G4GammaTransition.cc

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// $Id: G4GammaTransition.cc 85659 2014-11-03 10:59:10Z vnivanch $
//
// -------------------------------------------------------------------
//      GEANT 4 class file 
//
//      CERN, Geneva, Switzerland
//
//      File name:     G4GammaTransition
//
//      Author V.Ivanchenko 27 February 2015
//
// -------------------------------------------------------------------

#include "G4GammaTransition.hh"
#include "G4AtomicShells.hh"
#include "Randomize.hh"
#include "G4Gamma.hh"
#include "G4Electron.hh"
#include "G4LorentzVector.hh"
#include "G4SystemOfUnits.hh"
#include "G4PhysicalConstants.hh"

G4GammaTransition::G4GammaTransition() 
{}

G4GammaTransition::~G4GammaTransition() 
{}
  
G4Fragment* 
G4GammaTransition::SampleTransition(G4Fragment* nucleus,
                                    G4double newExcEnergy,
                                    G4int, size_t shell,
                                    G4bool isGamma,
                                    G4bool isLongLived)
{
  G4Fragment* result = 0;
  G4double bond_energy = 0.;

  if (!isGamma) { 
    G4int Z = nucleus->GetZ_asInt();
    if(Z <= 100) {
      G4int idx = (G4int)shell;
      idx = std::min(idx, G4AtomicShells::GetNumberOfShells(Z)-1);
      bond_energy = G4AtomicShells::GetBindingEnergy(Z, idx);
    }
  }
  G4double etrans = nucleus->GetExcitationEnergy() - newExcEnergy 
    - bond_energy;
  //G4cout << "G4GammaTransition::GenerateGamma - Etrans(MeV)= " 
  //     << etrans << "  Eexnew= " << newExcEnergy 
  //     << " Ebond= " << bond_energy << G4endl; 
  if(etrans <= 0.0) { return result; }
  
  // Do complete Lorentz computation 
  G4LorentzVector lv = nucleus->GetMomentum();
  G4double mass = nucleus->GetGroundStateMass() + newExcEnergy;

  //G4double e0 = lv.e();

  // select secondary
  G4ParticleDefinition* part;

  if(isGamma) { part =  G4Gamma::Gamma(); }
  else {
    part = G4Electron::Electron();
    G4int ne = nucleus->GetNumberOfElectrons() - 1;
    if(ne < 0) { ne = 0; } 
    nucleus->SetNumberOfElectrons(ne);
    lv += G4LorentzVector(0.0,0.0,0.0,
                          CLHEP::electron_mass_c2 - bond_energy);
  }

  G4double cosTheta = 1. - 2. * G4UniformRand(); 
  G4double sinTheta = std::sqrt(1. - cosTheta * cosTheta);
  G4double phi = twopi * G4UniformRand();

  G4double emass = part->GetPDGMass();

  // 2-body decay in rest frame
  G4double ecm       = lv.mag();
  G4ThreeVector bst  = lv.boostVector();

  //G4cout << "Ecm= " << ecm << " mass= " << mass << " emass= " << emass 
  //     << "  isLongLived: " << isLongLived << G4endl;

  G4double energy = 0.5*((ecm - mass)*(ecm + mass) + emass*emass)/ecm;
  energy = std::max(energy, emass);

  G4double mom = std::sqrt((energy - emass)*(energy + emass));

  G4LorentzVector res4mom(mom * sinTheta * std::cos(phi),
                          mom * sinTheta * std::sin(phi),
                          mom * cosTheta, energy);

  // Lab system transform for short lived level
  if(!isLongLived) { 
    res4mom.boost(bst); 
    lv -= res4mom;
  } else {  
    // In exceptional case sample decay at rest at not correct position 
    // of stopping ion, 4-momentum balance is breaked but gamma energy
    // is correct
    lv -= res4mom;
    G4double E = lv.e();
    G4double P2= (E - mass)*(E + mass);
    G4ThreeVector v = lv.vect().unit();
    G4double p = 0.0;
    if(P2 > 0.0) { p = std::sqrt(P2); } 
    else { E = mass; }
    lv.set(v.x()*p, v.y()*p, v.z()*p, E);  
  }

  // modified primary fragment 
  nucleus->SetMomentum(lv);

  // gamma or e- are produced
  result = new G4Fragment(res4mom, part);

  //  G4cout << " DeltaE= " << e0 - lv.e() - res4mom.e() << G4endl;
  //G4cout << "G4GammaTransition::GenerateGamma : " << thePhoton << G4endl;
  //G4cout << "       Left nucleus: " << aNucleus << G4endl;
  return result;
}