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Question Unwanted beam dispersion in vacuum. 

Forum: Fast Simulation, Transportation & Others
Date: 19 Jun, 2013
From: Martin McHugh <Martin McHugh>

Hello fellow users,

I'm developing a simulation of a detector attached to a polarimeter however I'm facing a basic issue I can't find any other documentation of. I fire a monoenergetic from (3-8 MeV) beam of electrons from about 2' (0.6 m) away, they must pass through a circular aperture 0.38" (~0.9 cm) across. until they get to the aperture, they're passing through vacuum defined as such:

  //Vacuum
  G4Material* Air = G4NistManager::Instance()->FindOrBuildMaterial("G4_AIR");
  G4Material* Vacuum = new G4Material("Vacuum", density=1.e-5*g/cm3, 
				      nel=1, kStateGas, 
                                      temperature=STP_Temperature, 
				      pressure=2.e-2*bar);
  Vacuum->AddMaterial(Air, fractionmass=1.0);

The problem I encounter is this: At 3 MeV only ~50% of the electrons thrown actually make it through the aperature and into the detector which I believe is unphysical due to the typical beam spot size of O(1 mm) in this device. The relevant sections from the PrimaryGeneratorAction are shown below:

  G4int n_particle = 1;
  particleGun = new G4ParticleGun(n_particle);

  // Set default particle to electron

  G4ParticleTable* particleTable = G4ParticleTable::GetParticleTable();
  G4ParticleDefinition* particle = particleTable->FindParticle("e-");

  G4double energy = 5.0*MeV;
  G4double position = 0.0*cm;
  G4ThreeVector direction = G4ThreeVector(0.0, 0.0, 1.0);
  		direction.setTheta(172.7*deg);

  particleGun->SetParticleDefinition(particle);
  particleGun->SetParticlePosition(G4ThreeVector(0.*cm,0.*cm,position));
  particleGun->SetParticleMomentumDirection(direction);
  particleGun->SetParticleEnergy(energy);

And the primaries are generated in

void MottPrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
{   
  particleGun->GeneratePrimaryVertex(anEvent);
}

Any input at all would be welcome since I've been searching for a method of dealing with this for the last two weeks or so while doing other work on the simulation but now it's becoming a major stumbling block.

Best, Marty

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