Introduction to GEANT4

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== <span style="color:#000080"> Introduction </span> ==
== <span style="color:#000080"> Introduction </span> ==
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GEANT4 is a software toolkit based on C++. In your code you have to define:  
GEANT4 is a software toolkit based on C++. In your code you have to define:  
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* Your experimental setup - geometry, materials and primary particles.
* Your experimental setup - geometry, materials and primary particles.
* Which physics process you are interested in.  
* Which physics process you are interested in.  
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* You may take actions during the simulation - inspect and store results.  
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* Take actions during the simulation to inspect and store results.  
The interaction with GEANT4 is done via base classes.     
The interaction with GEANT4 is done via base classes.     
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; Manager class  
; Manager class  
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* <span style="color:#ff0000"> G4RunManager </span>: Manages the simulation process;
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* <span style="color:#ff0000"> G4RunManager </span>: Manages processing the run;
; Useful terminology
; Useful terminology
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* <span style="color:#ff0000"> Event </span>: a collection of information from tracks and trajectories
* <span style="color:#ff0000"> Event </span>: a collection of information from tracks and trajectories
* <span style="color:#ff0000"> Run </span>: a collection of events
* <span style="color:#ff0000"> Run </span>: a collection of events
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== <span style="color:#000080"> The function main() </span> ==
== <span style="color:#000080"> The function main() </span> ==
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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 <span style="color:#ff0000"> MyDetectorConstruction </span>, <span style="color:#ff0000"> MyPhysicsList </span>, <span style="color:#ff0000"> MyPrimaryGeneratorAction </span>, <span style="color:#ff0000"> MyEventAction </span> and <span style="color:#ff0000"> MyRunAction </span> are derived classes from the GEANT4 base classes:
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 <span style="color:#ff0000"> MyDetectorConstruction </span>, <span style="color:#ff0000"> MyPhysicsList </span>, <span style="color:#ff0000"> MyPrimaryGeneratorAction </span>, <span style="color:#ff0000"> MyEventAction </span> and <span style="color:#ff0000"> MyRunAction </span> are derived classes from the GEANT4 base classes:
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== <span style="color:#000080"> Experimental setup </span> ==
== <span style="color:#000080"> Experimental setup </span> ==
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You derive your own class from <span style="color:#ff0000"> G4VUserDetectorConstruction </span> base class. In the derived class you will:  
You derive your own class from <span style="color:#ff0000"> G4VUserDetectorConstruction </span> base class. In the derived class you will:  
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* Construct materials and electromagnetic fields using <span style="color:#ff0000"> G4Logical Volume </span>  
* Construct materials and electromagnetic fields using <span style="color:#ff0000"> G4Logical Volume </span>  
* Place volumes of your detector geometry using <span style="color:#ff0000"> G4VPhysical Volume </span>  
* Place volumes of your detector geometry using <span style="color:#ff0000"> G4VPhysical Volume </span>  
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<u> Simple example of class <span style="color:#ff0000"> MyDetectorConstruction </span> </u>:  
<u> Simple example of class <span style="color:#ff0000"> MyDetectorConstruction </span> </u>:  
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Now construct the detector. Your detector is always placed in a mother volume called the world volume.   
Now construct the detector. Your detector is always placed in a mother volume called the world volume.   
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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G4VPhysicalVolume* pWorldPhys = new G4PVPlacement(0,G4ThreeVector(),pWorldLog,"World",0,false,0);   
G4VPhysicalVolume* pWorldPhys = new G4PVPlacement(0,G4ThreeVector(),pWorldLog,"World",0,false,0);   
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// Water box  
// Water box  
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“WaterBox”, pWorldLog, false, copyNo); ... }  
“WaterBox”, pWorldLog, false, copyNo); ... }  
</pre>
</pre>
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The elements and materials are defined using classes <span style="color:#ff0000"> G4Element </span> and <span style="color:#ff0000"> G4Material </span>. For example water, hydrogen and oxygen are defined as:
The elements and materials are defined using classes <span style="color:#ff0000"> G4Element </span> and <span style="color:#ff0000"> G4Material </span>. For example water, hydrogen and oxygen are defined as:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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water->AddElement(O, natoms=1); ...}   
water->AddElement(O, natoms=1); ...}   
</pre>
</pre>
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__detectorConstruction.html Here] you can find more examples of DetectorConstruction classes.
[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__detectorConstruction.html Here] you can find more examples of DetectorConstruction classes.
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== <span style="color:#000080"> Physics processes </span> ==  
== <span style="color:#000080"> Physics processes </span> ==  
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You can build your own physics list or chose from already built physics lists. To build your own physics lists, you can use two base physics list classes: <span style="color:#ff0000"> G4VUserPhysicsList </span> and <span style="color:#ff0000"> G4ModularPhysicsList </span>. The class <span style="color:#ff0000"> G4VUserPhysicsList </span> is used for simple physics lists while <span style="color:#ff0000"> G4ModularPhysicsList </span> is used to build more complex physics lists. There exist also already built pre-packaged physics lists.
You can build your own physics list or chose from already built physics lists. To build your own physics lists, you can use two base physics list classes: <span style="color:#ff0000"> G4VUserPhysicsList </span> and <span style="color:#ff0000"> G4ModularPhysicsList </span>. The class <span style="color:#ff0000"> G4VUserPhysicsList </span> is used for simple physics lists while <span style="color:#ff0000"> G4ModularPhysicsList </span> is used to build more complex physics lists. There exist also already built pre-packaged physics lists.
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=== <span style="color:#000080"> Simple physics lists </span> ===   
=== <span style="color:#000080"> Simple physics lists </span> ===   
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If the particles in your simulation undergo a descrete number of physics processes you can use the class <span style="color:#ff0000"> G4VUserPhysicsList </span> to define them. This class has three methods:
If the particles in your simulation undergo a descrete number of physics processes you can use the class <span style="color:#ff0000"> G4VUserPhysicsList </span> to define them. This class has three methods:
* ConstructParticles() : Define all necessary particles;   
* ConstructParticles() : Define all necessary particles;   
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* ConstructProcesses() : Define all necessary processes and assign them to proper particles;
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* ConstructProcesses() : Define all necessary processes and assign them to corresponding particles;
* SetCuts() : Define production thresholds in terms of range;  
* SetCuts() : Define production thresholds in terms of range;  
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<u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>:  
<u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>:  
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
   
   
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Now implement the methods ConstructParticle(), ConstructProcess() and SetCuts():
Now implement the methods ConstructParticle(), ConstructProcess() and SetCuts():
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
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GEANT4 provides a variety of physics processes. These processes are decoupled from one another and you can select those which are relevant to your simulation. The processes are grouped in seven categories  
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GEANT4 provides a variety of physics processes. These processes are decoupled from one another and you can select those which are relevant to your simulation. The processes are grouped in seven categories and their list is available [http://geant4.cern.ch/support/proc_mod_catalog/processes/ here]:
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<div style="column-count:2;-moz-column-count:2;-webkit-column-count:2">
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</div>
</div>
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and their list is available [http://geant4.cern.ch/support/proc_mod_catalog/processes/ here]. For each particle in ConstructParticle() assign all the physics processes you want to consider in your simulation:  
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For each particle defined in ConstructParticle() assign all the physics processes that you want to
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consider in your simulation:  
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
void MyPhysicsList::ConstructProcess() {  
void MyPhysicsList::ConstructProcess() {  
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ConstructGeneral(); // Other processes }
ConstructGeneral(); // Other processes }
</pre>
</pre>
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In methods ConstructEM() and ConstructGeneral() assign the physics processes to the corresponding particles:  
In methods ConstructEM() and ConstructGeneral() assign the physics processes to the corresponding particles:  
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>   
</pre>   
    
    
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
void MyPhysicsList::ConstructGeneral() {  
void MyPhysicsList::ConstructGeneral() {  
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</pre>
</pre>
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This is the full [http://geant4.cern.ch/support/proc_mod_catalog/particles/ list] of physics processes available for every  particle. Finally, in method SetCuts() you can define cuts on the particles:
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This is the full [http://geant4.cern.ch/support/proc_mod_catalog/particles/ list] of physics processes available for every  particle. Finally, method SetCuts() is defined as:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
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See the [[Computed tomography]] tutorial to learn more about simple physics lists .
=== <span style="color:#000080"> Detailed physics lists </span> ===  
=== <span style="color:#000080"> Detailed physics lists </span> ===  
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If you want to build more realistic physics list you have to use the class <span style="color:#ff0000"> G4VModularPhysicsList </span>. In <span style="color:#ff0000"> G4VModularPhysicsList </span> you can group the physics processes into separate modules which are already pre-build physics list and later chose one of those modules.  
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If you want to build more realistic physics list you have to use the class <span style="color:#ff0000"> G4VModularPhysicsList </span>. For example, the photon from the example above can undergo compton scattering apart from conversion. In <span style="color:#ff0000"> G4VModularPhysicsList </span> you can group the physics processes into separate modules: EM physics, hadronic physics, decay physics etc.  
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<u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>:
<u> Simple example of class <span style="color:#ff0000"> MyPhysicsList </span> </u>:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
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Now, build the physics lists:
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Now we can build the physics lists:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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} ... }  
} ... }  
</pre>
</pre>
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and  
and  
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
    
    
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See the [[Monoenergetic proton pencil beam]] tutorial to learn more about detailed physics lists.
=== <span style="color:#000080"> Pre-packaged physics lists </span> ===
=== <span style="color:#000080"> Pre-packaged physics lists </span> ===
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Some built-in pre-packaged physics lists are available [http://geant4.web.cern.ch/geant4/support/proc_mod_catalog/physics_lists/referencePL.shtml here]. You can use them as a starting point of your simulation.     
Some built-in pre-packaged physics lists are available [http://geant4.web.cern.ch/geant4/support/proc_mod_catalog/physics_lists/referencePL.shtml here]. You can use them as a starting point of your simulation.     
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<u> Simple example of <span style="color:#ff0000"> pre-packaged physics list </span> </u>:
<u> Simple example of <span style="color:#ff0000"> pre-packaged physics list </span> </u>:
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In function <span style="color:#ff0000"> main() </span>:  
In function <span style="color:#ff0000"> main() </span>:  
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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runManager->SetUserInitialization(physicsList);  
runManager->SetUserInitialization(physicsList);  
</pre>
</pre>
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Most of the pre-packaged physics lists use "standard" electromagnetic physics processes. The "standard" EM processes are defined with classes: G4EmStandardPhysics, G4EmStandardPhysics_option1, G4EmStandardPhysics_option2 and G4EmStandardPhysics_option3. If you want to use "low energy" electromagnetic physics processes (defined with classes G4EmLivermorePhysics, G4EmLivermorePolarizedPhysics, G4EmPenelopePhysics and G4EmDNAPhysics) in the pre-packaged physics list you have to define them in your physics list class (see example ProtonPencilBeam).
 
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For example, if you want to simulate clinical proton beam of energy 150 MeV you can use pre-packaged physics list e.g. QGSP_BIC, QGSP_BERT and FTFP_BERT. If you are interested in Bragg curve physics, use a physics list ending in "EMV" or "EMX" e.g. QGSP_BERT_EMV.  
For example, if you want to simulate clinical proton beam of energy 150 MeV you can use pre-packaged physics list e.g. QGSP_BIC, QGSP_BERT and FTFP_BERT. If you are interested in Bragg curve physics, use a physics list ending in "EMV" or "EMX" e.g. QGSP_BERT_EMV.  
   
   
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== <span style="color:#000080"> Generate primary particles </span> ==  
== <span style="color:#000080"> Generate primary particles </span> ==  
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You derive your own class from <span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span> base class.
You derive your own class from <span style="color:#ff0000"> G4VUserPrimaryGeneratorAction </span> base class.
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* <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"> 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:#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.  
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<u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using particle gun </u>:
<u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using particle gun </u>:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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G4ParticleGun*  fParticleGun; };  
G4ParticleGun*  fParticleGun; };  
</pre>
</pre>
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<u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using general particle source </u>:
<u> Simple example of class <span style="color:#ff0000"> MyPrimaryGeneratorAction </span> using general particle source </u>:
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<pre style="color: #800000; background-color: #dcdcdc">
<pre style="color: #800000; background-color: #dcdcdc">
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</pre>
</pre>
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__primaryGenerator.html Here] you can find how to implement MyPrimaryGeneratorAction class in your code.
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__primaryGenerator.html Here] you can find how to implement MyPrimaryGeneratorAction class defined above in your code.
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The [[Monoenergetic photon pencil beam]] example uses the '''G4ParticleGun''' class. The [[Proton beam with realistic geometry]] tutorial uses '''G4GeneralParticleSource'''. 
    
    
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== <span style="color:#000080"> Optional user classes </span> ==
== <span style="color:#000080"> Optional user classes </span> ==
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=== <span style="color:#000080"> Run </span> ===
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You can derive your own class from <span style="color:#ff0000"> G4UserRunAction </span> base class where
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you may book results of the run. The run starts with "Beam On" and within a run you can not change the detector geometry and physics processes. The run is represented by class <span style="color:#ff0000"> G4Run </span>. 
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__userActions.html Here] you can find examples of user defined RunAction classes.
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=== <span style="color:#000080"> Event </span> ===
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Event is the basic GEANT4 unit. At the beginning of an event are generated primary tracks which are pushed into a stack. The tracks from the stack are analyzed one by one. The primary tracks might lead to secondary tracks which are also pushed into the stack. When the stack becomes empty the processing of the event is over. The event is represented by class <span style="color:#ff0000"> G4Event </span> which gives hits and trajectory collections as output.   
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You can derive your own class from <span style="color:#ff0000"> G4UserEventAction </span> base class where
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you can select and analize data.
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__userActions.html Here] you can find examples of user defined EventAction classes.
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=== <span style="color:#000080"> Track </span> ===
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Track is a snapshot of a particle and it is represented by <span style="color:#ff0000"> G4Track </span> class. At the end of the event the track objects do not exist. Tracks are pushed step by step. Moving by one step is called "stepping" and this can be controlled by the <span style="color:#ff0000"> G4UserSteppingAction </span> class (see below). At the end of each step the state of a track can be changed (killed, suspended, postponed).   
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=== <span style="color:#000080"> Step </span> ===
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Step is defined by two points, it contains also information about the particle e.g. energy loss on the step. A step is represented by <span style="color:#ff0000"> G4Step </span> and <span style="color:#ff0000"> G4StepPoint </span> classes. <span style="color:#ff0000"> G4UserSteppingAction </span> is optional class where you can kill, suspend, postpone a track.
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Status is attached to each <span style="color:#ff0000"> G4StepPoint </span> to show how step was determined. You can use ''PostStepPoint'' to get status of current step and ''PreStepPoint'' to get status of previous step. For example to get the "x" coordinate of a step you do the following:
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<pre style="color: #800000; background-color: #dcdcdc">
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G4StepPoint* prePoint  = step->GetPreStepPoint();
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G4double x1 = prePoint->GetPosition().x();
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</pre>
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=== <span style="color:#000080"> Trajectory </span> ===
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[http://geant4.web.cern.ch/geant4/UserDocumentation/Doxygen/examples_doc/html/group__extended__common__userActions.html Here] you can find examples of user defined RunAction and EventAction classes.
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Trajectories are represented by classes <span style="color:#ff0000"> G4Trajectory </span> and <span style="color:#ff0000"> G4TrajectoryPoint </span>. <span style="color:#ff0000"> G4Trajectory </span> class copies some of the <span style="color:#ff0000"> G4Track </span> class information. <span style="color:#ff0000"> G4TrajectoryPoint </span> is the class which copies some of the <span style="color:#ff0000"> G4Step </span> information.

Latest revision as of 17:00, 4 September 2014

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