A Handson introduction to Geant4

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A Hands-on introduction to Geant4

• Andrea DellAcqua
• CERN-EP/ATC

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Monday, Feb. 26th

• 130pm-530pm
• Introduction
• Installation
• conventions, types, main()
• DAWN display features, VRML
• Rotation Matrices
• 3-vectors
• if enough time
• Geometry (part I)
• Simple solids
• Logical Volumes
• Positioning

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Introduction
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What is detector simulation?

• A detector simulation program must provide the
  possibility of describing accurately an
  experimental setup (both in terms of materials
  and geometry)
• The program must provide the possibility of
  generating physics events (kinematics) and
  efficiently track particles through the simulated
  detector
• The interactions between particles and matter
  must be simulated by taking into account all
  possible physics processes, for the whole energy
  range
• The possibility of recording at run time all
  quantities need for reproducing the experiment
  functioning must be provided
• Some graphic support must be in place
• and much more...

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Geant4 - Why?

• Geant3 was a detector simulation program
  developed for the LEP era
• Fortran, ZEBRA
• Electromagnetic physics directly from EGS
• Hadronic physics added as an afterthought (and
  always by interfacing with external packages)
• Powerful but simplistic geometry model
• Physics processes very often limited to LEP
  energy range (100 Gev)
• (Painfully) debugged with the help and the
  collaboration of 100s of physicist from all over
  the world
• LHC detectors need powerful simulation tools for
  the next 20 years
• reliability, extensibility, maintainability,
  openness
• good physics, with the possibility of extending

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Geant4 - Why?

• Geant3
• The geometry model is limited to a pre-defined
  set of basic shapes. Adding a new shape requires
  changing the code of several (20) routines.
  Interface to CAD systems in not possible
• Tracking is the result of several iterations (by
  different people) and as such it has become a
  garbled mess. Not for the fainthearted
• EM physics is built in, but several processes are
  missing and their implementation would be very
  hard
• Hadronic physics is implemented via external (and
  obsolete) packages. Modifications require the
  authors intervention

• Geant4
• The geometry has been based since the beginning
  on a CAD-oriented model. The introduction of a
  new shape does not influence tracking
• Tracking has been made independent from
  geometrical navigation, tracking in
  electromagnetic fields (or any field) has been
  improved
• EM and hadronic physics implemented in terms of
  processes. A process can be easily added or
  modified by the user and assigned to the relevant
  particles with no change in the tracking. The cut
  philosophy has been changed so as to make result
  less dependent on the cuts used for the
  simulation. Framework for physics
  parameterisation in place

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Geant4 - How?

• The principle points behind the Geant4
  development have been
• ease of maintainability
• openness of the design
• performance
• development by a rather substantial group of
  physicists
• An Object Oriented approach facilitates the
  achievement of these goals
• the problem domain is naturally split into
  categories which can be tackled by different
  groups (with different expertise)
• the process (if done properly) is
  self-documenting. The components are relatively
  independent from each other and can easily be
  replaced without affecting the whole program too
  much

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Geant4 - How? (2)

• Geant4 is a toolkit
• the framework part in Geant4 has been reduced to
  a minimum, so as to allow the user to implement
  his/her own program structure
• bits and pieces of Geant4 can be used
  independently from the rest
• no main program is provided
• libraries have been made as granular as possible,
  in order to reduce (re-) compilation time and to
  allow the user to only link against those parts
  of G4 which are needed
• C as implementation language
• de-facto standard for what concerns OO
  programming these days
• high performance
• big commercial support, well known in the
  scientific world
• there is practically no Fortran in Geant4
  anymore, hence the user must know some elements
  of C in order to get the best from G4

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Geant4 - How? (3)

• Make Geant4 as independent as possible from
  commercial packages
• You dont need an Objectivity license to run
  Geant4!
• Interfaces to commercial products have been
  provided whenever it was felt appropriate (so
  you can use Objectivity to store your data)
• The only commercial package which was needed to
  run Geant4 was Rogue Waves Tools.h which did
  provide utility classes (as vectors, strings,
  containers and iterators), waiting for the
  Standard Template Library to become available. An
  interface to STL has been made available with the
  4.0.1 release, hence Tools.h in not needed
  anymore
• The Geant4 code is written in reasonable C
• migration to ISO/ANSI C to follow when most of
  the compilers will deal with it
• for the moment use Stroustrups 2nd edition...

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Geant4 documentation

• For a complete documentation set go to
• http//wwwinfo.cern.ch/asd/geant4/geant4.html
• There youll find
• Information on how to install a Geant4
  application
• Documentation on how to get started with Geant4
  and how to build your detector simulation program
• Class diagrams which document the G4 design
• A complete Class Documentation set
• Useful links
• Best reference system I found until now is at
• http//purdue-cdf.fnal.gov/CdfCode and follow the
  geant4 link

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Geant4 status

• In terms of performance and potentiality, Geant4
  should be at least at the same level of Geant3
• First production version (geant4.0.0) released
  at the end of 1998
• Full of bugs
• Several part still under heavy development
• Many improvements proposed, AD cycles still
  undergoing
• Physics part still being improved
• Several new versions released after that (running
  4.3.0R3 now)
• Interface to STL in place and running well
• Migration to ISO/ANSI C is undergoing
• Still lots of improvements needed (IMHO)
• Physics evaluation programmes going on with some
  of the major experiments involved
• G4 is growing up!

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A word on STL

• STL provides a set of utility classes (vectors,
  lists, maps) to be used as tools
• No compiler (in 1994-5) could provide a
  consistent set
• Tools.h was chosen (when the G4 project was
  started) as an interim replacement for STL, with
  the idea of migrating to STL as soon as a
  standard implementation would become available
• 4.0.1 provides an interface to STL but
• using the ObjectSpace STL on HP and SUN
• using the native STL on Linux
• STL has been wrapped into a Tools-like interface
  (
• The interface seems to be stable and robust, now
  the problem is to migrate away from the
  Tools-like interface.