G4DipBustGenerator.cc

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// $Id: G4DipBustGenerator.cc 110415 2018-05-23 06:44:31Z gcosmo $
//
// -------------------------------------------------------------------
//
// GEANT4 Class file
//
//
// File name:     G4DipBustGenerator
//
// Author:  Vladimir Grichine     
// 
// Creation date: 17 May 2011
//
// Modifications: 
// 
//
// Class Description: 
//
// Bremsstrahlung Angular Distribution Generation 
// suggested  the dipole approximation in the rest frame of electron 
// busted in the laboratory frame.
//
// Class Description: End 
//
// -------------------------------------------------------------------
//

#include "G4DipBustGenerator.hh"
#include "Randomize.hh"
#include "G4Log.hh"
#include "G4Exp.hh"
#include <CLHEP/Units/PhysicalConstants.h>

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G4DipBustGenerator::G4DipBustGenerator(const G4String&)
  : G4VEmAngularDistribution("DipBustGen")
{}    

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G4DipBustGenerator::~G4DipBustGenerator() 
{}

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G4double G4DipBustGenerator::SampleCosTheta(G4double kinEnergy)
{
  G4double c = 4. - 8.*G4UniformRand();
  G4double delta  = 0.5*(std::sqrt(c*c+4.) + std::abs(c));
  G4double signc = (c < 0.) ? -1.0 : 1.0; 
  G4double cofA = -signc*G4Exp(G4Log(delta)/3.0);
  G4double cosTheta = cofA - 1./cofA;
  G4double tau = kinEnergy/CLHEP::electron_mass_c2;
  G4double beta = std::sqrt(tau*(tau + 2.))/(tau + 1.);

  return (cosTheta + beta)/(1 + cosTheta*beta);
}

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G4ThreeVector& 
G4DipBustGenerator::SampleDirection(const G4DynamicParticle* dp,
            G4double, G4int, const G4Material*)
{
  G4double cosTheta = SampleCosTheta(dp->GetKineticEnergy());

  G4double sinTheta = std::sqrt((1 - cosTheta)*(1 + cosTheta));
  G4double phi  = CLHEP::twopi*G4UniformRand(); 

  fLocalDirection.set(sinTheta*std::cos(phi), sinTheta*std::sin(phi),cosTheta);
  fLocalDirection.rotateUz(dp->GetMomentumDirection());

  return fLocalDirection;
}

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G4double G4DipBustGenerator::PolarAngle(G4double eTkin,
          G4double, // final_energy
          G4int ) // Z
{
  G4double cosTheta = SampleCosTheta(eTkin);
  G4double theta = std::acos(cosTheta);

  theta = std::min(std::max(theta, 0.), CLHEP::pi);
  return theta;
}

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void G4DipBustGenerator::SamplePairDirections(const G4DynamicParticle* dp,
                G4double elecKinEnergy,
                G4double posiKinEnergy,
                G4ThreeVector& dirElectron,
                G4ThreeVector& dirPositron,
                G4int, const G4Material*)
{
  G4double phi  = CLHEP::twopi * G4UniformRand();
  G4double sinp = std::sin(phi);
  G4double cosp = std::cos(phi);

  G4double cost = SampleCosTheta(elecKinEnergy);
  G4double sint = std::sqrt((1. - cost)*(1. + cost));

  dirElectron.set(sint*cosp, sint*sinp, cost);
  dirElectron.rotateUz(dp->GetMomentumDirection());

  cost = SampleCosTheta(posiKinEnergy);
  sint = std::sqrt((1. - cost)*(1. + cost));

  dirPositron.set(-sint*cosp, -sint*sinp, cost);
  dirPositron.rotateUz(dp->GetMomentumDirection());
}

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void G4DipBustGenerator::PrintGeneratorInformation() const
{
  G4cout << "\n" << G4endl;
  G4cout << "Angular Generator based on classical formula from" << G4endl;
  G4cout << "J.D. Jackson, Classical Electrodynamics, Wiley, New York 1975" 
   << G4endl;
} 

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