An Introduction to Geant4 - Nuclear Physics - University of Liverpool

An Introduction to Geant4 

David Oxley 

University of Liverpool 

Nuclear Physics 

David Oxley 

An Introduction to Geant4

Introduction: GEANT4 

• Geant4 is monte-carlo simulation toolkit 

– http://geant4.web.cern.ch/geant4 

• Simulates radiation interactions with matter 

• GEometry ANd Tracking 

• HEP background 

• C++ based / Object orientated 

Be aware when to use Geant 

Freedom to do anything you want* 

vs 

Not being able to take anything for 

granted 

*providing you know how to do it 

David Oxley 


Where to go for help? 

Other users within the group. 

Geant4 report/guide my website or Laura’s website 

http://geant4.cern.ch/G4UsersDocuments/Overview/html/index.html 

Geant4 Application Developer’s Guide: 

http://geant4.cern.ch/G4UsersDocuments/UsersGuides/ForApplicationD 

eveloper/html/index.html 

You will need this, this course does not replace it. 

HyperNews Forum 

http://hypernews.slac.stanford.edu/HyperNews/geant4/cindex 

Self-help forum, where users and developers help each other. 

Existing topics may include your answers. 

Explain your problem, what you have done: either another user, or a 

developer will help. 

All developers I have had to ask have been very helpful. 

David Oxley 


Setup 

Geant4 should be installed across the network and useable on any of 

the systems. 

Set environment variables each time you open xterm. 

Open an xterm, and enter the three commands below. 

Entering the first command should result in a list of text outputs 

being shown in the xterm. 

Geant4 will then run in this (and only this) xterm. 

If you close the xterm and open a new one, you will need to execute 

the commands again. 

• source /usr/share/geant4/env.csh 

• setenv G4LEDATA “/usr/local/geant4/data/G4EMLOW4.3” 

• source /usr/local/aida/openscientist/v16rl/aida-setup.csh 

If this does not happen computer support may need to install a 

package on your machine. 

David Oxley 


Checking the geant4 setup 

• I provide an example code on my website (3 versions) 

• http://ns.ph.liv.ac.uk/imaging-group/group-members/dave-oxley.php 

• V3: One smartPET detector, 

– data stream read out, 

– source defined in a macros 

• V4: Two SmartPET detectors in coincidence 

– Two separate singles data streams and one coincidence data stream 

– Source defined in a macros 

• V5: Two SmartPET detectors in PET configuration 

– Coincidence between the two 

– Motion of detectors at 10deg angular steps (controlable in macros) 

– One coincidence output reading out angle as well 

• Copy any of these to a directory where geant4 should run 

• Type “make” to build the simulation 

• Type “g4run ” i.e. g4run V3 to execute 

• The command line should say “Idle>” this means geant4 is 

running 

David Oxley 


Units in Geant4 

• Users Guide section 3.3 

• Geant4 kernel uses a fixed set of units. 

• Define your variables in a unit by multiplying. 

Example: 

int length = 5*cm; 

double mass = 68*kg; 

• Extract values from geant4. 

• Change them into the necessary unit by dividing. 

Example: 

int xside = length; // will be in Geant standard units (mm’s for length) 

Int xside2 = length/cm; // will be in cm 

David Oxley 


Some Basics 

• Simplest Geant simulation needs three classes and a main to run 

– Geometry 

– Primary Generator Action 

– Physics List 

Main 

PrimaryGenerator Geometry Physics 

Running Geant4: 

Use the command line 

Hard code your commands into your main 

Use a macros a script containing a list of commands executed 

sequentially 

Use a GUI (I have never done) 

David Oxley 


Defining your materials 

• Materials are defined based upon their chemical structure 

• First define your element(s) 

• Then define your material 

• Assigned weighted quantities of element to material 

• Two examples: Argon (element), Water (composite) 

• Atomic number, molar mass, density 

Example: Water H 2 

O 

• Two ingredients : Two hydrogen , one oxygen 

• Define hydrogen 

• Define Oxygen 

• Define Water 

• Assign two hydrogen atoms and one oxygen atom to water 

vid Oxley 

An Introduction to Geant

Defining your materials in C++ 

Define liquid Argon: 

G4double density = 1.390*g/cm3; 

G4double molMass = 39.95*g/mole; 

G4Material* lAr = new G4Material(name="liquidArgon", z=18.,molMass, 

density); 

Define Water: 

molMass = 1.01*g/mole; 

G4Element* elH = new G4Element(name="Hydrogen",symbol="H" , 

z= 1., molMass); 

molMass = 16.00*g/mole; 

G4Element* elO = new G4Element(name="Oxygen" ,symbol="O" , z= 

8., molMass); 

density = 1.000*g/cm3; 

G4Material* H2O = new 

G4Material(name="Water",density,ncomponents=2); 

H2O->AddElement(elH, natoms=2); 

H2O->AddElement(elO, natoms=1); 

User Guide section 4.2

Designing your world 

• Define all objects within the world 

– Shape 

– Size 

SOLID (G4SOLID) 

– Material 

– Position 

LOGIC (G4LOGIC) 

PHYSICAL (G4PVPLACEMENT) 

To start, use the classes provided 

– Box, Cylinder, Sphere 

– Many more existing classes, User’s Guide (4.1.2) 

David Oxley 


Defining your world in C++ 

• Define a cube of liquid water (3x4x8)cm at p1 = (5,2,3)cm 

– We need a cube or box: Look at the G4Box constructor 

G4Box (const G4String& pName, 

G4double halfX, 

G4double halfY, 

G4double halfZ) 

G4LogicalVolume(G4VSolid pSolid, 

G4Material pMaterial, 

const G4String& Name) 

G4PVPlacement( G4RotationMatrix* pRot, 

const G4ThreeVector& tlate, 

G4LogicalVolume* pCurrentLogical, 

const G4String& pName, 

G4LogicalVolume* pMotherLogical, 

G4bool pSurfChk=false, 

G4int pCopyNo)

Defining your world in C++ 

• Define a cube of water (3x4x8)cm at p1 = (5,2,3)cm 

– We need a cube or box: Look at the G4Box constructor 

(1)G4Box *thisBox = new G4Box(“ThisBoxsName”,1.5*cm,2*cm,4*cm); 

creates a box with these dimensions, it is not made of anything or put 

anywhere yet! 

(2) G4LogicalVolume *thisBox_log = 

new G4LogicalVolume(thisBox,H20,”BoxLogName”); 

tells Geant4 that this box is to be made of water 

(3) G4PVPlacement *thisBox_phys = new G4PVPlacement 

(0, 

G4ThreeVector(5*cm,2*cm,3*cm), 

thisBox_log, 

”BoxPhysName”, 

world, // its mother 

false, 

0);

Summary: Geometry 

Solid Volume 

Shape , Size 

G4Box (box(!)), G4Tubs(cylinder) 

G4Orb (sphere) 

Logical Volume 

Material 

G4LogicalVolume 

H 2 

O 

Physical Volume 

Rotation, Translation 

G4PVPlacement 

y 

z 

H 2 

O 

x

Defining a source 

• Two types covered here 

– Beams 

– Stationary sources 

• Beams are defined in a source file in the Primary Generator Action 

• Stationary sources can be defined there using random numbers to 

generate isotropic distributions 

• Or use pre-defined class in a macro G4GeneralParticleSource 

Primary 

Secondary 

e- 

Source 

David Oxley 


Primary Generator Action 

• Generate particles using G4GeneralParticleSource() or 

G4GeneralParticleGun(G4int NumberofParticles) 

• Source for stationary sources 

/gps/source/intensity 1 # intensity, relative to other sources 

/gps/particle gamma # particle type 

/gps/pos/type Point # source shape 

/gps/pos/centre 0. 0. 0. cm # source central position 

/gps/ang/typ iso 

# distribution 

/gps/energy 200 keV # energy 

http://reat.space.qinetiq.com/gps/new_gps_sum_files/gps_sum.htm 

• Gun for a beam experiment (see attachment or User’s 

Guide 2.6.1): 

– Assign a particle name (type α,β,γ) 

– Assign a kinetic energy 

– Assign a momentum direction 

– Assign a position the particles originate from 

David Oxley 


Physics List 

• Geant4 doesn’t “automatically” include any physics 

• You need to tell Geant4 what physical interactions you are 

interested in 

• Originates from particles physics background where “new 

physics” would need implementing 

• Include: 

– Particles you want (bosons, mesons etc.) 

– Interactions you want (photoelectric, Compton, Pair production 

– The correct energy range: different for the energy range you are 

interested in 

– Standard, low energy, very low energy 

– Beware of your cut values 

• Example physics list attached 

David Oxley 


Physics List: Cuts 

• Cut value defines the extent to which a particle is tracked 

• Cuts are defined in distance 

• Converted into energy based on the material 

Cut value = 1mm 

SOLID GERMANIUM 

GAS 

100keV 

1mm 

e - 

100keV 

Electron won’t be tracked 

Distance < Cut 

Electron will be tracked 

Distance > Cut

The main 

• The main file loads in these three classes 

• See attached example 

• See User Guide for example , section 2.1.1 

David Oxley 


• Visualize 

Run an example

Running a useful geant4 

simulation 

• Although the simulation described above will run, it won’t 

produce an output 

• Geant4 has objects where interactions are not readout (i.e. 

the frame of an array or some lead shielding 

• If the user is interested in the interactions, the volume must 

be made “sensitive” 

NaI 

Sensitive 

David Oxley 


Running a useful geant4 

simulation 

• Although the simulation described above will run, it won’t 

produce an output 

• Geant4 has objects where interactions are not readout (i.e. 

the frame of an array or some lead shielding) 

• If the user is interested in the interactions, the volume must 

be made “sensitive” 

Gamma Camera 

Sensitive 

David Oxley 


Extracting information from Geant 

• Two additional classes (minimum): 

– Sensitive Detector 

– Hit 

– (Event Action) is useful, but not essential 

• Which volumes are made sensitive is defined in the geometry class 

– At logical stage 

• Hit defines the object of an interaction 

– Energy deposit, Position, Interaction type, Detector segment 

• Hits will only be defined in sensitive detectors (not in passive 

volumes) 

• Hits are assigned their attributes (Energy, position) not in the hit 

class, but in the sensitive detector class 

Geometry 

Sensitive 

Detector 

Hit 

Output 

David Oxley 


Compare C++ codes 

• Geometry: assigns a volume to be sensitive 

• Sensitive detector: 

– builds a Hit 

– assigns it to HitCollection 

• Hit class defines attributes of the hit 

• Sensitive Detector either: 

– outputs HitCollection 

– or sends HitCollection to EventAction file 

• EventAction: 

– necessary to process several HitCollections 

– examine several HitCollections, establish HitCollection 

David Oxley 


Other, simpler Option 

• Scoring 

• Easier than sensitive detector/ hit. 

• Envolves less coding 

• Never used it myself not in these lectures or my codes 

• Good presentation 

• geant4.slac.stanford.edu/Space06/presentations/05_Friday/ 

03-Aso_SpaceScorer.ppt 

David Oxley 


Analysing the output 

• Build root tree within Geant 

• Personally, output .csv-file analyse offline in root, java, C, 

matlab 

• Write data to external file 

ofstream file(“/home/dco/geant4/files/output.csv”); 

// declares the output stream name (in this case “file” and 

assigns it a location 

file

DO’s 

Finally… 

DON’T’s 

Consult the user guide with any 

problems or for more detail 

Consult the HyperNews forum if 

you get stuck 

Just start playing around with one 

of the codes 

If you get a bug in it you can’t fix, 

you can always download it again 

Always think about what you 

need in terms of physics and 

cut values 

Do discard physics and events you don’t 

need: 

you are the expert of your work 

Treat this lecture as a 

replacement of the User Guide: 

Geant4 is very complex and 

cannot be explained fully in two 

hours 

Don’t wait until you are a C++ 

expert before you start 

Don’t read the whole user guide 

before you start 

Don’t assume Geant4 will “just 

get it right” 

Don’t let too much detail slow 

you down

An Introduction to Geant4 - Nuclear Physics - University of Liverpool

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