introduction to geant4: basic concepts y pedro arce dubois ciemat, 5 th july 2012

Introduction to GEANT4:

Basic concepts http://geant4.cern.ch

Pedro Arce Dubois

CIEMAT, 5th July 2012

Pedro Arce Introduction to GEANT4 2

OutlineGeometry

Magnetic field

Particle generator

G4Run/G4Event/G4Track/G4Step /G4Trajectory

Sensitive detector

Electromagnetic physics: standard

Electromagnetic physics: low energy

Hadronic physics. Neutrons

Production cuts

Geometry


Materials G4Isotope

G4Element

Simple (Symbol, Z, A)

Mixture of isotopes

G4Material Simple (Z, A, density)

Mixture of elements

Mixture of materials

G4NistMaterialMgr Predefined list of elements and materials (accesible by name)

G4NistManager::Instance()->FindOrBuildElement(name)

G4NistManager::Instance()->FindOrBuildMaterial(name)

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

All simple materials (Z=1,98)

Many common materials (most from medical physics domain)


G4VSolid = 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

STEP interface: to import BREPs from CAD systems


G4LogicalVolume

Contains all information of a detector element except position

Minimum: solid + material Sensitive detector Visualisation Magnetic field User limits Parameterisations of physics ...


G4VPhysicalVolume

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


Individual copies of a volume

How to identify a volume uniquely?

Example: - one LV A placed in 5 positions (5 PV) inside World- one LV B placed in 12 positions (12 PV) inside A

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

And even a PV can represent multiple copies (Parameterisations or

Replicas)

- How can I have access to the 60 different copies of B (for example, for finding where is a hit)?

ANSWER: each of the 60 volumes B will be a distinct G4VTouchableeach of the 60 volumes B will be a distinct G4VTouchable

But, for efficiency reasons, G4VTouchable´s are only created at tracking time, when a particle enters the corresponding volume

Magnetic and ElectricFields


Field types

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)


Magnetic field: chords

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

Particle Generator


Primary Particles Generator

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

G4Run /G4Event / G4Track / G4StepG4Trajectory


Step• 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


Trajectory

• 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....

Sensitive Detector


Detector sensitivityDetector sensitivity

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

A sensitive detector eitherconstructs 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”.


Sensitive detector and HitSensitive detector and Hit

Each “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


Hit classHit class

Hit is a user-defined class derived from G4VHit. 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

Electromagnetic Physics: Standard


Physics Process

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


Standard e.m. Physics Processes

Cover physics from 10 keV up to PeV

Common to all charged particles• ionization• Coulomb scattering from nuclei• Cerenkov effect• scintillation• transition radiationElectrons• bremsstrahlung• nuclear interactionsPositrons• bremsstrahlung• annihilation• nuclear interactions

Muons• bremsstrahlung• e+e- pair production• nuclear interactionsPhotons• gamma conversion• Compton scattering• Rayleigh scattering• photo electric effect• nuclear interactionsOptical photons• reflection and refraction• absorption• Rayleigh scattering


Features of Standard e.m. processes

Multiple scatteringMultiple scattering several models computes mean free path length

and lateral displacement includes single scattering

IonisationIonisation optimise the generation of rays

near boundaries

Variety of modelsVariety of models for ionisation and energy loss including the PhotoAbsorption

Interaction model

Differential and Integral approachDifferential and Integral approach for ionisation, Bremsstrahlung,

positron annihilation, energy loss and multiple scattering

Multiple scattering

6.56 MeV proton , 92.6 mm Si

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

Electromagnetic physics: Low energy


Electrons and Photons

Validity range: 250 eV – 100 GeV250 eV – 100 GeV250 eV is a “suggested” limit

Data library down to 10 eV1 < Z < 100

Exploits evaluated data libraries 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, 1m Pb

shell effects

GaAs linesFe lines

fluorescence Compton scattering Rayleigh scattering Photoelectric effect Pair production Bremsstrahlung Ionization

+ atomic relaxation


Photon attenuation: comparison with NIST data

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

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


Procesos de hadrones e iones

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

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 HadronsPositively charged Hadrons

Positively charged IonsPositively charged Ions

Negatively charged HadronsNegatively charged Hadrons

Scale:

Parameteritations 0.01 < < 0.05, Bragg peak

based on Ziegler and ICRU revisions

< 0.01: free electron gas model

Parameterization of available experimental data Quantum Harmonic Oscilator model

Modelo original de Geant4


Some results: protons

Straggling

Pode de frenado Dependencia en Z a varias energías

Modelos Ziegler e ICRU Ziegler e ICRU, Fe Ziegler e ICRU, Si

Poder de frenado nuclearPico de Bragg (con interacciones

hadrónicas)


Some results: ions & antiprotons

antiprotons

protons

Energy lost in Silicon

Ions Ar y C

Deuterons

Hadronic Physics


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 process

At rest: Stopped muon, pion, kaon, anti-proton Radioactive decay

Elastic: Same process for all long-lived hadrons

Inelastic: Different process for each hadron Photo-nuclear Electro-nuclear

Capture: Pion- and kaon- in flight

Fission


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)


Alternative cross sections

Low energy neutrons 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


Different types of hadronic shower models

Data driven models

Parametrisation driven models

Theory driven models


Low energy (< 20MeV) neutrons physics

• High Precision Neutron Models (and Cross Section Data Sets)• G4NDL

• ENDF

• Elastic

• Inelastic

• Capture

• Fission

• NeutronHPorLEModel(s)


G4NDL (Geant4 Neutron Data Library)

The neutron data files for High Precision Neutron models The data are including both cross sections and final

states The data are derived evaluations based on the following

evaluated data libraries Brond-2.1 CENDL2.2 EFF-3 ENDF/B-VI.0, 1, 4 FENDL/E2.0 JEF2.2 JENDL-FF JENDL-3.1,2 MENDL-2

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


Radioactive Decay

Decay of radioactive nuclei by , -, +, 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

Production cuts


What are the (production) cuts?

- 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


cut, step length and number of 2ary particles

Secondary particles are only produced above the energy cut 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)


Other Cuts in GEANT4 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 processesFor 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)

User Commands


GEANT4 commands

Commands control what your job will doo /run/initializeo /run/beamOno /tracking/verbose o /run/particle/dumpCutValueso ...o /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)

Examples of Applications


GEANT4 Application examples

ray telescopeBrachytherapy 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


User applications

Courtesy of S. Magni, Borexino Courtesy of A. Howard, UKDM

ZEPLIN IIIDark Matter, Boulby mine

No time to mention them all!

Solar system explorations

Courtesy SOHO EIT

Cosmic rays,jovian electrons

Solar X-rays, e, p

Courtesy P.Truscott, DERA

Courtesy of R. Nartallo, ESA

XMMX-ray telescope

-4-2024

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10

Z (cm)

Y (

cm

)

RGS EPIC

Brachytherapy

Bepi Colombo


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

Radiotherapy: GEANT4 and com-mercial treatment planifiers

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%

Plano del hazHueso craneal

Tumor


Radfet #2Radfet #4

Radfet #1#3

S300/50G300/50D300/50

D690/15

DG300/50

G690/15

S690/15

DG690/15

Bulk

BulkDiode

Geant4 interface to microelectronic component radiation models

An

a K

eati

ng

(ES

A-E

ST

EC

)


523 MeV 20Ne in a FinFET

Nuclear reaction fragments generated in an overlayer

TCAD (geometry) Geant4

Geant4 (edep) TCAD

introduction to geant4: basic concepts y pedro arce dubois ciemat, 5 th july 2012

Documents

geant4pedro arceintroduction

simple elements z

isotopesall simple materials

volume instance

solid geometry

field typesseveral field

volumes b

low energyhadronic physics