G4LivermoreNuclearGammaConversionModel.cc

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// $Id$
//
// Authors: G.Depaola & F.Longo
//

#include "G4LivermoreNuclearGammaConversionModel.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"

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using namespace std;

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G4LivermoreNuclearGammaConversionModel::G4LivermoreNuclearGammaConversionModel(const G4ParticleDefinition*,
                                 const G4String& nam)
  :G4VEmModel(nam),fParticleChange(0),smallEnergy(2.*MeV),
   isInitialised(false),
   crossSectionHandler(0),meanFreePathTable(0)
{
  lowEnergyLimit = 2.0*electron_mass_c2;
  highEnergyLimit = 100 * GeV;
  SetHighEnergyLimit(highEnergyLimit);
     
  verboseLevel= 0;
  // Verbosity scale:
  // 0 = nothing 
  // 1 = warning for energy non-conservation 
  // 2 = details of energy budget
  // 3 = calculation of cross sections, file openings, sampling of atoms
  // 4 = entering in methods

  if(verboseLevel > 0) {
    G4cout << "Livermore Nuclear Gamma conversion is constructed " << G4endl
       << "Energy range: "
       << lowEnergyLimit / MeV << " MeV - "
       << highEnergyLimit / GeV << " GeV"
       << G4endl;
  }
}

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G4LivermoreNuclearGammaConversionModel::~G4LivermoreNuclearGammaConversionModel()
{  
  if (crossSectionHandler) delete crossSectionHandler;
}

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void 
G4LivermoreNuclearGammaConversionModel::Initialise(const G4ParticleDefinition*,
                        const G4DataVector&)
{
  if (verboseLevel > 3)
    G4cout << "Calling G4LivermoreNuclearGammaConversionModel::Initialise()" << G4endl;

  if (crossSectionHandler)
  {
    crossSectionHandler->Clear();
    delete crossSectionHandler;
  }

  // Read data tables for all materials
  
  crossSectionHandler = new G4CrossSectionHandler();
  crossSectionHandler->Initialise(0,lowEnergyLimit,100.*GeV,400);
  G4String crossSectionFile = "pairdata/pp-pair-cs-"; // here only pair in nuclear field cs should be used
  crossSectionHandler->LoadData(crossSectionFile);

  //
  
  if (verboseLevel > 0) {
    G4cout << "Loaded cross section files for Livermore GammaConversion" << G4endl;
    G4cout << "To obtain the total cross section this should be used only " << G4endl 
       << "in connection with G4ElectronGammaConversion " << G4endl;
  }

  if (verboseLevel > 0) { 
    G4cout << "Livermore Nuclear Gamma Conversion model is initialized " << G4endl
       << "Energy range: "
       << LowEnergyLimit() / MeV << " MeV - "
       << HighEnergyLimit() / GeV << " GeV"
       << G4endl;
  }

  if(isInitialised) return;
  fParticleChange = GetParticleChangeForGamma();
  isInitialised = true;
}

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G4double 
G4LivermoreNuclearGammaConversionModel::ComputeCrossSectionPerAtom(const G4ParticleDefinition*,
                                G4double GammaEnergy,
                                G4double Z, G4double,
                                G4double, G4double)
{
  if (verboseLevel > 3) {
    G4cout << "Calling ComputeCrossSectionPerAtom() of G4LivermoreNuclearGammaConversionModel" 
       << G4endl;
  }
  if (GammaEnergy < lowEnergyLimit || GammaEnergy > highEnergyLimit) return 0;

  G4double cs = crossSectionHandler->FindValue(G4int(Z), GammaEnergy);
  return cs;
}

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void G4LivermoreNuclearGammaConversionModel::SampleSecondaries(std::vector<G4DynamicParticle*>* fvect,
                          const G4MaterialCutsCouple* couple,
                          const G4DynamicParticle* aDynamicGamma,
                          G4double,
                          G4double)
{

// The energies of the e+ e- secondaries are sampled using the Bethe - Heitler
// cross sections with Coulomb correction. A modified version of the random
// number techniques of Butcher & Messel is used (Nuc Phys 20(1960),15).

// Note 1 : Effects due to the breakdown of the Born approximation at low
// energy are ignored.
// Note 2 : The differential cross section implicitly takes account of
// pair creation in both nuclear and atomic electron fields. However triplet
// prodution is not generated.

  if (verboseLevel > 3)
    G4cout << "Calling SampleSecondaries() of G4LivermoreNuclearGammaConversionModel" << G4endl;

  G4double photonEnergy = aDynamicGamma->GetKineticEnergy();
  G4ParticleMomentum photonDirection = aDynamicGamma->GetMomentumDirection();

  G4double epsilon ;
  G4double epsilon0Local = electron_mass_c2 / photonEnergy ;

  // Do it fast if photon energy < 2. MeV
  if (photonEnergy < smallEnergy )
    {
      epsilon = epsilon0Local + (0.5 - epsilon0Local) * G4UniformRand();
    }
  else
    {
      // Select randomly one element in the current material
      //const G4Element* element = crossSectionHandler->SelectRandomElement(couple,photonEnergy);
      const G4ParticleDefinition* particle =  aDynamicGamma->GetDefinition();
      const G4Element* element = SelectRandomAtom(couple,particle,photonEnergy);

      if (element == 0)
    {
      G4cout << "G4LivermoreNuclearGammaConversionModel::SampleSecondaries - element = 0" 
         << G4endl;
      return;
    }
      G4IonisParamElm* ionisation = element->GetIonisation();
      if (ionisation == 0)
    {
      G4cout << "G4LivermoreNuclearGammaConversionModel::SampleSecondaries - ionisation = 0" 
         << G4endl;
      return;
    }

      // Extract Coulomb factor for this Element
      G4double fZ = 8. * (ionisation->GetlogZ3());
      if (photonEnergy > 50. * MeV) fZ += 8. * (element->GetfCoulomb());

      // Limits of the screening variable
      G4double screenFactor = 136. * epsilon0Local / (element->GetIonisation()->GetZ3()) ;
      G4double screenMax = std::exp ((42.24 - fZ)/8.368) - 0.952 ;
      G4double screenMin = std::min(4.*screenFactor,screenMax) ;

      // Limits of the energy sampling
      G4double epsilon1 = 0.5 - 0.5 * std::sqrt(1. - screenMin / screenMax) ;
      G4double epsilonMin = std::max(epsilon0Local,epsilon1);
      G4double epsilonRange = 0.5 - epsilonMin ;

      // Sample the energy rate of the created electron (or positron)
      G4double screen;
      G4double gReject ;

      G4double f10 = ScreenFunction1(screenMin) - fZ;
      G4double f20 = ScreenFunction2(screenMin) - fZ;
      G4double normF1 = std::max(f10 * epsilonRange * epsilonRange,0.);
      G4double normF2 = std::max(1.5 * f20,0.);

      do {
    if (normF1 / (normF1 + normF2) > G4UniformRand() )
      {
        epsilon = 0.5 - epsilonRange * std::pow(G4UniformRand(), 0.3333) ;
        screen = screenFactor / (epsilon * (1. - epsilon));
        gReject = (ScreenFunction1(screen) - fZ) / f10 ;
      }
    else
      {
        epsilon = epsilonMin + epsilonRange * G4UniformRand();
        screen = screenFactor / (epsilon * (1 - epsilon));
        gReject = (ScreenFunction2(screen) - fZ) / f20 ;
      }
      } while ( gReject < G4UniformRand() );

    }   //  End of epsilon sampling

  // Fix charges randomly

  G4double electronTotEnergy;
  G4double positronTotEnergy;

  if (G4int(2*G4UniformRand()))    
    {
      electronTotEnergy = (1. - epsilon) * photonEnergy;
      positronTotEnergy = epsilon * photonEnergy;
    }
  else
    {
      positronTotEnergy = (1. - epsilon) * photonEnergy;
      electronTotEnergy = epsilon * photonEnergy;
    }

  // Scattered electron (positron) angles. ( Z - axis along the parent photon)
  // Universal distribution suggested by L. Urban (Geant3 manual (1993) Phys211),
  // derived from Tsai distribution (Rev. Mod. Phys. 49, 421 (1977)

  G4double u;
  const G4double a1 = 0.625;
  G4double a2 = 3. * a1;
  //  G4double d = 27. ;

  //  if (9. / (9. + d) > G4UniformRand())
  if (0.25 > G4UniformRand())
    {
      u = - std::log(G4UniformRand() * G4UniformRand()) / a1 ;
    }
  else
    {
      u = - std::log(G4UniformRand() * G4UniformRand()) / a2 ;
    }

  G4double thetaEle = u*electron_mass_c2/electronTotEnergy;
  G4double thetaPos = u*electron_mass_c2/positronTotEnergy;
  G4double phi  = twopi * G4UniformRand();

  G4double dxEle= std::sin(thetaEle)*std::cos(phi),dyEle= std::sin(thetaEle)*std::sin(phi),dzEle=std::cos(thetaEle);
  G4double dxPos=-std::sin(thetaPos)*std::cos(phi),dyPos=-std::sin(thetaPos)*std::sin(phi),dzPos=std::cos(thetaPos);
  
  
  // Kinematics of the created pair:
  // the electron and positron are assumed to have a symetric angular 
  // distribution with respect to the Z axis along the parent photon
  
  G4double electronKineEnergy = std::max(0.,electronTotEnergy - electron_mass_c2) ;
  
  // SI - The range test has been removed wrt original G4LowEnergyGammaconversion class

  G4ThreeVector electronDirection (dxEle, dyEle, dzEle);
  electronDirection.rotateUz(photonDirection);
      
  G4DynamicParticle* particle1 = new G4DynamicParticle (G4Electron::Electron(),
                                electronDirection,
                                electronKineEnergy);

  // The e+ is always created (even with kinetic energy = 0) for further annihilation
  G4double positronKineEnergy = std::max(0.,positronTotEnergy - electron_mass_c2) ;

  // SI - The range test has been removed wrt original G4LowEnergyGammaconversion class

  G4ThreeVector positronDirection (dxPos, dyPos, dzPos);
  positronDirection.rotateUz(photonDirection);   
  
  // Create G4DynamicParticle object for the particle2 
  G4DynamicParticle* particle2 = new G4DynamicParticle(G4Positron::Positron(),
                               positronDirection, positronKineEnergy);
  // Fill output vector

  fvect->push_back(particle1);
  fvect->push_back(particle2);

  // kill incident photon
  fParticleChange->SetProposedKineticEnergy(0.);
  fParticleChange->ProposeTrackStatus(fStopAndKill);   

}

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G4double G4LivermoreNuclearGammaConversionModel::ScreenFunction1(G4double screenVariable)
{
  // Compute the value of the screening function 3*phi1 - phi2

  G4double value;
  
  if (screenVariable > 1.)
    value = 42.24 - 8.368 * std::log(screenVariable + 0.952);
  else
    value = 42.392 - screenVariable * (7.796 - 1.961 * screenVariable);
  
  return value;
} 

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G4double G4LivermoreNuclearGammaConversionModel::ScreenFunction2(G4double screenVariable)
{
  // Compute the value of the screening function 1.5*phi1 - 0.5*phi2
  
  G4double value;
  
  if (screenVariable > 1.)
    value = 42.24 - 8.368 * std::log(screenVariable + 0.952);
  else
    value = 41.405 - screenVariable * (5.828 - 0.8945 * screenVariable);
  
  return value;
}