G4Transportation.cc

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//
// $Id: G4Transportation.cc 2011/06/10 16:19:46 japost Exp japost $
// 
// ------------------------------------------------------------
//  GEANT 4  include file implementation
//
// ------------------------------------------------------------
//
// This class is a process responsible for the transportation of 
// a particle, ie the geometrical propagation that encounters the 
// geometrical sub-volumes of the detectors.
//
// It is also tasked with the key role of proposing the "isotropic safety",
//   which will be used to update the post-step point's safety.
//
// =======================================================================
// Modified:   
//   28 Oct  2011, P.Gumpl./J.Ap: Detect gravity field, use magnetic moment 
//   20 Nov  2008, J.Apostolakis: Push safety to helper - after ComputeSafety
//    9 Nov  2007, J.Apostolakis: Flag for short steps, push safety to helper
//   19 Jan  2006, P.MoraDeFreitas: Fix for suspended tracks (StartTracking)
//   11 Aug  2004, M.Asai: Add G4VSensitiveDetector* for updating stepPoint.
//   21 June 2003, J.Apostolakis: Calling field manager with 
//                     track, to enable it to configure its accuracy
//   13 May  2003, J.Apostolakis: Zero field areas now taken into
//                     account correclty in all cases (thanks to W Pokorski).
//   29 June 2001, J.Apostolakis, D.Cote-Ahern, P.Gumplinger: 
//                     correction for spin tracking   
//   20 Febr 2001, J.Apostolakis:  update for new FieldTrack
//   22 Sept 2000, V.Grichine:     update of Kinetic Energy
// Created:  19 March 1997, J. Apostolakis
// =======================================================================

#include "G4Transportation.hh"
#include "G4TransportationProcessType.hh"

#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4ProductionCutsTable.hh"
#include "G4ParticleTable.hh"

#include "G4ChargeState.hh"
#include "G4EquationOfMotion.hh"

#include "G4FieldManagerStore.hh"

class G4VSensitiveDetector;

//
// Constructor

G4Transportation::G4Transportation( G4int verbosity )
  : G4VProcess( G4String("Transportation"), fTransportation ),
    fTransportEndPosition( 0.0, 0.0, 0.0 ),
    fTransportEndMomentumDir( 0.0, 0.0, 0.0 ),
    fTransportEndKineticEnergy( 0.0 ),
    fTransportEndSpin( 0.0, 0.0, 0.0 ),
    fMomentumChanged(true),
    fEndGlobalTimeComputed(false), 
    fCandidateEndGlobalTime(0.0),
    fParticleIsLooping( false ),
    fGeometryLimitedStep(true),
    fPreviousSftOrigin( 0.,0.,0. ),
    fPreviousSafety( 0.0 ),
    // fParticleChange(),
    fEndPointDistance( -1.0 ), 
    fThreshold_Warning_Energy( 100 * MeV ),  
    fThreshold_Important_Energy( 250 * MeV ), 
    fThresholdTrials( 10 ), 
    fNoLooperTrials( 0 ),
    fSumEnergyKilled( 0.0 ), fMaxEnergyKilled( 0.0 ), 
    fShortStepOptimisation( false ), // Old default: true (=fast short steps)
    fUseMagneticMoment( false ),
    fVerboseLevel( verbosity )
{
  // set Process Sub Type
  SetProcessSubType(static_cast<G4int>(TRANSPORTATION));

  G4TransportationManager* transportMgr ; 

  transportMgr = G4TransportationManager::GetTransportationManager() ; 

  fLinearNavigator = transportMgr->GetNavigatorForTracking() ; 

  fFieldPropagator = transportMgr->GetPropagatorInField() ;

  fpSafetyHelper =   transportMgr->GetSafetyHelper();  // New 

  // Cannot determine whether a field exists here, as it would 
  //  depend on the relative order of creating the detector's 
  //  field and this process. That order is not guaranted.
  // Instead later the method DoesGlobalFieldExist() is called

  static G4ThreadLocal G4TouchableHandle* pNullTouchableHandle = 0;
  if ( !pNullTouchableHandle)  { pNullTouchableHandle = new G4TouchableHandle; }
  fCurrentTouchableHandle = *pNullTouchableHandle;
    // Points to (G4VTouchable*) 0

#ifdef G4VERBOSE
  if( fVerboseLevel > 0) 
  { 
     G4cout << " G4Transportation constructor> set fShortStepOptimisation to "; 
     if ( fShortStepOptimisation )  G4cout << "true"  << G4endl;
     else                           G4cout << "false" << G4endl;
  } 
#endif
}


G4Transportation::~G4Transportation()
{
  if( (fVerboseLevel > 0) && (fSumEnergyKilled > 0.0 ) )
  { 
    G4cout << " G4Transportation: Statistics for looping particles " << G4endl;
    G4cout << "   Sum of energy of loopers killed: " <<  fSumEnergyKilled << G4endl;
    G4cout << "   Max energy of loopers killed: " <<  fMaxEnergyKilled << G4endl;
  } 
}

//
// Responsibilities:
//    Find whether the geometry limits the Step, and to what length
//    Calculate the new value of the safety and return it.
//    Store the final time, position and momentum.

G4double G4Transportation::
AlongStepGetPhysicalInteractionLength( const G4Track&  track,
                                             G4double, //  previousStepSize
                                             G4double  currentMinimumStep,
                                             G4double& currentSafety,
                                             G4GPILSelection* selection )
{
  G4double geometryStepLength= -1.0, newSafety= -1.0; 
  fParticleIsLooping = false ;

  // Initial actions moved to  StartTrack()   
  // --------------------------------------
  // Note: in case another process changes touchable handle
  //    it will be necessary to add here (for all steps)   
  // fCurrentTouchableHandle = aTrack->GetTouchableHandle();

  // GPILSelection is set to defaule value of CandidateForSelection
  // It is a return value
  //
  *selection = CandidateForSelection ;

  // Get initial Energy/Momentum of the track
  //
  const G4DynamicParticle*    pParticle  = track.GetDynamicParticle() ;
  const G4ParticleDefinition* pParticleDef   = pParticle->GetDefinition() ;
  G4ThreeVector startMomentumDir       = pParticle->GetMomentumDirection() ;
  G4ThreeVector startPosition          = track.GetPosition() ;

  // G4double   theTime        = track.GetGlobalTime() ;

  // The Step Point safety can be limited by other geometries and/or the 
  // assumptions of any process - it's not always the geometrical safety.
  // We calculate the starting point's isotropic safety here.
  //
  G4ThreeVector OriginShift = startPosition - fPreviousSftOrigin ;
  G4double      MagSqShift  = OriginShift.mag2() ;
  if( MagSqShift >= sqr(fPreviousSafety) )
  {
     currentSafety = 0.0 ;
  }
  else
  {
     currentSafety = fPreviousSafety - std::sqrt(MagSqShift) ;
  }

  // Is the particle charged or has it a magnetic moment?
  //
  G4double particleCharge = pParticle->GetCharge() ; 
  G4double magneticMoment = pParticle->GetMagneticMoment() ;
  G4double       restMass = pParticleDef->GetPDGMass() ;

  fGeometryLimitedStep = false ;
  // fEndGlobalTimeComputed = false ;

  // There is no need to locate the current volume. It is Done elsewhere:
  //   On track construction 
  //   By the tracking, after all AlongStepDoIts, in "Relocation"

  // Check if the particle has a force, EM or gravitational, exerted on it
  //
  G4FieldManager* fieldMgr=0;
  G4bool          fieldExertsForce = false ;

  G4bool gravityOn = false;
  G4bool fieldExists= false;  // Field is not 0 (null pointer)

  fieldMgr = fFieldPropagator->FindAndSetFieldManager( track.GetVolume() );
  if( fieldMgr != 0 )
  {
     // Message the field Manager, to configure it for this track
     fieldMgr->ConfigureForTrack( &track );
     // Is here to allow a transition from no-field pointer 
     //   to finite field (non-zero pointer).

     // If the field manager has no field ptr, the field is zero 
     //     by definition ( = there is no field ! )
     const G4Field* ptrField= fieldMgr->GetDetectorField();
     fieldExists = (ptrField!=0) ;
     if( fieldExists ) 
     {
        gravityOn= ptrField->IsGravityActive();

        if(  (particleCharge != 0.0) 
            || (fUseMagneticMoment && (magneticMoment != 0.0) )
            || (gravityOn          && (restMass != 0.0) )
          )
        {
           fieldExertsForce = fieldExists; 
        }
     }
  }
  // G4cout << " G4Transport:  field exerts force= " << fieldExertsForce
  //        << "  fieldMgr= " << fieldMgr << G4endl;
  
  if( !fieldExertsForce ) 
  {
     G4double linearStepLength ;
     if( fShortStepOptimisation && (currentMinimumStep <= currentSafety) )
     {
       // The Step is guaranteed to be taken
       //
       geometryStepLength   = currentMinimumStep ;
       fGeometryLimitedStep = false ;
     }
     else
     {
       //  Find whether the straight path intersects a volume
       //
       linearStepLength = fLinearNavigator->ComputeStep( startPosition, 
                                                         startMomentumDir,
                                                         currentMinimumStep, 
                                                         newSafety) ;
       // Remember last safety origin & value.
       //
       fPreviousSftOrigin = startPosition ;
       fPreviousSafety    = newSafety ; 
       fpSafetyHelper->SetCurrentSafety( newSafety, startPosition);

       currentSafety = newSafety ;
          
       fGeometryLimitedStep= (linearStepLength <= currentMinimumStep); 
       if( fGeometryLimitedStep )
       {
         // The geometry limits the Step size (an intersection was found.)
         geometryStepLength   = linearStepLength ;
       } 
       else
       {
         // The full Step is taken.
         geometryStepLength   = currentMinimumStep ;
       }
     }
     fEndPointDistance = geometryStepLength ;

     // Calculate final position
     //
     fTransportEndPosition = startPosition+geometryStepLength*startMomentumDir ;

     // Momentum direction, energy and polarisation are unchanged by transport
     //
     fTransportEndMomentumDir   = startMomentumDir ; 
     fTransportEndKineticEnergy = track.GetKineticEnergy() ;
     fTransportEndSpin          = track.GetPolarization();
     fParticleIsLooping         = false ;
     fMomentumChanged           = false ; 
     fEndGlobalTimeComputed     = false ;
  }
  else   //  A field exerts force
  {
     G4double       momentumMagnitude = pParticle->GetTotalMomentum() ;
     G4ThreeVector  EndUnitMomentum ;
     G4double       lengthAlongCurve ;

     G4ChargeState chargeState(particleCharge,             // The charge can change (dynamic)
                               pParticleDef->GetPDGSpin(),
                               magneticMoment); 

     G4EquationOfMotion* equationOfMotion = 
     (fFieldPropagator->GetChordFinder()->GetIntegrationDriver()->GetStepper())
     ->GetEquationOfMotion();

//     equationOfMotion->SetChargeMomentumMass( particleCharge,
     equationOfMotion->SetChargeMomentumMass( chargeState,
                                              momentumMagnitude,
                                              restMass);
      
     G4ThreeVector spin        = track.GetPolarization() ;
     G4FieldTrack  aFieldTrack = G4FieldTrack( startPosition, 
                                               track.GetMomentumDirection(),
                                               0.0, 
                                               track.GetKineticEnergy(),
                                               restMass,
                                               track.GetVelocity(),
                                               track.GetGlobalTime(), // Lab.
                                               track.GetProperTime(), // Part.
                                               &spin                  ) ;
     if( currentMinimumStep > 0 ) 
     {
        // Do the Transport in the field (non recti-linear)
        //
        lengthAlongCurve = fFieldPropagator->ComputeStep( aFieldTrack,
                                                          currentMinimumStep, 
                                                          currentSafety,
                                                          track.GetVolume() ) ;
        fGeometryLimitedStep= lengthAlongCurve < currentMinimumStep; 
        if( fGeometryLimitedStep )
        {
           geometryStepLength   = lengthAlongCurve ;
        }
        else
        {
           geometryStepLength   = currentMinimumStep ;
        }

        // Remember last safety origin & value.
        //
        fPreviousSftOrigin = startPosition ;
        fPreviousSafety    = currentSafety ;         
        fpSafetyHelper->SetCurrentSafety( currentSafety, startPosition);
     }
     else
     {
        geometryStepLength   = lengthAlongCurve= 0.0 ;
        fGeometryLimitedStep = false ;
     }
      
     // Get the End-Position and End-Momentum (Dir-ection)
     //
     fTransportEndPosition = aFieldTrack.GetPosition() ;

     // Momentum:  Magnitude and direction can be changed too now ...
     //
     fMomentumChanged         = true ; 
     fTransportEndMomentumDir = aFieldTrack.GetMomentumDir() ;

     fTransportEndKineticEnergy  = aFieldTrack.GetKineticEnergy() ; 

     if( fFieldPropagator->GetCurrentFieldManager()->DoesFieldChangeEnergy() )
     {
        // If the field can change energy, then the time must be integrated
        //    - so this should have been updated
        //
        fCandidateEndGlobalTime   = aFieldTrack.GetLabTimeOfFlight();
        fEndGlobalTimeComputed    = true;

        // was ( fCandidateEndGlobalTime != track.GetGlobalTime() );
        // a cleaner way is to have FieldTrack knowing whether time is updated.
     }
     else
     {
        // The energy should be unchanged by field transport,
        //    - so the time changed will be calculated elsewhere
        //
        fEndGlobalTimeComputed = false;

        // Check that the integration preserved the energy 
        //     -  and if not correct this!
        G4double  startEnergy= track.GetKineticEnergy();
        G4double  endEnergy= fTransportEndKineticEnergy; 

        static G4ThreadLocal G4int no_inexact_steps=0, no_large_ediff;
        G4double absEdiff = std::fabs(startEnergy- endEnergy);
        if( absEdiff > perMillion * endEnergy )
        {
          no_inexact_steps++;
          // Possible statistics keeping here ...
        }
        if( fVerboseLevel > 1 )
        {
          if( std::fabs(startEnergy- endEnergy) > perThousand * endEnergy )
          {
            static G4ThreadLocal G4int no_warnings= 0, warnModulo=1,
                                       moduloFactor= 10; 
            no_large_ediff ++;
            if( (no_large_ediff% warnModulo) == 0 )
            {
               no_warnings++;
               G4cout << "WARNING - G4Transportation::AlongStepGetPIL() " 
                      << "   Energy change in Step is above 1^-3 relative value. " << G4endl
                      << "   Relative change in 'tracking' step = " 
                      << std::setw(15) << (endEnergy-startEnergy)/startEnergy << G4endl
                      << "     Starting E= " << std::setw(12) << startEnergy / MeV << " MeV " << G4endl
                      << "     Ending   E= " << std::setw(12) << endEnergy   / MeV << " MeV " << G4endl;       
               G4cout << " Energy has been corrected -- however, review"
                      << " field propagation parameters for accuracy."  << G4endl;
               if( (fVerboseLevel > 2 ) || (no_warnings<4) || (no_large_ediff == warnModulo * moduloFactor) )
               {
                 G4cout << " These include EpsilonStepMax(/Min) in G4FieldManager "
                        << " which determine fractional error per step for integrated quantities. " << G4endl
                        << " Note also the influence of the permitted number of integration steps."
                        << G4endl;
               }
               G4cerr << "ERROR - G4Transportation::AlongStepGetPIL()" << G4endl
                      << "        Bad 'endpoint'. Energy change detected"
                      << " and corrected. " 
                      << " Has occurred already "
                      << no_large_ediff << " times." << G4endl;
               if( no_large_ediff == warnModulo * moduloFactor )
               {
                  warnModulo *= moduloFactor;
               }
            }
          }
        }  // end of if (fVerboseLevel)

        // Correct the energy for fields that conserve it
        //  This - hides the integration error
        //       - but gives a better physical answer
        fTransportEndKineticEnergy= track.GetKineticEnergy(); 
     }

     fTransportEndSpin = aFieldTrack.GetSpin();
     fParticleIsLooping = fFieldPropagator->IsParticleLooping() ;
     fEndPointDistance   = (fTransportEndPosition - startPosition).mag() ;
  }

  // If we are asked to go a step length of 0, and we are on a boundary
  // then a boundary will also limit the step -> we must flag this.
  //
  if( currentMinimumStep == 0.0 ) 
  {
      if( currentSafety == 0.0 )  { fGeometryLimitedStep = true; }
  }

  // Update the safety starting from the end-point,
  // if it will become negative at the end-point.
  //
  if( currentSafety < fEndPointDistance ) 
  {
      if( particleCharge != 0.0 ) 
      {
         G4double endSafety =
               fLinearNavigator->ComputeSafety( fTransportEndPosition) ;
         currentSafety      = endSafety ;
         fPreviousSftOrigin = fTransportEndPosition ;
         fPreviousSafety    = currentSafety ; 
         fpSafetyHelper->SetCurrentSafety( currentSafety, fTransportEndPosition);

         // Because the Stepping Manager assumes it is from the start point, 
         //  add the StepLength
         //
         currentSafety     += fEndPointDistance ;

#ifdef G4DEBUG_TRANSPORT 
         G4cout.precision(12) ;
         G4cout << "***G4Transportation::AlongStepGPIL ** " << G4endl  ;
         G4cout << "  Called Navigator->ComputeSafety at " << fTransportEndPosition
                << "    and it returned safety=  " << endSafety << G4endl ; 
         G4cout << "  Adding endpoint distance   " << fEndPointDistance  
                << "    to obtain pseudo-safety= " << currentSafety << G4endl ; 
      }
      else
      {
         G4cout << "***G4Transportation::AlongStepGPIL ** " << G4endl  ;
         G4cout << "  Avoiding call to ComputeSafety : " << G4endl;
         G4cout << "    charge     = " << particleCharge     << G4endl;
         G4cout << "    mag moment = " << magneticMoment     << G4endl;
#endif
      }
  }            

  fParticleChange.ProposeTrueStepLength(geometryStepLength) ;

  return geometryStepLength ;
}

//
//   Initialize ParticleChange  (by setting all its members equal
//                               to corresponding members in G4Track)

G4VParticleChange* G4Transportation::AlongStepDoIt( const G4Track& track,
                                                    const G4Step&  stepData )
{
  static G4ThreadLocal G4int noCalls=0;
  noCalls++;

  fParticleChange.Initialize(track) ;

  //  Code for specific process 
  //
  fParticleChange.ProposePosition(fTransportEndPosition) ;
  fParticleChange.ProposeMomentumDirection(fTransportEndMomentumDir) ;
  fParticleChange.ProposeEnergy(fTransportEndKineticEnergy) ;
  fParticleChange.SetMomentumChanged(fMomentumChanged) ;

  fParticleChange.ProposePolarization(fTransportEndSpin);
  
  G4double deltaTime = 0.0 ;

  // Calculate  Lab Time of Flight (ONLY if field Equations used it!)
  // G4double endTime   = fCandidateEndGlobalTime;
  // G4double delta_time = endTime - startTime;

  G4double startTime = track.GetGlobalTime() ;
  
  if (!fEndGlobalTimeComputed)
  {
     // The time was not integrated .. make the best estimate possible
     //
     G4double initialVelocity = stepData.GetPreStepPoint()->GetVelocity();
     G4double stepLength      = track.GetStepLength();

     deltaTime= 0.0;  // in case initialVelocity = 0 
     if ( initialVelocity > 0.0 )  { deltaTime = stepLength/initialVelocity; }

     fCandidateEndGlobalTime   = startTime + deltaTime ;
     fParticleChange.ProposeLocalTime(  track.GetLocalTime() + deltaTime) ;
  }
  else
  {
     deltaTime = fCandidateEndGlobalTime - startTime ;
     fParticleChange.ProposeGlobalTime( fCandidateEndGlobalTime ) ;
  }


  // Now Correct by Lorentz factor to get delta "proper" Time
  
  G4double  restMass       = track.GetDynamicParticle()->GetMass() ;
  G4double deltaProperTime = deltaTime*( restMass/track.GetTotalEnergy() ) ;

  fParticleChange.ProposeProperTime(track.GetProperTime() + deltaProperTime) ;
  //fParticleChange. ProposeTrueStepLength( track.GetStepLength() ) ;

  // If the particle is caught looping or is stuck (in very difficult
  // boundaries) in a magnetic field (doing many steps) 
  //   THEN this kills it ...
  //
  if ( fParticleIsLooping )
  {
      G4double endEnergy= fTransportEndKineticEnergy;

      if( (endEnergy < fThreshold_Important_Energy) 
          || (fNoLooperTrials >= fThresholdTrials ) )
      {
        // Kill the looping particle 
        //
        fParticleChange.ProposeTrackStatus( fStopAndKill )  ;

        // 'Bare' statistics
        fSumEnergyKilled += endEnergy; 
        if( endEnergy > fMaxEnergyKilled) { fMaxEnergyKilled= endEnergy; }

#ifdef G4VERBOSE
        if( (fVerboseLevel > 1) && 
            ( endEnergy > fThreshold_Warning_Energy )  )
        { 
          G4cout << " G4Transportation is killing track that is looping or stuck "
                 << G4endl
                 << "   This track has " << track.GetKineticEnergy() / MeV
                 << " MeV energy." << G4endl;
          G4cout << "   Number of trials = " << fNoLooperTrials 
                 << "   No of calls to AlongStepDoIt = " << noCalls 
                 << G4endl;
        }
#endif
        fNoLooperTrials=0; 
      }
      else
      {
        fNoLooperTrials ++; 
#ifdef G4VERBOSE
        if( (fVerboseLevel > 2) )
        {
          G4cout << "   G4Transportation::AlongStepDoIt(): Particle looping -  "
                 << "   Number of trials = " << fNoLooperTrials 
                 << "   No of calls to  = " << noCalls 
                 << G4endl;
        }
#endif
      }
  }
  else
  {
      fNoLooperTrials=0; 
  }

  // Another (sometimes better way) is to use a user-limit maximum Step size
  // to alleviate this problem .. 

  // Introduce smooth curved trajectories to particle-change
  //
  fParticleChange.SetPointerToVectorOfAuxiliaryPoints
    (fFieldPropagator->GimmeTrajectoryVectorAndForgetIt() );

  return &fParticleChange ;
}

//
//  This ensures that the PostStep action is always called,
//  so that it can do the relocation if it is needed.
// 

G4double G4Transportation::
PostStepGetPhysicalInteractionLength( const G4Track&,
                                            G4double, // previousStepSize
                                            G4ForceCondition* pForceCond )
{ 
  *pForceCond = Forced ; 
  return DBL_MAX ;  // was kInfinity ; but convention now is DBL_MAX
}

//

G4VParticleChange* G4Transportation::PostStepDoIt( const G4Track& track,
                                                   const G4Step& )
{
   G4TouchableHandle retCurrentTouchable ;   // The one to return
   G4bool isLastStep= false; 

  // Initialize ParticleChange  (by setting all its members equal
  //                             to corresponding members in G4Track)
  // fParticleChange.Initialize(track) ;  // To initialise TouchableChange

  fParticleChange.ProposeTrackStatus(track.GetTrackStatus()) ;

  // If the Step was determined by the volume boundary,
  // logically relocate the particle

  if(fGeometryLimitedStep)
  {  
    // fCurrentTouchable will now become the previous touchable, 
    // and what was the previous will be freed.
    // (Needed because the preStepPoint can point to the previous touchable)

    fLinearNavigator->SetGeometricallyLimitedStep() ;
    fLinearNavigator->
    LocateGlobalPointAndUpdateTouchableHandle( track.GetPosition(),
                                               track.GetMomentumDirection(),
                                               fCurrentTouchableHandle,
                                               true                      ) ;
    // Check whether the particle is out of the world volume 
    // If so it has exited and must be killed.
    //
    if( fCurrentTouchableHandle->GetVolume() == 0 )
    {
       fParticleChange.ProposeTrackStatus( fStopAndKill ) ;
    }
    retCurrentTouchable = fCurrentTouchableHandle ;
    fParticleChange.SetTouchableHandle( fCurrentTouchableHandle ) ;

    // Update the Step flag which identifies the Last Step in a volume
    isLastStep =  fLinearNavigator->ExitedMotherVolume() 
                       | fLinearNavigator->EnteredDaughterVolume() ;

#ifdef G4DEBUG_TRANSPORT
    //  Checking first implementation of flagging Last Step in Volume
    //
    G4bool exiting =  fLinearNavigator->ExitedMotherVolume();
    G4bool entering = fLinearNavigator->EnteredDaughterVolume();
    if( ! (exiting || entering) )
    { 
      G4cout << " Transport> :  Proposed isLastStep= " << isLastStep 
             << " Exiting "  << fLinearNavigator->ExitedMotherVolume()          
             << " Entering " << fLinearNavigator->EnteredDaughterVolume() 
             << G4endl;
    } 
#endif
  }
  else                 // fGeometryLimitedStep  is false
  {                    
    // This serves only to move the Navigator's location
    //
    fLinearNavigator->LocateGlobalPointWithinVolume( track.GetPosition() ) ;

    // The value of the track's current Touchable is retained. 
    // (and it must be correct because we must use it below to
    // overwrite the (unset) one in particle change)
    //  It must be fCurrentTouchable too ??
    //
    fParticleChange.SetTouchableHandle( track.GetTouchableHandle() ) ;
    retCurrentTouchable = track.GetTouchableHandle() ;

    isLastStep= false;

#ifdef G4DEBUG_TRANSPORT
    //  Checking first implementation of flagging Last Step in Volume
    //
    G4cout << " Transport> Proposed isLastStep= " << isLastStep
           << " Geometry did not limit step. " << G4endl;
#endif
  }         // endif ( fGeometryLimitedStep ) 

  fParticleChange.ProposeLastStepInVolume(isLastStep);    

  const G4VPhysicalVolume* pNewVol = retCurrentTouchable->GetVolume() ;
  const G4Material* pNewMaterial   = 0 ;
  const G4VSensitiveDetector* pNewSensitiveDetector   = 0 ;
                                                                                       
  if( pNewVol != 0 )
  {
    pNewMaterial= pNewVol->GetLogicalVolume()->GetMaterial();
    pNewSensitiveDetector= pNewVol->GetLogicalVolume()->GetSensitiveDetector();
  }

  // ( <const_cast> pNewMaterial ) ;
  // ( <const_cast> pNewSensitiveDetector) ;

  fParticleChange.SetMaterialInTouchable( (G4Material *) pNewMaterial ) ;
  fParticleChange.SetSensitiveDetectorInTouchable( (G4VSensitiveDetector *) pNewSensitiveDetector ) ;

  const G4MaterialCutsCouple* pNewMaterialCutsCouple = 0;
  if( pNewVol != 0 )
  {
    pNewMaterialCutsCouple=pNewVol->GetLogicalVolume()->GetMaterialCutsCouple();
  }

  if( pNewVol!=0 && pNewMaterialCutsCouple!=0 && pNewMaterialCutsCouple->GetMaterial()!=pNewMaterial )
  {
    // for parametrized volume
    //
    pNewMaterialCutsCouple =
      G4ProductionCutsTable::GetProductionCutsTable()
                             ->GetMaterialCutsCouple(pNewMaterial,
                               pNewMaterialCutsCouple->GetProductionCuts());
  }
  fParticleChange.SetMaterialCutsCoupleInTouchable( pNewMaterialCutsCouple );

  // temporarily until Get/Set Material of ParticleChange, 
  // and StepPoint can be made const. 
  // Set the touchable in ParticleChange
  // this must always be done because the particle change always
  // uses this value to overwrite the current touchable pointer.
  //
  fParticleChange.SetTouchableHandle(retCurrentTouchable) ;

  return &fParticleChange ;
}

// New method takes over the responsibility to reset the state of G4Transportation
//   object at the start of a new track or the resumption of a suspended track. 

void 
G4Transportation::StartTracking(G4Track* aTrack)
{
  G4VProcess::StartTracking(aTrack);

  // The actions here are those that were taken in AlongStepGPIL
  //   when track.GetCurrentStepNumber()==1

  // reset safety value and center
  //
  fPreviousSafety    = 0.0 ; 
  fPreviousSftOrigin = G4ThreeVector(0.,0.,0.) ;
  
  // reset looping counter -- for motion in field
  fNoLooperTrials= 0; 
  // Must clear this state .. else it depends on last track's value
  //  --> a better solution would set this from state of suspended track TODO ? 
  // Was if( aTrack->GetCurrentStepNumber()==1 ) { .. }

  // ChordFinder reset internal state
  //
  if( DoesGlobalFieldExist() )
  {
     fFieldPropagator->ClearPropagatorState();   
       // Resets all state of field propagator class (ONLY)
       //  including safety values (in case of overlaps and to wipe for first track).

     // G4ChordFinder* chordF= fFieldPropagator->GetChordFinder();
     // if( chordF ) chordF->ResetStepEstimate();
  }

  // Make sure to clear the chord finders of all fields (ie managers)
  //
  G4FieldManagerStore* fieldMgrStore = G4FieldManagerStore::GetInstance();
  fieldMgrStore->ClearAllChordFindersState(); 

  // Update the current touchable handle  (from the track's)
  //
  fCurrentTouchableHandle = aTrack->GetTouchableHandle();
}