Introduction to GEANT4
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== <span style="color:#000080"> Introduction </span> == GEANT4 is a software toolkit based on C++. In your code you have to define: * Your experimental setup - geometry, materials and primary particles. * Which physics process you are interested in. * You may take actions during the simulation - inspect and store results. The interaction with GEANT4 is done via base classes. ; Mandatory classes: * <span style="color:#ff0000"> G4VUserDetectorConstruction </span>: Describe the experimental setup, geometry and materials; * <span style="color:#ff0000"> G4VUserPhysicsList </span>: Define particles, physics processes and range cuts; * <span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span>: Describe particle source, source dimensions, initial position, energy spectrum, angular distributions; ; Optional classes: * <span style="color:#ff0000"> G4UserRunAction </span>: Define and store histograms; * <span style="color:#ff0000"> G4UserEventAction </span>: Event selection and analysis of simulation data; * <span style="color:#ff0000"> G4UserStackingAction </span>: Customize priority of tracks; * <span style="color:#ff0000"> G4UserTrackingAction </span>: Decide whether a trajectory should be stored or not; * <span style="color:#ff0000"> G4UserSteppingAction </span>: Kill, suspend, postpone a track; ; Manager class * <span style="color:#ff0000"> G4RunManager </span>: Manages the simulation process; == <span style="color:#000080"> The function main() </span> == The function <span style="color:#ff0000"> main() </span> defines the skeleton of your simulation code. Inside the function you instantiate <span style="color:#ff0000"> G4RunManager </span> and notify it of your mandatory and optional classes. This is example <span style="color:#ff0000"> main() </span> function, where <span style="color:#ff0000"> MyDetectorConstruction </span>, <span style="color:#ff0000"> MyPhysicsList </span>, <span style="color:#ff0000"> MyPrimaryGeneratorAction </span>, <span style="color:#ff0000"> MyEventAction </span> and <span style="color:#ff0000"> MyRunAction </span> are derived classes from the GEANT4 base classes: <pre style="color: #800000; background-color: #dcdcdc"> int main() { // Run manager construction G4RunManager* runManager = new G4RunManager; // mandatory user initialization classes runManager->SetUserInitialization(new MyDetectorConstruction); runManager->SetUserInitialization(new MyPhysicsList); // mandatory user action classes runManager->SetUserAction(new MyPrimaryGeneratorAction); // optional user action classes runManager->SetUserAction(new MyEventAction); runManager->SetUserAction(new MyRunAction); ... } </pre> == <span style="color:#000080"> Experimental setup </span> == You derive your own class from <span style="color:#ff0000"> G4VUserDetectorConstruction </span> base class. In the derived class you will: * Describe the shape and the size of your detector using <span style="color:#ff0000"> G4VSolid </span> * Construct materials and electromagnetic fields using <span style="color:#ff0000"> G4Logical Volume </span> * Place volumes of your detector geometry using <span style="color:#ff0000"> G4VPhysical Volume </span> <u> Simple example of class <span style="color:#ff0000"> MyDetectorConstruction </span> </u>: <pre style="color: #800000; background-color: #dcdcdc"> class MyDetectorConstruction:public G4VUserDetectorConstruction { public: MyDetectorConstruction(); ~MyDetectorConstruction(); virtual G4VPhysicalVolume* Construct(); private: void DefineMaterials(); }; </pre> Now construct the detector. Your detector is always placed in a mother volume called the world volume. <pre style="color: #800000; background-color: #dcdcdc"> G4PhysicalVolume* MyDetectorConstruction::Construct() { ... // World volume G4VSolid* pWorld = new G4Box("World",5*m,5*m,5*m); G4LogicalVolume* pWorldLog = new G4LogicalVolume(pWorld,vacuum, "World"); G4VPhysicalVolume* pWorldPhys = new G4PVPlacement(0,G4ThreeVector(),pWorldLog,"World",0,false,0); </pre> <pre style="color: #800000; background-color: #dcdcdc"> // Water box </span> G4VSolid* pBoxSolid = new G4Box(“WaterBox”, 1.*m, 2.*m, 3.*m); G4LogicalVolume* pBoxLog = new G4LogicalVolume( pBoxSolid, water, “WaterBox”); G4VPhysicalVolume* aBoxPhys = new G4PVPlacement( pRotation, G4ThreeVector(posX, posY, posZ), pBoxLog, “WaterBox”, pWorldLog, false, copyNo); ... } </pre> The elements and materials are defined using classes <span style="color:#ff0000"> G4Element </span> and <span style="color:#ff0000"> G4Material </span>. For example water, hydrogen and oxygen are defined as: <pre style="color: #800000; background-color: #dcdcdc"> void MyDetectorConstruction::DefineMaterials() { ... G4Element* H = new G4Element("Hydrogen","H",z=1.,a= 1.01*g/mole); G4Element* O = new G4Element("Oxygen","O",z=8.,a=16.00*g/mole); density = 1.000*g/cm3; G4Material* water = new G4Material("Water",density,ncomp=2); water->AddElement(H, natoms=2); water->AddElement(O, natoms=1); ...} </pre> [http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__detectorConstruction.html Here] you can find more examples of DetectorConstruction classes. == <span style="color:#000080"> Physics processes </span> == You can build your own physics list or chose from already built physics lists. To build your own physics lists, you can use two base physics list classes: <span style="color:#ff0000"> G4VUserPhysicsList </span> and <span style="color:#ff0000"> G4ModularPhysicsList </span>. The class <span style="color:#ff0000"> G4VUserPhysicsList </span> is used for simple physics lists while <span style="color:#ff0000"> G4ModularPhysicsList </span> is used to build more complex physics lists. There exist also already built pre-packaged physics lists. === <span style="color:#000080"> Simple physics lists </span> === If the particles in your simulation undergo a descrete number of physics processes you can use the class <span style="color:#ff0000"> G4VUserPhysicsList </span> to define them. This class has three methods: * ConstructParticles() : Define all necessary particles; * ConstructProcesses() : Define all necessary processes and assign them to proper particles; * SetCuts() : Define production thresholds in terms of range; <u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>: <pre style="color: #800000; background-color: #dcdcdc"> class MyPhysicsList:public G4VUserPhysicsList() { public: MyPhysicsList(); ~MyPhysicsList(); void ConstructParticle(); void ConstructProcess(); void SetCuts(); } </pre> Now implement the methods ConstructParticle(), ConstructProcess() and SetCuts(): <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::ConstructParticle() { // Define the particles G4Electron::ElectronDefinition(); G4Positron::PositronDefinition(); G4Proton::ProtonDefinition(); G4Neutron::NeutronDefinition(); G4Gamma::GammaDefinition(); ... } </pre> GEANT4 provides a variety of physics processes. These processes are decoupled from one another and you can select those which are relevant to your simulation. The processes are grouped in seven categories and their list is available [http://geant4.cern.ch/support/proc_mod_catalog/processes/ here]: <div style="column-count:2;-moz-column-count:2;-webkit-column-count:2"> * electromagnetic * hadronic * decay * photolepton-hadron * optical * parameterization * transportation </div> For each particle defined in ConstructParticle() assign all the physics processes that you want to consider in your simulation: <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::ConstructProcess() { AddTransportation(); // Assign transportation process to all particles ConstructEM(); // Electromagnetic processes ConstructGeneral(); // Other processes } </pre> In methods ConstructEM() and ConstructGeneral() assign the physics processes to the corresponding particles: <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::ConstructEM() { aParticleIterator->reset(); while((*aParticleIterator)()){ G4ParticleDefinition* particle = aParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); G4String particleName = particle->GetParticleName(); if (particleName == "gamma") { pmanager->AddDiscreteProcess(new G4GammaConversion); ...} </pre> <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::ConstructGeneral() { G4Decay* theDecayProcess = new G4Decay() aParticleIterator->reset(); while((*aParticleIterator)()) { G4ParticleDefinition* particle = aParticleIterator->value(); G4ProcessManager* pmanager = particle->GetProcessManager(); if theDecayProcess->IsApplicable(*particle)) { pmanager->AddProcess(theDecayProcess); pmanager->SetProcessOrdering(theDecayProcess,idxPostStep); pmanager->SetProcessOrdering(theDecayProcess,idxAtRest); }}} </pre> This is the full [http://geant4.cern.ch/support/proc_mod_catalog/particles/ list] of physics processes available for every particle. Finally, method SetCuts() is defined as: <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::SetCuts() { </span> defaultCutValue = 1.0*mm; SetCutValue(defaultCutValue, "gamma"); SetCutValue(defaultCutValue, "e+"); SetCutValue(defaultCutValue, "e-"); </pre> === <span style="color:#000080"> Detailed physics lists </span> === If you want to build more realistic physics list you have to use the class <span style="color:#ff0000"> G4VModularPhysicsList </span>. For example, the photon from the example above can undergo compton scattering apart from conversion. In <span style="color:#ff0000"> G4VModularPhysicsList </span> you can group the physics processes into separate modules: EM physics, hadronic physics, decay physics etc. <u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>: <pre style="color: #800000; background-color: #dcdcdc"> class MyPhysicsList:public G4VModularPhysicsList { public: MyPhysicsList(); ~MyPhysicsList(); virtual void ConstructParticle(); virtual void SetCuts(); void AddPhysicsList(const G4String& name); virtual void ConstructProcess(); private: G4String fEmName; G4VPhysicsConstructor* fEmPhysicsList; G4VPhysicsConstructor* fDecPhysicsList; std::vector<G4VPhysicsConstructor*> fHadronPhysicsList; }; </pre> Now we can build the physics lists: <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::AddPhysicsList(const G4String& name) { ... if (name == "emstandard_opt3") { fEmName = name; delete fEmPhysicsList; fEmPhysicsList = new G4EmStandardPhysics_option3(); } else if (name == "emlivermore") { fEmName = name; delete fEmPhysicsList; fEmPhysicsList = new G4EmLivermorePhysics(); } else if (name == "empenelope") { fEmName = name; delete fEmPhysicsList; fEmPhysicsList = new G4EmPenelopePhysics(); } else if (name == "HElastic") { fHadronPhysicsList.push_back( new G4HadronHElasticPhysics()); } else if (name == "HInelastic") { fHadronPhysicsList.push_back(new G4HadronInelasticQBBC()); } ... } </pre> and <pre style="color: #800000; background-color: #dcdcdc"> void MyPhysicsList::ConstructProcess() { AddTransportation(); // transportation fEmPhysicsList->ConstructProcess(); // electromagnetic physics list fDecPhysicsList->ConstructProcess(); // decay physics list for(size_t i=0; i<fHadronPhys.size(); i++) { // hadronic physics lists fHadronPhys[i]->ConstructProcess(); } } </pre> === <span style="color:#000080"> Pre-packaged physics lists </span> === Some built-in pre-packaged physics lists are available [http://geant4.web.cern.ch/geant4/support/proc_mod_catalog/physics_lists/referencePL.shtml here]. You can use them as a starting point of your simulation. <u> Simple example of <span style="color:#ff0000"> pre-packaged physics list </span> </u>: In function <span style="color:#ff0000"> main() </span>: <pre style="color: #800000; background-color: #dcdcdc"> G4PhysListFactory factory* physListFactory = new G4PhysListFactory(); G4VUserPhysicsList* physicsList = physListFactory->GetReferencePhysList(“FTFP_BERT”); runManager->SetUserInitialization(physicsList); </pre> Most of the pre-packaged physics lists use "standard" electromagnetic physics processes. The "standard" EM processes are defined with classes: G4EmStandardPhysics, G4EmStandardPhysics_option1, G4EmStandardPhysics_option2 and G4EmStandardPhysics_option3. If you want to use "low energy" electromagnetic physics processes (defined with classes G4EmLivermorePhysics, G4EmLivermorePolarizedPhysics, G4EmPenelopePhysics and G4EmDNAPhysics) in the pre-packaged physics list you have to define them in your physics list class (see example ProtonPencilBeam). For example, if you want to simulate clinical proton beam of energy 150 MeV you can use pre-packaged physics list e.g. QGSP_BIC, QGSP_BERT and FTFP_BERT. If you are interested in Bragg curve physics, use a physics list ending in "EMV" or "EMX" e.g. QGSP_BERT_EMV. == <span style="color:#000080"> Generate primary particles </span> == You derive your own class from <span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span> base class. Actual generation of primary particles is done via classes <span style="color:#ff0000"> G4ParticleGun </span> and <span style="color:#ff0000"> G4GeneralParticleSource </span>. * <span style="color:#ff0000"> G4ParticleGun </span> is used to simulate a beam of particles. It shoots a primary particle of a certain energy and direction from a given point at a given time. * <span style="color:#ff0000"> G4GeneralParticleSource </span> simulates a beam of particles and the primary vertex is randomly chosen on surface of a given volume with pre-defined energy spectra, spatial and angular distribution. <u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using particle gun </u>: <pre style="color: #800000; background-color: #dcdcdc"> class MyPrimaryGeneratorAction:public G4VUserPrimaryGeneratorAction { public: MyPrimaryGeneratorAction( const G4String& particleName = "proton", G4double energy = 1.*CLHEP::MeV, G4ThreeVector position= G4ThreeVector(0,0,0), G4ThreeVector momentumDirection = G4ThreeVector(0,0,1)); ~MyPrimaryGeneratorAction(); virtual void GeneratePrimaries(G4Event*); private: G4ParticleGun* fParticleGun; }; </pre> <u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using general particle source </u>: <pre style="color: #800000; background-color: #dcdcdc"> class MyPrimaryGeneratorAction:public G4VUserPrimaryGeneratorAction { public: MyPrimaryGeneratorAction(); ~MyPrimaryGeneratorAction(); virtual void GeneratePrimaries(G4Event*); private: static const G4String ParticleName; static const G4double ParticleEnergy; G4GeneralParticleSource* fGeneralParticleSource; }; </pre> [http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__primaryGenerator.html Here] you can find how to implement MyPrimaryGeneratorAction class defined above in your code. == <span style="color:#000080"> Optional user classes </span> == [http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__userActions.html Here] you can find examples of user defined RunAction and EventAction classes.
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