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Part I. The motivations for Geant4. 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. Once upon a time there was a X-ray telescope. Chandra scheme. Chandra CCDs.

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part i

Part I

The motivations for Geant4

part i outline
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
an excerpt of a press release
An excerpt of a press release

Chandra X-ray Observatory Status Update

September 14, 1999



Normally every complex space facility encounters a few problems during its checkout period; even though Chandra’s 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 instrument’s back-side illuminated chips have shown no reduction in capability and continue to perform flawlessly.

geant4 simulations of chandra and xmm
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, e+e- 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

CCD displacement damage: front vs. back-illuminated.

30 mm Si  ~1.5 MeV p+

Active layerPassive layer

2 mm

30 mm

2 mm

30 mm

the role of simulation
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
domains of application
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
requirements for physics validation
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

  • 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
the world of simulation packages
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
  • LHI
  • TART...
  • etc.
integrated suites vs specialised codes
Integrated suites vs specialised codes
  • Specialised packages cover a specific simulation domain


    • 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


    • a typical experiment covers many domains, not just one
    • domains are often inter-connected
  • Integrated packages cover all/many simulation domains


    • the same environment provides all the functionality


    • 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
fast and full simulation
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 integratedin the same environment
    • complexity of maintenance and evolution
    • possibility of controversial results
the toolkit approach
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
geant a historical overview
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 R&D 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
the role of geant
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)
the past geant3
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
new simulation requirements
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
new simulation requirements physics
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.
new simulation requirements computing
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.
what is geant4
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
motivations for a redesign of geant
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.
geant4 history the r d phase
Geant4 history: the R&D phase
  • Approved as R&D end 1994 (RD44)
    • >100 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 R&D phase
  • All milestones have been met by RD44
geant4 history the production phase
Geant4 history: the production phase
  • Reconfiguration at the end of the R&D phase
    • International Geant4 Collaboration sincel 1/1/1999
    • 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