G4TransportationWithMsc.cc

Go to the documentation of this file.

//
// ********************************************************************
// * License and Disclaimer                                           *
// *                                                                  *
// * The  Geant4 software  is  copyright of the Copyright Holders  of *
// * the Geant4 Collaboration.  It is provided  under  the terms  and *
// * conditions of the Geant4 Software License,  included in the file *
// * LICENSE and available at  http://cern.ch/geant4/license .  These *
// * include a list of copyright holders.                             *
// *                                                                  *
// * Neither the authors of this software system, nor their employing *
// * institutes,nor the agencies providing financial support for this *
// * work  make  any representation or  warranty, express or implied, *
// * regarding  this  software system or assume any liability for its *
// * use.  Please see the license in the file  LICENSE  and URL above *
// * for the full disclaimer and the limitation of liability.         *
// *                                                                  *
// * This  code  implementation is the result of  the  scientific and *
// * technical work of the GEANT4 collaboration.                      *
// * By using,  copying,  modifying or  distributing the software (or *
// * any work based  on the software)  you  agree  to acknowledge its *
// * use  in  resulting  scientific  publications,  and indicate your *
// * acceptance of all terms of the Geant4 Software license.          *
// ********************************************************************
//
// G4TransportationWithMsc
//
// Class Description:
//
// It is a generic process of transportation with multiple scattering included
// in the step limitation and propagation.
//
// Original author: Jonas Hahnfeld, 2022
 
// -------------------------------------------------------------------
//
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
#include "G4TransportationWithMsc.hh"
 
#include "G4DynamicParticle.hh"
#include "G4Electron.hh"
#include "G4EmConfigurator.hh"
#include "G4EmDataHandler.hh"
#include "G4LossTableBuilder.hh"
#include "G4LossTableManager.hh"
#include "G4ParticleChangeForGamma.hh"
#include "G4ParticleChangeForMSC.hh"
#include "G4PhysicalConstants.hh"
#include "G4PhysicsTableHelper.hh"
#include "G4PhysicsVector.hh"
#include "G4ProductionCutsTable.hh"
#include "G4Step.hh"
#include "G4StepPoint.hh"
#include "G4StepStatus.hh"
#include "G4Track.hh"
#include "G4VMscModel.hh"
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
static constexpr G4double kLowestKinEnergy = 10 * CLHEP::eV;
static constexpr G4double kGeomMin = 0.05 * CLHEP::nm;
static constexpr G4double kMinDisplacement2 = kGeomMin * kGeomMin;
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
G4TransportationWithMsc::G4TransportationWithMsc(ScatteringType type, G4int verbosity)
  : G4Transportation(verbosity, "TransportationWithMsc"), fType(type)
{
  SetVerboseLevel(1);
 
  fEmManager = G4LossTableManager::Instance();
  fModelManager = new G4EmModelManager;
 
  if (type == ScatteringType::MultipleScattering) {
    fParticleChangeForMSC = new G4ParticleChangeForMSC;
  }
  else if (type == ScatteringType::SingleScattering) {
    fParticleChangeForSS = new G4ParticleChangeForGamma;
    fSecondariesSS = new std::vector<G4DynamicParticle*>;
  }
 
  G4ThreeVector zero;
  fSubStepDynamicParticle = new G4DynamicParticle(G4Electron::Definition(), zero);
  fSubStepTrack = new G4Track(fSubStepDynamicParticle, 0, zero);
  fSubStep = new G4Step;
  fSubStepTrack->SetStep(fSubStep);
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
G4TransportationWithMsc::~G4TransportationWithMsc()
{
  delete fModelManager;
  delete fParticleChangeForMSC;
  delete fEmData;
  delete fParticleChangeForSS;
  delete fSecondariesSS;
 
  // fSubStepDynamicParticle is owned and also deleted by fSubStepTrack!
  delete fSubStepTrack;
  delete fSubStep;
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
void G4TransportationWithMsc::AddMscModel(G4VMscModel* mscModel, G4int order,
                                          const G4Region* region)
{
  if (fType != ScatteringType::MultipleScattering) {
    G4Exception("G4TransportationWithMsc::AddMscModel", "em0051", FatalException,
                "not allowed unless type == MultipleScattering");
  }
 
  fModelManager->AddEmModel(order, mscModel, nullptr, region);
  mscModel->SetParticleChange(fParticleChangeForMSC);
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
void G4TransportationWithMsc::AddSSModel(G4VEmModel* model, G4int order, const G4Region* region)
{
  if (fType != ScatteringType::SingleScattering) {
    G4Exception("G4TransportationWithMsc::AddSSModel", "em0051", FatalException,
                "not allowed unless type == SingleScattering");
  }
 
  fModelManager->AddEmModel(order, model, nullptr, region);
  model->SetPolarAngleLimit(0.0);
  model->SetParticleChange(fParticleChangeForSS);
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
void G4TransportationWithMsc::PreparePhysicsTable(const G4ParticleDefinition& part)
{
  if (nullptr == fFirstParticle) {
    fFirstParticle = &part;
    G4VMultipleScattering* ptr = nullptr;
    auto emConfigurator = fEmManager->EmConfigurator();
    emConfigurator->PrepareModels(&part, ptr, this);
  }
 
  if (fFirstParticle == &part) {
    G4bool master = fEmManager->IsMaster();
    G4LossTableBuilder* bld = fEmManager->GetTableBuilder();
    G4bool baseMat = bld->GetBaseMaterialFlag();
    const auto* theParameters = G4EmParameters::Instance();
 
    if (master) {
      SetVerboseLevel(theParameters->Verbose());
    }
    else {
      SetVerboseLevel(theParameters->WorkerVerbose());
    }
 
    const G4int numberOfModels = fModelManager->NumberOfModels();
    if (fType == ScatteringType::MultipleScattering) {
      for (G4int i = 0; i < numberOfModels; ++i) {
        auto msc = static_cast<G4VMscModel*>(fModelManager->GetModel(i));
        msc->SetMasterThread(master);
        msc->SetPolarAngleLimit(theParameters->MscThetaLimit());
        G4double emax = std::min(msc->HighEnergyLimit(), theParameters->MaxKinEnergy());
        msc->SetHighEnergyLimit(emax);
        msc->SetUseBaseMaterials(baseMat);
      }
    }
    else if (fType == ScatteringType::SingleScattering) {
      if (master) {
        if (fEmData == nullptr) {
          fEmData = new G4EmDataHandler(2);
        }
 
        fLambdaTable = fEmData->MakeTable(0);
        bld->InitialiseBaseMaterials(fLambdaTable);
      }
    }
 
    fCuts = fModelManager->Initialise(fFirstParticle, G4Electron::Electron(), verboseLevel);
  }
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
void G4TransportationWithMsc::BuildPhysicsTable(const G4ParticleDefinition& part)
{
  if (fFirstParticle == &part) {
    fEmManager->BuildPhysicsTable(fFirstParticle);
 
    if (fEmManager->IsMaster()) {
      if (fType == ScatteringType::SingleScattering) {
        const auto* theParameters = G4EmParameters::Instance();
        G4LossTableBuilder* bld = fEmManager->GetTableBuilder();
        const G4ProductionCutsTable* theCoupleTable =
          G4ProductionCutsTable::GetProductionCutsTable();
        std::size_t numOfCouples = theCoupleTable->GetTableSize();
 
        G4double emin = theParameters->MinKinEnergy();
        G4double emax = theParameters->MaxKinEnergy();
 
        G4double scale = emax / emin;
        G4int nbin = theParameters->NumberOfBinsPerDecade() * G4lrint(std::log10(scale));
        scale = nbin / G4Log(scale);
 
        G4int bin = G4lrint(scale * G4Log(emax / emin));
        bin = std::max(bin, 5);
 
        for (std::size_t i = 0; i < numOfCouples; ++i) {
          if (!bld->GetFlag(i)) continue;
 
          // Create physics vector and fill it
          const G4MaterialCutsCouple* couple = theCoupleTable->GetMaterialCutsCouple((G4int)i);
 
          auto* aVector = new G4PhysicsLogVector(emin, emax, bin, /*splineFlag*/ true);
          fModelManager->FillLambdaVector(aVector, couple, /*startNull*/ false);
          aVector->FillSecondDerivatives();
          G4PhysicsTableHelper::SetPhysicsVector(fLambdaTable, i, aVector);
        }
      }
    }
    else {
      const auto masterProcess = static_cast<const G4TransportationWithMsc*>(GetMasterProcess());
 
      // Initialisation of models.
      const G4int numberOfModels = fModelManager->NumberOfModels();
      if (fType == ScatteringType::MultipleScattering) {
        for (G4int i = 0; i < numberOfModels; ++i) {
          auto msc = static_cast<G4VMscModel*>(fModelManager->GetModel(i));
          auto msc0 = static_cast<G4VMscModel*>(masterProcess->fModelManager->GetModel(i));
          msc->SetCrossSectionTable(msc0->GetCrossSectionTable(), false);
          msc->InitialiseLocal(fFirstParticle, msc0);
        }
      }
      else if (fType == ScatteringType::SingleScattering) {
        this->fLambdaTable = masterProcess->fLambdaTable;
      }
    }
  }
 
  if (!G4EmParameters::Instance()->IsPrintLocked() && verboseLevel > 0) {
    G4cout << G4endl;
    G4cout << GetProcessName() << ": for " << part.GetParticleName();
    if (fMultipleSteps) {
      G4cout << " (multipleSteps: 1)";
    }
    G4cout << G4endl;
    fModelManager->DumpModelList(G4cout, verboseLevel);
  }
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
 
void G4TransportationWithMsc::StartTracking(G4Track* track)
{
  auto* currParticle = track->GetParticleDefinition();
  fIonisation = fEmManager->GetEnergyLossProcess(currParticle);
 
  fSubStepDynamicParticle->SetDefinition(currParticle);
 
  const G4int numberOfModels = fModelManager->NumberOfModels();
  if (fType == ScatteringType::MultipleScattering) {
    for (G4int i = 0; i < numberOfModels; ++i) {
      auto msc = static_cast<G4VMscModel*>(fModelManager->GetModel(i));
      msc->StartTracking(track);
      msc->SetIonisation(fIonisation, currParticle);
    }
  }
 
  // Ensure that field propagation state is also cleared / prepared
  G4Transportation::StartTracking(track);
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
 
G4double G4TransportationWithMsc::AlongStepGetPhysicalInteractionLength(const G4Track& track,
                                                                        G4double previousStepSize,
                                                                        G4double currentMinimumStep,
                                                                        G4double& proposedSafety,
                                                                        G4GPILSelection* selection)
{
  *selection = NotCandidateForSelection;
 
  const G4double physStepLimit = currentMinimumStep;
 
  switch (fType) {
    case ScatteringType::MultipleScattering: {
      // Select the MSC model for the current kinetic energy.
      G4VMscModel* mscModel = nullptr;
      const G4double ekin = track.GetKineticEnergy();
      const auto* couple = track.GetMaterialCutsCouple();
      const auto* particleDefinition = track.GetParticleDefinition();
      if (physStepLimit > kGeomMin) {
        G4double ekinForSelection = ekin;
        G4double pdgMass = particleDefinition->GetPDGMass();
        if (pdgMass > CLHEP::GeV) {
          ekinForSelection *= proton_mass_c2 / pdgMass;
        }
 
        if (ekinForSelection >= kLowestKinEnergy) {
          mscModel = static_cast<G4VMscModel*>(
            fModelManager->SelectModel(ekinForSelection, couple->GetIndex()));
          if (mscModel == nullptr) {
            G4Exception("G4TransportationWithMsc::AlongStepGPIL", "em0052", FatalException,
                        "no MSC model found");
          }
          if (!mscModel->IsActive(ekinForSelection)) {
            mscModel = nullptr;
          }
        }
      }
 
      // Call the MSC model to potentially limit the step and convert to
      // geometric path length.
      if (mscModel != nullptr) {
        mscModel->SetCurrentCouple(couple);
 
        // Use the provided track for the first step.
        const G4Track* currentTrackPtr = &track;
 
        G4double currentSafety = proposedSafety;
        G4double currentEnergy = ekin;
 
        G4double stepLimitLeft = physStepLimit;
        G4double totalGeometryStepLength = 0, totalTruePathLength = 0;
        G4bool firstStep = true, continueStepping = fMultipleSteps;
 
        do {
          G4double gPathLength = stepLimitLeft;
          G4double tPathLength =
            mscModel->ComputeTruePathLengthLimit(*currentTrackPtr, gPathLength);
          G4bool mscLimitsStep = (tPathLength < stepLimitLeft);
          if (!fMultipleSteps && mscLimitsStep) {
            // MSC limits the step.
            *selection = CandidateForSelection;
          }
 
          if (!firstStep) {
            // Move the navigator to where the previous step ended.
            fLinearNavigator->LocateGlobalPointWithinVolume(fTransportEndPosition);
          }
 
          G4GPILSelection transportSelection;
          G4double geometryStepLength = G4Transportation::AlongStepGetPhysicalInteractionLength(
            *currentTrackPtr, previousStepSize, gPathLength, currentSafety, &transportSelection);
          if (geometryStepLength < gPathLength) {
            // Transportation limits the step, ie the track hit a boundary.
            *selection = CandidateForSelection;
            continueStepping = false;
          }
          if (fTransportEndKineticEnergy != currentEnergy) {
            // Field propagation changed the energy, it's not possible to
            // estimate the continuous energy loss and continue stepping.
            continueStepping = false;
          }
 
          if (firstStep) {
            proposedSafety = currentSafety;
          }
          totalGeometryStepLength += geometryStepLength;
 
          // Sample MSC direction change and displacement.
          const G4double range = mscModel->GetRange(particleDefinition, currentEnergy, couple);
 
          tPathLength = mscModel->ComputeTrueStepLength(geometryStepLength);
 
          // Protect against wrong t->g->t conversion.
          tPathLength = std::min(tPathLength, stepLimitLeft);
 
          totalTruePathLength += tPathLength;
          if (*selection != CandidateForSelection && !mscLimitsStep) {
            // If neither MSC nor transportation limits the step, we got the
            // distance we want - make sure we exit the loop.
            continueStepping = false;
          }
          else if (tPathLength >= range) {
            // The particle will stop, exit the loop.
            continueStepping = false;
          }
          else {
            stepLimitLeft -= tPathLength;
          }
 
          // Do not sample scattering at the last or at a small step.
          if (tPathLength < range && tPathLength > kGeomMin) {
            static constexpr G4double minSafety = 1.20 * CLHEP::nm;
            static constexpr G4double sFact = 0.99;
 
            // The call to SampleScattering() *may* directly fill in the changed
            // direction into fParticleChangeForMSC, so we have to:
            // 1) Make sure the momentum direction is initialized.
            fParticleChangeForMSC->ProposeMomentumDirection(fTransportEndMomentumDir);
            // 2) Call SampleScattering(), which *may* change it.
            const G4ThreeVector displacement =
              mscModel->SampleScattering(fTransportEndMomentumDir, minSafety);
            // 3) Get the changed direction.
            fTransportEndMomentumDir = *fParticleChangeForMSC->GetProposedMomentumDirection();
 
            const G4double r2 = displacement.mag2();
            if (r2 > kMinDisplacement2) {
              G4bool positionChanged = true;
              G4double dispR = std::sqrt(r2);
              G4double postSafety =
                sFact * fpSafetyHelper->ComputeSafety(fTransportEndPosition, dispR);
 
              // Far away from geometry boundary
              if (postSafety > 0.0 && dispR <= postSafety) {
                fTransportEndPosition += displacement;
 
                // Near the boundary
              }
              else {
                // displaced point is definitely within the volume
                if (dispR < postSafety) {
                  fTransportEndPosition += displacement;
 
                  // reduced displacement
                }
                else if (postSafety > kGeomMin) {
                  fTransportEndPosition += displacement * (postSafety / dispR);
 
                  // very small postSafety
                }
                else {
                  positionChanged = false;
                }
              }
              if (positionChanged) {
                fpSafetyHelper->ReLocateWithinVolume(fTransportEndPosition);
              }
            }
          }
 
          if (continueStepping) {
            // Update safety according to the geometry distance.
            if (currentSafety < fEndPointDistance) {
              currentSafety = 0;
            }
            else {
              currentSafety -= fEndPointDistance;
            }
 
            // Update the kinetic energy according to the continuous loss.
            currentEnergy = mscModel->GetEnergy(particleDefinition, range - tPathLength, couple);
 
            // From now on, use the track that we can update below.
            currentTrackPtr = fSubStepTrack;
 
            fSubStepDynamicParticle->SetKineticEnergy(currentEnergy);
            fSubStepDynamicParticle->SetMomentumDirection(fTransportEndMomentumDir);
            fSubStepTrack->SetPosition(fTransportEndPosition);
 
            G4StepPoint& subPreStepPoint = *fSubStep->GetPreStepPoint();
            subPreStepPoint.SetMaterialCutsCouple(couple);
            subPreStepPoint.SetPosition(fTransportEndPosition);
            subPreStepPoint.SetSafety(currentSafety);
            subPreStepPoint.SetStepStatus(fAlongStepDoItProc);
          }
          firstStep = false;
        } while (continueStepping);
 
        // Note: currentEnergy is only updated if continueStepping is true.
        // In case field propagation changed the energy, this flag is
        // immediately set to false and currentEnergy is still equal to the
        // initial kinetic energy stored in ekin.
        if (currentEnergy != ekin) {
          // If field propagation didn't change the energy and we potentially
          // did multiple steps, reset the energy that G4Transportation will
          // propose to not subtract the energy loss twice.
          fTransportEndKineticEnergy = ekin;
          // Also ask for the range again with the initial energy so it is
          // correctly cached in the G4VEnergyLossProcess.
          // FIXME: Asking for a range should never change the cached values!
          (void)mscModel->GetRange(particleDefinition, ekin, couple);
        }
 
        fParticleChange.ProposeTrueStepLength(totalTruePathLength);
 
        // Inform G4Transportation that the momentum might have changed due
        // to scattering. We do this unconditionally to avoid the situation
        // where the last step is done without MSC and G4Transportation reset
        // the flag, for example when running without field.
        fMomentumChanged = true;
 
        return totalGeometryStepLength;
      }
      break;
    }
 
    case ScatteringType::SingleScattering: {
      // Select the model for the current kinetic energy.
      const G4double ekin = track.GetKineticEnergy();
      const auto* couple = track.GetMaterialCutsCouple();
      const auto* particleDefinition = track.GetParticleDefinition();
 
      G4double ekinForSelection = ekin;
      G4double pdgMass = particleDefinition->GetPDGMass();
      if (pdgMass > CLHEP::GeV) {
        ekinForSelection *= proton_mass_c2 / pdgMass;
      }
 
      G4VEmModel* currentModel = fModelManager->SelectModel(ekinForSelection, couple->GetIndex());
      if (currentModel == nullptr) {
        G4Exception("G4TransportationWithMsc::AlongStepGPIL", "em0052", FatalException,
                    "no scattering model found");
      }
      if (!currentModel->IsActive(ekinForSelection)) {
        currentModel = nullptr;
      }
 
      if (currentModel != nullptr) {
        currentModel->SetCurrentCouple(couple);
        G4int coupleIndex = couple->GetIndex();
 
        // Compute mean free path.
        G4double logEkin = track.GetDynamicParticle()->GetLogKineticEnergy();
        G4double lambda = ((*fLambdaTable)[coupleIndex])->LogVectorValue(ekin, logEkin);
        if (lambda > 0.0) {
          // Assume that the mean free path and dE/dx are constant along the
          // step, which is a valid approximation for most cases.
          G4double meanFreePath = 1.0 / lambda;
          G4double dedx = fIonisation->GetDEDX(ekin, couple);
 
          G4double currentSafety = proposedSafety;
          G4double currentEnergy = ekin;
 
          // Use the provided track for the first step.
          const G4Track* currentTrackPtr = &track;
 
          G4double stepLimitLeft = physStepLimit;
          G4double totalStepLength = 0;
          G4bool firstStep = true, continueStepping = fMultipleSteps;
 
          do {
            G4double interactionLength = meanFreePath * -G4Log(G4UniformRand());
 
            G4bool ssLimitsStep = (interactionLength < stepLimitLeft);
            G4double gPathLength = stepLimitLeft;
            if (ssLimitsStep) {
              if (!fMultipleSteps) {
                // Scattering limits the step.
                *selection = CandidateForSelection;
              }
              gPathLength = interactionLength;
            }
 
            if (!firstStep) {
              // Move the navigator to where the previous step ended.
              fLinearNavigator->LocateGlobalPointWithinVolume(fTransportEndPosition);
            }
 
            G4GPILSelection transportSelection;
            G4double geometryStepLength = G4Transportation::AlongStepGetPhysicalInteractionLength(
              *currentTrackPtr, previousStepSize, gPathLength, currentSafety, &transportSelection);
            if (geometryStepLength < gPathLength) {
              // Transportation limits the step, ie the track hit a boundary.
              *selection = CandidateForSelection;
              ssLimitsStep = false;
              continueStepping = false;
            }
            if (fTransportEndKineticEnergy != currentEnergy) {
              // Field propagation changed the energy, it's not possible to
              // estimate the continuous energy loss and continue stepping.
              continueStepping = false;
            }
 
            if (firstStep) {
              proposedSafety = currentSafety;
            }
            totalStepLength += geometryStepLength;
 
            if (*selection != CandidateForSelection && !ssLimitsStep) {
              // If neither scattering nor transportation limits the step, we
              // got the distance we want - make sure we exit the loop.
              continueStepping = false;
            }
            else {
              stepLimitLeft -= geometryStepLength;
            }
 
            // Update the kinetic energy according to the continuous loss.
            G4double energyAfterLinearLoss =
              fTransportEndKineticEnergy - geometryStepLength * dedx;
 
            if (ssLimitsStep) {
              fSubStepDynamicParticle->SetKineticEnergy(energyAfterLinearLoss);
 
              // The call to SampleSecondaries() directly fills in the changed
              // direction into fParticleChangeForSS, so we have to:
              // 1) Set the momentum direction in dynamic particle.
              fSubStepDynamicParticle->SetMomentumDirection(fTransportEndMomentumDir);
              // 2) Call SampleSecondaries(), which changes the direction.
              currentModel->SampleSecondaries(fSecondariesSS, couple, fSubStepDynamicParticle,
                                              (*fCuts)[coupleIndex]);
              // 3) Get the changed direction.
              fTransportEndMomentumDir = fParticleChangeForSS->GetProposedMomentumDirection();
 
              // Check that the model neither created secondaries nor proposed
              // a local energy deposit because this process does not know how
              // to handle these cases.
              if (fSecondariesSS->size() > 0) {
                G4Exception("G4TransportationWithMsc::AlongStepGPIL", "em0053", FatalException,
                            "scattering model created secondaries");
              }
              if (fParticleChangeForSS->GetLocalEnergyDeposit() > 0) {
                G4Exception("G4TransportationWithMsc::AlongStepGPIL", "em0053", FatalException,
                            "scattering model proposed energy deposit");
              }
            }
 
            if (continueStepping) {
              // Update safety according to the geometry distance.
              if (currentSafety < fEndPointDistance) {
                currentSafety = 0;
              }
              else {
                currentSafety -= fEndPointDistance;
              }
 
              // Update the energy taking continuous loss into account.
              currentEnergy = energyAfterLinearLoss;
 
              // From now on, use the track that we can update below.
              currentTrackPtr = fSubStepTrack;
 
              fSubStepDynamicParticle->SetKineticEnergy(currentEnergy);
              fSubStepDynamicParticle->SetMomentumDirection(fTransportEndMomentumDir);
              fSubStepTrack->SetPosition(fTransportEndPosition);
 
              G4StepPoint& subPreStepPoint = *fSubStep->GetPreStepPoint();
              subPreStepPoint.SetMaterialCutsCouple(couple);
              subPreStepPoint.SetPosition(fTransportEndPosition);
              subPreStepPoint.SetSafety(currentSafety);
              subPreStepPoint.SetStepStatus(fAlongStepDoItProc);
            }
            firstStep = false;
          } while (continueStepping);
 
          // Note: currentEnergy is only updated if continueStepping is true.
          // In case field propagation changed the energy, this flag is
          // immediately set to false and currentEnergy is still equal to the
          // initial kinetic energy stored in ekin.
          if (currentEnergy != ekin) {
            // If field propagation didn't change the energy and we potentially
            // did multiple steps, reset the energy that G4Transportation will
            // propose to not subtract the energy loss twice.
            fTransportEndKineticEnergy = ekin;
          }
 
          fParticleChange.ProposeTrueStepLength(totalStepLength);
 
          // Inform G4Transportation that the momentum might have changed due
          // to scattering, even if there is no field.
          fMomentumChanged = true;
 
          return totalStepLength;
        }
      }
      break;
    }
  }
 
  // If we get here, no scattering has happened.
  return G4Transportation::AlongStepGetPhysicalInteractionLength(
    track, previousStepSize, currentMinimumStep, proposedSafety, selection);
}
 
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....