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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

Part i

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