Response simulation with Geant4

Martin Ljunggren [[email protected]]

TPC/Sim/AliTPCv2.cxx

@@ -2136,7 +2136,9 @@ void AliTPCv2::StepManager()
   //
   const Float_t kprim = 14.35; // number of primary collisions per 1 cm
   const Float_t kpoti = 20.77e-9; // first ionization potential for Ne/CO2
-  const Float_t kwIon = 35.97e-9; // energy for the ion-electron pair creation 
+  const Float_t kwIon = 35.97e-9; // energy for the ion-electron pair creation
+  const Float_t kScalewIonG4 = 0.85; // scale factor to tune kwIon for Geant4 
+  const Float_t kFanoFactorG4 = 0.7; // parameter for smearing the number of ionizations (nel) using Geant4
   const Int_t   kMaxDistRef =15;     // maximal difference between 2 stored references 
 
   const Float_t kbig = 1.e10;
@@ -2264,6 +2266,8 @@ void AliTPCv2::StepManager()
       nel = (Int_t)(((gMC->Edep())-kpoti)/kwIon) + 1;
     }
     else {
+
+      /*
       static Double_t deForNextStep = 0.;
       // Geant4 (the meaning of Edep as in Geant3) - wrong
       //nel = (Int_t)(((gMC->Edep())-kpoti)/kwIon) + 1;
@@ -2272,6 +2276,11 @@ void AliTPCv2::StepManager()
       Double_t eAvailable = gMC->Edep() + deForNextStep;
       nel = (Int_t)(eAvailable/kwIon);
       deForNextStep = eAvailable - nel*kwIon;
+      */
+
+      //new Geant4-approach
+      Double_t meanIon = gMC->Edep()/(kwIon*kScalewIonG4);
+      nel = (Int_t) ( kFanoFactorG4*AliMathBase::Gamma(meanIon/kFanoFactorG4)); // smear nel using gamma distr w mean = meanIon and variance = meanIon/kFanoFactorG4
     }
     nel=TMath::Min(nel,300); // 300 electrons corresponds to 10 keV
     //