G4FermiPhaseSpaceDecay.cc

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//
// Hadronic Process: Phase space decay for the Fermi BreakUp model
// by V. Lara
//
// Modifications:
// 01.04.2011 General cleanup by V.Ivanchenko: 
//          - IsotropicVector is inlined
//          - Momentum computation return zero or positive value
//          - DumpProblem method is added providing more information
//          - Reduced usage of exotic std functions  

#include "G4FermiPhaseSpaceDecay.hh"

#include "G4RandomDirection.hh"
#include "G4Pow.hh"

#include <CLHEP/Units/SystemOfUnits.h>
#include <CLHEP/Units/PhysicalConstants.h>
#include <CLHEP/Random/RandomEngine.h>

G4FermiPhaseSpaceDecay::G4FermiPhaseSpaceDecay()
{
  g4calc = G4Pow::GetInstance();
}

G4FermiPhaseSpaceDecay::~G4FermiPhaseSpaceDecay()
{}

std::vector<G4LorentzVector*>* G4FermiPhaseSpaceDecay::Decay(G4double M, 
                               const std::vector<G4double>& mr) const
  // Calculates momentum for N fragments (Kopylov's method of sampling is used)
{
  size_t N = mr.size();

  std::vector<G4LorentzVector*>* P = 
    new std::vector<G4LorentzVector*>(N, nullptr);

  G4double mtot = 0.0;
  for(size_t k=0; k<N; ++k) { mtot += mr[k]; }

  G4double mu = mtot;
  G4double PFragMagCM = 0.0;

  // Primary mass is above the sum of mass of components
  G4double Mass = std::max(M, mtot + CLHEP::eV);
  G4double T = Mass-mtot;

  G4LorentzVector PFragCM(0.0,0.0,0.0,0.0);
  G4LorentzVector PRestCM(0.0,0.0,0.0,0.0);
  G4LorentzVector PRestLab(0.0,0.0,0.0,Mass);

  CLHEP::HepRandomEngine* rndmEngine = G4Random::getTheEngine();

  for (size_t k = N-1; k>0; --k)
    {
      mu -= mr[k];
      if (k>1) { T *= BetaKopylov(k, rndmEngine); }
      else     { T = 0.0; }
      
      G4double RestMass = mu + T;

      PFragMagCM = PtwoBody(Mass,mr[k],RestMass);
      
      // Create a unit vector with a random direction isotropically distributed
      G4ThreeVector RandVector = PFragMagCM*G4RandomDirection();

      PFragCM.setVect(RandVector);
      PFragCM.setE(std::sqrt(PFragMagCM*PFragMagCM + mr[k]*mr[k]));

      PRestCM.setVect(-RandVector);
      PRestCM.setE(std::sqrt(PFragMagCM*PFragMagCM + RestMass*RestMass));

      G4ThreeVector BoostV = PRestLab.boostVector();

      PFragCM.boost(BoostV);
      (*P)[k] = new G4LorentzVector(PFragCM);

      PRestCM.boost(BoostV);
      PRestLab = PRestCM;
      
      Mass = RestMass;
    }

  (*P)[0] = new G4LorentzVector(PRestLab);

  return P;
}

G4double G4FermiPhaseSpaceDecay::BetaKopylov(G4int K, 
                     CLHEP::HepRandomEngine* rndmEngine) const
{
  G4int N = 3*K - 5;
  G4double xN = (G4double)N;
  G4double xN1= (G4double)(N + 1);
  G4double F;
  // VI variant
  G4double Fmax = std::sqrt(g4calc->powN(xN/xN1,N)/xN1); 
  G4double chi;
  do {
    chi = rndmEngine->flat();
    F = std::sqrt(g4calc->powN(chi,N)*(1-chi));      
    // Loop checking, 05-Aug-2015, Vladimir Ivanchenko
   } while ( Fmax*rndmEngine->flat() > F);  
  return chi;
}