Geant4

Geant4 is a toolkit for the simulation of the passage of particles through matter. It is used, e. g. by Mokka. The central installation is available at /afs/desy.de/group/it/ilcsoft/geant4, which is also used by the central Mokka installation. But if you need to change / add something, you have to get your own installation. The latest version and some documentation is available at http://cern.ch/geant4/.

Contents

  1. Geant4
    1. Installing Geant4
    2. Creating the setup script
    3. Running Geant4 with Mokka
    4. Modifying Geant4
      1. Adding new Particles
      2. Modifying the physics list

Installing Geant4

First of all download the latest Geant4 version (currently 4.9.1 patch-02 released 9 May 2008) as Unix tar format, compressed using gzip. You will most probably also need the data packages

Best is to make a new directory for Geant4, and one for the data files. 

$ cd myPathToGeant
$ mkdir Geant4
$ cd Geant4
$ mkdir G4Data

Save the geant4.9.1.p02.tar.gz in Geant4 and the data packages in G4Data.

The next step is to unpack all the packages: 

$ tar -xzvf geant4.9.1.p02.tar.gz

which will create a directory geant4.9.1.p02. You also need to unpack the data packages.

Before proceeding, make sure you haven’t set any other Geant4 environment in your shell. This can be done by 

printenv | grep G4

If you find something, please unset it.

Now you can start the installation. This will take some time and you will have to answere some questions. 

$ cd myPathToGeant/Geant4/geant4.9.1.p02
$ ./Configure -build

When it says: 

Could not find CLHEP installed on this system!

Please, specify where CLHEP is installed. The central installation is /opt/products/CLHEP/2.0.3.2.

You should be able to pick the default for almost all questions. Anyhow, it doesn’t harm to actually read the question before hitting Enter. One point you may want to change is 

G4VIS_BUILD_OPENGLX_DRIVER
G4VIS_USE_OPENGLX

The default here is no. Typing “y” will allow you to visualize what is going on.

Once all of the questions have been answered, you will be told: 

WARNING: the generated configuration file
can be edited if necessary!
You can introduce any change to the configuration file
...
Press [Enter] to start installation or use a shell escape to edit config.sh:

So hit Enter one last time and do something else for the next hour or so while your computer is busy with the installation. It should start with 

Now starting Geant4 libraries build...

and end with 

####################################################
# Your Geant4 installation seems to be successful!  
# To be sure please have a look into the log file:  
# Geant4/geant4.9.1.p02/.config/bin/Linux-g++/g4make.log   
####################################################

Creating the setup script

Before actually running Geant4, you have to set some environment variables. Fortunately, Geant4 is generating the environment script itself. Just type 

$ ./Configure

once more (without -build this time). This will create an env.sh and env.csh file in your Geant4/geant4.9.1.p02 directory.

Running Geant4 with Mokka

To introduce your Geant4 installation to Mokka adopt the Mokka environment script 

export LD_LIBRARY_PATH=${CLHEP_LIB_DIR}:${LD_LIBRARY_PATH}

export G4INSTALL="myPathtoGeant/Geant4/geant4.9.1.p02"
export G4WORKDIR=$PWD
source ${G4INSTALL}/env.sh

This will automatically set the correct variables for Geant4 if you initialize Mokka.

Modifying Geant4

Adding new Particles

Geant4 knows the particles as defined in geant4.9.1.p02/source/particles. There are subdevided in bosons, hadrons, and leptons. Chose the type of particle you would like to add and add a class in the corresponding subdirectory. For example, I have decided to create a Neutralino, which is a supersymmetric partner of a boson, and therefore a lepton. 

$ cd myPathToGeant/Geant4/geant4.9.1.p02/source/particles/leptons/include

Create a file G4Neutralino.hh, which is derived from G4ParticleDefinition.hh: 

#ifndef G4Neutralino
#define G4Neutralino_h 1

#include "globals.hh"
#include "G4ios.hh"
#include "G4ParticleDefinition.hh"

class G4Neutralino : public G4ParticleDefinition
{
 private:
   static G4Neutralino *theInstance;
   G4Neutralino() {}
   ~G4Neutralino() {}

 public:
   static G4Neutralino *Definition();
   static G4Neutralino *NeutralinoDefinition();
   static G4Neutralino *Neutralino();
};

#endif

Now you have to assign your particles properties in 

$ cd myPathToGeant/Geant4/geant4.9.1.p02/source/particles/leptons/src

Where you should add a file G4Neutralino.cc, which should look like 

#include "G4Neutralino.hh"
#include "G4ParticleTable.hh"
#include "G4PhaseSpaceDecayChannel.hh"
#include "G4DecayTable.hh"

G4Neutralino *G4Neutralino::theInstance = 0;

G4Neutralino *G4Neutralino::Definition()
{
  if (theInstance != 0) return theInstance;
  const G4String name = "neu1";
  // search in particle table
  G4ParticleTable *pTable = G4ParticleTable::GetParticleTable();
  G4ParticleDefinition *anInstance = pTable->FindParticle(name);
  if (anInstance == 0)
  {
  // create particle
  //
  //    Arguments for constructor are as follows
  //               name             mass          width         charge
  //             2*spin           parity  C-conjugation
  //          2*Isospin       2*Isospin3       G-parity
  //               type    lepton number  baryon number   PDG encoding
  //             stable         lifetime    decay table
  //             shortlived      subType    anti_encoding
   anInstance = new G4ParticleDefinition(
                 name,      151.01*GeV, 1.601e-12*MeV,         0.0,
                    1,               0,             0,
                    0,               0,             0,
             "lepton",               0,             0,     1000022,
                false,      0.20285*ns,          NULL,
                false,    "neutralino"
                 );
  }
  //create Decay Table
  G4DecayTable *table = new G4DecayTable();

  // create a decay channel
  G4VDecayChannel *mode;
  // neu1 -> gravitino + gamma
  mode = new G4PhaseSpaceDecayChannel("neu1", 1.0, 2, "grav", "gamma");
  table->Insert(mode);

  anInstance->SetDecayTable(table);

  theInstance = reinterpret_cast<G4Neutralino*>(anInstance);
  return theInstance;
}

G4Neutralino *G4Neutralino::NeutralinoDefinition()
{
  return Definition();
}

G4Neutralino *G4Neutralino::Neutralino()
{
  return Definition();
}

This corresponds to a spin 1/2 lepton with a mass of 151 GeV and a width of 1.601e-6 eV, that will decay within 0.20285 ns and a branching ratio of 100% into a Gravitino and a photon. One also has to introdice the Gravitino in this example.

Of course you will have to re-compile Geant4 once you changed it.

Modifying the physics list

Before accessing the new particle, you will have to introduce it to the physics list you are using. Usually this will be LCPhys as defined in your Mokka version. 

$ cd myPathToMokka/Mokka/Mokka_06-06_p03/source/LCPhys/src

Open the particle class you have changed with your favourite text editor, e. g. 

$ emacs LCLeptonPhysics.cc&

and add 

...
#include "G4Neutralino.hh"
...
void LCLeptonPhysics::ConstructParticle()
{
  ...
  G4Neutralino::NeutralinoDefinition();
}
...

Afterwards re-compile Mokka.

Geant4 (last edited 2009-06-16 18:32:15 by localhost)