Geant4 - a simulation toolkit -

1
Geant4- a simulation toolkit -

• Makoto Asai (SLAC Computing Services)
• On behalf of the Geant4 Collaboration
• December 1st, 2003
• ACAT03 _at_ KEK

2
Contents

• General introduction and brief history
• Geant4 kernel
• Geometry
• Physics
• Highlights of the new developments
• Highlights of user applications
• User support processes
• Summary

3
General introduction and brief history
4
What is Geant4?

• Geant4 is an object-oriented toolkit for
  simulation of elementary particles passing
  through and interacting with matter. It includes
  a complete range of functionality including
  tracking, geometry, physics models and hits.
• Geant4's application area includes high energy
  and nuclear physics experiments, accelerator and
  shielding studies, space engineering, medical
  physics and several other fields.
• Geant4 is the successor of GEANT3, the
  world-standard toolkit for HEP detector
  simulation. Geant4 is one of the first successful
  attempt to re-design a major package of HEP
  software for the next generation of experiments
  using an Object-Oriented environment.

5
Flexibility of Geant4

• In order to meet wide variety of requirements
  from various application fields, a large degree
  of functionality and flexibility are provided.
• Geant4 has many types of geometrical descriptions
  to describe most complicated and realistic
  geometries
• CSG, BREP, Boolean
• XML interface
• The physics processes offered cover a
  comprehensive range including electromagnetic,
  hadronic and optical processes for a large set of
  long-lived particles in materials and elements,
  over a wide energy range. The applicable energy
  range begins, in some cases, from 100 eV and
  extends up to the TeV energy range.

6
Physics in Geant4

• It is rather unrealistic to develop a uniform
  physics model to cover wide variety of particles
  and/or wide energy range.
• Much wider coverage of physics comes from mixture
  of theory-driven, parameterized, and empirical
  formulae. Thanks to polymorphism mechanism, both
  cross-sections and models (final state
  generation) can be combined in arbitrary manners
  into one particular process.
• Standard EM processes
• Low energy EM processes
• Hadronic processes
• Photon/lepton-hadron processes
• Optical photon processes
• Decay processes
• Shower parameterization
• Event biasing technique

7
Geant4 Its history and future

• Dec 94 - Project start
• Apr 97 - First alpha release
• Jul 98 - First beta release
• Dec 98 - Geant4 0.0 (first public) release
• Jul 99 - Geant4 0.1 release
• Jun 03 - Geant4 5.2 release
• Dec 12th, 03 - Geant4 6.0 release (planned)
• We currently provide two to three public releases
  and bimonthly beta releases in between public
  releases every year.

8
Geant4 Collaboration
HARP
PPARC
Univ. Barcelona
Collaborators also from non-member institutions,
including Budker Inst. of Physics IHEP
Protvino MEPHI Moscow Pittsburg University
Lebedev
Helsinki Inst. Ph.
9
Geant4 kernel
10
Geant4 kernel

• Geant4 consists of 17 categories.
• Independently developed and maintained by WG(s)
  responsible to each category.
• Interfaces between categories (e.g. top level
  design) are maintained by the global architecture
  WG.
• Geant4 Kernel
• Handles run, event, track, step, hit, trajectory.
• Provides frameworks of geometrical representation
  and physics processes.

Geant4
Inter
Readout
Visuali
faces
zation
Persis
Run
tency
Event
Tracking
Digits
Processes
Hits
Track
Geometry
Particle
Graphic
Material
_reps
Intercoms
Global
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Tracking and processes

• Geant4 tracking is general.
• It is independent to
• the particle type
• the physics processes involving to a particle
• It gives the chance to all processes
• To contribute to determining the step length
• To contribute any possible changes in physical
  quantities of the track
• To generate secondary particles
• To suggest changes in the state of the track
• e.g. to suspend, postpone or kill it.

12
Processes in Geant4

• In Geant4, particle transportation is a process
  as well, by which a particle interacts with
  geometrical volume boundaries and field of any
  kind.
• Because of this, for example, shower
  parameterization process can take over from the
  ordinary transportation without modifying the
  transportation process.
• Each particle has its own list of applicable
  processes. At each step, all processes listed are
  invoked to get proposed physical interaction
  lengths.
• The process which requires the shortest
  interaction length (in space-time) limits the
  step.

13
Cuts in Geant4

• A Cut in Geant4 is a production threshold.
• Only for physics processes that have infrared
  divergence
• Not tracking cut, which does not exist in Geant4
• Energy threshold must be determined at which
  discrete energy loss is replaced by continuous
  loss
• Old way
• Track primary particle until cut-off energy is
  reached, calculate continuous loss and dump it at
  that point, stop tracking primary
• Create secondaries only above cut-off energy, or
  add to continuous loss of primary for less
  energetic secondaries
• Geant4 way
• Specify range (which is converted to energy for
  each material) at which continuous loss begins,
  track primary particle one more step to make it
  down to zero range
• Create secondaries only above specified range, or
  add to continuous loss of primary for less
  energetic secondaries

14
Energy cut vs. range cut

• 500 MeV/c proton in liq.Ar (4mm) / Pb (4mm)
  sampling calorimeter

• Geant3 (energy cut)
• Ecut 450 keV

liq.Ar
Pb
liq.Ar
Pb

• Geant4 (range cut)
• Rcut 1.5 mm
• Corresponds to Ecut in liq.Ar
  450 keV, Ecut in Pb 2 MeV

15
Geometry
16
Volume

• Three conceptual layers
• G4VSolid -- shape, size
• G4LogicalVolume -- daughter physical volumes,
• material,
  sensitivity, user limits, etc.
• G4VPhysicalVolume -- position, rotation
• Hierarchal three layers of geometry description
  allows maximum reuse of information to minimize
  the use of memory space.

17
Solid

• Geant4 geometry module supports variety of
  representations of shapes.
• CSG (Constructed Solid Geometry) solids
• G4Box, G4Tubs, G4Cons, G4Trd,
• Analogous to simple GEANT3 CSG solids
• Specific solids (CSG like)
• G4Polycone, G4Polyhedra, G4Hype,
• BREP (Boundary REPresented) solids
• G4BREPSolidPolycone, G4BSplineSurface,
• Any order surface
• Boolean solids
• G4UnionSolid, G4SubtractionSolid,

18
Physical volume

• G4PVPlacement 1 Placement One
  Volume
• A volume instance positioned once in its mother
  volume
• G4PVParameterised 1 Parameterized Many
  Volumes
• Parameterized by the copy number
• Shape, size, material, position and rotation can
  be parameterized, by implementing a concrete
  class of G4VPVParameterisation.
• Reduction of memory consumption
• G4PVReplica 1 Replica Many
  Volumes
• Slicing a volume into smaller pieces (if it has a
  symmetry)
• G4ReflectionFactory 1 Placement a set of
  Volumes
• Generating a pair of placements of a volume and
  its reflected volume
• Useful typically for end-cap calorimeter
• G4AssemblyVolume 1 Placement a set of
  Placements
• Position a group of volumes

19
Smart voxelization

• In case of Geant 3.21, the user had to carefully
  implement his/her geometry to maximize the
  performance of geometrical navigation.
• While in Geant4, users geometry is automatically
  optimized to most suitable to the navigation. -
  "Voxelization"
• For each mother volume, one-dimensional virtual
  division is performed.
• Subdivisions (slices) containing same volumes are
  gathered into one.
• Additional division again using second and/or
  third Cartesian axes, if needed.
• "Smart voxels" are computed at initialisation
  time
• When the detector geometry is closed
• Does not require large memory or computing
  resources
• At tracking time, searching is done in a
  hierarchy of virtual divisions

20
Geometry checking tools

• An overlapping volume is a contained volume which
  actually protrudes from its mother volume
• Volumes are also often positioned in a same
  volume with the intent of not provoking
  intersections between themselves. When volumes in
  a common mother actually intersect themselves are
  defined as overlapping
• Geant4 does not allow for malformed geometries
• The problem of detecting overlaps between volumes
  is bounded by the complexity of the solid models
  description
• Utilities are provided for detecting wrong
  positioning
• Graphical tools
• Kernel run-time commands

21
Physics
22
Physics processes in Geant4

• Each process can act at any of three space-time
  intervals
• In time - At rest (e.g. decay at rest)
• Continuously along a step (e.g. Cherenkov
  radiation)
• At a point - at the end of the step (e.g. decay
  in flight)
• Along step actions are applied cumulatively,
  while others are selectively applied.
• A process can have more than one types of action
  according to its nature.
• For example, Ionization process has Along and End
  step actions
• Tracking handles each type of action in turn.
• It loops over all processes with such a type of
  action.
• The motivation for creating these categories of
  actions is to keep the tracking independent of
  the physics processes.
• All seven combinations of actions are possible.
• The traditional Continuous, Continuous-Discrete,
  Discrete and AtRest are found in these cases.
• The ordering of processes is important in some
  cases.
• e.g. Multiple scattering affects the step length.

23
Electromagnetic processes

• Gammas
• Gamma-conversion, Compton scattering,
  Photo-electric effect
• Leptons (e, m), charged hadrons, ions
• Energy loss (Ionisation, Bremstrahlung) or PAI
  model energy loss, Multiple scattering,
  Transition radiation, Synchrotron radiation,
• Optical photons
• Cerenkov, Rayleigh, Reflection, Refraction,
  Absorption, Scintillation
• High energy m
• Alternative implementation
• Standard EM package ignores the binding energy of
  electron to an atom, while Low Energy EM package
  takes it into account.
• Standard for applications that do not need to
  go below 1 KeV
• Low Energy down to 250eV (e/g), O(0.1mm) for
  hadrons

24
Multiple scattering

• Examples of comparisons
• 15.7 MeV e-
• on gold foil
• Modelling comparisons
• L. Urban

Angle (deg)
25
Hadronic and Photolepton-hadron processes

• Each hadronic process may have one or more cross
  section data sets, and final state production
  models associated with it. Each one has its own
  energy range of applicability.
• The term data set is meant in a broad sense to
  be an object that encapsulates methods and data
  for calculating total cross sections.
• The term model is meant in a broad sense to be
  an object that encapsulates methods and data for
  calculating final state products.

26
Parameterization and data driven models

• Parameterization models on the fly
• high energy inelastic (Aachen, CERN)
• low energy inelastic, elastic, fission, capture
  (TRIUMF, UBC, CERN, SLAC)
• Parameterization models for stopping particles
• base line (TRIUMF, CHAOS)
• mu- (TRIUMF, FIDUNA)
• pi- (INFN, CERN, TRIUMF)
• K- (Crystal Barrel, TRIUMF)
• anti-protons (JLAB, CERN)
• Electromagnetic transitions of the exotic atom
  prior to capture effects of atomic binding.
  (Novosibirsk, ESA)
• Data driven models
• Low energy neutron transport (neutron_hp),
• Radioactive decay (DERA, ESA)
• photon evaporation (INFN)
• elastic scattering (TRIUMF, U.Alberta, CERN)
• internal conversion (ESA)
• etc..

27
Theory driven models

• Ultra-high energy models
• Parton transport model (U.Frankfurt, in
  discussion)
• High energy models
• Fritjof type string model (CERN)
• Quark gluon String model (CERN)
• Pythia(7) interface (Lund, CERN)
• Intra-nuclear transport models (or replacements)
• Hadronic cascadepre-equilibrium model (HIP,
  CERN)
• Binary and Bertini cascade models (HIP, CERN,
  Novosibirsk, SLAC)
• QMD type models (CERN, Inst.Th.Phys. Frankfurt)
• Chiral invariant phase-space decay model (JLAB,
  CERN, ITEP)
• Partial Mars rewrite (Kyoto, Uvic, in
  collaboration with FNAL)
• De-excitation
• Evaporation, fission, multi-fragmentation,
  fermi-break-up (CMS)

28
Hadronic Model Inventory
CHIPS
At rest Absorption m, p, K, anti-p
CHIPS (gamma)
Photo-nuclear, electro-nuclear
High precision neutron
Evaporation
FTF String (up to 20 TeV)
Fermi breakup
Pre- compound
Multifragment
Bertini cascade
QG String (up to 100 TeV)
Photon Evap
Binary cascade
Fission
Rad. decay
MARS
HEP ( up to 20 TeV)
LE pp, pn
LEP
1 MeV 10 MeV 100 MeV 1 GeV
10 GeV 100 GeV 1 TeV
29
Verification

• The verification effort of the geant4 hadronic
  working group is grouped into several sections
• Inclusive cross-sections
• Thin target comparisons
• Verification of model components
• Code comparisons (least effective)
• Complete application tests
• Robustness.
• A few examples of each are given in the following
  slides.

30
Proton reaction total cross-section
J.P.Wellisch
31
Pion production examples, QGSRapidity
distributions and invariant cross-section
predictions in quark gluon string model

400GeV protons on Lithium
100 GeV pi on Gold
32
Forward peaks in proton induced neutron
production
Lead
Beryllium
256 MeV data Neutrons at 7.5deg.

Iron
Aluminum
33
Production from 730 MeV p (Bertini Model)
34
Low energy neutron capturegammas from 14 MeV
capture on Uranium
35
Nuclear interactions with Geant4 versus experiment
10-9 pC/incident proton
Channel
Phantom and experimental results from
H.Paganetti, B.Gottschalk, Medical physics Vol.
30, No.7, 2003
36
Atlas HEC (e/p ratio)
37
CMS ECAL HCAL testbeam GEANT3 - GEANT4
comparison
100 GeV pi ECALHCAL
38
"Educated guess" physics lists

• In Geant4, the physics lists serve the same
  purpose as the "packages" (GHEISHA, FLUKA,
  GCALOR) in geant3.
• Conceptually, the two are identical.
• Both ways provide the physics and its modeling to
  an application.
• Each "package" is built of a complete and
  consistent set of models
• In Geant4, the number of "packages" is quite
  large. Each option comes with trade-offs in
  descriptive power and performance.
• It simply became clear that writing a good
  physics list is not trivial, in particular when
  hadronic physics is involved.
• It is nice to be able to exploit the full power
  in the flexibility and variety of hadronic
  physics modeling in geant4, but being forced to
  do so is not what we want.
• It is also nice to have the physics transparently
  in front of the user and to exploit it in the
  best possible way, but being forced to understand
  everything is (very understandably) not what
  people want, either.
• We have systematically accumulated experience
  with various combinations of cross-section and
  models over the past years. Today we provide a
  set of physics lists institutionalizing this
  knowledge.
• "Educated guess" physics lists

39
Use-case driven packages

• LCG simulation project.
• HEP calorimetry.
• HEP trackers.
• 'Average' collider detector
• Low energy dosimetric applications with neutrons
• low energy nucleon penetration shielding
• linear collider neutron fluxes
• high energy penetration shielding
• medical and other life-saving neutron
  applications
• low energy dosimetric applications

• high energy production targets
• e.g. 400GeV protons on C or Be
• medium energy production targets
• e.g. 15-50 GeV p on light targets
• LHC neutron fluxes
• low background experiments
• Air shower applications (still working on this)
• Each package has several physics lists suitable
  to the use-case

40
Decay

• A decay table is associated to the definition of
  each particle type.
• A track can have a decay channel. If it has, it
  exactly decays through this channel without
  randomizing by the decay ratio.
• This allows the user to import decay chains
  generated by physics generators such as Pythia,
  and rely on Geant4 tracking for such unstable
  particles.

Primary particle list
G4Track
B0
B0
K0L
pre-defined decay channel
K0L
41
Optical processes

• Geant4 has a particle named Optical Photon,
  which is distinguished from gamma. It interacts
  by optical processes.
• Geant4 is an ideal framework for modeling the
  optics of scintillation and Cerenkov detectors
  and their associated light guides. This is
  founded in the toolkit's unique capability of
  commencing the simulation with the propagation of
  a charged particle and completing it with the
  detection of the ensuing optical photons on photo
  sensitive areas, all within the same event loop.
• This functionality is now employed world-wide in
  experimental simulations as diverse as ALICE,
  ANTARES, AMANDA, Borexino, Icarus, LHCb, HARP,
  KOPIO, the Pierre Auger Observatory, and the GATE
  (Imaging in Nuclear Medicine) Collaboration.

42
Optical processes

• Optical photons are generated if one or more of
  following processes are activated.
• Cerenkov radiation,
• Transition radiation,
• Scintillation
• Optical processes built in Geant4
• Absorption,
• Rayleigh scattering
• Boundary Processes (reflection, refraction)
• Optical properties, e.g. dielectric coefficient
  and surface smoothness, can be set to a volume.

43
Shower parameterization framework

• Geant4 includes a built-in framework for shower
  parameterization scheme. Currently, the user has
  to concrete his/her own parameterization assigned
  to a logical volume, which is then called as an
  envelop.
• Regardless of the existence of granular daughter
  geometry, a particle comes into the envelop can
  be fully treated by the shower parameterization
  process.
• The user still have a dynamic choice to take
  his/her parameterization or to follow the
  ordinary tracking in the granular geometry.
• The shower parameterization process can directly
  contact to a sensitive detector associating to
  the volume to produce more than one distributed
  hits.

44
Highlights ofnew developments
45
Event biasing in Geant4

• Event biasing (variance reduction) technique is
  one of the most important requirements, which
  Geant4 collaboration is aware of.
• This feature could be utilized by many
  application fields such as
• Radiation shielding
• Dosimetry
• Since Geant4 is a toolkit and also all source
  code is open, the user can do whatever he/she
  wants.
• CMS, ESA, Alice, and some other experiments have
  already had their own implementations of event
  biasing options.
• Its much better and convenient for the user if
  Geant4 itself provides most commonly used event
  biasing techniques.

46
Current features in Geant4

• Partial MARS migration
• n, p, pi, K (lt 5 GeV)
• Since Geant4 0.0
• General particle source module
• Primary particle biasing
• Since Geant4 3.0
• Radioactive decay module
• Physics process biasing in terms of decay
  products and momentum distribution
• Since Geant4 3.0
• Cross-section biasing (partial) for hadronic
  physics
• Since Geant4 3.0
• Leading particle biasing
• Since Geant4 4.0
• Geometry based biasing
• Weight associating with real volume or artificial
  volume
• Since Geant4 5.0

47
Geometrical importance biasing

• Define importance for each geometrical region
• Duplicate a track with half (or relative) weight
  if it goes toward more important region.
• Russian-roulette in another direction.
• Scoring particle flux with weights
• At the surface of volumes

48
Cuts per Region

• Geant4 has had a unique and uniform production
  threshold (cut) expressed in length (range of
  secondary).
• For all volumes
• One cut in range for each particle
• By default is the same cut for all particles.
• Consistency of the physics simulated
• A volume with dense material will not dominate
  the simulation time at the expense of sensitive
  volumes with light material.
• Yet appropriate length scales can vary greatly
  between different areas of a large detector
• E.g. a vertex detector (5 mm) and a muon detector
  (2.5 cm).
• Having a unique (low) cut can create a
  performance penalty.
• Requests from ATLAS, BABAR, CMS, LHCb, , to
  allow several cuts
• Enabling the tuning of production thresholds at
  the level of a sub-detector, i.e. region.
• Cuts are applied only for gamma, electron and
  positron.
• Full release in Geant4 5.1 (end April, 2003)
• Comparable run-time performance compared to
  global cuts.

49
Region

• Introducing the concept of region.
• Set of geometry volumes, typically of a
  sub-system
• Or any group of volumes
• A cut in range is associated to a region
• a different range cut for each particle is
  allowed in a region.
• Typical Uses
• barrel end-caps of the calorimeter can be a
  region
• Deep areas of support structures can be a
  region.

50
Highlights ofUsers Applications
51
Geant4 in HEP

• ATLAS (CERN-LHC)
• 22 x 22 x 44 m3
• 15,000 ton
• 4 million channels
• 40 MHz readout

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53
CMS
Sliced view of CMS barrel detectors
View of CMS muon system
View of 180 Higgs event simulated in CMS Tracker
detector
54
LHCb
A Typical event in the Testbeam
Red lines Charged particle Green lines
Optical Photons.
55
Geant4 for beam transportation
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58
INTEGRAL

• Gamma ray astronomy from 15 keV to 10 MeV
• Launched 17 October 2002
• Length 5 m, diameter 3.7 m, mass 4 tons

59
INTEGRAL Geant4 model byUniversity of
Southampton
INTEGRAL in the ESA/ESTEC test center
60
International Space Station (ISS)
Space Environments and Effects Section
61
Space Radiation Solar Events of October-November
2003!
Images by the ESA/NASA SOHO spacecraft
The effects of space radiation on spacecraft and
on astronauts can be simulated with Geant4
62
ESA Space Environment Effects Analysis Section

• DESIRE (Dose Estimation by Simulation of the ISS
  Radiation Environment)
• KTH Stockholm, ESTEC, EAC, NASA Johnson
• Prediction of the ambient energetic particle
  environment (SPENVIS additional models)
• Construction of COLUMBUS geometry in Geant4
• Radiation transport, including secondary particle
  production, through the geometry
• Calculation of astronaut radiation doses

63
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65
User Support
66
User Support

• Geant4 Collaboration offers extensive user
  supports.
• Documents
• Examples
• Users workshops
• Tutorial courses
• HyperNews and mailing list
• Bug reporting system
• Requirements tracking system
• Daily private communications
• New implementation - Technical Forum

67
Documents for users

• One introduction and Five user manuals
• http//wwwasd.web.cern.ch/wwwasd/geant4/G4UsersDoc
  uments/ Overview/html/index.html
• Installation guide
• User's guide for application developers
• For a user who develops a simulation application
  using Geant4
• User's guide for toolkit developers
• For a user who develops a module which alternates
  or enhances some of geant4 functionalities
• Physics reference manual
• Detailed description of each physics process with
  information of references
• Software reference manual
• LXR source code browser maintained by TRIUMF and
  KEK.
• Materials of past tutorials / presentations,
  HyperNews and Web pages maintained by developers
  also available via Geant4 official Web page.
• "Geant4 general paper" - NIM A 506.

68
Examples

• Along the code releases, Geant4 provides examples
  which help user's understanding of
  functionalities of Geant4 and are reusable as
  "skeletons" of user's application.
• Three levels of examples
• Novice examples
• Demonstrate most basic features
• Extended examples
• Highlight some functionalities / use-cases in
  detail
• Some examples require external package(s)
• Advanced examples
• Most realistic applications
• User's contributions

69
Geant4 users workshop

• Users workshops were held or are going to be held
  hosted by several institutes for various user
  communities.
• KEK - Dec.2000, Jul.2001, Mar.2002, Jul.2002,
  Mar.2003, Jul.2003
• SLAC - Feb.2002
• Spain (supported by INFN) - Jul.2002
• CERN - Nov.2002
• ESA/NASA - Jan.2003, May.2004
• dedicated to space-related users
• Helsinki - Oct.2003
• Local workshops of one or two days were held or
  are planned at several places.

70
Geant4 tutorials / lectures

• In addition to the users workshops, many tutorial
  courses and lectures with some discussion time
  slots were held for various user communities.
• CERN School of Computing
• Italian National School for HEP/Nuclear
  Physicists
• MC2000, MCNEG workshop, IEEE NSS/MIC
• KEK, SLAC, DESY, FNAL, INFN, Frascati,
  Karolinska, GranSasso, etc.
• ATLAS, CMS, LHCb
• Tutorials/lectures at universities
• U.K. - Imperial
• Italy - Genoa, Bologna, Udine, Roma, Trieste
• Near future tutorial courses
• KEK (Dec. 8th-11th, 2003)
• Vanderbilt Univ. TN. USA (Jan. 11th-13th, 2004)
• SLAC (Mar. 8th-10th, 2004)

71
HyperNews

• HyperNews system was set up in April 2001

72
HyperNews

• 19 categories
• Not only user-developer, but also user-user
  information exchanges are quite intensive.

73
HyperNews is quite active
74
Technical Forum

• In the Technical Forum, the Geant4 Collaboration,
  its user community and resource providers
  discuss
• major user and developer requirements, user and
  developer priorities, software implementation
  issues, prioritized plans, physics validation
  issues, user support issues
• The Technical Forum is open to all interested
  parties
• To be held at least 4 times per year (in at least
  two locales)
• The purpose of the forum is to
• Achieve, as much as possible, a mutual
  understanding of the needs and plans of users and
  developers.
• Provide the Geant4 Collaboration with the
  clearest possible understanding of the needs of
  its users.
• Promote the exchange of information about physics
  validation performed by Geant4 Collaborators and
  Geant4 users.
• Promote the exchange of information about user
  support provided by Geant4 Collaborators and
  Geant4 user communities.
• Next Technical Forum meeting _at_ CERN on February
  5th, 2004.

75
Summary

• Geant4 is a worldwide collaboration providing a
  tool for simulation of particles interacting with
  matter.
• Geant4s object-oriented modular structure allows
  a large degree of functionality and flexibility.
• Geant4 can handle most complicated and realistic
  geometries.
• Geant4 provides sets of alternative physics
  models so that users can choose appropriate
  models.
• Geant4 is being used by not only high energy and
  nuclear physics but also accelerator physics,
  astrophysics, space science and medical and other
  applications.
• Geant4 Collaboration offers extensive user
  supports.
• http//cern.ch/geant4/