Introduction to GEANT4

== <span style="color:#000080"> Introduction </span> ==

GEANT4 is a software toolkit based on C++. In your code you have to define: 

* Your experimental setup - geometry, materials and primary particles.
* Which physics process you are interested in. 
* You may take actions during the simulation - inspect and store results. 

The interaction with GEANT4 is done via base classes.    

; Mandatory classes:   

* <span style="color:#ff0000"> G4VUserDetectorConstruction </span>: Describe the experimental setup, geometry and materials
* <span style="color:#ff0000"> G4VUserPhysicsList </span>: Define particles, physics processes and range cuts
* <span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span>: Describe particle source, source dimensions, initial position, energy spectrum, angular distributions

; Optional classes: 

* <span style="color:#ff0000"> G4UserRunAction </span>: Define and store histograms 
* <span style="color:#ff0000"> G4UserEventAction </span>: Event selection and analysis of simulation data
* <span style="color:#ff0000"> G4UserStackingAction </span>: Customize priority of tracks
* <span style="color:#ff0000"> G4UserTrackingAction </span>: Decide whether a trajectory should be stored or not
* <span style="color:#ff0000"> G4UserSteppingAction </span>: Kill, suspend, postpone a track

; Manager class 

* <span style="color:#ff0000"> G4RunManager </span>: Manages the simulation process
  
== <span style="color:#000080"> The function main() </span> ==

The function <span style="color:#ff0000"> main() </span> defines the skeleton of your simulation code. Inside the function you instantiate <span style="color:#ff0000"> G4RunManager </span> and notify it of your mandatory and optional classes. This is example <span style="color:#ff0000"> main() </span> function, where MyDetectorConstruction, MyPhysicsList, MyPrimaryGeneratorAction, MyEventAction and MyRunAction are derived classes from the GEANT4 base classes:

<span style="color:#800000"> { </span>

<span style="color:#800000"> ... </span>

<span style="color:#800000"> // Run manager construction </span> 

<span style="color:#800000"> G4RunManager* runManager = new G4RunManager; </span> 


<span style="color:#800000"> // mandatory user initialization classes </span> 

<span style="color:#800000"> runManager->SetUserInitialization(new MyDetectorConstruction); </span> 

<span style="color:#800000"> runManager->SetUserInitialization(new MyPhysicsList); </span> 


<span style="color:#800000"> // mandatory user action classes  </span> 

<span style="color:#800000"> runManager->SetUserAction(new MyPrimaryGeneratorAction); </span> 


<span style="color:#800000"> // optional user action classes </span>  

<span style="color:#800000"> runManager->SetUserAction(new MyEventAction); </span> 

<span style="color:#800000"> runManager->SetUserAction(new MyRunAction); </span> 

<span style="color:#800000"> ... </span> 

<span style="color:#800000"> } </span>
 
== <span style="color:#000080"> Experimental setup </span> ==

You can define your experiment by using three base classes:

* Describing the shape and the size of your detector: <span style="color:#ff0000"> G4VSolid </span> 
* Adding properties - material and electromagnetic field: <span style="color:#ff0000"> G4Logical Volume </span> 
* Placing it in another volume - in one or many positions: <span style="color:#ff0000"> G4VPhysical Volume </span> 

For example if you want to make an experiment using a water box detector it ca be defined in the following way:


<span style="color:#800000"> G4VSolid* pBoxSolid = new G4Box(“WaterBox”, 1.*m, 2.*m, 3.*m); </span> 

<span style="color:#800000"> G4LogicalVolume* pBoxLog = new G4LogicalVolume( pBoxSolid, water, “WaterBox”); </span> 

<span style="color:#800000"> G4VPhysicalVolume* aBoxPhys = new G4PVPlacement( pRotation, G4ThreeVector(posX, posY, posZ), pBoxLog, “WaterBox”, pWorldLog,
false, copyNo); </span> 

   
Your detector is always placed in a mother volume called the world volume.  
The world volume is defined in a similar way:


<span style="color:#800000"> G4VSolid* pWorld = new G4Box("World",5*m,5*m,5*m); </span>
                                         
<span style="color:#800000"> G4LogicalVolume* pWorldLog = new G4LogicalVolume(pWorld,vacuum, "World"); </span>   
                                                                           
<span style="color:#800000"> G4VPhysicalVolume* pWorldPhys = new G4PVPlacement(0,G4ThreeVector(),pWorldLog,"World",0,false,0); </span> 


The elements and materials used in the experiment are defined using classes <span style="color:#ff0000"> G4Element </span> and <span style="color:#ff0000"> G4Material </span>. For example material water and elements hydrogen and oxygen are defined as:


<span style="color:#800000"> G4Element* H = new G4Element("Hydrogen","H",z=1.,a= 1.01*g/mole); </span>
 
<span style="color:#800000"> G4Element* O = new G4Element("Oxygen","O",z=8.,a=16.00*g/mole); </span> 


<span style="color:#800000"> density = 1.000*g/cm3; </span> 

<span style="color:#800000"> G4Material* H2O = new G4Material("Water",density,ncomp=2); </span> 

<span style="color:#800000"> H2O->AddElement(H, natoms=2); </span> 

<span style="color:#800000"> H2O->AddElement(O, natoms=1); </span> 

== <span style="color:#000080"> Physics processes </span> == 

Two kinds of base physics list classes are available for users to derive from <span style="color:#ff0000"> G4VUserPhysicsList </span> and <span style="color:#ff0000"> G4ModularPhysicsList </span>. There exist also prepackaged physics list provided by GEANT4. 


=== <span style="color:#000080"> Simple physics lists </span> ===  

The class <span style="color:#ff0000"> G4VUserPhysicsList </span> is used for simple physics lists. It has three methods:


* ConstructParticles(): define all necessary particles;  
* ConstructProcesses(): define all necessary processes and assign them to proper particles;
* SetCuts(): define production thresholds in terms of range; 


This example shows how to define your own class derived from the <span style="color:#ff0000"> G4VUserPhysicsList </span> base class.


<span style="color:#800000"> class MyPhysicsList:public G4VUserPhysicsList() </span>

<span style="color:#800000"> { public: </span>

<span style="color:#800000"> MyPhysicsList(); </span>

<span style="color:#800000"> ~MyPhysicsList(); </span>

<span style="color:#800000"> void ConstructParticle(); </span>

<span style="color:#800000"> void ConstructProcess(); </span>

<span style="color:#800000"> void SetCuts(); } </span>
 

Now you must implement the methods ConstructParticle(), ConstructProcess() and SetCuts().

 
<span style="color:#800000"> void MyPhysicsList :: ConstructParticles() </span>

<span style="color:#800000"> { //Define the particles </span>

<span style="color:#800000"> G4Electron::ElectronDefinition(); </span>

<span style="color:#800000"> G4Positron::PositronDefinition(); </span>

<span style="color:#800000"> G4Proton::ProtonDefinition(); </span>

<span style="color:#800000"> G4Neutron::NeutronDefinition(); </span>

<span style="color:#800000"> G4Gamma::GammaDefinition(); ... } </span>


GEANT4 provides a variety of physics processes. These processes are decoupled from one another and the user can select those processes which are relevant to his/her simulation. There are seven categories of processes provided by GEANT4:

# electromagnetic 
#* standard
#* low Energy (Livermore Library / Penelope)
# hadronic
# decay
# photolepton-hadron 
# optical
# parameterization - "fast simulation" physics
# transportation  


In method ConstructProcess() you have to define your physics processes:
  

<span style="color:#800000"> void MyPhysicsList :: ConstructProcess() </span>

<span style="color:#800000"> { // Assign transportation process to all particles </span>

<span style="color:#800000"> AddTransportation(); </span>

<span style="color:#800000"> // Electromagnetic processes </span>

<span style="color:#800000"> ConstructEM(); </span>

<span style="color:#800000"> // Other processes </span>

<span style="color:#800000"> ConstructGeneral(); ... } </span>


where the method ConstructEM() is defined as: 


<span style="color:#800000"> void MyPhysicsList::ConstructEM() </span>

<span style="color:#800000"> { aParticleIterator->reset(); </span>

<span style="color:#800000"> while( (*aParticleIterator)() ){ </span>

<span style="color:#800000"> G4ParticleDefinition* particle = aParticleIterator->value(); </span>
 
<span style="color:#800000"> G4ProcessManager* pmanager = particle->GetProcessManager(); </span>

<span style="color:#800000"> G4String particleName = particle->GetParticleName(); </span>
     
<span style="color:#800000"> if (particleName == "gamma") { </span>
     
<span style="color:#800000"> pmanager->AddDiscreteProcess(new G4GammaConversion); ...} </span> 
    

and the method ConstructGeneral() is defined as:



<span style="color:#800000"> void MyPhysicsList::ConstructGeneral() </span> 






The method SetCuts() is defined as:


<span style="color:#800000"> void MyPhysicsList :: SetCuts() </span>

<span style="color:#800000"> { defaultCutValue = 1.0*mm; </span>

<span style="color:#800000"> SetCutValue(defaultCutValue, "gamma"); </span>

<span style="color:#800000"> SetCutValue(defaultCutValue, "e+"); </span>

<span style="color:#800000"> SetCutValue(defaultCutValue, "e-"); </span>


=== <span style="color:#000080"> Detailed physics lists </span> === 



== <span style="color:#000080"> Generate primary particles </span> == 

<span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span> is a mandatory user action class to control the generation of primary particles. 
The particle generation is done via classes <span style="color:#ff0000"> G4ParticleGun </span> and <span style="color:#ff0000"> G4GeneralParticleSource </span>. 

* <span style="color:#ff0000"> G4ParticleGun </span> is used to simulate a beam of particles. It shoots a primary particle of a certain energy and direction from a given point at a given time.
* <span style="color:#ff0000"> G4GeneralParticleSource </span> simulates a beam of particles and the primary vertex is randomly chosen on surface of a given volume with pre-defined energy spectra, spatial and angular distribution. 

== <span style="color:#000080"> Optional user classes </span> ==