User:
Structure of the main program
Description 
The simplest main program has the structure:
```
int main(int argc, char** argv){
G4RunManager* runManager = new G4RunManager;
MyGeometry* geom = new MyGeometry();
runManager->SetUserInitialization(geom);
MyPhysicsList* physics = new MyPhysicsList();
runManager->SetUserInitialization(physics);
MyPrimaryGeneratorAction* generator = new MyPrimaryGeneratorAction();
runManager->SetUserAction(generator);
runManager->Initialize();
runManager->BeamOn(1);
delete runManager;
return 0;
}
```
where:
> control is initialized
> geometry is created
> the particles are created
> the particle generator is created
> a particle is fired
> the program is closed
ID:(9412, 0)
Creation of geometry
Description
The minimum to create the geometry is a volume, which can be done through
```
Class MyGeometry : public G4VUserDetectorConstruction{
G4VPhysicalVolume* Construct();
}
```
what is inherited from G4VUserDetectorConstruction.
The volume will include all the remaining volumes of the model and is called the 'world'.
ID:(9411, 0)
Creation of particles
Description
The easiest way to create the particles is to use the default list that Gean4 brings, which is done through
```
G4VUserPhysicsList* physics = new FTFP_BERT();
runManager->SetUserInitialization(physics);
```
what is inherited from G4VUserPhysicsList.
The other alternative is the customized creation by the user that must include the particle constructor (ConstructParticle ()), processes (ConstructProcess ()) and cuts (SetCuts ()) by:
```
class MyPhysics : public G4VUserPhysicsList{
void ConstructParticle();
void ConstructProcess();
void SetCuts();
}
```
ID:(9413, 0)
Shot of particles
Description
To generate the particles a generator is created with
```
class myGenerator : public G4VUserPrimaryGeneratorAction{
void GeneratePrimaries(G4Event*);
}
```
which is inherited from G4VUserPrimaryGeneratorAction and must implement GeneratePrimaries(G4Event*).
The particles can be generated by G4ParticleGun on which you can define key parameters such as:
```
G4ParticleGun* myGun = new G4ParticleGun(int n_particle = 1);
myGun->SetMomentumDirection(G4ThreeVector(1,0,0));
myGun->SetKineticEnergy(50.*MeV);
myGun->GeneratePrimaryVertex(G4Event* anEvent);
```
ID:(9414, 0)