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Introduction to GEANT4: Basic concepts. Pedro Arce Dubois CIEMAT, 5 th July 2012. Outline. Geometry Magnetic field Particle generator G4Run/G4Event/G4Track/G4Step /G4Trajectory Sensitive detector Electromagnetic physics: standard

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Introduction to geant4 basic concepts

Introduction to GEANT4: Basic concepts


Pedro Arce Dubois

CIEMAT, 5th July 2012


  • Geometry

  • Magnetic field

  • Particle generator

  • G4Run/G4Event/G4Track/G4Step /G4Trajectory

  • Sensitive detector

  • Electromagnetic physics: standard

  • Electromagnetic physics: low energy

  • Hadronic physics. Neutrons

  • Production cuts

Introduction to GEANT4




Simple (Symbol, Z, A)

Mixture of isotopes


Simple (Z, A, density)

Mixture of elements

Mixture of materials


Predefined list of elements and materials (accesible by name)



All simple elements (Z=1,107) with all isotopes

All simple materials (Z=1,98)

Many common materials (most from medical physics domain)


Introduction to GEANT4


= geometrical shape + dimensions

CSG (Constructed Solid Geometry): G4Box, G4Cons, G4Trap, G4Sphere, G4Polycone, etc.

BREP (Boundary REPresented): G4BREPSolidPolycone, G4BSplineSurface, etc. (much slower navigation)

BOOLEAN: a solid is made adding, subtracting or intersecting two

TESSELATED: a solid is made with a set of triangular or quadrangular facets

STEPinterface: to import BREPs from CAD systems


Introduction to GEANT4


Contains all information of a detector element except position

Minimum: solid + material

Sensitive detector


Magnetic field

User limits

Parameterisations of physics



Introduction to GEANT4

G4VPhysicalVolume position

  • Information about placement of a volume

  • G4PVPlacement

    • Is is a volume instance positioned once in a mother

  • G4PVParameterized

    • Parameterized by the copy number

    • Shape, size, material, position and rotation can be parameterized

  • G4PVReplica

    • Slicing a volume into smaller pieces (if it has a symmetry)

  • G4PVDivision

    • Slicing a volume into smaller pieces (if it has a symmetry)

    • Internally implemented as parameterization (no G4ReplicaNavigation)

    • Allows offset

    • Allows constructor with only number of divisions or size of division

  • G4PVAssembly

    • Assembly of volumes without a mother volume

Introduction to GEANT4

Individual copies of a volume
Individual copies of a volume position

Howtoidentify a volumeuniquely?


- one LV A placed in 5 positions (5 PV) insideWorld

- one LV B placed in 12 positions (12 PV) insideA

GEANT4 constructs 5+12 PV, not 5 PVs of A and 60 (=5x12) PVs of B

And even a PV can representmultiple copies (Parameterisationsor Replicas)

- How can I haveaccesstothe 60 different copies of B (forexample, forfindingwhereis a hit)?


each of the 60 volumes B willbe a distinct G4VTouchable

But, forefficiencyreasons, G4VTouchable´s are onlycreated at tracking time, when a particleentersthecorrespondingvolume

Introduction to GEANT4

Magnetic and electric fields
Magnetic and Electric positionFields

Field types
Field types position

  • Several field types can be defined in Geant4:

    • Electric fields

    • Magnetic fields

    • Electromagnetic fields

    • Gravity fields

    • Fields can be assigned only to a few volumes

    • Fields can vary with time

  • - In order to propagate a particle inside a field, the equation of motion of the particle in field is integrated (Runge-Kutta methods or others)

Introduction to GEANT4

Magnetic field chords
Magnetic field: chords position

The path is calculated using the chosen integration method and then it is broken into linear chord segments that closely approximate the curved path

The chords are used to interrogate the Navigator, to see whether the track has crossed a volume boundary

Introduction to GEANT4

Primary particles generator
Primary Particles Generator position

  • G4Event has a list of G4PrimaryVertex’s

    • G4double X0, Y0, Z0;

    • G4double T0;

    • G4double Weight0;

  • G4PrimaryVertex has a list of G4PrimaryParticle’s

    • G4int PDGcode;

    • G4ParticleDefinition * G4code;

    • G4double Px, Py, Pz;

    • G4int trackID;

    • G4double charge;

    • G4double polX, polY, polZ;

    • G4double Weight0;

    • G4double properTime;

  • Geant4 provides some concrete implementations of

  • G4VPrimaryGenerator

    • G4ParticleGun: one particle

    • G4HEPEvtInterface: reading event particles from text files

    • G4GeneralParticleSource: big flexibility

Introduction to GEANT4

G4run g4event g4track g4step g4trajectory
G4Run /G4Event / G4Track / G4Step positionG4Trajectory

  • Step position

  • Step has two points and also ´delta´information of a particle (energy loss on the step, time-of-flight spent in the step, etc.)

  • Each point knows the volume. In case a step is limited by a volume boundary, the end point physically stands on the boundary, and it logically belongs to the next volume

  • Current volume: G4Track::GetNextVolume(); =

  • G4Step::GetPostStepPoint()->GetPhysicalVolume();

  • Previous volume: G4Track::GetVolume(); =

  • G4Step::GetPreStepPoint()->GetPhysicalVolume();

  • What you see with ‘/tracking/verbose 1’ is the current volume

Introduction to GEANT4

  • Trajectory position

  • Trajectory is a record of a track history. It stores some information of all steps done by the track as objects of G4VTrajectoryPoint class

  • The user can create its own trajectory class deriving from G4VTrajectory and G4VTrajectoryPoint base classes for storing any aditional information

  • While Tracks are killed when its tracking finishes, Trajectories are kept for an event lifetime:

    • Think of your favorite application....

Introduction to GEANT4

Detector sensitivity

A logical volume becomes sensitive if it has a pointer to a concrete class derived from G4VSensitiveDetector.

A sensitive detector either

constructs one or more hit objects or

accumulates values to existing hits

using information given in a G4Step object.

NOTE: you must get the volume information from the “PreStepPoint”.

Detector sensitivity

Introduction to GEANT4

Sensitive detector and hit

Each concrete class derived from G4VSensitiveDetector.“Logical Volume” can have a pointer to a sensitive detector

Hit is a snapshot of the physical interaction of a track or an accumulation of interactions of tracks in the sensitive region of your detector

A sensitive detector creates hit(s) using the information given in G4Step object. The user has to provide his/her own implementation of the detector response

Hit objects, which still are the user’s class objects, are collected in a G4Event object at the end of an event

Sensitive detector and Hit

Introduction to GEANT4

Hit class

Hit is a user-defined class derived from G4VHit. concrete class derived from G4VSensitiveDetector.

You can store various types information by implementing your own concrete Hit class.

For example:

Position and time of the step

Momentum and energy of the track

Energy deposition of the step

Geometrical information

or any combination of above

Hit class

Introduction to GEANT4

Electromagnetic physics standard
Electromagnetic Physics: concrete class derived from G4VSensitiveDetector.Standard

Physics process
Physics Process concrete class derived from G4VSensitiveDetector.

  • OOD (Object-Oriented Design) allows to implement or modify any physics process without affecting other parts of the software

  • Tracking is independent from physics processes (Transportation is also a process)

  • The generation of the final state is independent from the access and use of cross sections

  • Transparent access via virtual functions to

    • cross sections (formulas, data sets, etc.)

    • models underlying physics processes

      G4VProcess: base class for all processes

Introduction to GEANT4

Standard e m physics processes
Standard e.m. Physics Processes concrete class derived from G4VSensitiveDetector.

  • Common to all charged particles

  • ionization

  • Coulomb scattering from nuclei

  • Cerenkov effect

  • scintillation

  • transition radiation

  • Electrons

  • bremsstrahlung

  • nuclear interactions

  • Positrons

  • bremsstrahlung

  • annihilation

  • nuclear interactions

  • Muons

  • bremsstrahlung

  • e+e- pair production

  • nuclear interactions

  • Photons

  • gamma conversion

  • Compton scattering

  • Rayleigh scattering

  • photo electric effect

  • nuclear interactions

  • Optical photons

  • reflection and refraction

  • absorption

  • Rayleigh scattering

Cover physics from 10 keV up to PeV

Introduction to GEANT4

Features of standard e m processes
Features of Standard e.m. processes concrete class derived from G4VSensitiveDetector.

Multiple scattering

6.56 MeV proton , 92.6 mm Si

  • Multiple scattering

    • several models

    • computes mean free path length and lateral displacement

    • includes single scattering

  • Ionisation

    • optimise the generation of d-rays near boundaries

  • Variety of models for ionisation and energy loss

    • including the PhotoAbsorption Interaction model

  • Differential and Integral approach

    • for ionisation, Bremsstrahlung, positron annihilation, energy loss and multiple scattering

J.Vincour and P.Bem Nucl.Instr.Meth. 148. (1978) 399

Introduction to GEANT4

Introduction to GEANT4 concrete class derived from G4VSensitiveDetector.

Electromagnetic physics low energy
Electromagnetic physics: concrete class derived from G4VSensitiveDetector.Low energy

Electrons and photons

Validity range: concrete class derived from G4VSensitiveDetector.250 eV – 100 GeV

250 eV is a “suggested” limit

Data library down to 10 eV

1 < Z < 100

Exploits evaluated data libraries

EADL (Evaluated Atomic Data Library)

EEDL (Evaluated Electron Data Library)

EPDL97 (Evaluated Photon Data Library)

For the calculation of total cross sections and the final state generation

Photon transmission, 1mm Pb

shell effects


GaAs lines

Fe lines

Electrons and Photons

  • Compton scattering

  • Rayleigh scattering

  • Photoelectric effect

  • Pair production

  • Bremsstrahlung

  • Ionization

    • + atomic relaxation

Introduction to GEANT4

Photon attenuation comparison with nist data
Photon attenuation: comparison with NIST data concrete class derived from G4VSensitiveDetector.

Test and validation by IST - Natl. Inst. for Cancer Research, Genova

Introduction to GEANT4

Courtesy of S. Agostinelli, R. Corvo, F. Foppiano, S. Garelli, G. Sanguineti, M. Tropeano

Procesos de hadrones e iones
Procesos de hadrones e iones concrete class derived from G4VSensitiveDetector.

Variety of models, depending on the energy range, particle type and charge

Positively charged Hadrons

  • Bethe-Bloch model for energy lost, E > 2 MeV

  • 5 parameterized models, E < 2 MeV

    • based on Ziegler and ICRU revisions

  • 3 model of energy lost fluctuationss

  • Density corrections at high energy

  • Shell correction term for intermediate energies

  • Independent term for spin

  • Barkas and Block terms

  • Chemical effect for compound materials

  • Nuclear stopping power

  • Effective charged model

Positively charged Ions

  • Scale:

  • Parameteritations 0.01 < b < 0.05, Bragg peak

    • based on Ziegler and ICRU revisions

  • b < 0.01: free electron gas model

Negatively charged Hadrons

  • Parameterization of available experimental data

  • Quantum Harmonic Oscilator model

  • Modelo original de Geant4

Introduction to GEANT4

Some results protons

Pode de frenado concrete class derived from G4VSensitiveDetector.

Dependencia en Z a varias energías

Modelos Ziegler e ICRU

Ziegler e ICRU, Fe

Ziegler e ICRU, Si


Poder de frenado nuclear

Pico de Bragg (con interacciones hadrónicas)

Some results: protons

Introduction to GEANT4

Some results ions antiprotons

protons concrete class derived from G4VSensitiveDetector.

Energy lost in Silicon



Some results: ions & antiprotons

Ions Ar y C

Introduction to GEANT4

Hadronic physics
Hadronic Physics concrete class derived from G4VSensitiveDetector.

Hadronic physics challenge

Even though there is an underlying theory (QCD), applying it is much more difficult than applying QED for EM physics

We must deal with at least three energy regimes:

Chiral perturbation theory (< 100 MeV)

Resonance and cascade region (100 MeV – 20 GeV)

QCD strings (> 20 GeV)

Within each regime there are several models:

Many of these are phenomenological

Hadronic physics challenge

Introduction to GEANT4

Hadronic process

At rest is much more difficult than applying QED for EM physics :

Stopped muon, pion, kaon, anti-proton

Radioactive decay


Same process for all long-lived hadrons


Different process for each hadron




Pion- and kaon- in flight


Hadronic process

Introduction to GEANT4

Cross sections

Default cross section sets are provided for each type of hadronic process:

Fission, capture, elastic, inelastic

Can be overridden or completely replaced

Different types of cross section sets:

Some contain only a few numbers to parameterize cross section

Some represent large databases (data driven models)

Cross sections

Introduction to GEANT4

Alternative cross sections

Low energy neutrons hadronic process:

G4NDL available as Geant4 distribution data files

Available with or without thermal cross sections

Neutron and proton reaction cross sections

20 MeV < E < 20 GeV

Ion-nucleus reaction cross sections

Good for E/A < 1 GeV

Isotope production data

E < 100 MeV

Alternative cross sections

Introduction to GEANT4

Different types of hadronic shower models

Data driven models hadronic process:

Parametrisation driven models

Theory driven models

Different types of hadronic shower models

Introduction to GEANT4

Low energy 20mev neutrons physics

High Precision Neutron Models hadronic process: (and Cross Section Data Sets)








Low energy (< 20MeV) neutrons physics

Introduction to GEANT4

G4ndl geant4 neutron data library

The neutron data files for High Precision Neutron models hadronic process:

The data are including both cross sections and final states

The data are derived evaluations based on the following evaluated data libraries




ENDF/B-VI.0, 1, 4






The data format is similar ENDF, however it is not equal to.

G4NDL (Geant4 Neutron Data Library)

Introduction to GEANT4

Radioactive decay

Decay of radioactive nuclei by hadronic process:, -, +, electron capture and isomeric transitions

The simulation model is empirical and data-driven, and uses the Evaluated Nuclear Structure Data File (ENSDF)

nuclear half-lives,

nuclear level structure for the parent or daughter nuclide,

decay branching ratios

the energy of the decay process

Application of variance reduction techniques

bias decays to occur within user-defined times of observations

split radionuclei to increase sampling

apply minimum bias limit to ensure adequate sampling of low-probability channels which have high impact

Radioactive Decay

Introduction to GEANT4

Production cuts
Production cuts hadronic process:

What are the production cuts

- hadronic process:Some electromagnetic processes have diverging cross sections at low energy

Ionisation: producing delta rays

Bremsstrahlung: producing gammas

 Need to put a cut: produce only secondaries from some energy up

- GEANT3/MCNP/EGS/Penelope: cuts per energy

- GEANT4: cuts per range

 more uniform treatment in different materials

- But cuts are converted to energy in each material and always used in energy

What are the (production) cuts?

Introduction to GEANT4

Cut step length and number of 2ary particles

Secondary particles are only produced above the energy cut hadronic process: Primary gives the step in which it would loose enough energy to produce a secondary

GEANT4: secondaries that would live for a length above range cut

Example: Tracking of a muon with a cut of 1 mm in iron.

Energy of secondary electron/positron to live 1mm in iron: 1 GeV

Energy of secondary gamma to live 1mm in iron: 10 MeV

Calculate in which step length the sum of the energies of all delta rays produced by the muon (ionisation is in reality a ‘continuous’ process = ocurring at atomic lengths) is enough to produce an electron of 1 GeV

Same for gammas from bremmstrahlung adding up to 10 MeV

Same for e+e- from pair production adding up to 1 GeV

Choose between the three the smallest step length: make a step of this length

Bigger cut  bigger step ( logarithmically)

cut, step length and number of 2ary particles

Introduction to GEANT4

Other cuts in geant4
Other Cuts in GEANT4 hadronic process:

  • All cuts are always set by particle type

  • UserLimits / G4UserSpecialCuts ‘process’:

    • Define the step length

    • Kill particle if: track length too big, time of flight too big, energy too small, range too small

      • User can define other conditions

    • An extra process that is attached to a G4LogicalVolume

      • BUT: just proposes an step, that competes with other processes

    • For example: if in a volume there is an small electron cut (= produce delta rays every small step) and in the same volume a UserLimits selects a bigger step, this UserLimit have no effect, because ionisation proposes smaller steps than UserLimits process (and always the smallest step is chosen)

Introduction to GEANT4

User commands
User Commands hadronic process:

Geant4 commands
GEANT4 commands hadronic process:

  • Commands control what your job will do

    • /run/initialize

    • /run/beamOn

    • /tracking/verbose

    • /run/particle/dumpCutValues

    • ...

    • /control/manual prints all available commands

  • Usually they are put in a file and given as name to the executable:

    • myg4prog mycommands.lis

  • All commands are processed through the singleton class G4UImanager

    •  You can apply any command at any point in your code

    • G4UImanager* UI = G4UImanager::GetUIpointer();

    • UI->ApplyCommand(“run/beamOn”);

  • New commands are easily created, creating a messenger and an action (see the many examples in OSCAR)

Introduction to GEANT4

Examples of applications
Examples of hadronic process:Applications

Geant4 application examples
GEANT4 Application examples hadronic process:

  • g ray telescope

  • Brachytherapy

  • X ray telecope

  • Underground physics and background radiation

  • X ray Fluorescence and PIXE

  • Gamma therapy

  • DICOM reading

Full applications that show the physics and advanced interactive facilities in realistic set-ups

Introduction to GEANT4

Introduction to GEANT4 hadronic process:

User applications

Solar system explorations hadronic process:

Cosmic rays,

jovian electrons

Courtesy P.Truscott, DERA


Bepi Colombo


Courtesy SOHO EIT

Solar X-rays, e, p

Courtesy of R. Nartallo, ESA

X-ray telescope




Dark Matter, Boulby mine

Courtesy of S. Magni, Borexino

Courtesy of A. Howard, UKDM

User applications

No time to mention them all!

Introduction to GEANT4

Radiotherapy geant4 and com mercial treatment planifiers
Radiotherapy: GEANT4 and com-mercial treatment planifiers hadronic process:

Plano del haz

Hueso craneal


M. C. Lopes1, L. Peralta2, P. Rodrigues2, A. Trindade2

1 IPOFG-CROC Coimbra Oncological Regional Center - 2 LIP - Lisbon

Head and neck with two opposite beams of field size 5x5 y 10x10

One “off-axis” depth dose taken in one of the slices near the isocentre

PLATO fails in the air cavities and bone structures and cannot predict with exactnes the dose in tissue when surrounded by air

Deviations are up to 25-30%

Introduction to GEANT4

Introduction to GEANT4 hadronic process:

Geant4 interface to microelectronic component radiation models

Radfet #2 hadronic process:


Radfet #4





Radfet #1#3







Ana Keating (ESA-ESTEC)

Geant4 interface to microelectronic component radiation models

Introduction to GEANT4

Introduction to GEANT4 hadronic process:

523 mev 20 ne in a finfet

Nuclear reaction fragments generated in an overlayer hadronic process:

523 MeV 20Ne in a FinFET

TCAD (geometry) Geant4

Geant4 (edep) TCAD

Introduction to GEANT4