G4ChargeExchange.cc

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//
// G4 Model: Charge and strangness exchange based on G4LightMedia model
//           28 May 2006 V.Ivanchenko
//
// Modified:
// 07-Jun-06 V.Ivanchenko fix problem of rotation of final state
// 25-Jul-06 V.Ivanchenko add 19 MeV low energy, below which S-wave is sampled
// 12-Jun-12 A.Ribon fix warnings of shadowed variables
// 06-Aug-15 A.Ribon migrating to G4Exp, G4Log and G4Pow
//
 
#include "G4ChargeExchange.hh"
#include "G4ChargeExchangeXS.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
#include "G4IonTable.hh"
#include "Randomize.hh"
#include "G4NucleiProperties.hh"
#include "G4DecayTable.hh"
#include "G4VDecayChannel.hh"
#include "G4DecayProducts.hh"
#include "G4NistManager.hh"
#include "G4Fragment.hh"
#include "G4ExcitationHandler.hh"
#include "G4ReactionProductVector.hh"
 
#include "G4Exp.hh"
#include "G4Log.hh"
#include "G4Pow.hh"
 
#include "G4HadronicParameters.hh"
#include "G4PhysicsModelCatalog.hh"
 
namespace
{
  constexpr G4int maxN = 1000;
  constexpr G4double emin = 2*136.9*CLHEP::MeV;
}
 
G4ChargeExchange::G4ChargeExchange(G4ChargeExchangeXS* ptr)
  : G4HadronicInteraction("ChargeExchange"),
    fXSection(ptr), fXSWeightFactor(1.0)
{
  lowEnergyLimit = 1.*CLHEP::MeV;
  secID = G4PhysicsModelCatalog::GetModelID( "model_ChargeExchange" );
  nist = G4NistManager::Instance();
  fHandler = new G4ExcitationHandler();
  if (nullptr != fXSection) {
    fXSWeightFactor = 1.0/fXSection->GetCrossSectionFactor();
  }
}
 
G4ChargeExchange::~G4ChargeExchange()
{
  delete fHandler;
}
 
G4HadFinalState* G4ChargeExchange::ApplyYourself(
         const G4HadProjectile& aTrack, G4Nucleus& targetNucleus)
{
  theParticleChange.Clear();
  auto part = aTrack.GetDefinition();
  G4double ekin = aTrack.GetKineticEnergy();
 
  G4int A = targetNucleus.GetA_asInt();
  G4int Z = targetNucleus.GetZ_asInt();
 
  if (ekin <= lowEnergyLimit) {
    return &theParticleChange;
  }
  theParticleChange.SetWeightChange(fXSWeightFactor);
 
  G4int projPDG = part->GetPDGEncoding();
 
  // for hydrogen targets and positive projectile change exchange
  // is not possible on proton, only on deuteron
  if (1 == Z && (211 == projPDG || 321 == projPDG)) { A = 2; } 
  
  if (verboseLevel > 1)
    G4cout << "G4ChargeExchange for " << part->GetParticleName()
       << " PDGcode= " << projPDG << " on nucleus Z= " << Z
       << " A= " << A << " N= " << A - Z
       << G4endl;
 
  G4double mass1 = G4NucleiProperties::GetNuclearMass(A, Z);
  G4LorentzVector lv0 = aTrack.Get4Momentum();
  G4double etot = mass1 + lv0.e();
 
  // select final state
  const G4ParticleDefinition* theSecondary =
    fXSection->SampleSecondaryType(part, Z, A);
  G4int pdg = theSecondary->GetPDGEncoding();
 
  // omega(782) and f2(1270)
  G4bool isShortLived = (pdg == 223 || pdg == 225);
 
  // atomic number of the recoil nucleus
  if (projPDG == -211) { --Z; }
  else if (projPDG == 211) { ++Z; }
  else if (projPDG == -321) { --Z; }
  else if (projPDG == 321) { ++Z; }
  else if (projPDG == 130) {
    if (theSecondary->GetPDGCharge() > 0.0) { --Z; }
    else { ++Z; }
  } else {
    // not ready for other projectile
    return &theParticleChange;
  }
 
  // recoil nucleus
  const G4ParticleDefinition* theRecoil = nullptr;
  if (Z == 0 && A == 1) { theRecoil = G4Neutron::Neutron(); }
  else if (Z == 1 && A == 1) { theRecoil = G4Proton::Proton(); }
  else if (Z == 1 && A == 2) { theRecoil = G4Deuteron::Deuteron(); }
  else if (Z == 1 && A == 3) { theRecoil = G4Triton::Triton(); }
  else if (Z == 2 && A == 3) { theRecoil = G4He3::He3(); }
  else if (Z == 2 && A == 4) { theRecoil = G4Alpha::Alpha(); }
  else if (nist->GetIsotopeAbundance(Z, A) > 0.0) {
    theRecoil = G4ParticleTable::GetParticleTable()
      ->GetIonTable()->GetIon(Z, A, 0.0);
  }
 
  // check if there is enough energy for the final state
  // and sample mass of produced state
  const G4double mass0 = theSecondary->GetPDGMass();
  G4double mass3 = (nullptr == theRecoil) ?
    G4NucleiProperties::GetNuclearMass(A, Z) : theRecoil->GetPDGMass();
  G4double mass2 = mass0;
  if (isShortLived &&
      !SampleMass(mass2, theSecondary->GetPDGWidth(), etot - mass3)) {
    return &theParticleChange;
  }
 
  // not possible kinematically
  if (etot <= mass2 + mass3) {
    return &theParticleChange;
  }
 
  // sample kinematics
  G4LorentzVector lv1(0.0, 0.0, 0.0, mass1);
  G4LorentzVector lv = lv0 + lv1;
  G4ThreeVector bst = lv.boostVector();
  G4double ss = lv.mag2();
 
  // tmax = 4*momCMS^2
  G4double e2 = ss + mass2*mass2 - mass3*mass3;
  G4double tmax = e2*e2/ss - 4*mass2*mass2;
 
  G4double t = SampleT(theSecondary, A, tmax);
  
  G4double phi  = G4UniformRand()*CLHEP::twopi;
  G4double cost = 1. - 2.0*t/tmax;
 
  if (cost > 1.0) { cost = 1.0; }
  else if(cost < -1.0) { cost = -1.0; } 
 
  G4double sint = std::sqrt((1.0-cost)*(1.0+cost));
 
  if (verboseLevel>1) {
    G4cout << " t= " << t << " tmax(GeV^2)= " << tmax/(GeV*GeV) 
       << " cos(t)=" << cost << " sin(t)=" << sint << G4endl;
  }
  G4double momentumCMS = 0.5*std::sqrt(tmax);
  G4LorentzVector lv2(momentumCMS*sint*std::cos(phi),
              momentumCMS*sint*std::sin(phi),
              momentumCMS*cost,
              std::sqrt(momentumCMS*momentumCMS + mass2*mass2));
 
  // kinematics in the final state, may be a warning should be added if 
  lv2.boost(bst);
  if (lv2.e() < mass2) {
    lv2.setE(mass2);
  }
  lv -= lv2;
  if (lv.e() < mass3) {
    lv.setE(mass3);
  }
 
  // prepare secondary particles
  theParticleChange.SetStatusChange(stopAndKill);
  theParticleChange.SetEnergyChange(0.0);
 
  if (!isShortLived) {
    auto aSec = new G4DynamicParticle(theSecondary, lv2);
    theParticleChange.AddSecondary(aSec, secID);
  } else {
    auto channel = theSecondary->GetDecayTable()->SelectADecayChannel(mass2);
    auto products = channel->DecayIt(mass2);
    G4ThreeVector bst1 = lv2.boostVector();
    G4int N = products->entries();
    for (G4int i=0; i<N; ++i) {
      auto p = (*products)[i];
      auto lvp = p->Get4Momentum();
      lvp.boost(bst1);
      p->Set4Momentum(lvp);
      theParticleChange.AddSecondary(p, secID);
    }
    delete products;
  }
 
  // recoil is a stable isotope
  if (nullptr != theRecoil) {
    auto aRec = new G4DynamicParticle(theRecoil, lv);
    theParticleChange.AddSecondary(aRec, secID);
  } else {
    // recoil is an unstable fragment
    G4Fragment frag(A, Z, lv);
    auto products = fHandler->BreakItUp(frag);
    for (auto & prod : *products) {
      auto dp = new G4DynamicParticle(prod->GetDefinition(), prod->GetMomentum());
      theParticleChange.AddSecondary(dp, secID);
      delete prod;
    }
    delete products;
  }
  return &theParticleChange;
}
 
G4double G4ChargeExchange::SampleT(const G4ParticleDefinition*,
                                   const G4int A, const G4double tmax) const
{
  G4double aa, bb, cc, dd;
  G4Pow* g4pow = G4Pow::GetInstance();
  if (A <= 62.) {
    aa = g4pow->powZ(A, 1.63);
    bb = 14.5*g4pow->powZ(A, 0.66);
    cc = 1.4*g4pow->powZ(A, 0.33);
    dd = 10.;
  } else {
    aa = g4pow->powZ(A, 1.33);
    bb = 60.*g4pow->powZ(A, 0.33);
    cc = 0.4*g4pow->powZ(A, 0.40);
    dd = 10.;
  }
  G4double x1 = (1.0 - G4Exp(-tmax*bb))*aa/bb;
  G4double x2 = (1.0 - G4Exp(-tmax*dd))*cc/dd;
  
  G4double t;
  G4double y = bb;
  if(G4UniformRand()*(x1 + x2) < x2) y = dd;
 
  for (G4int i=0; i<maxN; ++i) {
    t = -G4Log(G4UniformRand())/y;
    if (t <= tmax) { return t; }
  }
  return 0.0;
}
 
G4bool G4ChargeExchange::SampleMass(G4double& M, const G4double G, const G4double elim)
{
  // +- 4 width but above 2 pion mass
  const G4double e1 = std::max(M - 4*G, emin);
  const G4double e2 = std::min(M + 4*G, elim) - e1;
  if (e2 <= 0.0) { return false; }
  const G4double M2 = M*M;
  const G4double MG2 = M2*G*G;
 
  // sampling Breit-Wigner function
  for (G4int i=0; i<maxN; ++i) {
    G4double e = e1 + e2*G4UniformRand();
    G4double x = e*e - M2;
    G4double y = MG2/(x*x + MG2);
    if (y >= G4UniformRand()) {
      M = e;
      return true;
    }
  }
  return false;
}