Overview of the Geant4 OO Simulation Toolkit

1
Overview of the Geant4 OO Simulation Toolkit

• Maria Grazia Pia
• INFN Genova, Italy and CERN/IT
• Maria.Grazia.Pia_at_cern.ch
• S. Agostinelli, S. Chauvie, F. Foppiano, P.
  Nieminen, S. Garelli, V. Rolando
• 32nd Meeting of the Proton Therapy Cooperative
  Group
• Uppsala, 16 April 2000
• http//www.ge.infn.it/geant4/talks/Uppsala/index.h
  tml

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Outline

• What is Geant?
• Status of Geant3 and motivations for Geant4
• The Geant4 RD phase RD44
• The role of software engineering and OO
  technology
• Main features of the Geant4 toolkit
• the kernel
• physics
• other tools
• Performance
• The Geant4 Collaboration
• A selection of Geant4 applications
• Conclusions

3
The role of Geant

• Geant is a simulation tool, that provides a
  general infrastructure for
• the description of geometry and materials
• particle transport and interaction with matter
• the description of detector response
• visualisation of geometries, tracks and hits
• The user develops the specific code for
• the primary event generator
• the geometrical description of the set-up
• the digitisation of the detector response

4
The past Geant3

• Geant 3
• has been used by most major HEP experiments
• Frozen since March 1994 (Geant3.21)
• 200K lines of code
• equivalent of 50 man-years, along 15 years
• used also in nuclear physics experiments, medical
  physics, radiation background studies, space
  applications etc.
• The result is a complex system
• because its problem domain is complex
• because it requires flexibility for a variety of
  applications
• because its management and maintenance are
  complex
• It is not self-sufficient
• hadronic physics is not native, it is handled
  through the interface to external packages

5
New simulation requirements

• Very high statistics to be simulated
• robustness and reliability for large scale
  production
• Exchange of CAD detector descriptions
• Transparent physics for validation of physics
  results
• Physics extensions to high energies
• LHC, cosmic ray experiments...
• Physics extensions to low energies
• space applications, medical physics, X-ray
  analysis, astrophysics, nuclear and atomic
  physics...
• Reliable hadronic physics
• not only for calorimetry, but also for PID
  applications (CP violation experiments)
• ...etc.
• User requirements formally collected and coded
  according to PSS05 standard
• Geant4 User Requirements Document

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What is Geant4?

• OO toolkit for the simulation of next generation
  HEP detectors
• ...of the current generation too
• ...not only of HEP detectors
• already used also in nuclear physics, medical
  physics, space applications, radiation background
  studies etc.
• It is also an experiment of distributed software
  production and management, as a large
  international collaboration with the
  participation of various experiments, labs and
  institutes
• It is also an experiment of application of
  rigorous software engineering and Object Oriented
  technologies to the HEP environment

7
A bit of history...

• Approved as RD end 1994 (RD44)
• gt100 physicits and software engineers
• 40 institutes, international collaboration
• responded to DRCC/LCB
• Milestones end 1995
• OO methodology, problem domain analysis, full
  OOAD
• tracking prototype, performance evaluation
• Milestones spring 1997
• ?-release with the same functionality as Geant
  3.21
• persistency (hits), ODBMS
• transparency of physics models

• Milestone July 1998
• public ?-release
• Milestone end 1998
• production release Geant4.0, end of the RD
  phase
• All milestones have been met by RD44
• Reconfiguration at the end of the RD phase
• International Geant4 Collaboration sincel
  1/1/1999
• Management of the production phase
• Continuing RD also in the production phase

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Software Engineering
Software Engineering plays a fundamental role in
Geant4

• Software process
• based on Booch methodology
• spiral type, with cycles of design-implementation
  iterations
• OO Analysis
• OO Design
• Evolution
• Maintenance
• in a worldwide collaboration!

• Software process
• User requirements
• OOAD
• Quality Assurance
• User Requirements
• Collected initially and systematically updated
• Coded according to ESA PSS-05 standard

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OO technologies

• OO design fundamental for distributed parallel
  approach
• every part can be developed, refined, maintained
  independently
• Problem domain decomposition and OOAD result
  into a unidirectional dependency of class
  categories

• Open to evolution
• extensibility, implementation of new models and
  algorithms without interfering with existing
  software
• the user can extend the toolkit with his/her
  model and data

• Transparency
• decoupling from implementation
• Flexibility
• alternative models and implementations
• Interface to external software, without
  dependencies
• databases for persistency
• visualisation libraries
• tools for UI
• etc.

10
Quality Assurance

• Testing
• Unit testing
• in most cases down to class level granularity
• Integration testing
• sets of logically connected classes
• Test-bench for each category
• eg. test-suite of 375 tests for hadronic physics
  parameterised models
• System testing
• exercising all Geant4 functionalities in
  realistic set-ups
• Physics testing
• comparisons with experimental data
• Performance Benchmarks

• Extensive use of Quality Assurance systems
• fundamental for a toolkit of wide public use
• Commercial tools
• Insure, Logiscope etc.
• C coding guidelines
• scripts to verify their applications
  automatically
• Code inspections
• within working groups and across groups

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Standards
Based on standards, ISO e de facto

• OpenGL e VRML for graphics
• CVS for code management
• C as programming language

• STEP
• engineering and CAD systems
• ODMG
• RD45

• Units
• Geant4 is independent from the system of units
• all numerical quantities expressed with their
  units explicitly
• user not constrained to use any specific system
  of units
• have you heard of the accident with NASAs Mars
  Climate Orbiter (125 million)?

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What is needed to run Geant4

• Platforms
• AIX, HP, DEC, Sun, (SGI) native compilers, , g
• Linux g
• Windows-NT Visual C
• Commercial software
• ObjectStore STL (optional)
• Free software
• CVS
• gmake, g
• CLHEP

• Graphics
• OpenGL, X11, OpenInventor, DAWN, VRML...
• OPACS, GAG, MOMO...
• Persistence
• it is possible to run in transient mode
• in persistent mode use a HepDB interface, ODMG
  standard

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The Geant4 kit

• Code
• 1M lines of code, 2000 classes
• (continuously growing)
• publicly available from the web
• Documentation
• 6 manuals
• publicly available from the web
• Examples
• distributed with the code
• navigation between documentation and examples code

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The kernel

• Run and event
• the Run Manager can handle multiple events
• possibility to handle the pile-up
• multiple runs in the same job
• with different geometries, materials etc.
• powerful stacking mechanism
• three levels by default handle trigger studies,
  loopers etc.
• Tracking
• decoupled from physics all processes handled
  through the same abstract interface
• tracking is independent from particle type
• it is possible to add new physics processes
  without affecting the tracking
• Geant4 has only production thresholds , no
  tracking cuts
• all particles are tracked down to zero range
• energy, TOF ... cuts can be defined by the user

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Geometry

• Role detailed detector description and efficient
  navigation
• CAD exchange
• interface through ISO STEP (Standard for the
  Exchange of Product Model Data)
• Multiple representations
• CGS (Constructed Solid Geometries)
• simple solids
• STEP extensions
• polyhedra,, spheres, cylinders, cones, toroids,
  etc.
• BREPS (Boundary REPresented Solids)
• volumes defined by boundary surfaces
• include solids defined by NURBS (Non-Uniform
  Rational B-Splines)
• External tool for g3tog4 geometry conversion
• Fields
• of variable non-uniformity and differentiability
• use of various integrators, beyond Runge-Kutta
• time of flight correction along particle transport

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Processes

• Processes describe how particles interact with
  material or with a volume itself
• Three basic types
• At rest process
• (e.g. decay at rest)
• Continuous process
• (e.g. ionization)
• Discrete process
• (e.g. decay in flight)
• Transportation is a process
• interacting with volume boundary
• A process which requires the shortest interaction
  length limits the step

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Physics

• From the Minutes of LCB (LHCC Computing Board)
  meeting on 21 October, 1997
• It was noted that experiments have requirements
  for independent, alternative physics models. In
  Geant4 these models, differently from the concept
  of packages, allow the user to understand how the
  results are produced, and hence improve the
  physics validation. Geant4 is developed with a
  modular architecture and is the ideal framework
  where existing components are integrated and new
  models continue to be developed.

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The approach to physics

• Ample variety of independent, alternative physics
  models available in Geant4
• No more black boxes of packages
• The users are directly exposed to the physics
  they use in their simulation
• This approach is fundamental for the validation
  of the experiments physics results

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Transparency of Geant4 physics

• No hard coded numbers
• Explicit use of units throughout the code
• Separation between the calculation of cross
  sections and the generation of the final state
• Calculation of cross-sections independent from
  the way they are accessed (data files, analytical
  formulae etc.)
• Distinction between processes and models
• Cuts in range (rather than in energy, as usual)
• consistent treatment of interactions near
  boundaries between materials
• Modular design, at a fine granularity, to expose
  the physics
• physics independent from tracking
• Public distribution of the code, from one
  reference repository worldwide

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Physics general features

• Abstract interface to physics processes
• tracking independent from processes
• Distinction between processes and models
• often multiple models for the same process
• Data encapsulation and polymorfism
• Transparent access to cross sections, from files,
  interpolation from tables, analytical formulae
  etc.
• Distinction between the calculation of cross
  sections and their use
• Calculation of the final state independent from
  tracking
• Uniform treatment of electromagnetic and hadronic
  physics
• Open system
• Users can easily create and use their own models

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Data libraries

• Systematic collection and evaluation of
  experimental data from many sources worldwide
• Databases
• ENDF/B, JENDL, FENDL, CENDL, ENSDF,JEF, BROND,
  EFF, MENDL, IRDF, SAID, EPDL, EEDL, EADL, SANDIA,
  ICRU etc.
• Collaborating distribution centres
• NEA, LLNL, BNL, KEK, IAEA, IHEP, TRIUMF, FNAL,
  Helsinki, Durham, Japan etc.
• The use of evaluated data is important for the
  validation of physics results of the experiments

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Electromagnetic physics

• Comparable to Geant3 and EGS already in the
  ?-release
• Substantial further extensions
• Multiple alternatives for various processes
• High energy extensions
• models for ? up to PeV
• fundamental for LHC experiments, cosmic ray
  experiments etc.
• Low energy extensions
• e,? down to 250 eV
• (EGS, ITS etc. to 1 keV, Geant3 to 10 keV))
• low energy hadrons and ions models based on
  Ziegler and ICRU data and parametrisations
• models for antiprotons
• fundamental for space and medical applications,
  neutrino experiments, antimatter spectroscopy etc.

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E.M. processes in Geant4

• multiple scattering
• energy loss
• Bremsstrahlung
• ionisation
• annihilation
• photoelectric effect
• Compton scattering
• pair production
• synchrotron radiation

• transition radiation
• Cherenkov
• Rayleigh effect
• rifraction
• reflection
• absorption
• scintillation
• fluorescence
• Auger (in progress)

24
Hadronic physics

• Completely different approach w.r.t. the past
• transparent
• native
• no longer interface to external packages
• clear separation between data and their use in
  algorithms
• Cross section data sets
• transparent and interchangeable
• Final state calculation
• models by particle, energy, material

25
Completeness of Geant4 hadronic physics

• Ample variety of models
• the most complete hadronic simulation kit on the
  market
• alternative and complementary models
• it is possible to mix-and-match, with fine
  granularity
• data-driven, parameterised and theoretical models
• Consequences for the users
• no more confined to the black box of one package
• the user has control on the physics used in the
  simulation, which contributes to the validation
  of physics results

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Hadronic physicsParameterised and data-driven
models

• Based on experimental data
• Some models originally from GHEISHA
• completely reengineered into OO design
• refined physics parameterisations
• New parameterisations
• pp, elastic differential cross section
• nN, total cross section
• pN, total cross section
• np, elastic differential cross section
• ?N, total cross section
• ?N, coherent elastic scattering

• Other models are completely new, such as
• stopping particles (?- , K- )
• neutron transport
• isotope production
• All databases existing worldwide used in neutron
  transport
• Brond, CENDL, EFF, ENDFB, JEF, JENDL, MENDL etc.

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Hadronic physicsTheoretical models

• They fall into different parts
• the evaporation phase
• the low energy range, pre-equilibrium, O(100
  MeV),
• the intermediate energy range, O(100 MeV) to O(5
  GeV), intra-nuclear transport
• the high energy range, hadronic generator
  régime
• Geant4 provides complementary theoretical models
  to cover all the various parts
• Geant4 provides alternative models within the
  same part
• All this is made possible by the powerful Object
  Oriented design of Geant4 hadronic physics
• Easy evolution new models can be easily added,
  existing models can be extended

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Event biasing

• Geant4 provides facilities for event biasing
• The effect consists in producing a small number
  of secondaries, which are artificially recognized
  as a huge number of particles by their
  statistical weights
• Event biasing can be used, for instance, for the
  transportation of slow neutrons or in the
  radioactive decay simulation

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Other components

• Materials
• elements, isotopes, compounds, chemical formulae
• Particles
• all PDG data
• and more, for specific Geant4 use, like ions
• Hits Digi
• to describe detector response
• Persistency
• possibility to run in transient or persistent
  mode
• no dependence on any specific persistency model
• persistency handled through abstract interfaces
  to ODBMS

• Visualisation
• Various drivers
• OpenGL, OpenInventor, X11, Postscript, DAWN,
  OPACS, VRML
• User Interfaces
• Command-line, Tcl/Tk, Tcl/Java, batchmacros,
  OPACS, GAG, MOMO
• automatic code generation for geometry and
  materials
• Interface to Event Generators
• through ASCII file for generators supporting
  /HEPEVT/
• abstract interface to Lund

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ESA projects in Geant4

• Low energy extensions of electromagnetic
  physics
• Source Particle Module
• Radioactivity Decay Module
• Sector Shielding Analysis Tool
• CAD Tool Front-End

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Fast simulation

• Geant4 allows to perform full simulation and
  fast simulation in the same environment
• Geant4 parameterisation produces a direct
  detector response, from the knowledge of particle
  and volume properties
• hits, digis, reconstructed-like objects (tracks,
  clusters etc.)
• Great flexibility
• activate fast /full simulation by detector
• example full simulation for inner detectors,
  fast simulation per calorimeters
• activate fast /full simulation by geometry
  region
• example fast simulation in central areas and
  full simulation near cracks
• activate fast /full simulation by particle type
• example in e.m. calorimeter e/?
  parameterisation and full simulation of hadrons
• parallel geometries in fast/full simulation
• example inner and outer tracking detectors
  distinct in full simulation, but handled together
  in fast simulation

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Performance

• Various Geant4 - Geant3.21 comparisons
• realistic detector configurations
• results and plots in
• Geant4 Web Gallery (from Geant4 homepage)
• RD44 Status Report, 1995
• Benchmark in liquid Argon/Pb calorimeter
• at comparable physics performance Geant4 is
  faster than (fully optimised) Geant3.21 by
• a factor gt3 using exactly the same cuts
• a factor gt10 optimising Geant4 cuts, while
  keeping the same physics performance
• at comparable speed Geant4 physics performance is
  greatly superior to Geant3.21
• Benchmark in thin silicon layer
• at comparable physics performance Geant4 is 25
  faster than Geant3.21 (single volume, single
  material)

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Geant4 Collaboration

• New organization for the production phase, MoU
  based
• Geant4 distribution, development and User Support
• Atlas, BaBar, CMS, LHCB
• CERN, KEK, SLAC, TRIUMF, JNL
• ESA, Frankfurt Univ, INFN, IN2P3, Karolinska
  Inst., Lebedev, TERA
• COMMON (Serpukov, Novosibirsk, Pittsburg etc.)
• other memberships currently being discussed

• Collaboration Board
• manages resources and responsibilities
• Technical Steering Board
• manages scientific and technical matters
• manages the Production Service and User Support
• Working Groups
• do maintenance, development, QA, user support etc.

Members of National Institutes, laboratories and
experiments participating in Geant4 Collaboration
acquire the right to the Production Service and
User Support For others free code and user
support on best effort basis
34
Bragg peak, Magic cube data and Geant4
Experimental data Bragg peak of a 270 MeV/u
carbon ion beam
Geant4 and experimental data, PSI test with
proton beam
distance(cm)
35
Brachyterapy at Nat. Inst. Cancer Research, Genova
S. Agostinelli, R. Corvo, F. Foppiano, S.
Garelli, G. Sanguineti

• The source holder is a standard endobronchial
  treatment catheter, the chamber is a 0.6 cc
  Capintec chamber connected to a Capintec 192
  electrometer
• The IST group follows the direction of Basic
  Dosimetry on Radiotherapy with Brachytherapy
  Source of the Italian Association of Biomedical
  Physics (AIFB)

The custom calibration plexiglas jig, used for in
air measurements.
36
CT interface and treatment planning

• Two possible approaches
• CT interface Geant4 full simulation
• CT interface Geant4 fast simulation
  (physics processes
  parameterised through an analytical treatment)
• Geant-based tools for
• inverse planning
• technique of active dose delivery
• Software interface for Geant4 that reads input
  data in DICOM3 format developed at Medical Dept.,
  University of Piemonte Orientale and INFN Torino

37
Geant4 for scatter compensation in Megavoltage 3D
CT

• Use GEANT4 to obtain digitally reconstructed
  radiographs (DRRs), including full scatter
  simulation
• This represents a great improvement over
  approaches based on ray-casting

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In vivo dosimetry for mammography

• TLD characterization for mammography screening
• simulation of dose deposition and glow curve
• Mammography simulation
• Goal minimize dose on patient
• Comparison between experimental data TLD in vivo
  dosimetry and Geant4 simulation
• Activity at Medical Physics Dept., Umberto I
  Hospital of Ordine Mauriziano, Torino
• in progress

39
An ambitious project
What if the geometry to describe with Geant4 were
DNA and the process were mutagenesis?

• Multi-disciplinary Collaboration of
• astrophysicists and space scientists
• particle physicists
• medical physicists
• biologists
• physicians
• First phase of the project User
  Requirements
• Other applications (not only in space programmes)

• Study of radiation damage at the cellular and DNA
  level in the space radiation environment
• ESA-sponsored project, in
  collaboration with INFN
• INFN (Genova, Torino, Cosenza)
• Istituto Nazionale per la Ricerca sul Cancro
• Università del Piemonte Orientale
• ESA
• CERN
• Karolinska Institute

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Geant4 features relevant for medical applications

• The transparency of physics
• Advanced functionalities in every domain
  geometry, physics, visualisation etc.
• Extensibility in any domain to satisfy new user
  requirements
• thanks to OO technology
• open design new physics, new features can be
  easily added, without any perturbation to the
  existing code
• Adopts standards wherever available (de jure or
  de facto)
• Use of evaluated data libraries
• Quality Assurance based on sound software
  engineering
• Subject to independent validation by a large user
  community worldwide
• User support organization by a large
  international Collaboration of experts

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Documentation
http//wwwinfo.cern.ch/asd/geant4/geant4.html

• User Documentation
• Introduction to Geant4
• Installation Guide
• Geant4 Users Guide - For Application Developers
• for those wishing to use Geant4
• Geant4 Users Guide - For Toolkit Developers
• for those wishing to extend Geant4 functionality
• Software Reference Manual
• documentation of the public interface of all
  Geant4 classes
• Physics Reference Manual
• extended documentation on Geant4 physics

• Examples
• a set of Novice, Extended and Advanced examples
  illustrating the main functionalities of Geant4
  in realistic set-ups
• The Gallery
• a web collection of performance and physics
  evaluations
• http//wwwinfo.cern.ch/asd/geant4/reports/gallery/
  
• Publication and Results web page
• http//wwwinfo.cern.ch/asd/geant4/reports/reports.
  html