Geant4 Quickstart Guide
Step 2: Defining the particles source
The previous step demonstrated how to define the experimental setup to simulate. This section will
demonstrate how to create energetic particles which will interact with the setup.
In order to start off the simulation, the user must provide a particle source by creating a class that
inherits from G4VUserPrimaryGeneratorAction and overrides the abstract
method
void GeneratePrimaries(G4Event*).
class BasicPrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction{
public:
void GeneratePrimaries(G4Event* anEvent){
The easiest way to generate primary particles in Geant4 is to use the helper class
G4ParticleGun.
void GeneratePrimaries(G4Event* anEvent){
G4ParticleGun* myGun = new G4ParticleGun();
//Tell the gun to emit protons
myGun->SetParticleDefinition(G4Proton::ProtonDefinition());
//Set the kinetic energy of the protons to 50 keV
//and tell the gun to emit them along the x-axis
myGun->SetParticleEnergy(50.*keV);
myGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0,0));
//Generate the primary particle
myGun->GeneratePrimaryVertex(anEvent);
}
Note that the proton definition has been obtained from a static constructor
G4Proton::ProtonDefinition(). The reasonfor this is that particle definitions in Geant4 always
exist only oncein memory. In this way, if a million protons are created, attributes like proton mass
and charge only need to be held in memory once.
The class BasicPrimaryGeneretorAction defined above will be passed to the
Geant4 kernel during initialization. At the beginning of the simulation the Geant4 kernel will call the
void GeneratePrimaries(G4Event*) method in order to generate a 50 keV proton
travelling in the direction of the x-axis.
Link to the complete BasicPrimaryGeneratorAction class.
Go to page 3: Writing the main-method
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