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Geant4 Physics Models


  • pg 1
									                         Geant4 Physics Models

                       Geant4 Workshop at Helsinki 30.-31.10.2003
                               T. Lampén, HIP / Helsinki

T. Lampén 30.10.2003
         Outlook of Presentation
    I.          Overview of Geant4 MC simulation
    II.         Physics processes and models
    III.        Use of physics processes in practice
    IV.         Geant4 physics validation

T. Lampén 30.10.2003
 Geant4 Collaboration

                                 Univ. Barcelona               PPARC
                                                   Collaborators also from non-
                                                   member institutions, including
                                                     Budker Inst. of Physics
                                                          IHEP Protvino
                       Lebedev                          MEPHI Moscow
                                                       Pittsburg University
T. Lampén 30.10.2003
     Requirements for Geant4 simulation
       ●   Geant4 : general purpose Monte Carlo toolkit for the simulation of
           the passage of particles through matter
       ●   Variety of requirements from: heavy ion physics, CP violation
           physics, cosmic ray physics, astrophysics, space science, medical
           applications, …
           => large degree of functionality and flexibility are needed
       ●   Everything open to the user
            – physics processes as well as
            – geometry
            – visualization
            – GUI
            – persistency
            – histogramming
       ●   OO methods and C++
T. Lampén 30.10.2003
  Overview of Geant4 kernel                                        Geant4

                                              Visuali              Readout                    Inter
                                               zation                                          faces
Geant4 consists of 17 categories
  Independently developed and maintained                    Run                    Persis

  Working Groups
  Interfaces between categories maintained
  by the global architecture WG                  Digits +                    Processes
Geant4 Kernel
  Handles run, event, track, step, hit,
  trajectory                                      Geometry                    Particle

  Provides frameworks of geometrical
  representation and physics processes            Graphic



  T. Lampén 30.10.2003
     Tracking in Geant4
       ●   Tracking, i.e. calculating the path of particle, is the
           fundamental part in detector simulation
       ●   In Geant4, tracking is independent to particle type and
           physics processes
       ●   Tracking consists of steps
       ●   All physics processes may contribute to step length,
           interactions, generation of secondary particles, etc.
       ●   Cut in range: production thresholds for secondaries expressed
           in range instead of energy
                                                        End of step point
                       Begin of step point
T. Lampén 30.10.2003
     Physics in Geant4, Basic Principles
     ●   Uniform physics models not realistic (covering wide variety of
         particle / energy range)
     ●   Instead, mixture of models used for wide (and appropriate)
         coverage of physics
     ●   Modular physics architecture also allows individual universities /
         groups to contribute their models to Geant4 and get credit
     ●   i.e. HIP has participated to implementation of INUCL and HETC
         models to Geant4
     ●   Use of polymorphism allows generic use and combination of
         different models (i.e. cross-sections and theory-driven models)

T. Lampén 30.10.2003
     Physics Processes and Models
       ●   Distinguish between process and model
       ●   Each particle has to be assigned with process(es)
       ●   Processes take care of interactions and decays (also
       ●   Process may consist of several
           models, cross-sections etc.
           with different energy ranges           particle

                       Gamma              process 1       process 2

                                                model 1         cross section set 1
            Compton        Pair
                                                model 2         cross section set 2
            scattering     production
                                                …               cross section set 3
T. Lampén 30.10.2003
     Process Properties
       ●   Processes all inherit from G4VProcess
       ●   Processes do have for tracking:
              –   method GetPhysicalInteractionLength()
                  to know when interaction might happen (space-time)
              –   method DoIt()
                  to generate final state (momentum, secondaries etc.)
       ●   Processes may be:
              –   in time (decay at rest)
              –   continuously along step (Cherenkov radiation)
              –   at a point (Compton scattering)
              –   or combinations (bremsstrahlung, ionization)
T. Lampén 30.10.2003
     Selecting Process in Simulation Step
          Stepping            Physics           Particle      Step        Track   Logical   Sensitive
           Manager             Process            Change                           Volume    Detector



T. Lampén 30.10.2003
     Different Physics Processes and Models
       ●    7 major process categories
             – electromagnetic
             – hadronic
             – photolepton-hadron
             – decay
             – optical
             – parameterization
             – transportation
       ●    Three categories of modeling approaches:
             – theory-driven
             – parametrized
             – empirical formulae

T. Lampén 30.10.2003
     In Practice

       ●   User’s responsibility to choose physics processes,
           models and cross-section tables, no default
       ●   Hadronic part most difficult (multiple models and
           cross-sections per process)
       ●   Examples and ready-made hadronic physics lists
           available ( G4 home page / site index / physics lists )
       ●   (a physics list can contain ”sub” physics lists)

T. Lampén 30.10.2003
     Physics Lists - architecture

T. Lampén 30.10.2003
     Examples of Available Hadronic Physics
      •    LCG simulation project           • Low energy dosimetric
      •    HEP calorimetry                    applications
      •    HEP trackers                     • High energy production targets
      •    'Average' collider detector         e.g. 400GeV protons on C or Be
      •    Low energy dosimetric            • Medium energy production
           applications with neutrons         targets
      •    Low energy nucleon                  e.g. 15-50 GeV p on light targets
           penetration shielding
      •    Linear collider neutron fluxes   • LHC neutron fluxes
      •    High energy penetration          • Air shower applications (under
           shielding                          development)
      •    Medical and other life-saving    • Low background experiments
           neutron applications               (underground experiments)

T. Lampén 30.10.2003
     Physics Lists – How-to

       ●   Class has to be derived from G4VUserPhysicsList
       ●   Methods to implement:
              –   ConstructParticle() // creates particles
              –   ConstructProcess() // assigns processes
              –   SetCuts()           // set range cuts for secondaries
       ●   Register to Geant4 run manager in main program

T. Lampén 30.10.2003
     Physics Lists – example of code
       void MyPhysicsList::ConstructParticle() // take particles into use

       void MyPhysicsList::ConstructEM() // construct em process
             G4ProcessManager* pm = particle->GetProcessManager();
             if (particle->GetParticleName() == ”e-”){
                 pm->AddProcess(new G4MultipleScattering, -1, 1, 1);
                 pm->AddProcess(new G4eIonisation, -1, 2,2);
                 pm->AddProcess(new G4eBremsstrahlung, -1, -1, 3);
T. Lampén 30.10.2003
     Physics Lists – example of code

void MyPhysicsList::SetCuts()
      defaultCutValue = 1.0 *cm;      //1.0 mm G4 recommendation

// Finally register physics list to Geant4 with this line in main program
runManager->SetUserInitialization( new MyPhysicsList );

// NOTE: also cross sections have to be registered. One can also set
   energy limits for models
T. Lampén 30.10.2003
     Physics Lists – For More Information

     ●   Complete EM physics list (complete std EM) in novice
         example N03 (see and modify)
     ●   For hadronic physics lists, several models available,
         choosing right one requires care and experience
         => ready-made hadronic physics lists warmly suggested
         contains also small EM list…
     ●   Physics list derived from G4VModularPhysicsList can
         contain several lists

T. Lampén 30.10.2003
     Physics Processes in Detail:
     EM Processes

       ●   Two sets of EM processes:
              –   Standard (e- binding energy ignored)
              –   Low energy (e- binding energy taken into account)
                       ●   applicable to same energy(std goes beyond 1TeV) range as
                           standard EM
                       ●   slower than standard EM, but more detailed at low energy
                       ●   uses atomic shell cross sections from DB, not parametrized
                       ●   developed for medical and space applications
                       ●   some photon and electron processes go down to 250 eV(100
                           eV from June),
                           for protons, ions and antiprotons down to about 1 keV
T. Lampén 30.10.2003
     Standard EM Processes
     • Gammas                         • Muons
        – photo-electric effect          – mu ionization, energy loss
        – Compton scattering             – mu bremsstrahlung
        – e-, muon pair production       – e+e- pair production
     • Electrons                      • Charged hadrons
        – e Ionization, energy loss      – ionization, energy loss
        – e Bremsstrahlung            • All charged particles
        – e+e- -annihilation             – multiple scattering
        – sychrotron radiation           – transition radiation
                                         – scintillation
                                         – Cerenkov radiation
T. Lampén 30.10.2003
     Additional Low Energy EM processes

     • Not covered in standard EM • Improved in low energy
       – Rayleigh scattering         – hadron and ion energy loss
       – Compton scattering by       – electron ionization
         linearly polarized gammas
                                     – Bremsstrahlung
       – Fluorescence
       – Auger process
                                     – photo-electric effect

T. Lampén 30.10.2003
     Hadronic and Photolepton-hadron
     processes I
       ●   Hadronic processes:
              –   At rest:
                    ● Stopped muons, pions, kaons, anti-proton

                    ● Radioactive decay

              –   Elastic
                    ● One process for all long-lived hadrons

              –   Inelastic
                    ● Specific process for each hadron

                    ● Photo-nuclear

                    ● Electro-nuclear

              –   Capture
                    ● Π , K in flight
                        -   -

              –   Fission
T. Lampén 30.10.2003
     Hadronic and Photolepton-hadron
     processes II
       ●   Each hadronic process may have one or more
            – cross section data sets
            – final state production models
           (with different energy ranges)
       ●   “data set” encapsulates methods and data for calculating
           total cross sections.
       ●   “model” encapsulates methods and data for calculating
           final state products
       ●   5-level architecture

T. Lampén 30.10.2003
     The Decay Process

       ●   Applied to all unstable long-lived particles
       ●   Mean free path for decay λ=γβcτ,
           for most other processes λ=Nρσ/A
       ●   Same process for all particles,
           BR and decay modes from the particle’s decay table
       ●   Decay modes for heavy flavor particles not included
           in Geant4, external event generators can be used if
           needed (i.e. Pythia)

T. Lampén 30.10.2003
     Optical processes
       ●   Optical photon distinguished from gamma (wavelength >>
           atomic spacing)
       ●   Optical photons are generated by:
            – Cherenkov radiation
            – Transition radiation
            – Scintillation
            – Optical processes
            – Absorption
            – Rayleigh scattering
            – Boundary processes (reflection, refraction)
       ●   Polarization taken into account, phase not (no
       ●   Warning: no energy conservation in these processes
25     ●   Optical properties can be set for materials and volumes
T. Lampén 30.10.2003
     Shower parametrization framework

       ●   Built-in framework for shower parameterization
           available, if the ordinary tracking is not suitable
       ●   User has to concrete the parametrization process and
           assign it to a geometrical volume
       ●   Shower parameterization process can directly contact
           to sensitive detector to produce hits

T. Lampén 30.10.2003
     Optional Data to Use in Geant4

       ●   G4NDL version 3.7 neutron data files (26.04 Mb)
       ●   low energy EM process data files (7.13 Mb, version
       ●   photon evaporation data files (7.16 Mb)
       ●   radio-active decay hadronic process data files
           (0.63 Mb)

T. Lampén 30.10.2003
     Examples to Study

       ●   Good example to study physics lists and settings is
           novice example N02
       ●   N02 contains electromagnetic processes in uniform
           magnetic field
       ●   This will be studied in tomorrow’s session

       ● > Download > examples

T. Lampén 30.10.2003
     Event Biasing Techniques
     To facilitate computation, event biasing
     techniques can be used:
           –   leading particle biasing
           –   primary event biasing
           –   physics / geometry based biasing
           –   forced interaction
           –   cuts per region
           –   …

                                             Leading particle biasing –
                                        only most energetic particle followed
T. Lampén 30.10.2003
     Geant4 Physics Validation
       ●   EM validation
            – thin target tests comparing Geant4 and data
              –   Comparison (not validation)Geant4 often compared to well established
                  EM simulation code EGS4
       ●   Hadronic validation
            – thin target tests comparing Geant4 and data
            – full setup HEP tests (full physics and full geometry)
       ●   Validation done extensively in large HEP experiments:
           ATLAS, CMS, BaBar, HARP
       ●   CMS just taking Geant4 into use as official simulation tool
       ●   ATLAS: Geant4 simulates relevant features of muons,
           electrons and pions usually better than Geant3
       ●   Expanded validation suites planned for users
T. Lampén 30.10.2003
     User Documents at
       ●   Introduction and 5 user manuals:
            – Installation guide
            – User's guide for application developers
            – User's guide for toolkit developers
            – Physics reference manual
                       ●   detailed description of each physics process with information
                           of references
              –   Software reference manual
                       ●   description of classes and methods
       ●   Examples and code
       ●   LXR source code browser
       ●   Materials of past tutorials & presentations, HyperNews and
           Web pages maintained by developers
       ●   "Geant4 general paper" - NIM A 506
T. Lampén 30.10.2003
       ●   General purpose Monte Carlo toolkit for the simulation
           of the passage of particles through matter
       ●   Particles have to be assigned with processes, which are
           responsible for physical interactions and transportation
       ●   A process can consist of several models and cross-
           sections, which have different applicability (particles,
           energy range)
       ●   Ready-made physics lists are provided especially for
           hadronic processes

T. Lampén 30.10.2003

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