Calibrating the output(output factor) of a smaller source using Geant4/GATE MonteCarlo simulation

Question

I am using GATE(which uses Geant4) to do MC studies on dosimetric output. I am using a cylindrical cobalt source at 80 cm SAD to measure the PDD in a water phantom and dose at depth of 10 cm.

I now want to simulate a smaller source (say, r/2 and h/2) and compare the dosimetric output at a depth of 10 cm. Besides the geometry, I see that I am able to control the number of particles and time of the simulation. What would be the best way to change these two parameters to mimic the lower output from a smaller source? Or is there any other parameter that can be changed to mimic a smaller source? I am trying to calculate the output factor of the smaller source w.r.t. to the original source.

Answer 1

Not sure if it helps, this is cylindrical source with Co60

Source::Source():
    _particleGun{nullptr},
    _sourceMessenger{nullptr},
    _radius{-1.0},
    _halfz{-1.0},
    _nof_particles{10}
{
    _particleGun = new G4ParticleGun( 1 );

    G4ParticleTable* particleTable = G4ParticleTable::GetParticleTable();
    G4String particleName = "gamma"; // "geantino"
    _particleGun->SetParticleDefinition(particleTable->FindParticle(particleName));

    _particleGun->SetParticlePosition(G4ThreeVector(0., 0., 0.));
    _particleGun->SetParticleMomentumDirection(G4ThreeVector(0., 0., 1.));
    _particleGun->SetParticleEnergy(1000.0*MeV);

    _sourceMessenger = new SourceMessenger(this);
}


Source::~Source()
{
    delete _particleGun;
    delete _sourceMessenger;
}


troika Source::sample_direction()
{
    double phi   = 2.0 * M_PI * G4UniformRand();

    double cos_z = 2.0 * G4UniformRand() - 1.0;
    double sin_z = sqrt( (1.0 - cos_z) * (1.0 + cos_z) );

    return troika{ sin_z * cos(phi), sin_z * sin(phi), cos_z };
}


double Source::sample_energy()
{
    return (G4UniformRand() < P_lo) ? E_lo : E_hi;
}


void Source::GeneratePrimaries(G4Event* anEvent)
{
    for(int k = 0; k != _nof_particles; ++k) // we generate _nof_particles at once
    {
        // here we sample spatial decay vertex uniformly in the cylinder
        double z   = _halfz * ( 2.0*G4UniformRand() - 1.0 );
        double phi = 2.0 * M_PI * G4UniformRand();
        double r   = _radius * sqrt(G4UniformRand());

        auto x = r * cos(phi);
        auto y = r * sin(phi);
        _particleGun->SetParticlePosition(G4ThreeVector(x, y, z));

        // now uniform-on-the-sphere direction
        auto dir = sample_direction();
        _particleGun->SetParticleMomentumDirection(G4ThreeVector(dir._wx, dir._wy, dir._wz));

        // energy 50/50 1.17 or 1.33
        auto e = sample_energy();
        _particleGun->SetParticleEnergy(e);

        // all together in a vertex
        _particleGun->GeneratePrimaryVertex(anEvent);
    }
}