[u/mrichter/AliRoot.git] / TRD / AliTRDv2.cxx

/**************************************************************************
 * Copyright(c) 1998-1999, ALICE Experiment at CERN, All rights reserved. *
 *                                                                        *
 * Author: The ALICE Off-line Project.                                    *
 * Contributors are mentioned in the code where appropriate.              *
 *                                                                        *
 * Permission to use, copy, modify and distribute this software and its   *
 * documentation strictly for non-commercial purposes is hereby granted   *
 * without fee, provided that the above copyright notice appears in all   *
 * copies and that both the copyright notice and this permission notice   *
 * appear in the supporting documentation. The authors make no claims     *
 * about the suitability of this software for any purpose. It is          *
 * provided "as is" without express or implied warranty.                  *
 **************************************************************************/

/*
$Log$
Revision 1.14  1999/10/04 14:48:07  fca
Avoid warnings on non-ansi compiler HP-UX CC

Revision 1.13  1999/09/29 09:24:35  fca
Introduction of the Copyright and cvs Log

*/

///////////////////////////////////////////////////////////////////////////////
//                                                                           //
//  Transition Radiation Detector version 2 -- detailed simulation           //
//                                                                           //
//Begin_Html
/*
<img src="picts/AliTRDv2Class.gif">
*/
//End_Html
//                                                                           //
//                                                                           //
///////////////////////////////////////////////////////////////////////////////

#include <stdlib.h>

#include <TMath.h>
#include <TVector.h>
#include <TRandom.h>

#include "AliTRDv2.h"
#include "AliTRDmatrix.h"
#include "AliRun.h"
#include "AliMC.h"
#include "AliConst.h"

ClassImp(AliTRDv2)

//_____________________________________________________________________________
AliTRDv2::AliTRDv2(const char *name, const char *title) 
         :AliTRD(name, title) 
{
  //
  // Standard constructor for Transition Radiation Detector version 2
  //

  fIdSens       = 0;

  fIdSpace1     = 0;
  fIdSpace2     = 0;
  fIdSpace3     = 0;

  fIdChamber1   = 0;
  fIdChamber2   = 0;
  fIdChamber3   = 0;

  fSensSelect   = 0;
  fSensPlane    = 0;
  fSensChamber  = 0;
  fSensSector   = 0;

  Int_t iplan;

  for (iplan = 0; iplan < kNplan; iplan++) {
    for (Int_t icham = 0; icham < kNcham; icham++) {
      fRowMax[iplan][icham] = 0;
    }
    fColMax[iplan] = 0;
  }
  fTimeMax      = 0;

  fRowPadSize   = 0;
  fColPadSize   = 0;
  fTimeBinSize  = 0;

  fGasGain      = 0;
  fNoise        = 0;
  fChipGain     = 0;
  fADCoutRange  = 0;
  fADCinRange   = 0;
  fADCthreshold = 0;

  fDiffusionT   = 0;
  fDiffusionL   = 0;

  fDeltaE       = NULL;

  SetBufferSize(128000);

}

//_____________________________________________________________________________
AliTRDv2::~AliTRDv2()
{

  if (fDeltaE) delete fDeltaE;

}
 
//_____________________________________________________________________________
void AliTRDv2::CreateGeometry()
{
  //
  // Create the GEANT geometry for the Transition Radiation Detector - Version 2
  // This version covers the full azimuth. 
  //
  // Author:  Christoph Blume (C.Blume@gsi.de) 20/07/99 
  //

  Float_t xpos, ypos, zpos;

  // Check that FRAME is there otherwise we have no place where to put the TRD
  AliModule* FRAME = gAlice->GetModule("FRAME");
  if (!FRAME) return;

  // Define the chambers
  AliTRD::CreateGeometry();

  // Position the the TRD-sectors in all TRD-volumes in the spaceframe
  xpos     = 0.;
  ypos     = 0.;
  zpos     = 0.;
  gMC->Gspos("TRD ",1,"BTR1",xpos,ypos,zpos,0,"ONLY");
  gMC->Gspos("TRD ",2,"BTR2",xpos,ypos,zpos,0,"ONLY");
  gMC->Gspos("TRD ",3,"BTR3",xpos,ypos,zpos,0,"ONLY");

}

//_____________________________________________________________________________
void AliTRDv2::CreateMaterials()
{
  //
  // Create materials for the Transition Radiation Detector version 2
  //

  AliTRD::CreateMaterials();

}

//_____________________________________________________________________________
void AliTRDv2::Diffusion(Float_t driftlength, Float_t *xyz)
{
  //
  // Applies the diffusion smearing to the position of a single electron
  //

  if ((driftlength >        0) && 
      (driftlength < kDrThick)) {
    Float_t driftSqrt = TMath::Sqrt(driftlength);
    Float_t sigmaT = driftSqrt * fDiffusionT;
    Float_t sigmaL = driftSqrt * fDiffusionL;
    xyz[0] = gRandom->Gaus(xyz[0], sigmaL);
    xyz[1] = gRandom->Gaus(xyz[1], sigmaT);
    xyz[2] = gRandom->Gaus(xyz[2], sigmaT);
  }
  else {
    xyz[0] = 0.0;
    xyz[1] = 0.0;
    xyz[2] = 0.0;
  }

}

//_____________________________________________________________________________
void AliTRDv2::Hits2Digits()
{
  //
  // Creates TRD digits from hits. This procedure includes the following:
  //      - Diffusion
  //      - Gas gain including fluctuations
  //      - Pad-response (simple Gaussian approximation)
  //      - Electronics noise
  //      - Electronics gain
  //      - Digitization
  //      - ADC threshold
  // The corresponding parameter can be adjusted via the various Set-functions.
  // If these parameters are not explicitly set, default values are used (see
  // Init-function).
  // To produce digits from a galice.root file with TRD-hits use the
  // digitsCreate.C macro.
  //

  printf(" Start creating digits\n");

  ///////////////////////////////////////////////////////////////
  // Parameter 
  ///////////////////////////////////////////////////////////////

  // Converts number of electrons to fC
  const Float_t el2fC = 1.602E-19 * 1.0E15; 

  ///////////////////////////////////////////////////////////////

  Int_t nBytes = 0;

  AliTRDhit *TRDhit;

  Int_t iplan;
  Int_t iRow;

  // Position of pad 0,0,0 
  // 
  // chambers seen from the top:
  //     +----------------------------+
  //     |                            |
  //     |                            |     ^
  //     |                            | rphi|
  //     |                            |     |
  //     |0                           |     | 
  //     +----------------------------+     +------>
  //                                             z 
  // chambers seen from the side:           ^
  //     +----------------------------+ time|
  //     |                            |     |
  //     |0                           |     |
  //     +----------------------------+     +------>
  //                                             z
  //                                             
  // The pad row (z-direction)
  Float_t row0[kNplan][kNcham];
  for (iplan = 0; iplan < kNplan; iplan++) {
    row0[iplan][0] = -fClengthI[iplan]/2. - fClengthM[iplan] - fClengthO[iplan] 
                   + kCcthick; 
    row0[iplan][1] = -fClengthI[iplan]/2. - fClengthM[iplan]                    
                   + kCcthick;
    row0[iplan][2] = -fClengthI[iplan]/2.                                       
                   + kCcthick;
    row0[iplan][3] =  fClengthI[iplan]/2.                                       
                   + kCcthick; 
    row0[iplan][4] =  fClengthI[iplan]/2. + fClengthM[iplan]                    
                   + kCcthick; 
  }
  // The pad column (rphi-direction)  
  Float_t col0[kNplan];
  for (iplan = 0; iplan < kNplan; iplan++) {
    col0[iplan]    = -fCwidth[iplan]/2. + kCcthick;
  }
  // The time bucket
  Float_t time0[kNplan];
  for (iplan = 0; iplan < kNplan; iplan++) {
    time0[iplan]   = kRmin + kCcframe/2. + kDrZpos - 0.5 * kDrThick
                           + iplan * (kCheight + kCspace);
  } 

  // Get the pointer to the hit tree
  TTree *HitTree    = gAlice->TreeH();
  // Get the pointer to the digits tree
  TTree *DigitsTree = gAlice->TreeD();

  // Get the number of entries in the hit tree
  // (Number of primary particles creating a hit somewhere)
  Int_t nTrack = (Int_t) HitTree->GetEntries();

  Int_t chamBeg = 0;
  Int_t chamEnd = kNcham;
  if (fSensChamber) chamEnd = chamBeg = fSensChamber;
  Int_t planBeg = 0;
  Int_t planEnd = kNplan;
  if (fSensPlane)   planEnd = planBeg = fSensPlane;
  Int_t sectBeg = 0;
  Int_t sectEnd = kNsect;
  if (fSensSector)  sectEnd = sectBeg = fSensSector;

  // Loop through all the chambers
  for (Int_t icham = chamBeg; icham < chamEnd; icham++) {
    for (iplan = planBeg; iplan < planEnd; iplan++) {
      for (Int_t isect = sectBeg; isect < sectEnd; isect++) {

        printf(" Digitizing chamber %d, plane %d, sector %d\n"
              ,icham+1,iplan+1,isect+1);

        // Create a detector matrix to keep the signal and track numbers
        AliTRDmatrix *matrix = new AliTRDmatrix(fRowMax[iplan][icham]
                                               ,fColMax[iplan]
                                               ,fTimeMax
                                               ,isect+1,icham+1,iplan+1);

        // Loop through all entries in the tree
        for (Int_t iTrack = 0; iTrack < nTrack; iTrack++) {

          gAlice->ResetHits();
          nBytes += HitTree->GetEvent(iTrack);

          // Get the number of hits in the TRD created by this particle
          Int_t nHit = fHits->GetEntriesFast();

          // Loop through the TRD hits  
          for (Int_t iHit = 0; iHit < nHit; iHit++) {

            if (!(TRDhit = (AliTRDhit *) fHits->UncheckedAt(iHit))) 
              continue;

            Float_t x       = TRDhit->fX;
            Float_t y       = TRDhit->fY;
            Float_t z       = TRDhit->fZ;
            Float_t q       = TRDhit->fQ;
            Int_t   track   = TRDhit->fTrack;
            Int_t   plane   = TRDhit->fPlane;
            Int_t   sector  = TRDhit->fSector;
            Int_t   chamber = TRDhit->fChamber;        

            if ((sector  != isect+1) ||
                (plane   != iplan+1) ||
                (chamber != icham+1)) 
              continue;

            // Rotate the sectors on top of each other
            Float_t phi  = 2.0 * kPI /  (Float_t) kNsect 
                               * ((Float_t) sector - 0.5);
            Float_t xRot = -x * TMath::Cos(phi) + y * TMath::Sin(phi);
            Float_t yRot =  x * TMath::Sin(phi) + y * TMath::Cos(phi);
            Float_t zRot =  z;

            // The hit position in pad coordinates (center pad)
            // The pad row (z-direction)
            Int_t rowH  = (Int_t) ((zRot -  row0[iplan][icham]) / fRowPadSize);
            // The pad column (rphi-direction)  
            Int_t colH  = (Int_t) ((yRot -  col0[iplan]       ) / fColPadSize);
            // The time bucket
            Int_t timeH = (Int_t) ((xRot - time0[iplan]       ) / fTimeBinSize);

            // Array to sum up the signal in a box surrounding the
            // hit postition
            const Int_t timeBox = 5;
            const Int_t  colBox = 7;
            const Int_t  rowBox = 5;
            Float_t signalSum[rowBox][colBox][timeBox];
            for (iRow  = 0;  iRow <  rowBox; iRow++ ) {
              for (Int_t iCol  = 0;  iCol <  colBox; iCol++ ) {
                for (Int_t iTime = 0; iTime < timeBox; iTime++) {
                  signalSum[iRow][iCol][iTime] = 0;
		}
	      }
	    }

            // Loop over all electrons of this hit
            Int_t nEl = (Int_t) q;
            for (Int_t iEl = 0; iEl < nEl; iEl++) {

              // Apply the diffusion smearing
              Float_t driftlength = xRot - time0[iplan];
              Float_t xyz[3];
              xyz[0] = xRot;
              xyz[1] = yRot;
              xyz[2] = zRot;
              Diffusion(driftlength,xyz);

              // At this point absorption effects that depend on the 
	      // driftlength could be taken into account.              

              // The electron position and the distance to the hit position
	      // in pad units
              // The pad row (z-direction)
              Int_t  rowE = (Int_t) ((xyz[2] -  row0[iplan][icham]) / fRowPadSize);
              Int_t  rowD =  rowH -  rowE;
              // The pad column (rphi-direction)
              Int_t  colE = (Int_t) ((xyz[1] -  col0[iplan]       ) / fColPadSize);
              Int_t  colD =  colH -  colE;
              // The time bucket
              Int_t timeE = (Int_t) ((xyz[0] - time0[iplan]       ) / fTimeBinSize);
              Int_t timeD = timeH - timeE;

              // Apply the gas gain including fluctuations
              Int_t signal = (Int_t) (-fGasGain * TMath::Log(gRandom->Rndm()));

	      // The distance of the electron to the center of the pad 
	      // in units of pad width
              Float_t dist = (xyz[1] - col0[iplan] - (colE + 0.5) * fColPadSize) 
                           / fColPadSize;

              // Sum up the signal in the different pixels
              // and apply the pad response
              Int_t  rowIdx =  rowD + (Int_t) ( rowBox / 2);
              Int_t  colIdx =  colD + (Int_t) ( colBox / 2);
              Int_t timeIdx = timeD + (Int_t) (timeBox / 2);
              signalSum[rowIdx][colIdx-1][timeIdx] += PadResponse(dist-1.) * signal;
              signalSum[rowIdx][colIdx  ][timeIdx] += PadResponse(dist   ) * signal;
              signalSum[rowIdx][colIdx+1][timeIdx] += PadResponse(dist+1.) * signal;

            }
            
            // Add the padcluster to the detector matrix
            for (iRow  = 0;  iRow <  rowBox; iRow++ ) {
              for (Int_t iCol  = 0;  iCol <  colBox; iCol++ ) {
                for (Int_t iTime = 0; iTime < timeBox; iTime++) {

                  Int_t  rowB =  rowH + iRow  - (Int_t) ( rowBox / 2); 
                  Int_t  colB =  colH + iCol  - (Int_t) ( colBox / 2);
                  Int_t timeB = timeH + iTime - (Int_t) (timeBox / 2);

                  Float_t signalB = signalSum[iRow][iCol][iTime];
                  if (signalB > 0.0) {
                    matrix->AddSignal(rowB,colB,timeB,signalB);
                    if (!(matrix->AddTrack(rowB,colB,timeB,track))) 
                      printf("More than three tracks in a pixel!\n");
		  }

		}
	      }
	    }

          }

	}

        // Create the hits for this chamber
        for (Int_t iRow  = 0; iRow  <  fRowMax[iplan][icham]; iRow++ ) {
          for (Int_t iCol  = 0; iCol  <  fColMax[iplan]         ; iCol++ ) {
            for (Int_t iTime = 0; iTime < fTimeMax                ; iTime++) {         

              Float_t signalAmp = matrix->GetSignal(iRow,iCol,iTime);

              // Add the noise
              signalAmp  = TMath::Max(gRandom->Gaus(signalAmp,fNoise),(Float_t) 0.0);
	      // Convert to fC
              signalAmp *= el2fC;
              // Convert to mV
              signalAmp *= fChipGain;
	      // Convert to ADC counts
              Int_t adc  = (Int_t) (signalAmp * (fADCoutRange / fADCinRange));

	      // Apply threshold on ADC value
              if (adc > fADCthreshold) {

                Int_t trackSave[3];
                for (Int_t ii = 0; ii < 3; ii++) {
                  trackSave[ii] = matrix->GetTrack(iRow,iCol,iTime,ii);
	        }

                Int_t digits[7];
                digits[0] = matrix->GetSector();
                digits[1] = matrix->GetChamber();
                digits[2] = matrix->GetPlane();
                digits[3] = iRow;
                digits[4] = iCol;
                digits[5] = iTime;
                digits[6] = adc;

		// Add this digit to the TClonesArray
                AddDigit(trackSave,digits);

	      }

	    }
	  }
	}

	// Clean up
        delete matrix;

      }
    }
  }

  // Fill the digits-tree
  DigitsTree->Fill();

}

//_____________________________________________________________________________
void AliTRDv2::Init() 
{
  //
  // Initialise Transition Radiation Detector after geometry has been built.
  // Includes the default settings of all parameter for the digitization.
  //

  printf("**************************************"
	 "  TRD  "
	 "**************************************\n");
  printf("\n     Version 2 of TRD initialing, "
	 "symmetric TRD\n\n");

  AliTRD::Init();


  //
  // Check that FRAME is there otherwise we have no place where to
  // put TRD
  AliModule* FRAME=gAlice->GetModule("FRAME");
  if(!FRAME) {
    Error("Ctor","TRD needs FRAME to be present\n");
    exit(1);
  } else 
    if(FRAME->IsVersion()!=1) {
      Error("Ctor","FRAME version 1 needed with this version of TRD\n");
      exit(1);
    }

  if (fSensPlane)
    printf("          Only plane %d is sensitive\n",fSensPlane);
  if (fSensChamber)   
    printf("          Only chamber %d is sensitive\n",fSensChamber);
  if (fSensSector)
    printf("          Only sector %d is sensitive\n",fSensSector);

  for (Int_t i = 0; i < 80; i++) printf("*");
  printf("\n");

  // First ionization potential (eV) for the gas mixture (90% Xe + 10% CO2)
  const Float_t kPoti = 12.1;
  // Maximum energy (50 keV);
  const Float_t kEend = 50000.0;
  // Ermilova distribution for the delta-ray spectrum
  Float_t Poti = TMath::Log(kPoti);
  Float_t Eend = TMath::Log(kEend);
  fDeltaE  = new TF1("deltae",Ermilova,Poti,Eend,0);

  // Identifier of the sensitive volume (drift region)
  fIdSens     = gMC->VolId("UL05");

  // Identifier of the TRD-spaceframe volumina
  fIdSpace1   = gMC->VolId("B028");
  fIdSpace2   = gMC->VolId("B029");
  fIdSpace3   = gMC->VolId("B030");

  // Identifier of the TRD-driftchambers
  fIdChamber1 = gMC->VolId("UCIO");
  fIdChamber2 = gMC->VolId("UCIM");
  fIdChamber3 = gMC->VolId("UCII");

  // The default pad dimensions
  if (!(fRowPadSize))  fRowPadSize  = 4.5;
  if (!(fColPadSize))  fColPadSize  = 1.0;
  if (!(fTimeBinSize)) fTimeBinSize = 0.1;

  // The maximum number of pads
  for (Int_t iplan = 0; iplan < kNplan; iplan++) {
    // Rows 
    fRowMax[iplan][0] = 1 + TMath::Nint((fClengthO[iplan] - 2. * kCcthick) 
                                                          / fRowPadSize - 0.5);
    fRowMax[iplan][1] = 1 + TMath::Nint((fClengthM[iplan] - 2. * kCcthick) 
                                                          / fRowPadSize - 0.5);
    fRowMax[iplan][2] = 1 + TMath::Nint((fClengthI[iplan] - 2. * kCcthick) 
                                                          / fRowPadSize - 0.5);
    fRowMax[iplan][3] = 1 + TMath::Nint((fClengthM[iplan] - 2. * kCcthick) 
                                                          / fRowPadSize - 0.5);
    fRowMax[iplan][4] = 1 + TMath::Nint((fClengthO[iplan] - 2. * kCcthick) 
                                                          / fRowPadSize - 0.5);
    // Columns
    fColMax[iplan]    = 1 + TMath::Nint((fCwidth[iplan]   - 2. * kCcthick) 
                                                          / fColPadSize - 0.5);
  }
  // Time buckets
  fTimeMax = 1 + TMath::Nint(kDrThick / fTimeBinSize - 0.5);

  // The default parameter for the digitization
  if (!(fGasGain))      fGasGain      = 2.0E3;
  if (!(fNoise))        fNoise        = 3000.;
  if (!(fChipGain))     fChipGain     = 10.;
  if (!(fADCoutRange))  fADCoutRange  = 255.;
  if (!(fADCinRange))   fADCinRange   = 2000.;
  if (!(fADCthreshold)) fADCthreshold = 0;

  // Transverse and longitudinal diffusion coefficients (Xe/Isobutane)
  if (!(fDiffusionT))   fDiffusionT   = 0.060;
  if (!(fDiffusionL))   fDiffusionL   = 0.017;

  printf("**************************************"
	 "  TRD  "
	 "**************************************\n");
}

//_____________________________________________________________________________
void AliTRDv2::MakeBranch(Option_t* option)
{
  //
  // Create Tree branches for the TRD digits.
  //

  Int_t  buffersize = 4000;
  Char_t branchname[10];

  sprintf(branchname,"%s",GetName());

  AliDetector::MakeBranch(option); 

  Char_t *D = strstr(option,"D");
  if (fDigits && gAlice->TreeD() && D) {
    gAlice->TreeD()->Branch(branchname,&fDigits, buffersize);
    printf("Making Branch %s for digits\n",branchname);
  }

}

//_____________________________________________________________________________
Float_t AliTRDv2::PadResponse(Float_t x)
{
  //
  // The pad response for the chevron pads. 
  // We use a simple Gaussian approximation which should be good
  // enough for our purpose.
  //

  // The parameters for the response function
  const Float_t aa  =  0.8872;
  const Float_t bb  = -0.00573;
  const Float_t cc  =  0.454;
  const Float_t cc2 =  cc*cc;

  Float_t pr = aa * (bb + TMath::Exp(-x*x / (2. * cc2)));

  //TF1 *funPR = new TF1("funPR","[0]*([1]+exp(-x*x /(2.*[2])))",-3,3);
  //funPR->SetParameter(0,aa );
  //funPR->SetParameter(1,bb );
  //funPR->SetParameter(2,cc2);
  //
  //Float_t pr = funPR->Eval(distance);
  //
  //delete funPR;

  return (pr);

}

//_____________________________________________________________________________
void AliTRDv2::SetSensPlane(Int_t iplane)
{
  //
  // Defines the hit-sensitive plane (1-6)
  //

  if ((iplane < 0) || (iplane > 6)) {
    printf("Wrong input value: %d\n",iplane);
    printf("Use standard setting\n");
    fSensPlane  = 0;
    fSensSelect = 0;
    return;
  }

  fSensSelect = 1;
  fSensPlane  = iplane;

}

//_____________________________________________________________________________
void AliTRDv2::SetSensChamber(Int_t ichamber)
{
  //
  // Defines the hit-sensitive chamber (1-5)
  //

  if ((ichamber < 0) || (ichamber > 5)) {
    printf("Wrong input value: %d\n",ichamber);
    printf("Use standard setting\n");
    fSensChamber = 0;
    fSensSelect  = 0;
    return;
  }

  fSensSelect  = 1;
  fSensChamber = ichamber;

}

//_____________________________________________________________________________
void AliTRDv2::SetSensSector(Int_t isector)
{
  //
  // Defines the hit-sensitive sector (1-18)
  //

  if ((isector < 0) || (isector > 18)) {
    printf("Wrong input value: %d\n",isector);
    printf("Use standard setting\n");
    fSensSector = 0;
    fSensSelect = 0;
    return;
  }

  fSensSelect = 1;
  fSensSector = isector;

}

//_____________________________________________________________________________
void AliTRDv2::StepManager()
{
  //
  // Called at every step in the Transition Radiation Detector version 2.
  // Slow simulator. Every charged track produces electron cluster as hits 
  // along its path across the drift volume. The step size is set acording
  // to Bethe-Bloch. The energy distribution of the delta electrons follows
  // a spectrum taken from Ermilova et al.
  //

  Int_t    iIdSens, icSens;
  Int_t    iIdSpace, icSpace;
  Int_t    iIdChamber, icChamber;
  Int_t    vol[3]; 
  Int_t    iPid;

  Int_t    secMap1[10] = {  3,  7,  8,  9, 10, 11,  2,  1, 18, 17 };
  Int_t    secMap2[ 5] = { 16, 15, 14, 13, 12 };
  Int_t    secMap3[ 3] = {  5,  6,  4 };

  Float_t  hits[4];
  Float_t  random[1];
  Float_t  charge;
  Float_t  aMass;

  Double_t pTot;
  Double_t qTot;
  Double_t eDelta;
  Double_t betaGamma, pp;

  TLorentzVector pos, mom;
  TClonesArray  &lhits = *fHits;

  const Double_t kBig = 1.0E+12;

  // Ionization energy
  const Float_t kWion    = 22.04;
  // Maximum energy for e+ e- g for the step-size calculation
  const Float_t kPTotMax = 0.002;
  // Plateau value of the energy-loss for electron in xenon
  // taken from: Allison + Comb, Ann. Rev. Nucl. Sci. (1980), 30, 253
  //const Double_t kPlateau = 1.70;
  // the averaged value (26/3/99)
  const Float_t kPlateau = 1.55;
  // dN1/dx|min for the gas mixture (90% Xe + 10% CO2)
  const Float_t kPrim    = 48.0;
  // First ionization potential (eV) for the gas mixture (90% Xe + 10% CO2)
  const Float_t kPoti    = 12.1;

  // Set the maximum step size to a very large number for all 
  // neutral particles and those outside the driftvolume
  gMC->SetMaxStep(kBig); 

  // Use only charged tracks 
  if (( gMC->TrackCharge()       ) &&
      (!gMC->IsTrackStop()       ) && 
      (!gMC->IsTrackDisappeared())) {

    // Inside a sensitive volume?
    iIdSens = gMC->CurrentVolID(icSens);
    if (iIdSens == fIdSens) { 

      iIdSpace   = gMC->CurrentVolOffID(4,icSpace  );
      iIdChamber = gMC->CurrentVolOffID(1,icChamber);

      // Calculate the energy of the delta-electrons
      eDelta = TMath::Exp(fDeltaE->GetRandom()) - kPoti;
      eDelta = TMath::Max(eDelta,0.0);

      // The number of secondary electrons created
      qTot = (Double_t) ((Int_t) (eDelta / kWion) + 1);

      // The hit coordinates and charge
      gMC->TrackPosition(pos);
      hits[0] = pos[0];
      hits[1] = pos[1];
      hits[2] = pos[2];
      hits[3] = qTot;

      // The sector number
      if      (iIdSpace == fIdSpace1) 
        vol[0] = secMap1[icSpace-1];
      else if (iIdSpace == fIdSpace2) 
        vol[0] = secMap2[icSpace-1];
      else if (iIdSpace == fIdSpace3) 
        vol[0] = secMap3[icSpace-1];

      // The chamber number 
      //   1: outer left
      //   2: middle left
      //   3: inner
      //   4: middle right
      //   5: outer right
      if      (iIdChamber == fIdChamber1)
        vol[1] = (hits[2] < 0 ? 1 : 5);
      else if (iIdChamber == fIdChamber2)       
        vol[1] = (hits[2] < 0 ? 2 : 4);
      else if (iIdChamber == fIdChamber3)       
        vol[1] = 3;

      // The plane number
      vol[2] = icChamber - TMath::Nint((Float_t) (icChamber / 7)) * 6;

      // Check on selected volumes
      Int_t addthishit = 1;
      if (fSensSelect) {
        if ((fSensPlane)   && (vol[2] != fSensPlane  )) addthishit = 0;
        if ((fSensChamber) && (vol[1] != fSensChamber)) addthishit = 0;
        if ((fSensSector)  && (vol[0] != fSensSector )) addthishit = 0;
      }

      // Add this hit
      if (addthishit) {

        new(lhits[fNhits++]) AliTRDhit(fIshunt,gAlice->CurrentTrack(),vol,hits);

        // The energy loss according to Bethe Bloch
        gMC->TrackMomentum(mom);
        pTot = mom.Rho();
        iPid = gMC->TrackPid();
        if ( (iPid >  3) ||
	    ((iPid <= 3) && (pTot < kPTotMax))) {
          aMass     = gMC->TrackMass();
          betaGamma = pTot / aMass;
          pp        = kPrim * BetheBloch(betaGamma);
	  // Take charge > 1 into account
          charge = gMC->TrackCharge();
          if (TMath::Abs(charge) > 1) pp = pp * charge*charge;
        }
        // Electrons above 20 Mev/c are at the plateau
        else {
          pp = kPrim * kPlateau;
        }
      
        // Calculate the maximum step size for the next tracking step
        if (pp > 0) {
          do 
            gMC->Rndm(random,1);
          while ((random[0] == 1.) || (random[0] == 0.));
          gMC->SetMaxStep( - TMath::Log(random[0]) / pp);
	}

      }
      else {
        // set step size to maximal value
        gMC->SetMaxStep(kBig); 
      }

    }

  }

}

//_____________________________________________________________________________
Double_t AliTRDv2::BetheBloch(Double_t bg) 
{
  //
  // Parametrization of the Bethe-Bloch-curve
  // The parametrization is the same as for the TPC and is taken from Lehrhaus.
  //

  // This parameters have been adjusted to averaged values from GEANT
  const Double_t kP1 = 7.17960e-02;
  const Double_t kP2 = 8.54196;
  const Double_t kP3 = 1.38065e-06;
  const Double_t kP4 = 5.30972;
  const Double_t kP5 = 2.83798;

  // This parameters have been adjusted to Xe-data found in:
  // Allison & Cobb, Ann. Rev. Nucl. Sci. (1980), 30, 253
  //const Double_t kP1 = 0.76176E-1;
  //const Double_t kP2 = 10.632;
  //const Double_t kP3 = 3.17983E-6;
  //const Double_t kP4 = 1.8631;
  //const Double_t kP5 = 1.9479;

  if (bg > 0) {
    Double_t yy = bg / TMath::Sqrt(1. + bg*bg);
    Double_t aa = TMath::Power(yy,kP4);
    Double_t bb = TMath::Power((1./bg),kP5);
             bb = TMath::Log(kP3 + bb);
    return ((kP2 - aa - bb)*kP1 / aa);
  }
  else
    return 0;

}

//_____________________________________________________________________________
Double_t Ermilova(Double_t *x, Double_t *)
{
  //
  // Calculates the delta-ray energy distribution according to Ermilova.
  // Logarithmic scale !
  //

  Double_t energy;
  Double_t dpos;
  Double_t dnde;

  Int_t    pos1, pos2;

  const Int_t nV = 31;

  Float_t vxe[nV] = { 2.3026, 2.9957, 3.4012, 3.6889, 3.9120  
                    , 4.0943, 4.2485, 4.3820, 4.4998, 4.6052
                    , 4.7005, 5.0752, 5.2983, 5.7038, 5.9915
                    , 6.2146, 6.5221, 6.9078, 7.3132, 7.6009
                    , 8.0064, 8.5172, 8.6995, 8.9872, 9.2103
                    , 9.4727, 9.9035,10.3735,10.5966,10.8198
                    ,11.5129 };

  Float_t vye[nV] = { 80.0  , 31.0  , 23.3  , 21.1  , 21.0
                    , 20.9  , 20.8  , 20.0  , 16.0  , 11.0
                    ,  8.0  ,  6.0  ,  5.2  ,  4.6  ,  4.0
                    ,  3.5  ,  3.0  ,  1.4  ,  0.67 ,  0.44
                    ,  0.3  ,  0.18 ,  0.12 ,  0.08 ,  0.056
                    ,  0.04 ,  0.023,  0.015,  0.011,  0.01
		    ,  0.004 };

  energy = x[0];

  // Find the position 
  pos1 = pos2 = 0;
  dpos = 0;
  do {
    dpos = energy - vxe[pos2++];
  } 
  while (dpos > 0);
  pos2--; 
  if (pos2 > nV) pos2 = nV;
  pos1 = pos2 - 1;

  // Differentiate between the sampling points
  dnde = (vye[pos1] - vye[pos2]) / (vxe[pos2] - vxe[pos1]);

  return dnde;

}