G4ModifiedTsai.cc

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//
// -------------------------------------------------------------------
//
// GEANT4 Class file
//
//
// File name:     G4ModifiedTsai
//
// Author:        Andreia Trindade (andreia@lip.pt)
//                Pedro Rodrigues  (psilva@lip.pt)
//                Luis Peralta     (luis@lip.pt)
// 
// Creation date: 21 March 2003
//
// Modifications: 
// 21 Mar 2003 A.Trindade First implementation acording with new design
// 24 Mar 2003 A.Trindade Fix in Tsai generator in order to prevent theta 
//                        generation above pi
// 13 Oct 2010  V.Ivanchenko  Moved to standard and improved comment
// 26.04.2011   V.Grichine    Clean-up of PolarAngle method 
//
// Class Description: 
//
// Bremsstrahlung Angular Distribution Generation 
// suggested by L.Urban (Geant3 manual (1993) Phys211)
// Derived from Tsai distribution (Rev Mod Phys 49,421(1977))
//
// Class Description: End 
//
// -------------------------------------------------------------------
//

#include "G4ModifiedTsai.hh"
#include "Randomize.hh"
#include "G4Log.hh"
#include <CLHEP/Units/PhysicalConstants.h>

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G4ModifiedTsai::G4ModifiedTsai(const G4String&)
  : G4VEmAngularDistribution("ModifiedTsai")
{}    

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G4ModifiedTsai::~G4ModifiedTsai() 
{}

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G4ThreeVector& 
G4ModifiedTsai::SampleDirection(const G4DynamicParticle* dp,
                                G4double, G4int, const G4Material*)
{
  // Sample gamma angle (Z - axis along the parent particle).
  G4double cost = SampleCosTheta(dp->GetKineticEnergy());
  G4double sint = std::sqrt((1 - cost)*(1 + cost));
  G4double phi  = CLHEP::twopi*G4UniformRand(); 

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

  return fLocalDirection;
}

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G4double G4ModifiedTsai::SampleCosTheta(G4double kinEnergy)
{
  // Universal distribution suggested by L. Urban (Geant3 manual (1993) 
  // Phys211) derived from Tsai distribution (Rev Mod Phys 49,421(1977))

  G4double uMax = 2*(1. + kinEnergy/CLHEP::electron_mass_c2);   

  static const G4double a1     = 1.6;
  static const G4double a2     = a1/3.;
  static const G4double border = 0.25;
  G4double u;
  CLHEP::HepRandomEngine* rndmEngine = G4Random::getTheEngine();

  do {
    G4double uu = -G4Log(rndmEngine->flat()*rndmEngine->flat());
    u = (border > rndmEngine->flat()) ? uu*a1 : uu*a2;
    
    // Loop checking, 03-Aug-2015, Vladimir Ivanchenko
  } while(u > uMax);

  return 1.0 - 2.0*u*u/(uMax*uMax);
}

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void G4ModifiedTsai::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 G4ModifiedTsai::PrintGeneratorInformation() const
{
  G4cout << "\n" << G4endl;
  G4cout << "Bremsstrahlung Angular Generator is Modified Tsai" << G4endl;
  G4cout << "Distribution suggested by L.Urban (Geant3 manual (1993) Phys211)" 
         << G4endl;
  G4cout << "Derived from Tsai distribution (Rev Mod Phys 49,421(1977)) \n" 
         << G4endl;
} 

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