Geant4 in a Nutshell

Geant4 in a NutshellLCD group seminarPeter Speckmayer4. August 20094/3/20091GEANT4 seminar, Peter SpeckmayerHow does Geant4 work?

What is simulated in there?4/3/20092GEANT4 seminar, Peter SpeckmayerGeometryEventsTrackingStep lengthelectromagnetic fieldsrange cutPhysics processesElectromagnetic processesMultiple scatteringEnergy lossHadronic ProcessesModeling of final statesPhysics lists

Outline / elements of GEANT44/3/20093GEANT4 seminar, Peter SpeckmayerSimulation of interactions of particles with matter

A toolkit (user has to wrap code around)GeometryDigitization

Aim of G4 design:FlexibilityCustomizableextendableWhat is Geant4?4/3/20094GEANT4 seminar, Peter SpeckmayerDescription of the detectorefficient navigation of particles

Volumes defined by user with codetypically experiments wrap interface around (e.g. compact.xml)

sensitive logical volumes for readout linked with user defined class for detector response (digitization)Geometry4/3/20095GEANT4 seminar, Peter SpeckmayerGeometry (detector description)Logical volumeMother volumeLogical volumes become physical when placed into a mother volumeReadout structure can be different from detectorstructureReadout structure4/3/20096GEANT4 seminar, Peter SpeckmayerEvent main unit of simulation

Before being processed:Primary particlesVertices

Afterwards:HitsDigitizations (response of parts of the detector)(simulation truth)Events4/3/20097GEANT4 seminar, Peter SpeckmayerEvent consists of many tracksTrack a transported particleconsists of many stepsStep smallest unit

Particles not self movingParticles are transportedtransportation is a (physics)process like decay, pair production,

Tracking4/3/20098GEANT4 seminar, Peter SpeckmayerTracking4/3/20099GEANT4 seminar, Peter SpeckmayerTrack: many subsequent stepsSecondaries: put into a stackwhen one track is finished,next particle is taken fromthe stack(three stacks: prioritize tracksuninteresting tracks can be suppressed performance)

Tracking / Step4/3/200910GEANT4 seminar, Peter Speckmayer all processes propose step length shortest one is taken (e.g. decay)

post step action of the limiting step is called

for particles at rest a time is proposed

for processes where energy is lost continuously preservation of precision is the limiting factor

tracking decides which processes are invoked process can demand to be invoked always (e.g. transportation, multiple scattering)Physics processes are associated to every particleEach process proposes a step length

Tracking/ calculating the step length4/3/200911GEANT4 seminar, Peter Speckmayer

decay: .. mean lifetime .. velocity .. Lorentz factorprobability of not interacting within Linteraction with material:(composed of isotopes) .. density of the materialmi .. mass of the isotopexi .. the mass fractioni .. cross-section of process forthis isotope .. mean free pathrandom number [0,1] is thrown length until decay or interaction L is computed

transportation-process L = length to next boundary step as well limited by user defined maximum allowed stepelectromagnetic fieldsnon-linear propagation of charged particlestrajectories are computed numericallymethod used (e.g. Runge-Kutta, helix) depends on smoothness of fieldintersection with boundaries chord-approximationrange-cutparticles with short range (less than user defined) are not generated energy depositedavoids generating many soft photons and e-ranges in tables (calculated once at G4 startup)Tracking / electromagnetic fields / range-cut4/3/200912GEANT4 seminar, Peter Speckmayer

A real MC life example for one step4/3/200913GEANT4 seminar, Peter Speckmayerprocess which limited the stepproduces energy deposit along pathchanges path of hadron slightlymoves the particlethree actions which can be implemented by a process(not part of Geant4)

Visible and invisible energy4/3/200914GEANT4 seminar, Peter Speckmayer

visible energy Evisenergy of secondaries EsecEmeas=Etot+corrMass Emeas,nucleon=Ekin,nucleon

Einvisible=Ekin,pre-step-Evis-Emeas,prim-i,secEmeas,iPre-stepPost-stepcorrMass=-me- : electron+me- : positron-mp : p+mp : anti-p-mn : n+mn : anti-nProcess specific physical interaction of a particle initial state, final state, cross-section/mean lifetime

detailed interaction controlled by a modelsecondaries, kinematicsmodel can be changed easily

several processes assigned to particle type (energy range)

Types of processesDecayElectromagnetic processesHadronic processesOptical processesPhysics processes and models4/3/200915GEANT4 seminar, Peter Speckmayerdecay tables in GEANT4 from PDG

complex decays (e.g. B mesons) are not modeledinterface to external event generators providedGeant4 talks to external program, external program calculates decaypre-assigned decay modeevent generator simulates decaysG4 uses these data when needed

what we do?... is slightly different we provide G4 only with particles which it can handleParticle decays ...4/3/200916GEANT4 seminar, Peter Speckmayerfor interactions of: e-, e+, photons, charged hadronseffects of shell structure of atoms are averagedloss of precision (when shell effects are important)

photons:Compton scatteringconversion into electron and muon pairsphoto-electric effect

e- and e+:bremsstrahlungionization-ray productione+ annihilationsynchrotron radiationElectromagnetic processes4/3/200917GEANT4 seminar, Peter Speckmayeradditions in the low energy region (examples)photoelectric effectCompton scatteringRayleigh scatteringflourescence emission from excited atomsAuger effectcorrections due to the molecular structure of materialscorrections due to the effect of the nuclear stopping powerBarkas effect

electromagnetic processes producing hadronsphotonuclear and electronuclear reactions can convert electromagnetical energy flow (e-, e+, photons) to energy flow of mesons, baryons and nuclear fracmentshadron production by nuclear interaction of muonsElectromagnetic processes (2)4/3/200918GEANT4 seminar, Peter Speckmayercalculated for one step angular deflection mean path length correction mean lateral displacement

simulated for all charged particles

Multiple scattering4/3/200919GEANT4 seminar, Peter Speckmayere+ and e- contributions are summed upcontinuous ionization, bremstrahlungdiscrete Moeller & Bhabha scattering, -ray production, hard bremsstrahlung

muonsionization, bremsstrahlung, pair production

charged hadronsionization (including hard -ray production)

mean energy loss is computed, fluctuations are added

ionization of ions and hadrons:model depends on energy range:>2MeV Bethe-Bloch formulaO(5GeV) )diffractive string excitation model (Fritiof like):the particles which are scattered only exchange momenta. For each of the scattered particles a string is formed where the quark content of the original hadron is randomly assigned to the string ends.quark gluon string model:the quark gluon string model splits a nucleon into a quark and a di-quark. Between those, strings are formed and hadroniyed (by adding a qqbar-pair). The color flow between partons from the interacting particles and the hadron-nucleon interactions are mediated by the exchange of Pomerons.Modeling of final states4/3/200924GEANT4 seminar, Peter Speckmayer

theory-based modelsintra-nuclear cascade models (ECMS atomic spacing are called opticaloptical properties of a medium can be given as a function of photon wavelengthimplemented processes:Cerenkov processScintillation: user can define light yield, photon emission spectrum, ...Absorption and Rayleigh scatteringReflection and Refraction optical boundaries can be defined by the userTransition radiation

Optical processes4/3/200926GEANT4 seminar, Peter SpeckmayerPhysics processes, overview4/3/200927GEANT4 seminar, Peter Speckmayer

G4 collaboration provides compilations of consistent sets of models physics listselectromagnetic simulations well known accurate simulationsmodeling of hadrons worse

LEP and HEP used as fallback solutions and for particles and regions where no other models are defined

High energy QGS, FTF

Transitions between models in different E-regions smoothed out continuous transition between models (e.g. QGS to Bertini cascade)

Physics lists to be mentioned:LHEPold G3 (GEISHA) parametrizations problematic (energy in processes not conserved, etc.)low energies: LEP, high energies: HEPLCPhysQGS (with precompound) above 12 GeV, LEP between 9.5 to 25 GeV, 0-9.9 GeV Bertini cascadeQGSP_BERTQGS (with precompound) above 25 GeV, LEP between 25 and 10 GeV, below that: Bertini cascadeFTF_BICFritiof with precompound above 5 GeV, below that: Binary cascade no LEP, no HEP for pi and pPhysics lists4/3/200928GEANT4 seminar, Peter SpeckmayerGEANT4 is very modular and flexiblechanges of models for physics processes possible but: should be done together with GEANT4 collaboration, since set of chosen models should be consistentGEANT4 performs good for electromagnetic processesGEANT4 gives you results for hadronic processescertainly less accurate than for the electromag. processeshas to be validated for the material which is usedphysics list? no clear favourite, ... again: validation necessaryWhich physics list to choose?QGSP_BERT is used the LHC-experimentsLCPhys is used in ILC community (seems to be rather similar to QGSP_BERT)FTF_BIC is complementary to QGSP_BERT good crosscheck

Summary4/3/200929GEANT4 seminar, Peter Speckmayer