Generating Primary Particles

1. Generating Primary Particles • Each Geant4 Event starts with generation of one or multiple primary particles • It is up to the user to define primary particle properties • Particle type, e.g. electron, gamma, ion • Initial kinetics, e.g. energy, momentum, origin and direction • Additional properties, e.g. polarization • These properties can be divided into a primary vertex: starting point in space and time • Primary particle: initial momentum, polarization, PDG code, list of daughters for decay chains • A primary particle can be a particle which can notusually be tracked by Geant4

2. The PrimaryGenerator • A primary generator is a class derived from G4VPrimaryGenerator which implements a GeneratePrimaryVertex() method • In this method the primary vertex and the primary particle are added to a Geant4 Event • Often it is practical to use an existing generator: • G4HEPEvtInterface • G4HEPMCInterface • G4GeneralParticleSource • G4ParticleGun Examples of experiment-specificgenerators. Control via text files More general purpose. Forvolume and surface sourcesAlso for beams. Can be used to produce a beam of particles

3. PrimaryGeneratorAction • Mandatory user action which controls the generation of primary particles • It should not generate primaries itself. The primary generator does this. • Implement your particle “shot”, “rail”, or machine gun here. It can also be a particle bomb if you like. • By using e.g. the G4ParticleGun • Repeatedly for a single event • Sampling particle type and direction randomly • Or using one of the other event generators

4. PrimaryGeneratorAction • Inherits from G4VUserPrimaryGeneratorAction • User should override GeneratePrimaries for particle generation PrimaryGeneratorAction::PrimaryGeneratorAction(const G4String & parName, G4double energy, G4ThreeVector pos, G4ThreeVector momDirection){ const G4int nParticles = 1; fParticleGun = new G4ParticleGun(nParticles); G4ParticleTable* parTable = G4ParticleTable::GetParticleTable(); G4ParticleDefinition* parDefinition = parTable->FindParticle(parName); fParticleGun->SetParticleDefinition(parDefinition); fParticleGun->SetParticleEnergy(energy); fParticleGun->SetParticlePosition(pos); fParticleGun->SetParticleMomentumDirection(momDirection); } The primary generator

5. Class PrimaryGeneratorAction • Inherits from G4VUserPrimaryGeneratorAction • User should override GeneratePrimaries for particle generation PrimaryGeneratorAction::GeneratePrimaries(G4Event* evt){ //some additional random sampling here fParticleGun->GeneratePrimaryVertex(evt); }

6. Alternative Method: GPS • The General Particle Source (GPS)1 provides a high-level interface to G4ParticleGun, mainly using macros • Define source types: point, beam, plane, surface, volume • Define angular distribution: isotropic, cosine-law, planar, 1d/2d beams, user defined • Define energy distribution: mono-energetic, linear, power-law, exponential, gaussian, Bremsstrahlung-spectrum, black body spectrum, cosmic diffuse gamma ray, user defined • Angular and energy distributions can be interpolated from histogrammed distributions • To use simply replace G4ParticleGun in PrimaryGeneratorAction with G4GeneralParticleSource

7. Alternative Method: GPS • For full documentation:http://reat.space.qinetiq.com/gps/new_gps_sum_files/gps_sum.htm