The motivations for Geant4

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Part I

• The motivations for Geant4

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Part I outline

• The role of simulation an example
• The role of simulation
• The market of simulation packages
• Geant what is and how it evolved
• Geant4 the motivations behind it

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Once upon a time there was a X-ray telescope...
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Chandra scheme
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Chandra CCDs
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An excerpt of a press release
Chandra X-ray Observatory Status
Update September 14, 1999 MSFC/CXC CHANDRA
CONTINUES TO TAKE SHARPEST IMAGES EVER TEAM
STUDIES INSTRUMENT DETECTOR CONCERN Normally
every complex space facility encounters a few
problems during its checkout period even though
Chandras has gone very smoothly, the science and
engineering team is working a concern with a
portion of one science instrument. The team is
investigating a reduction in the energy
resolution of one of two sets of X-ray detectors
in the Advanced Charge-coupled Device Imaging
Spectrometer (ACIS) science instrument. A series
of diagnostic activities to characterize the
degradation, identify possible causes, and test
potential remedial procedures is underway. The
degradation appeared in the front-side
illuminated Charge-Coupled Device (CCD) chips of
the ACIS. The instruments back-side illuminated
chips have shown no reduction in capability and
continue to perform flawlessly.
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Chandra in Geant4
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XMM
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Geant4 simulations of Chandra and XMM

• Simulations to study the response of the
  instruments to the radiation environment on orbit
  and the lifetime of detectors
• Hadron ionisation with d ray production, hadron
  multiple scattering, electron ionisation,
  electron Bremsstrahlung, ee- annihilation, m
  ionisation, m Bremsstrahlung, m pair production,
  photoelectric effect, Compton scattering, g
  conversion
• Protons of energies from hundreds keV to a few
  MeV can scatter at low angles through the mirror
  shells of X-ray astronomy missions, producing a
  high non-ionising dose in unshielded CCDs
• Experimental measurements of proton reflectivity
  of XMM grating and mirror samples are in good
  agreement with Geant4 simulation
• XMM was launched on 10 December 1999 from Kourou

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XMM
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CCDs
CCD displacement damage front vs.
back-illuminated.
30 mm Si ? 1.5 MeV p
Active layerPassive layer
30 mm
2 mm
30 mm
2 mm
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The role of simulation

• Simulation plays a fundamental role in various
  domains and phases of an experimental physics
  project
• design of the experimental set-up
• evaluation and definition of the potential
  physics output of the project
• evaluation of potential risks to the project
• assessment of the performance of the experiment
• development, test and optimisation of
  reconstruction and physics analysis software
• contribution to the calculation and validation of
  physics results
• The scope of these lectures (and of Geant4)
  encompasses the simulation of the passage of
  particles through matter
• there are other kinds of simulation components,
  such as physics event generators, electronics
  response generators, etc.
• often the simulation of a complex experiment
  consists of several of these components
  interfaced to one another

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Domains of application

• HEP and nuclear physics experiments
• the most traditional field of application
• used by nearly all experiments
• applications in astrophysics experiments too
• Radiation background studies
• evaluation of safety constraints and shielding
  for the experimental apparatus and human beings
• Medical applications
• radiotherapy
• design of instruments for therapeutic use
• Biological applications
• radiation damage (in human beings, food etc.)
• Space applications
• they encompass all the aspects of the other
  domains above
• In some of these areas simulation is mission
  critical

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Requirements for physics validation

• The validation of the overall physics results of
  an experiment impose some requirements on
  simulation
• Transparency
• the user has access to the code
• and he/she can understand its content and how it
  is used
• and he/she has control on what he/she uses in
  his/her physics application
• the data and their use are kept distinct
• Public distribution of the code
• the code is the same for all users and
  applications
• no hand-made specially tuned versions of the
  code
• the validation is done by independent users, not
  only by the authors of the code
• Use of evaluated databases and of published data
• no hard coded numbers or parameters of unknown
  source
• but use of the commonly accepted body of
  knowledge
• Geant4 implements all these guidelines

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Components

• Modeling of the experimental set-up
• Tracking of particles through matter
• Interaction of particles with matter
• Modeling of the detector response
• Run and event control
• Visualisation of the set-up, tracks and hits
• User interface
• Accessory utilities (random number generators,
  PDG particle information etc.)
• Interface to event generators
• Persistency

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The world of simulation packages

• Simulation of particle interaction with matter
  has been an active field for many years
• Many specialised and general purpose packages
  available on the market
• GEANT3
• EGS
• ITS
• HETC
• MCNP
• MORSE
• MICAP
• CALOR
• VENUS
• LHI
• CEPX-ONELD
• TRIM, SRIM
• TART...
• etc.

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Integrated suites vs specialised codes

• Specialised packages cover a specific simulation
  domain
• Pro
• the specific issue is treated in great detail
• often the package is based on a wealth of
  specific experimental data
• simple code, usually relatively easy to install
  and use
• Contra
• a typical experiment covers many domains, not
  just one
• domains are often inter-connected
• Integrated packages cover all/many simulation
  domains
• Pro
• the same environment provides all the
  functionality
• Contra
• it is more difficult to ensure detailed coverage
  of all the components at the same high quality
  level
• monolithic approach take all or nothing
• limited or no options for alternative models
• usually complex to install and use
• difficult maintenance and evolution

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Fast and full simulation

• Usually there are two types of simulations in a
  typical experiment
• Fast simulation
• mainly used for feasibility studies and quick
  evaluations
• coarse set-up description and physics modeling
• usually directly interfaced to event generators
• Full simulation
• used for precise physics and detector studies
• requires a detailed description of the
  experimental set-up and a complex physics
  modeling
• usually interfaced to event generators and event
  reconstruction
• Traditionally fast and full simulation are done
  by different programs and are not integrated in
  the same environment
• complexity of maintenance and evolution
• possibility of controversial results

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The Toolkit approach

• ...that is, how to get the best of all worlds
• A toolkit is a set of compatible components
• each component is specialised for a specific
  functionality
• each component can be refined independently to a
  great detail
• components can be integrated at any degree of
  complexity
• components can work together to handle
  inter-connected domains
• it is easy to provide (and use) alternative
  components
• the simulation application can be customised by
  the user according to his/her needs
• maintenance and evolution - both of the
  components and of the user application - is
  greatly facilitated
• ...but what is the price to pay?
• the user is invested of a greater responsibility
• he/she must critically evaluate and decide what
  he/she needs and wants to use

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Geant a historical overview

• GEANT comes from GEometry ANd Tracking
• Geant2 an attempt to build a first prototype in
  the late 70s
• the ideas behind memory management and
  integrated geometry/tracking/physics
• Geant3 the simulation tool of the 80s and 90s
• several versions (last Geant3.21 in 1994)
• New physics and software requirements for the LHC
  era triggered a RD project for Geant4
• With Geant4 there has been a technology
  transition
• before procedural software, FORTRAN
• Geant4 Object Oriented technology, C
• Geant3 was a CERN product
• developed, distributed, maintained and supported
  by the CERN DD/CN/IT Division
• a few external individuals contributed to its
  development
• Geant4 is the product of an international
  collaboration

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The role of Geant

• Geant is a simulation tool, that provides a
  general infrastructure for
• the description of the geometry and materials of
  an experimental set-up
• particle transport and interaction with matter
• the description of detector response
• visualisation of geometries, tracks and hits
• The experiment develops the specific code for
• the primary event generator
• the description of the experimental set-up
• the digitisation of the detector response
• It plays a fundamental role in various phases of
  the life-cycle of an experiment
• detector design
• development of reconstruction and analysis
  software
• physics studies
• Other roles in non-HEP fields (eg. treatment
  planning in radiotherapy)

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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

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New simulation requirements

• New simulation requirements derive from
• the specific features of the new generation of
  HEP experiments
• LHC, astroparticle physics etc.
• application of simulation tools to new domains
• space, medical, biological etc.
• New simulation requirements address
• the physics capabilities of the simulation tool
• the software/computing characteristics
• Geant4 was born from the user communites
• User requirements formally collected from the
  user communities and continuously updated
• Geant4 User Requirements Document

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New simulation requirements physics

• Transparent physics
• for the validation of physics results
• Physics extensions to high energies
• LHC experiments
• cosmic ray experiments
• etc. ...
• Physics extensions to low energies
• space applications
• medical physics
• X-ray analysis
• astrophysics experiments
• nuclear and atomic physics
• etc. ...
• Reliable hadronic physics
• not only for calorimetry, but also for PID
  applications (CP violation experiments)
• ...etc.

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New simulation requirements computing

• The very high statistics to be simulated requires
• robustness and reliability for large scale
  production
• The long lifetime of the new generation of
  experiments requires
• easy extension of the functionalities (new
  physics models, new data, new technologies etc.)
• easy maintenance and evolution
• Independence from external software products and
  specific technologies requires
• coupling to be managed through interfaces
• The connection between the physics design and the
  engineering design of the experiments requires
• exchange of CAD detector descriptions
• The wide range of expertise necessary for a new
  complex simulation tool requires
• software technologies suitable for distributed
  parallel development
• etc.

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What is Geant4?

• Geant4 is an 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

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Motivations for a redesign of Geant

• It had become too complicated
• to maintain the program
• to extend its functionality
• to improve the physics transparency and content
• Geant3 was not technically adequate to the new
  generation experiments
• the data structures are not adequate
• memory handling is not adequate
• Geant3 was not physically adequate to the new
  generation experiments
• either because of insufficient accuracy and
  reliability
• or because of incomplete coverage of the energy
  scale
• Data exchange and interface with other tools was
  too difficult or impossible
• A fundamental component (hadronic physics) was
  external to Geant3
• ...etc.

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Geant4 history the RD phase

• Approved as RD end 1994 (RD44)
• gt100 physicists 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
  Geant3.21
• 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

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Geant4 history the production phase

• Reconfiguration at the end of the RD phase
• International Geant4 Collaboration sincel
  1/1/1999
• CERN, JNL, KEK, SLAC, TRIUMF
• Atlas, BaBar, CMS, LHCB, TERA(IGD)
• ESA, Frankfurt Univ., IN2P3, INFN(IDG), Lebedev
• new membership applications being discussed
• Management of the production phase
• production service
• user support
• continuing development
• Production releases
• Geant4 0.0, December 1998
• Geant4 0.1, July 1999
• Geant4 1.0, December 1999
• ...more to come
• regular reference tags released for
  collaborating experiments