Compton scattering.

Activation of atomic deexcitation

The activation of atomic deexcitation in continuous processes in a user physics list can be done through the following G4EmProcessOptions class methods, respectively to activate deexcitation processes, Auger effect and deexcitation from ions (PIXE):

   G4EmProcessOptions::SetFluo(G4bool);
   G4EmProcessOptions::SetAuger(G4bool);
   G4EmProcessOptions::SetPIXE(G4bool);
 

Please note that in order to have Auger emission it is mandatory to activate Auger electron production for the region in which it is needed (World region included):

G4EmProcessOptions::SetDeexcitationActiveRegion(const G4String& , G4bool, G4bool, G4bool); 

The same methods can be accessed interactively via UI commands:

   /run/initialize
   /process/em/deexcitation region true true true
   /process/em/fluo true
   /process/em/auger true
   /process/em/pixe true
 

Note that fluorescence from photons and electrons is activated by default in Livermore and Penelope physics constructors, while Auger production and PIXE are not.

How to change ionisation cross section models ?

The user can also select which cross section model to use in order to calculate shell ionisation cross sections for generating PIXE. Again, it is possible to use methods of the G4EmProcessOptions class in the user Physics list:

    G4EmProcessOptions::SetPIXECrossSectionModel(const G4String&);
  

where the string can be "Empirical", "ECPSSR_FormFactor" or "ECPSSR_Analytical", or alternatively with the following UI commands:

    /process/em/pixeXSmodel value   
  

where value is equal to Empirical or ECPSSR_FormFactor or ECPSSR_Analytical.

Different shell cross sections models are available : "ECPSSR_Analytical" models derive from an analytical calculation of the ECPSSR theory (see A. Mantero et al., X-Ray Spec.40 (2011) 135-140) and it reproduces K and L shell cross sections over a wide range of energies; "ECPSSR_FormFactor" models derive from A. Taborda et al. calculations (see A. Taborda et al., X-Ray Spec. 40 (2011) 127-134) of ECPSSR values directly form Form Factors and it covers K, L shells on the range 0.1-100 MeV and M shells in the range 0.1-10 MeV; the "empirical" models are from Paul "reference values" (for protons and alphas for K-Shell) and Orlic empirical model for L shells (only for protons and ions with Z>2). The later ones are the models used by default. Out of the energy boundaries, "ECPSSR_Analytical" model is used. We recommend to use default settings if not sure what to use.

Example

The TestEm5 extended/electromagetic example shows how to simulate atomic deexcitation (see for eg. the pixe.mac macro).

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 highly integrated microelectronic components.

The Geant4-MuElec process and model classes apply to electrons, protons and heavy ions in silicon.

Electron processes and models

Proton processes and models

Heavy ion processes and models

A full list of publications regarding Geant4-MuElec is directly available from the Geant4-MuElec website.

A new Compton scattering model for unpolarised photons has been developed in the relativistic impulse approximation. The model was developed as an alternative to low energy electromagnetic Compton scattering models developed from Ribberfors' Compton scattering framework (Livermore, Penelope Compton models). The model class is named named G4LowEPComptonModel.

G4LowEPComptonModel has been added to the physics constructor G4EmStandardPhysics_option4, containing the most accurate models from the Standard and Low Energy Electromagnetic physics working groups.

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.

A prototype of the compact version of neutron cross sections derived from HP database are provided with new classes G4NeutronHPElasticData, G4NeutronCaptureXS, G4NeutronElasticXS, and G4NeutronInelasticXS.

List of implemented "Hadron at Rest" processes

The following process classes have been implemented:

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:

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:

What processes do you need?

For hadrons in motion, there are four physics process classes. Table 5.1 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 Example 5.1, where G4LEProtonInelastic.hh is the name of the include file:

 ----------------------------- 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.

Geant4 provides a G4Decay class for both ``at rest'' and ``in flight'' particle decays. G4Decay can be applied to all particles except:

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:

The G4Decay class calculates the PhysicalInteractionLength and boosts decay products created by G4VDecayChannel or event generators. See below for information on the determination of the decay modes.

An object of G4Decay can be shared by particles. Registration of the decay process to particles in the ConstructPhysics method of PhysicsList (see Section 2.5.3) is shown in Example 5.2.

#include "G4Decay.hh"
void MyPhysicsList::ConstructGeneral()
{
  // Add Decay Process
  G4Decay* theDecayProcess = new G4Decay();
  theParticleIterator->reset();
  while( (*theParticleIterator)() ){
    G4ParticleDefinition* particle = theParticleIterator->value();
    G4ProcessManager* pmanager = particle->GetProcessManager();
    if (theDecayProcess->IsApplicable(*particle)) { 
      pmanager ->AddProcess(theDecayProcess);
      // set ordering for PostStepDoIt and AtRestDoIt
      pmanager ->SetProcessOrdering(theDecayProcess, idxPostStep);
      pmanager ->SetProcessOrdering(theDecayProcess, idxAtRest);
    }
  }
}

Each particle has its G4DecayTable, which stores information on the decay modes of the particle. Each decay mode, with its branching ratio, corresponds to an object of various ``decay channel'' classes derived from G4VDecayChannel. Default decay modes are created in the constructors of particle classes. For example, the decay table of the neutral pion has G4PhaseSpaceDecayChannel and G4DalitzDecayChannel as follows:

  // create a decay channel
  G4VDecayChannel* mode;
  // pi0 -> gamma + gamma
  mode = new G4PhaseSpaceDecayChannel("pi0",0.988,2,"gamma","gamma");
  table->Insert(mode);
  // pi0 -> gamma + e+ + e-
  mode = new G4DalitzDecayChannel("pi0",0.012,"e-","e+");
  table->Insert(mode);

Decay modes and branching ratios defined in Geant4 are listed in Section 5.3.2.

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+

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.

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 step can also be limited with the SetMaxBetaChangePerStep method, where the argument is the allowed change in percent).

The frequently very large number of secondaries produced in a single step (about 300/cm in water), compelled the idea in GEANT3.21 of suspending the primary particle until all its progeny have been tracked. Despite the fact that GEANT4 employs dynamic memory allocation and thus does not suffer from the limitations of GEANT3.21 with its fixed large initial ZEBRA store, GEANT4 nevertheless provides for an analogous functionality with the public method SetTrackSecondariesFirst. An example of the registration of the Cerenkov process is given in Example 5.4.

#include "G4Cerenkov.hh"

void ExptPhysicsList::ConstructOp(){

  G4Cerenkov*   theCerenkovProcess = new G4Cerenkov("Cerenkov");

  G4int MaxNumPhotons = 300;

  theCerenkovProcess->SetTrackSecondariesFirst(true);
  theCerenkovProcess->SetMaxBetaChangePerStep(10.0);
  theCerenkovProcess->SetMaxNumPhotonsPerStep(MaxNumPhotons);

  theParticleIterator->reset();
  while( (*theParticleIterator)() ){
    G4ParticleDefinition* particle = theParticleIterator->value();
    G4ProcessManager* pmanager = particle->GetProcessManager();
    G4String particleName = particle->GetParticleName();
    if (theCerenkovProcess->IsApplicable(*particle)) {
      pmanager->AddProcess(theCerenkovProcess);
      pmanager->SetProcessOrdering(theCerenkovProcess,idxPostStep);
    }
  }
}

Every scintillating material has a characteristic light yield, SCINTILLATIONYIELD, and an intrinsic resolution, RESOLUTIONSCALE, which generally broadens the statistical distribution of generated photons. A wider intrinsic resolution is due to impurities which are typical for doped crystals like NaI(Tl) and CsI(Tl). On the other hand, the intrinsic resolution can also be narrower when the Fano factor plays a role. The actual number of emitted photons during a step fluctuates around the mean number of photons with a width given by ResolutionScale*sqrt(MeanNumberOfPhotons). The average light yield, MeanNumberOfPhotons, has a linear dependence on the local energy deposition, but it may be different for minimum ionizing and non-minimum ionizing particles.

A scintillator is also characterized by its photon emission spectrum and by the exponential decay of its time spectrum. In GEANT4 the scintillator can have a fast and a slow component. The relative strength of the fast component as a fraction of total scintillation yield is given by the YIELDRATIO. Scintillation may be simulated by specifying these empirical parameters for each material. It is sufficient to specify in the user's DetectorConstruction class a relative spectral distribution as a function of photon energy for the scintillating material. An example of this is shown in Example 5.5

  const G4int NUMENTRIES = 9;
  G4double Scnt_PP[NUMENTRIES] = { 6.6*eV, 6.7*eV, 6.8*eV, 6.9*eV,
                                   7.0*eV, 7.1*eV, 7.2*eV, 7.3*eV, 7.4*eV };

  G4double Scnt_FAST[NUMENTRIES] = { 0.000134, 0.004432, 0.053991, 0.241971, 
                                     0.398942, 0.000134, 0.004432, 0.053991,
                                     0.241971 };
  G4double Scnt_SLOW[NUMENTRIES] = { 0.000010, 0.000020, 0.000030, 0.004000,
                                     0.008000, 0.005000, 0.020000, 0.001000,
                                     0.000010 };

  G4Material* Scnt;
  G4MaterialPropertiesTable* Scnt_MPT = new G4MaterialPropertiesTable();

  Scnt_MPT->AddProperty("FASTCOMPONENT", Scnt_PP, Scnt_FAST, NUMENTRIES);
  Scnt_MPT->AddProperty("SLOWCOMPONENT", Scnt_PP, Scnt_SLOW, NUMENTRIES);

  Scnt_MPT->AddConstProperty("SCINTILLATIONYIELD", 5000./MeV);
  Scnt_MPT->AddConstProperty("RESOLUTIONSCALE", 2.0);
  Scnt_MPT->AddConstProperty("FASTTIMECONSTANT",  1.*ns);
  Scnt_MPT->AddConstProperty("SLOWTIMECONSTANT", 10.*ns);
  Scnt_MPT->AddConstProperty("YIELDRATIO", 0.8);

  Scnt->SetMaterialPropertiesTable(Scnt_MPT);

In cases where the scintillation yield of a scintillator depends on the particle type, different scintillation processes may be defined for them. How this yield scales to the one specified for the material is expressed with the ScintillationYieldFactor in the user's PhysicsList as shown in Example 5.6. 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.

  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.

When there are multiple scintillators in the simulation and/or when the scintillation yield is a non-linear function of the energy deposited, the user can also define an array of total scintillation light yields as a function of the energy deposited and particle type. The available particles are protons, electrons, deuterons, tritons, alphas, and carbon ions. These are the particles known to significantly effect the scintillation light yield, of for example, BC501A (NE213/EJ301) liquid organic scintillator and BC420 plastic scintillator as function of energy deposited.

The method works as follows:

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 energy. WLSCOMPONENT is the relative emission spectrum of the material as a function of the photon's energy, and WLSTIMECONSTANT accounts for any time delay which may occur between absorption and re-emission of the photon. An example is shown in Example 5.7.

 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.

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, dσ/dω, is proportional to 1+cos²(θ), where θ is the polar of the new polarization vector with respect to the old polarization vector. The G4OpRayleigh scattering process samples this angle accordingly and then calculates the scattered photon's new direction by requiring that it be perpendicular to the photon's new polarization in such a way that the final direction, initial and final polarizations are all in one plane. This process thus depends on the particle's polarization (spin). The photon's polarization is a data member of the G4DynamicParticle class.

A photon which is not assigned a polarization at production, either via the SetPolarization method of the G4PrimaryParticle class, or indirectly with the SetParticlePolarization method of the G4ParticleGun class, may not be Rayleigh scattered. Optical photons produced by the G4Cerenkov process have inherently a polarization perpendicular to the cone's surface at production. Scintillation photons have a random linear polarization perpendicular to their direction.

The process requires a G4MaterialPropertiesTable to be filled by the user with Rayleigh scattering length data. The Rayleigh scattering attenuation length is the average distance traveled by a photon before it is Rayleigh scattered in the medium and it is the distance returned by the GetMeanFreePath method. The G4OpRayleigh class provides a RayleighAttenuationLengthGenerator method which calculates the attenuation coefficient of a medium following the Einstein-Smoluchowski formula whose derivation requires the use of statistical mechanics, includes temperature, and depends on the isothermal compressibility of the medium. This generator is convenient when the Rayleigh attenuation length is not known from measurement but may be calculated from first principles using the above material constants. For a medium named Water and no Rayleigh scattering attenutation length specified by the user, the program automatically calls the RayleighAttenuationLengthGenerator which calculates it for 10 degrees Celsius liquid water.

Mie Scattering

Mie Scattering (or Mie solution) is an analytical solution of Maxwell's equations for scattering of optical photons by spherical particles. It is significant only when the radius of the scattering object is of order of the wave length. The analytical expressions for Mie Scattering are very complicated since they are a series sum of Bessel functions. One common approximation made is call Henyey-Greenstein (HG). The implementation in Geant4 follows the HG approximation (for details see the Physics Reference Manual) and the treatment of polarization and momentum are similar to that of Rayleigh scattering. We require the final polarization direction to be perpendicular to the momentum direction. We also require the final momentum, initial polarization and final polarization to be in the same plane.

The process requires a G4MaterialPropertiesTable to be filled by the user with Mie scattering length data (entered with the name: MIEHG) analogous to Rayleigh scattering. The Mie scattering attenuation length is the average distance traveled by a photon before it is Mie scattered in the medium and it is the distance returned by the GetMeanFreePath method. In practice, the user not only needs to provide the attenuation length of Mie scattering, but also needs to provide the constant parameters of the approximation: g_f, g_b, and r_f.  (with AddConstProperty and with the names: MIEHG_FORWARD, MIEHG_BACKWARD, and MIEHG_FORWARD_RATIO, respectively; see Novice Example N06.)

Boundary Process

Reference: E. Hecht and A. Zajac, Optics [ Hecht1974 ]

For the simple case of a perfectly smooth interface between two dielectric materials, all the user needs to provide are the refractive indices of the two materials stored in their respective G4MaterialPropertiesTable. In all other cases, the optical boundary process design relies on the concept of surfaces. The information is split into two classes. One class in the material category keeps information about the physical properties of the surface itself, and a second class in the geometry category holds pointers to the relevant physical and logical volumes involved and has an association to the physical class. Surface objects of the second type are stored in a related table and can be retrieved by either specifying the two ordered pairs of physical volumes touching at the surface, or by the logical volume entirely surrounded by this surface. The former is called a border surface while the latter is referred to as the skin surface. This second type of surface is useful in situations where a volume is coded with a reflector and is placed into many different mother volumes. A limitation is that the skin surface can only have one and the same optical property for all of the enclosed volume's sides. The border surface is an ordered pair of physical volumes, so in principle, the user can choose different optical properties for photons arriving from the reverse side of the same interface. For the optical boundary process to use a border surface, the two volumes must have been positioned with G4PVPlacement. The ordered combination can exist at many places in the simulation. When the surface concept is not needed, and a perfectly smooth surface exists beteen two dielectic materials, the only relevant property is the index of refraction, a quantity stored with the material, and no restriction exists on how the volumes were positioned.

The physical surface object also specifies which model the boundary process should use to simulate interactions with that surface. In addition, the physical surface can have a material property table all its own. The usage of this table allows all specular constants to be wavelength dependent. In case the surface is painted or wrapped (but not a cladding), the table may include the thin layer's index of refraction. This allows the simulation of boundary effects at the intersection between the medium and the surface layer, as well as the Lambertian reflection at the far side of the thin layer. This occurs within the process itself and does not invoke the G4Navigator. Combinations of surface finish properties, such as polished or ground and front painted or back painted, enumerate the different situations which can be simulated.

When a photon arrives at a medium boundary its behavior depends on the nature of the two materials that join at that boundary. Medium boundaries may be formed between two dielectric materials or a dielectric and a metal. In the case of two dielectric materials, the photon can undergo total internal reflection, refraction or reflection, depending on the photon's wavelength, angle of incidence, and the refractive indices on both sides of the boundary. Furthermore, reflection and transmission probabilites are sensitive to the state of linear polarization. In the case of an interface between a dielectric and a metal, the photon can be absorbed by the metal or reflected back into the dielectric. If the photon is absorbed it can be detected according to the photoelectron efficiency of the metal.

As expressed in Maxwell's equations, Fresnel reflection and refraction are intertwined through their relative probabilities of occurrence. Therefore neither of these processes, nor total internal reflection, are viewed as individual processes deserving separate class implementation. Nonetheless, an attempt was made to adhere to the abstraction of having independent processes by splitting the code into different methods where practicable.

One implementation of the G4OpBoundaryProcess class employs the UNIFIED model [A. Levin and C. Moisan, A More Physical Approach to Model the Surface Treatment of Scintillation Counters and its Implementation into DETECT, TRIUMF Preprint TRI-PP-96-64, Oct. 1996] of the DETECT program [G.F. Knoll, T.F. Knoll and T.M. Henderson, Light Collection Scintillation Detector Composites for Neutron Detection, IEEE Trans. Nucl. Sci., 35 (1988) 872.]. It applies to dielectric-dielectric interfaces and tries to provide a realistic simulation, which deals with all aspects of surface finish and reflector coating. The surface may be assumed as smooth and covered with a metallized coating representing a specular reflector with given reflection coefficient, or painted with a diffuse reflecting material where Lambertian reflection occurs. The surfaces may or may not be in optical contact with another component and most importantly, one may consider a surface to be made up of micro-facets with normal vectors that follow given distributions around the nominal normal for the volume at the impact point. For very rough surfaces, it is possible for the photon to inversely aim at the same surface again after reflection of refraction and so multiple interactions with the boundary are possible within the process itself and without the need for relocation by G4Navigator.

Figure 5.1.  Diagram of the UNIFIED Model for Optical Surfaces (courtesy A. Shankar)

The UNIFIED model (Figure 5.1) 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.

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.].

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 (Figure 5.1), 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. W. Moses, IEEE Trans. Nucl. Sci. 57 (3) (2010) 964-970.

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).

The Geant4 parameterization facilities allow you to shortcut the detailed tracking in a given volume and for given particle types in order for you to provide your own implementation of the physics and of the detector response.

Parameterisations are bound to a G4Region object, which, in the case of fast simulation is also called an envelope. Prior to release 8.0, parameterisations were bound to a G4LogicalVolume, the root of a volume hierarchy. These root volumes are now attributes of the G4Region. Envelopes often correspond to the volumes of sub-detectors: electromagnetic calorimeters, tracking chambers, etc. With GEANT4 it is also possible to define envelopes by overlaying a parallel or "ghost" geometry as discussed in Section 5.2.6.7.

In GEANT4, parameterisations have three main features. You must specify:

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:

The GEANT4 components which allow the implementation and control of parameterisations are:

Constructors:

The G4VFastSimulationModel class has two constructors. The second one allows you to get started quickly:

Virtual methods:

The G4VFastSimulationModel has three pure virtual methods which must be overriden in your concrete class:

G4FastSimulationManager functionnalities regarding the use of ghost volumes are explained in Section 5.2.6.7.

Constructor:

G4VFastSimulationModel object management:

The following two methods provide the usual management functions.

Interface with the G4FastSimulationManagerProcess:

This is described in the User's Guide for Toolkit Developers ( section 3.9.6 )

This G4VProcess serves as an interface between the tracking and the parameterisation. At tracking time, it collaborates with the G4FastSimulationManager of the current volume, if any, to allow the models to trigger. If no manager exists or if no model issues a trigger, the tracking goes on normally.

In the present implementation, you must set this process in the G4ProcessManager of the particles you parameterise to enable your parameterisation.

The processes ordering is:

    [n-3] ...
    [n-2] Multiple Scattering
    [n-1] G4FastSimulationManagerProcess
    [ n ] G4Transportation

This ordering is important if you use ghost geometries, since the G4FastSimulationManagerProcess will provide navigation in the ghost world to limit the step on ghost boundaries.

The G4FastSimulationManager must be added to the process list of a particle as a continuous and discrete process if you use ghost geometries for this particle. You can add it as a discrete process if you don't use ghosts.

The following code registers the G4FastSimulationManagerProcess with all the particles as a discrete and continuous process:

void MyPhysicsList::addParameterisation()
{
  G4FastSimulationManagerProcess*
    theFastSimulationManagerProcess = new G4FastSimulationManagerProcess();
  theParticleIterator->reset();
  while( (*theParticleIterator)() )
    {
      G4ParticleDefinition* particle = theParticleIterator->value();
      G4ProcessManager* pmanager = particle->GetProcessManager();
      pmanager->AddProcess(theFastSimulationManagerProcess, -1, 0, 0);
    }
}

This class is a singleton which can be accessed as follows:

#include "G4GlobalFastSimulationManager.hh"
...
...
G4GlobalFastSimulationManager* globalFSM;
globalFSM = G4GlobalFastSimulationManager::getGlobalFastSimulationManager();
...
...

Presently, you will mainly need to use the GlobalFastSimulationManager if you use ghost geometries.

In some cases, volumes of the tracking geometry do not allow envelopes to be defined. This may be the case with a geometry coming from a CAD system. Since such a geometry is flat, a parallel geometry must be used to define the envelopes.

Another interesting case involves defining an envelope which groups the electromagnetic and hadronic calorimeters of a detector into one volume. This may be useful when parameterizing the interaction of charged pions. You will very likely not want electrons to see this envelope, which means that ghost geometries have to be organized by particle flavours.

Using ghost geometries implies some more overhead in the parameterisation mechanism for the particles sensitive to ghosts, since navigation is provided in the ghost geometry by the G4FastSimulationManagerProcess. Usually, however, only a few volumes will be placed in this ghost world, so that the geometry computations will remain rather cheap.

In the existing implementation (temporary implementation with G4Region but before parallel geometry implementation), you may only consider ghost G4Regions with just one root G4LogicalVolume. The G4GlobalFastSimulationManager provides the construction of the ghost geometry by making first an empty "clone" of the world for tracking provided by the construct() method of your G4VUserDetectorConstruction concrete class. You provide the placement of the G4Region root G4LogicalVolume relative to the ghost world coordinates in the G4FastSimulationManager objects. A ghost G4Region is recognized by the fact that its associated G4FastSimulationManager retains a non-empty list of placements.

The G4GlobalFastSimulationManager will then use both those placements and the IsApplicable() methods of the models attached to the G4FastSimulationManager objects to build the flavour-dependant ghost geometries.

Then at the beginning of the tracking of a particle, the appropriate ghost world, if any, will be selected.

The steps required to build one ghost G4Region are:

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:

This section describes how to use the Gflash library. Gflash is a concrete parameterization which is based on the equations and parameters of the original Gflash package from H1(hep-ex/0001020, Grindhammer & Peters, see physics manual) and uses the "fast simulation" facilities of GEANT4 described above. Briefly, whenever a e-/e+ particle enters the calorimeter, it is parameterized if it has a minimum energy and the shower is expected to be contained in the calorimeter (or " parameterization envelope"). If this is fulfilled the particle is killed, as well as all secondaries, and the energy is deposited according to the Gflash equations. An example, provided in examples/extended/parametrisation/gflash/, shows how to interface Gflash to your application. The simulation time is measured, so the user can immediately see the speed increase resulting from the use of Gflash.

To use Gflash "out of the box" the following steps are necessary:

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.