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Part I

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

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  1. Part I The motivations for Geant4

  2. 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

  3. Once upon a time there was a X-ray telescope...

  4. Chandra scheme

  5. Chandra CCDs

  6. 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 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.

  7. Chandra in Geant4

  8. XMM

  9. 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

  10. XMM

  11. CCDs 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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.

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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)

  22. 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

  23. 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

  24. 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.

  25. 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.

  26. 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

  27. 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.

  28. 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

  29. Geant4 history: the production phase • Reconfiguration at the end of the R&D 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

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