Added a BirksLaw function from Geant4 G4EmSaturation into DDG4...

 Added a BirksLaw function from Geant4 G4EmSaturation into DDG4 Geant4StepHandler. It may be called within function 'totalEnery()' for each G4Step.

DDG4/include/DDG4/Geant4StepHandler.h

@@ -17,6 +17,7 @@
 #include "G4StepPoint.hh"
 #include "G4VTouchable.hh"
 #include "G4VSensitiveDetector.hh"
+#include "G4EmSaturation.hh"
 
 
 /// Namespace for the AIDA detector description toolkit
@@ -195,6 +196,24 @@ namespace DD4hep {
       Position localToGlobal(const G4ThreeVector& local)  const;
       /// Coordinate transformation to global coordinates in MM
       Position localToGlobal(double x, double y, double z)  const;
+      /// Apply BirksLaw
+      double BirkAttenuation(const G4Step* aStep) const
+      {
+	double energyDeposition = aStep->GetTotalEnergyDeposit();
+	double length = aStep->GetStepLength();
+	double niel   = aStep->GetNonIonizingEnergyDeposit();
+	const G4Track* track = aStep->GetTrack();
+	const G4ParticleDefinition* particle = track->GetDefinition();
+	const G4MaterialCutsCouple* couple = track->GetMaterialCutsCouple();
+	G4EmSaturation* emSaturation = new G4EmSaturation();
+	double engyVis = emSaturation->VisibleEnergyDeposition(particle,
+							       couple,
+							       length,
+							       energyDeposition,
+							       niel);
+	delete emSaturation; 
+	return engyVis;
+      }
     };
 
   }    // End namespace Simulation