Removed ClustersIndex - not used anymore

[u/mrichter/AliRoot.git] / TPC / AliTPCv3.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 1.7  1999/09/29 09:24:34  fca
Introduction of the Copyright and cvs Log

*/

///////////////////////////////////////////////////////////////////////////////
//                                                                           //
//  Time Projection Chamber version 3 -- detailed TPC and slow simulation    //
//                                                                           //
//Begin_Html
/*
<img src="picts/AliTPCv3Class.gif">
*/
//End_Html
//                                                                           //
//                                                                           //
///////////////////////////////////////////////////////////////////////////////

#include <stdlib.h>
#include <TMath.h>

#include "AliTPCv3.h"
#include "AliRun.h"
#include "AliConst.h"
#include "AliTPCD.h"
#include "AliTPCParam.h"

ClassImp(AliTPCv3)
 
//_____________________________________________________________________________
AliTPCv3::AliTPCv3(const char *name, const char *title) :
  AliTPC(name, title) 
{
  //
  // Standard constructor for Time Projection Chamber version 2
  //

  SetBufferSize(128000);
}
 
//_____________________________________________________________________________
void AliTPCv3::CreateGeometry()
{
  //
  // Creation of the TPC coarse geometry (version 0)
  // Origin Marek Kowalski Crakow
  //
  //Begin_Html
  /*
    <img src="picts/AliTPCv0.gif">
  */
  //End_Html
  //Begin_Html
  /*
    <img src="picts/AliTPCv0Tree.gif">
  */
  //End_Html

  AliTPCParam * fTPCParam = &(fDigParam->GetParam());

  Int_t *idtmed = fIdtmed->GetArray();

  Float_t dm[21];
  Int_t idrotm[120];

  Int_t nRotMat = 0;


  // ---------------------------------------------------- 
  //          FIELD CAGE WITH ENDCAPS - G10
  //          THIS IS ALSO A TPC MOTHER VOLUME 
  // ---------------------------------------------------- 

  dm[0] = 76.;
  dm[1] = 278.;
  dm[2] = 275.;

  gMC->Gsvolu("TPC ", "TUBE", idtmed[8], dm, 3); 

  //-----------------------------------------------------
  //  Endcap cover c-fibre 0.86% X0
  //-----------------------------------------------------

  dm[0] = 78.;
  dm[1] = 258.;
  dm[2] = 0.95;

  gMC->Gsvolu("TPEC","TUBE",idtmed[10],dm,3);

  //-----------------------------------------------------
  // Drift gas , leave 2 cm at the outer radius
  // and inner raddius
  //-----------------------------------------------------

  dm[0] = 78.;
  dm[1] = 258.;
  dm[2] = 250.;

  gMC->Gsvolu("TGAS", "TUBE", idtmed[4], dm, 3);


  //------------------------------------------------------
  //  membrane holder - carbon fiber
  //------------------------------------------------------


  gMC->Gsvolu("TPMH","TUBE",idtmed[6],dm,0);

  dm[0] = 252.;
  dm[1] = 258.;
  dm[2] = 0.2;

  gMC->Gsposp("TPMH",1,"TGAS",0.,0.,0.,0,"ONLY",dm,3);
 
  dm[0] = 78.;
  dm[1] = 82.;
  dm[2] = 0.1;

  gMC->Gsposp("TPMH",2,"TGAS",0.,0.,0.,0,"ONLY",dm,3);

  //----------------------------------------------------------
  //  HV membrane - 25 microns of mylar
  //----------------------------------------------------------

  dm[0] = 82.;
  dm[1] = 252.;
  dm[2] = 0.00125;

  gMC->Gsvolu("TPHV","TUBE",idtmed[5],dm,3);

  gMC->Gspos("TPHV",1,"TGAS",0.,0.,0.,0,"ONLY");

  gMC->Gspos("TGAS",1,"TPC ",0.,0.,0.,0,"ONLY");

  //----------------------------------------------------------
  // "side" gas volume, the same as the drift gas
  // the readout chambers are placed there.  
  //----------------------------------------------------------

  dm[0] = 78.;
  dm[1] = 258.;
  dm[2] = 0.5*(275. - 250.);
   
  gMC->Gsvolu("TPSG", "TUBE", idtmed[2], dm, 3);

  Float_t z_side = dm[2]; // 1/2 of the side gas thickness

  //-----------------------------------------------------------
  //   Readout chambers , 25% of X0, I use Al as the material
  //-----------------------------------------------------------

  Float_t InnerOpenAngle = fTPCParam->GetInnerAngle();
  Float_t OuterOpenAngle = fTPCParam->GetOuterAngle();

  Float_t InnerAngleShift = fTPCParam->GetInnerAngleShift();
  Float_t OuterAngleShift = fTPCParam->GetOuterAngleShift();


  Int_t nInnerSector = fTPCParam->GetNInnerSector()/2;
  Int_t nOuterSector = fTPCParam->GetNOuterSector()/2;


  Float_t InSecLowEdge = fTPCParam->GetInSecLowEdge();
  Float_t InSecUpEdge =  fTPCParam->GetInSecUpEdge();

  Float_t OuSecLowEdge = fTPCParam->GetOuSecLowEdge();
  Float_t OuSecUpEdge = fTPCParam->GetOuSecUpEdge();

  Float_t SecThick = 2.225; // Al

  Float_t edge = fTPCParam->GetEdge();

  //  S (Inner) sectors

  dm[0] = InSecLowEdge*TMath::Tan(0.5*InnerOpenAngle)-edge;
  dm[1] = InSecUpEdge*TMath::Tan(0.5*InnerOpenAngle)-edge;
  dm[2] = SecThick;
  dm[3] = 0.5*(InSecUpEdge-InSecLowEdge);

  Float_t xCenterS = InSecLowEdge+dm[3];

  gMC->Gsvolu("TRCS", "TRD1", idtmed[0], dm, 4); 

  //  L (Outer) sectors

  dm[0] = OuSecLowEdge*TMath::Tan(0.5*OuterOpenAngle)-edge;
  dm[1] = OuSecUpEdge*TMath::Tan(0.5*OuterOpenAngle)-edge;
  dm[2] = SecThick;
  dm[3] = 0.5*(OuSecUpEdge-OuSecLowEdge);

  Float_t xCenterL = OuSecLowEdge+dm[3];  

  gMC->Gsvolu("TRCL", "TRD1", idtmed[0], dm, 4);

  Float_t z1 = -z_side + SecThick*0.5;

  //------------------------------------------------------------------
  // Positioning of the S-sector readout chambers
  //------------------------------------------------------------------

  Int_t ns;
  Float_t theta1,theta2,theta3;
  Float_t phi1,phi2,phi3;
  Float_t alpha;
  Float_t x,y;

  for(ns=0;ns<nInnerSector;ns++){
    
    phi1 = ns * InnerOpenAngle + 270.*kDegrad + InnerAngleShift;
    phi1 *= kRaddeg; // in degrees

    phi1 = (Float_t)TMath::Nint(phi1);

    if (phi1 > 360.) phi1 -= 360.;
      
    theta1 = 90.;
    phi2   = 90.;
    theta2 = 180.;
    phi3   = ns * InnerOpenAngle + InnerAngleShift;
    phi3 *= kRaddeg; // in degrees

    phi3 = (Float_t)TMath::Nint(phi3);
      
    if(phi3 > 360.) phi3 -= 360.;

    theta3 = 90.;

    alpha = phi3*kDegrad;

    x = xCenterS * TMath::Cos(alpha);
    y = xCenterS * TMath::Sin(alpha); 
 
    AliMatrix(idrotm[nRotMat], theta1, phi1, theta2, phi2, theta3, phi3);  
     
    gMC->Gspos("TRCS", ns+1, "TPSG", x, y, z1, idrotm[nRotMat], "ONLY");

    nRotMat++;     

  }
    
  //-------------------------------------------------------------------
  //  Positioning of the L-sectors readout chambers
  //-------------------------------------------------------------------
    
  for(ns=0;ns<nOuterSector;ns++){
    phi1 = ns * OuterOpenAngle + 270.*kDegrad + OuterAngleShift;
    phi1 *= kRaddeg; // in degrees

    phi1 = (Float_t)TMath::Nint(phi1);
    

    if (phi1 > 360.) phi1 -= 360.;
      
    theta1 = 90.;
    phi2   = 90.;
    theta2 = 180.;
    phi3   = ns * OuterOpenAngle+OuterAngleShift;
    phi3 *= kRaddeg; // in degrees

    phi3 = (Float_t)TMath::Nint(phi3);

      
    if(phi3 > 360.) phi3 -= 360.;

    theta3 = 90.;

    alpha = phi3*kDegrad;

    x = xCenterL * TMath::Cos(alpha);
    y = xCenterL * TMath::Sin(alpha); 
 
    AliMatrix(idrotm[nRotMat], theta1, phi1, theta2, phi2, theta3, phi3);  
     

    gMC->Gspos("TRCL", ns+1, "TPSG", x, y, z1, idrotm[nRotMat], "ONLY"); 

    nRotMat++;   

  }

  Float_t z0 = z_side - 0.95;

  gMC->Gspos("TPEC",1,"TPSG",0.,0.,z0,0,"ONLY");

  // ========================================================== 
  //                  wheels 
  // ========================================================== 

  //
  //  auxilary structures
  //


  gMC->Gsvolu("TPWI","TUBE",idtmed[24],dm,0); // "air" 

  // ---------------------------------------------------------- 
  //       Large wheel -> positioned in the TPC 
  // ---------------------------------------------------------- 
  

  z0 = 263.5; // TPC length - 1/2 spoke wheel width

  dm[0] = 258.;
  dm[1] = 278.;
  dm[2] = 11.5;
  
  gMC->Gsvolu("TPWL", "TUBE", idtmed[0], dm, 3); 

  dm[0] = dm[0]+2.;
  dm[1] = 278.;
  dm[2] = dm[2]-2.;

  gMC->Gsposp("TPWI",1,"TPWL",0.,0.,0.,0,"ONLY",dm,3);

  gMC->Gspos("TPWL", 1, "TPC ", 0, 0, z0, 0, "ONLY");
  gMC->Gspos("TPWL", 2, "TPC ", 0, 0, -z0, 0, "ONLY");

  //
  //  Outer vessel + CO2 HV degrader
  //

  dm[0] = 260.;
  dm[1] = 278.;
  dm[2] = 252.;

  gMC->Gsvolu("TPCO","TUBE",idtmed[12],dm,3);

  dm[0] = 275.;
  dm[1] = 278.;
  
  gMC->Gsvolu("TPOV","TUBE",idtmed[10],dm,3);

  gMC->Gspos("TPOV",1,"TPCO",0.,0.,0.,0,"ONLY");


  // G10 plugs

  dm[0] = 258.;
  dm[1] = 260.;
  dm[2] = 1.;

  gMC->Gsvolu("TPG1","TUBE",idtmed[8],dm,3);
  gMC->Gspos("TPG1",1,"TPCO",0.,0.,251.,0,"ONLY");
  gMC->Gspos("TPG1",2,"TPCO",0.,0.,-251.,0,"ONLY");  

  gMC->Gspos("TPCO",1,"TPC ",0.,0.,0.,0,"ONLY");


  //----------------------------------------------------------
  //  Small wheel -> positioned in "side gas
  //----------------------------------------------------------

  dm[0] = 78.;
  dm[1] = 82.;
  dm[2] = 11.5;

  gMC->Gsvolu("TPWS", "TUBE", idtmed[0], dm, 3);

  dm[0] = 78.;
  dm[1] = dm[1]-2;
  dm[2] = dm[2]-2.;

  gMC->Gsvolu("TPW1", "TUBE", idtmed[2], dm, 3);
  
  gMC->Gspos("TPW1", 1, "TPWS", 0., 0., 0., 0, "ONLY");

  z0 = 1.; // spoke wheel is shifted w.r.t. center of the "side gas"

  gMC->Gspos("TPWS", 1, "TPSG", 0, 0, z0, 0, "ONLY");


  // to avoid overlaps

  dm[0] = 76.;
  dm[1] = 78.;
  dm[2] = 11.5;

  gMC->Gsvolu("TPS1","TUBE",idtmed[0],dm,3);

  dm[2] = 9.5;

  gMC->Gsvolu("TPS2","TUBE",idtmed[24],dm,3);

  gMC->Gspos("TPS2",1,"TPS1",0.,0.,0.,0,"ONLY");

  z0= 263.5;
  
  gMC->Gspos("TPS1",1,"TPC ",0.,0.,z0,0,"ONLY");
  gMC->Gspos("TPS1",2,"TPC ",0.,0.,-z0,0,"ONLY");

  // G10 plug

  dm[0] = 76.;
  dm[1] = 78.;
  dm[2] = 1.;

  gMC->Gsvolu("TPG2","TUBE",idtmed[8],dm,3);

  z0 = 251.;

  gMC->Gspos("TPG2",1,"TPC ",0.,0.,z0,0,"ONLY");
  gMC->Gspos("TPG2",2,"TPC ",0.,0.,-z0,0,"ONLY");


  //---------------------------------------------------------
  //  central wheel  6 (radial direction) x 4 (along z) cm2
  //---------------------------------------------------------

  dm[0] = 140.;
  dm[1] = 146.;
  dm[2] = 2.;

  gMC->Gsvolu("TPWC","TUBE",idtmed[0],dm,3);

  dm[0] = dm[0] + 2.;
  dm[1] = dm[1] - 2.;
  dm[2] = dm[2] - 1.;

  gMC->Gsposp("TPWI",2,"TPWC",0.,0.,0.,0,"ONLY",dm,3);

  z0 = z_side - 1.9 - 2.;

  gMC->Gspos("TPWC",1,"TPSG",0.,0.,z0,0,"ONLY");

  //

  gMC->Gsvolu("TPSE","BOX ",idtmed[24],dm,0); // "empty" part of the spoke 

 
  //---------------------------------------------------------
  //  inner spokes (nSectorInner)
  //---------------------------------------------------------

  dm[0] = 0.5*(139.9-82.1);
  dm[1] = 3.;
  dm[2] = 2.;

  Float_t x1 = dm[0]+82.;

  gMC->Gsvolu("TPSI","BOX",idtmed[0],dm,3);

  dm[1] = dm[1]-1.;
  dm[2] = dm[2]-1.;

  gMC->Gsposp("TPSE",1,"TPSI",0.,0.,0.,0,"ONLY",dm,3);

  for(ns=0;ns<nInnerSector;ns++){

    phi1 = 0.5*InnerOpenAngle + ns*InnerOpenAngle + InnerAngleShift;
    theta1=90.;
    phi1 *=kRaddeg;

    phi1 = (Float_t)TMath::Nint(phi1);

    phi2 = phi1+90.;
    if(phi2>360.) phi2 -= 360.;
    theta2=90.;
    phi3=0.;
    theta3=0.;

    alpha = phi1 * kDegrad;
    x     = x1 * TMath::Cos(alpha);
    y     = x1 * TMath::Sin(alpha);    

   AliMatrix(idrotm[nRotMat],theta1,phi1,theta2,phi2,theta3,phi3);

   gMC->Gspos("TPSI",ns+1,"TPSG",x,y,z0,idrotm[nRotMat],"ONLY");  

   nRotMat++;

  }

  //-------------------------------------------------------------
  // outer spokes (nSectorOuter)
  //-------------------------------------------------------------

  dm[0] = 0.5*(257.9-146.1);
  dm[1] = 3.;
  dm[2] = 2.;

  x1 = dm[0] + 146.;

  gMC->Gsvolu("TPSO","BOX ",idtmed[0],dm,3);

  dm[1] = dm[1] - 1.;
  dm[2] = dm[2] - 1.;

  gMC->Gsposp("TPSE",2,"TPSO",0.,0.,0.,0,"ONLY",dm,3);

  for(ns=0;ns<nOuterSector;ns++){

    phi1 = 0.5*OuterOpenAngle + ns*OuterOpenAngle + OuterAngleShift;
    theta1=90.;
    phi1 *=kRaddeg;

    phi1 = (Float_t)TMath::Nint(phi1);

    phi2 = phi1+90.;
    if(phi2>360.) phi2 -= 360.;
    theta2=90.;
    phi3=0.;
    theta3=0.;

    alpha = phi1 * kDegrad;
    x     = x1 * TMath::Cos(alpha);
    y     = x1 * TMath::Sin(alpha);    

   AliMatrix(idrotm[nRotMat],theta1,phi1,theta2,phi2,theta3,phi3);

   gMC->Gspos("TPSO",ns+1,"TPSG",x,y,z0,idrotm[nRotMat],"ONLY");  

   nRotMat++;

  }  
  

  
  // -------------------------------------------------------- 
  //         put the readout chambers into the TPC 
  // -------------------------------------------------------- 

  theta1 = 90.;
  phi1   = 0.;
  theta2 = 90.;
  phi2   = 270.;
  theta3 = 180.;
  phi3   = 0.;
  
  AliMatrix(idrotm[nRotMat], theta1, phi1, theta2, phi2, theta3, phi3);
  
  z0 = z_side + 250.;
  
  gMC->Gspos("TPSG", 1, "TPC ", 0, 0, z0, 0, "ONLY");
  gMC->Gspos("TPSG", 2, "TPC ", 0, 0, -z0, idrotm[nRotMat], "ONLY");
  
  gMC->Gspos("TPC ", 1, "ALIC", 0, 0, 0, 0, "ONLY");

  //----------------------------------------------------
  //  Inner vessel and HV degrader
  //----------------------------------------------------

  dm[0] = 0.;
  dm[1] = 360.;
  dm[2] = 4.;
  
  dm[3] = -250.;
  dm[4] = 74.4;
  dm[5] = 76.;

  dm[6] = -64.5;
  dm[7] = 50.;
  dm[8] = 76.;

  dm[9] = -64.5;
  dm[10] = 50.;
  dm[11] = 76.;

  dm[12] = 250.;
  dm[13] = 74.4;
  dm[14] = 76.;

  gMC->Gsvolu("TPVD", "PCON", idtmed[12], dm, 15); // CO2

  // cone parts

  dm[0] = 0.;
  dm[1] = 360.;
  dm[2] = 2.;

  dm[3] = 64.5;
  dm[4] = 50.;
  dm[5] = 51.6;
 
  dm[6] = 250.;
  dm[7] = 74.4;
  dm[8] = 76.;


  gMC->Gsvolu("TIVC","PCON",idtmed[11],dm,9); // C-fibre

  gMC->Gspos("TIVC",1,"TPVD",0.,0.,0.,0,"ONLY");
  gMC->Gspos("TIVC",2,"TPVD",0.,0.,0.,idrotm[nRotMat],"ONLY");

  // barrel part

  dm[0] = 50.;
  dm[1] = 50.5;
  dm[2] = 32.25;

  gMC->Gsvolu("TIVB","TUBE",idtmed[9],dm,3);

  gMC->Gspos("TIVB",1,"TPVD",0.,0.,0.,0,"ONLY");

  gMC->Gspos("TPVD",1,"ALIC",0.,0.,0.,0,"ONLY");

  

  

  // --------------------------------------------------- 
  //               volumes ordering 
  // --------------------------------------------------- 
  gMC->Gsord("TPSG", 6);
 
} // end of function

 
 
//_____________________________________________________________________________
void AliTPCv3::DrawDetector()
{
  //
  // Draw a shaded view of the Time Projection Chamber version 1
  //


  // Set everything unseen
  gMC->Gsatt("*", "seen", -1);
  // 
  // Set ALIC mother transparent
  gMC->Gsatt("ALIC","SEEN",0);
  //
  // Set the volumes visible
  gMC->Gsatt("TPC","SEEN",0);
  gMC->Gsatt("TGAS","SEEN",0);
  gMC->Gsatt("TPSG","SEEN",0);
  gMC->Gsatt("TPHV","SEEN",1);
  gMC->Gsatt("TPMH","SEEN",1);
  gMC->Gsatt("TPEC","SEEN",0);
  gMC->Gsatt("TRCS","SEEN",1);
  gMC->Gsatt("TRCL","SEEN",1);
  gMC->Gsatt("TPWL","SEEN",1);
  gMC->Gsatt("TPWI","SEEN",1);
  gMC->Gsatt("TPWS","SEEN",1);
  gMC->Gsatt("TPW1","SEEN",1);
  gMC->Gsatt("TPS1","SEEN",1);
  gMC->Gsatt("TPS2","SEEN",1);
  gMC->Gsatt("TPG1","SEEN",1);
  gMC->Gsatt("TPG2","SEEN",1);
  gMC->Gsatt("TPWC","SEEN",1);
  gMC->Gsatt("TPSI","SEEN",1); 
  gMC->Gsatt("TPSO","SEEN",1);
  gMC->Gsatt("TPCO","SEEN",1);
  gMC->Gsatt("TPOV","SEEN",1);
  gMC->Gsatt("TPVD","SEEN",1);
  //
  gMC->Gdopt("hide", "on");
  gMC->Gdopt("shad", "on");
  gMC->Gsatt("*", "fill", 7);
  gMC->SetClipBox(".");
  gMC->SetClipBox("*", 0, 1000, -1000, 1000, -1000, 1000);
  gMC->DefaultRange();
  gMC->Gdraw("alic", 40, 30, 0, 12, 9.5, .025, .025);
  gMC->Gdhead(1111, "Time Projection Chamber");
  gMC->Gdman(18, 4, "MAN");
  gMC->Gdopt("hide","off");
}

//_____________________________________________________________________________
void AliTPCv3::CreateMaterials()
{
  //
  // Define materials for version 2 of the Time Projection Chamber
  //


  //
  // Increase maximum number of steps
  gMC->SetMaxNStep(30000);
  //
  AliTPC::CreateMaterials();
}

//_____________________________________________________________________________
void AliTPCv3::Init()
{
  //
  // Initialises version 3 of the TPC after that it has been built
  //
  Int_t *idtmed = fIdtmed->GetArray()-399;

  AliTPC::Init();

  fIdSens1=gMC->VolId("TGAS"); // drift gas as a sensitive volume

  gMC->SetMaxNStep(30000); // max. number of steps increased

  gMC->Gstpar(idtmed[403],"LOSS",5);

  printf("*** TPC version 3 initialized ***\n");
  printf("Maximum number of steps = %d\n",gMC->GetMaxNStep());

  //
  
}

//_____________________________________________________________________________
void AliTPCv3::StepManager()
{
  //
  // Called for every step in the Time Projection Chamber
  //

  //
  // parameters used for the energy loss calculations
  //
  const Float_t prim = 14.35; // number of primary collisions per 1 cm
  const Float_t poti = 20.77e-9; // first ionization potential for Ne/CO2
  const Float_t w_ion = 35.97e-9; // energy for the ion-electron pair creation 
 
 
  const Float_t big = 1.e10;

  Int_t id,copy;
  TLorentzVector pos;
  Float_t hits[4];
  Int_t vol[2];  
  TClonesArray &lhits = *fHits;
  
  vol[1]=0;
  vol[0]=0;

  //

  gMC->SetMaxStep(big);
  
  if(!gMC->IsTrackAlive()) return; // particle has disappeared
  
  Float_t charge = gMC->TrackCharge();
  
  if(TMath::Abs(charge)<=0.) return; // take only charged particles
  
  
  id=gMC->CurrentVolID(copy);
  
  // Check the sensitive volume
  
  if (id != fIdSens1) return;
  
  //
  //  charged particle is in the sensitive volume
  //
  
  if(gMC->TrackStep() > 0) {

    
    Int_t nel = (Int_t)(((gMC->Edep())-poti)/w_ion) + 1;
    nel=TMath::Min(nel,300); // 300 electrons corresponds to 10 keV
    
    gMC->TrackPosition(pos);
    hits[0]=pos[0];
    hits[1]=pos[1];
    hits[2]=pos[2];

    //
    // check the selected side of the TPC
    //
 
    if(fSide && fSide*hits[2]<=0.) return;

    hits[3]=(Float_t)nel;
    
    // Add this hit
   
    new(lhits[fNhits++]) AliTPChit(fIshunt,gAlice->CurrentTrack(),vol,hits);
    
  } 
  
  // Stemax calculation for the next step
  
  Float_t pp;
  TLorentzVector mom;
  gMC->TrackMomentum(mom);
  Float_t ptot=mom.Rho();
  Float_t beta_gamma = ptot/gMC->TrackMass();
  
  if(gMC->IdFromPDG(gMC->TrackPid()) <= 3 && ptot > 0.002)
    { 
      pp = prim*1.58; // electrons above 20 MeV/c are on the plateau!
    }
  else
    {
      pp=prim*BetheBloch(beta_gamma);    
      if(TMath::Abs(charge) > 1.) pp *= (charge*charge);
    }
  
  Float_t random[1];
  gMC->Rndm(random,1); // good, old GRNDM from Geant3
  
  Double_t rnd = (Double_t)random[0];
  
  gMC->SetMaxStep(-TMath::Log(rnd)/pp);
  
}

//_____________________________________________________________________________
Float_t AliTPCv3::BetheBloch(Float_t bg)
{
  //
  // Bethe-Bloch energy loss formula
  //
  const Double_t p1=0.76176e-1;
  const Double_t p2=10.632;
  const Double_t p3=0.13279e-4;
  const Double_t p4=1.8631;
  const Double_t p5=1.9479;

  Double_t dbg = (Double_t) bg;

  Double_t beta = dbg/TMath::Sqrt(1.+dbg*dbg);

  Double_t aa = TMath::Power(beta,p4);
  Double_t bb = TMath::Power(1./dbg,p5);

  bb=TMath::Log(p3+bb);
  
  return ((Float_t)((p2-aa-bb)*p1/aa));
}