//defining a GPS
fGPS = new G4GeneralParticleSource();         

//Primary particles
G4ParticleDefinition* particle = 
	G4ParticleTable::GetParticleTable()->FindParticle("e-");
fGPS->SetParticleDefinition(particle);  

//Spatial distribution
G4SPSPosDistribution* vPosDist = fGPS->GetCurrentSource()->GetPosDist();
vPosDist->SetPosDisType("Plane");
vPosDist->SetPosDisShape("Circle");
vPosDist->SetRadius(50.*mm);   
vPosDist->SetCentreCoords(G4ThreeVector(0.,0.,0.));

//Angular distribution
G4SPSAngDistribution* vAngDist = fGPS->GetCurrentSource()->GetAngDist();
vAngDist->SetParticleMomentumDirection(G4ThreeVector(0., 0., 1.));  

//Energy distribution
G4SPSEneDistribution* vEneDist = fGPS->GetCurrentSource()->GetEneDist();
vEneDist->SetEnergyDisType("Mono");
vEneDist->SetMonoEnergy(400.*keV);