How to Generate a Primary Event
Generating Primary Events
G4VuserPrimaryGeneratorAction is one of the mandatory
classes available for deriving your own concrete class. In your
concrete class, you have to specify how a primary event should be
generated. Actual generation of primary particles will be done by
concrete classes of G4VPrimaryGenerator, explained in the
following sub-section. Your G4VUserPrimaryGeneratorAction
concrete class just arranges the way primary particles are generated.
An example of a <emphasis>G4VUserPrimaryGeneratorAction</emphasis>
concrete class using <emphasis>G4ParticleGun</emphasis>.
For the usage of <emphasis>G4ParticleGun</emphasis> refer to the
next subsection.
#ifndef ExN01PrimaryGeneratorAction_h
#define ExN01PrimaryGeneratorAction_h 1
#include "G4VUserPrimaryGeneratorAction.hh"
class G4ParticleGun;
class G4Event;
class ExN01PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
{
public:
ExN01PrimaryGeneratorAction();
~ExN01PrimaryGeneratorAction();
public:
void generatePrimaries(G4Event* anEvent);
private:
G4ParticleGun* particleGun;
};
#endif
#include "ExN01PrimaryGeneratorAction.hh"
#include "G4Event.hh"
#include "G4ParticleGun.hh"
#include "G4ThreeVector.hh"
#include "G4Geantino.hh"
#include "globals.hh"
ExN01PrimaryGeneratorAction::ExN01PrimaryGeneratorAction()
{
G4int n_particle = 1;
particleGun = new G4ParticleGun(n_particle);
particleGun->SetParticleDefinition(G4Geantino::GeantinoDefinition());
particleGun->SetParticleEnergy(1.0*GeV);
particleGun->SetParticlePosition(G4ThreeVector(-2.0*m,0.0*m,0.0*m));
}
ExN01PrimaryGeneratorAction::~ExN01PrimaryGeneratorAction()
{
delete particleGun;
}
void ExN01PrimaryGeneratorAction::generatePrimaries(G4Event* anEvent)
{
G4int i = anEvent->get_eventID() % 3;
switch(i)
{
case 0:
particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.0,0.0));
break;
case 1:
particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.1,0.0));
break;
case 2:
particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.0,0.1));
break;
}
particleGun->generatePrimaryVertex(anEvent);
}
Selection of the generator
In the constructor of your G4VUserPrimaryGeneratorAction,
you should instantiate the primary generator(s). If necessary, you
need to set some initial conditions for the generator(s).
In ,
G4ParticleGun is constructed to
use as the actual primary particle generator. Methods of
G4ParticleGun are described in the following section. Please
note that the primary generator object(s) you construct in your
G4VUserPrimaryGeneratorAction concrete class must be deleted
in your destructor.
Generation of an event
G4VUserPrimaryGeneratorAction has a pure virtual method
named generatePrimaries(). This method is invoked at the
beginning of each event. In this method, you have to invoke the
G4VPrimaryGenerator concrete class you instantiated via the
generatePrimaryVertex() method.
You can invoke more than one generator and/or invoke one
generator more than once. Mixing up several generators can produce
a more complicated primary event.
G4VPrimaryGenerator
Geant4 provides three G4VPrimaryGenerator concrete
classes. Among these G4ParticleGun and
G4GeneralParticleSource will be discussed here.
The third one is G4HEPEvtInterface, which will be
discussed in .
G4ParticleGun
G4ParticleGun is a generator provided by Geant4. This class
generates primary particle(s) with a given momentum and position.
It does not provide any sort of randomizing. The constructor of
G4ParticleGun takes an integer which causes the generation
of one or more primaries of exactly same kinematics. It is a rather
frequent user requirement to generate a primary with randomized
energy, momentum, and/or position. Such randomization can be
achieved by invoking various set methods provided by
G4ParticleGun. The invocation of these methods should be
implemented in the generatePrimaries() method of your
concrete G4VUserPrimaryGeneratorAction class before invoking
generatePrimaryVertex() of
G4ParticleGun.
Geant4 provides various random number generation methods with various
distributions (see ).
Public methods of <emphasis>G4ParticleGun</emphasis>
The following methods are provided by G4ParticleGun, and all
of them can be invoked from the generatePrimaries() method
in your concrete G4VUserPrimaryGeneratorAction class.
void SetParticleDefinition(G4ParticleDefinition*)
void SetParticleMomentum(G4ParticleMomentum)
void SetParticleMomentumDirection(G4ThreeVector)
void SetParticleEnergy(G4double)
void SetParticleTime(G4double)
void SetParticlePosition(G4ThreeVector)
void SetParticlePolarization(G4ThreeVector)
void SetNumberOfParticles(G4int)
G4GeneralParticleSource
For many applications G4ParticleGun is a suitable
particle generator. Howevr if you want to generate primary particles
in more sophisticated manner, you can utilize
G4GeneralParticleSource - Geant4 General Particle
Source module (GPS).
Using this tool, you can control the following characteristics of
primary particles:
Spectrum: linear, exponential, power-law, Gaussian, blackbody, or
piece-wise fits to data.
Angular distribution: unidirectional, isotropic, cosine-law,
beam or arbitrary (user defined).
Spatial sampling: on simple 2D or 3D surfaces such as discs, spheres,
and boxes.
Multiple sources: multiple independent sources can be used in the same run.
Details of information on the General Source Particle Module can be
found in the documents
Geant4 General Particle Source.