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// $Id: G4eeToTwoGammaModel.cc,v 1.15 2009/04/09 18:41:18 vnivanch Exp $
// GEANT4 tag $Name: geant4-09-03 $
//
// -------------------------------------------------------------------
//
// GEANT4 Class file
//
//
// File name:   G4eeToTwoGammaModel
//
// Author:        Vladimir Ivanchenko on base of Michel Maire code
//
// Creation date: 02.08.2004
//
// Modifications:
// 08-04-05 Major optimisation of internal interfaces (V.Ivanchenko)
// 18-04-05 Compute CrossSectionPerVolume (V.Ivanchenko)
// 06-02-06 ComputeCrossSectionPerElectron, ComputeCrossSectionPerAtom (mma)
// 29-06-06 Fix problem for zero energy incident positron (V.Ivanchenko) 
// 20-10-06 Add theGamma as a member (V.Ivanchenko)
//
//
// Class Description:
//
// Implementation of e+ annihilation into 2 gamma
//
// The secondaries Gamma energies are sampled using the Heitler cross section.
//
// A modified version of the random number techniques of Butcher & Messel
// is used (Nuc Phys 20(1960),15).
//
// GEANT4 internal units.
//
// Note 1: The initial electron is assumed free and at rest.
//
// Note 2: The annihilation processes producing one or more than two photons are
//         ignored, as negligible compared to the two photons process.



//
// -------------------------------------------------------------------
//
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#include "G4eeToTwoGammaModel.hh"
#include "G4TrackStatus.hh"
#include "G4Electron.hh"
#include "G4Positron.hh"
#include "G4Gamma.hh"
#include "Randomize.hh"
#include "G4ParticleChangeForGamma.hh"

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using namespace std;

G4eeToTwoGammaModel::G4eeToTwoGammaModel(const G4ParticleDefinition*,
                                         const G4String& nam)
  : G4VEmModel(nam),
    pi_rcl2(pi*classic_electr_radius*classic_electr_radius),
    isInitialised(false)
{
  theGamma = G4Gamma::Gamma();
}

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G4eeToTwoGammaModel::~G4eeToTwoGammaModel()
{}

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void G4eeToTwoGammaModel::Initialise(const G4ParticleDefinition*,
                                     const G4DataVector&)
{
  if(isInitialised) return;
  fParticleChange = GetParticleChangeForGamma();
  isInitialised = true;
}

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G4double G4eeToTwoGammaModel::ComputeCrossSectionPerElectron(
                                       const G4ParticleDefinition*,
                                       G4double kineticEnergy,
				       G4double, G4double)
{
  // Calculates the cross section per electron of annihilation into two photons
  // from the Heilter formula.
  
  G4double tau   = kineticEnergy/electron_mass_c2;
  G4double gam   = tau + 1.0;
  G4double gamma2= gam*gam;
  G4double bg2   = tau * (tau+2.0);
  G4double bg    = sqrt(bg2);

  G4double cross = pi_rcl2*((gamma2+4*gam+1.)*log(gam+bg) - (gam+3.)*bg)
                 / (bg2*(gam+1.));
  return cross;  
}

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G4double G4eeToTwoGammaModel::ComputeCrossSectionPerAtom(
                                    const G4ParticleDefinition* p,
                                    G4double kineticEnergy, G4double Z,
				    G4double, G4double, G4double)
{
  // Calculates the cross section per atom of annihilation into two photons
  
  G4double cross = Z*ComputeCrossSectionPerElectron(p,kineticEnergy);
  return cross;  
}

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G4double G4eeToTwoGammaModel::CrossSectionPerVolume(
					const G4Material* material,
					const G4ParticleDefinition* p,
					      G4double kineticEnergy,
					      G4double, G4double)
{
  // Calculates the cross section per volume of annihilation into two photons
  
  G4double eDensity = material->GetElectronDensity();
  G4double cross = eDensity*ComputeCrossSectionPerElectron(p,kineticEnergy);
  return cross;
}

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void G4eeToTwoGammaModel::SampleSecondaries(vector<G4DynamicParticle*>* vdp,
					    const G4MaterialCutsCouple*,
					    const G4DynamicParticle* dp,
					    G4double,
					    G4double)
{
  G4double PositKinEnergy = dp->GetKineticEnergy();

  // Case at rest
  if(PositKinEnergy == 0.0) {
    G4double cost = 2.*G4UniformRand()-1.;
    G4double sint = sqrt((1. - cost)*(1. + cost));
    G4double phi  = twopi * G4UniformRand();
    G4ThreeVector dir (sint*cos(phi), sint*sin(phi), cost);
    G4DynamicParticle* aGamma1 = new G4DynamicParticle(theGamma,
						       dir, electron_mass_c2);
    G4DynamicParticle* aGamma2 = new G4DynamicParticle(theGamma,
						       -dir, electron_mass_c2);
    vdp->push_back(aGamma1);
    vdp->push_back(aGamma2);

  } else {

    G4ThreeVector PositDirection = dp->GetMomentumDirection();

    G4double tau     = PositKinEnergy/electron_mass_c2;
    G4double gam     = tau + 1.0;
    G4double tau2    = tau + 2.0;
    G4double sqgrate = sqrt(tau/tau2)*0.5;
    G4double sqg2m1  = sqrt(tau*tau2);

    // limits of the energy sampling
    G4double epsilmin = 0.5 - sqgrate;
    G4double epsilmax = 0.5 + sqgrate;
    G4double epsilqot = epsilmax/epsilmin;

    //
    // sample the energy rate of the created gammas
    //
    G4double epsil, greject;

    do {
      epsil = epsilmin*pow(epsilqot,G4UniformRand());
      greject = 1. - epsil + (2.*gam*epsil-1.)/(epsil*tau2*tau2);
    } while( greject < G4UniformRand() );

    //
    // scattered Gamma angles. ( Z - axis along the parent positron)
    //

    G4double cost = (epsil*tau2-1.)/(epsil*sqg2m1);
    if(std::abs(cost) > 1.0) {
      G4cout << "### G4eeToTwoGammaModel WARNING cost= " << cost
	     << " positron Ekin(MeV)= " << PositKinEnergy
	     << " gamma epsil= " << epsil
	     << G4endl;
      if(cost > 1.0) cost = 1.0;
      else cost = -1.0; 
    }
    G4double sint = sqrt((1.+cost)*(1.-cost));
    G4double phi  = twopi * G4UniformRand();

    G4double dirx = sint*cos(phi) , diry = sint*sin(phi) , dirz = cost;

    //
    // kinematic of the created pair
    //

    G4double TotalAvailableEnergy = PositKinEnergy + 2.0*electron_mass_c2;
    G4double Phot1Energy = epsil*TotalAvailableEnergy;

    G4ThreeVector Phot1Direction (dirx, diry, dirz);
    Phot1Direction.rotateUz(PositDirection);
    G4DynamicParticle* aGamma1 = 
      new G4DynamicParticle (theGamma,Phot1Direction, Phot1Energy);
    vdp->push_back(aGamma1);

    G4double Phot2Energy =(1.-epsil)*TotalAvailableEnergy;
    G4double PositP= sqrt(PositKinEnergy*(PositKinEnergy+2.*electron_mass_c2));
    G4ThreeVector dir = PositDirection*PositP - Phot1Direction*Phot1Energy;
    G4ThreeVector Phot2Direction = dir.unit();

    // create G4DynamicParticle object for the particle2
    G4DynamicParticle* aGamma2= 
      new G4DynamicParticle (theGamma,Phot2Direction, Phot2Energy);
    vdp->push_back(aGamma2);
    /*
      G4cout << "Annihilation in fly: e0= " << PositKinEnergy
      << " m= " << electron_mass_c2
      << " e1= " << Phot1Energy 
      << " e2= " << Phot2Energy << " dir= " <<  dir 
      << " -> " << Phot1Direction << " " 
      << Phot2Direction << G4endl;
    */
  }
  fParticleChange->SetProposedKineticEnergy(0.);
  fParticleChange->ProposeTrackStatus(fStopAndKill);
}

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