Geant4: Electromagnetic Processes 1

1
Geant4Electromagnetic Processes 1

• Introduction
• Interfaces
• PhysicsList
• Optical process

2
Radiation effects

• Charged particles lost energy primary to
  ionization
• For high energy electrons radiation losses
  dominates
• Gammas convert to ee- or transform energy to
  atomic electrons
• Heavy particles interact with atomic nucleus

3
 What is tracked in Geant4 ?

• Propagated by the tracking,
• Snapshot of the particle state.

• Momentum, pre-assigned decay

• The  particle type 
• G4Electron,
• G4PionPlus

•  Hangs  the
• physics sensitivity

• The classes involved in the building the
   physics list  are
• The G4ParticleDefinition concrete classes
• The G4ProcessManager
• The processes

• The physics
• processes

4
Geant4 physics processes

• There are following processes
• Electromagnetic
• Hadronic
• Decay
• Optical
• Transportation
• Parameterized

• Subcategories of Electromagnetic domain
• Muons
• Lowenergy
• Standard
• Xrays
• Utils

5
Geant4 physics processes

• Physics is described via abstract interface
  called process associated with particles
• Process provides Interaction Lenths, StepLimits,
  and DoIt methods
• Process active AlongStep, PostStep, AtRest
• Distinction between process and model one
  process may includes many models
• Generation of final state is independent from the
  access and use of cross sections and from
  tracking

6
PhysicsList

• It is one of the  mandatory user classes 
• Defined in source/run
• Defines the three pure virtual methods
• ConstructParticles()
• ConstructProcesses()
• SetCuts()
• Concrete PhysicsList needs to inherit from
  G4VUserPhysicsList
• For interactivity G4UserPhysicsListMessenger can
  be used to handle PhysicsList parameters

7
Processes ordering

• Ordering of following processes is critical
• Assuming n processes, the ordering of the
  AlongGetPhysicalInteractionLength should be
• n-2
• n-1 multiple scattering
• n transportation
• Why ?
• Processes return a  true path length 
• The multiple scattering  virtually folds up 
  this
• true path length into a shorter  geometrical 
• path length
• Based on this new length, the transportation can
  geometrically limits the step.
• Other processes ordering usually do not matter.

?
8
AddTransportation

• void G4VUserPhysicsListAddTransportation()
• G4Transportation theTransportationProcess
  new G4Transportation()// loop over all
  particles in G4ParticleTable theParticleIterator-
  gtreset()while( (theParticleIterator)() )
  G4ParticleDefinition particle
  theParticleIterator-gtvalue()
  G4ProcessManager pmanager particle-gtGetProcessM
  anager() if (!particle-gtIsShortLived())
  if ( pmanager 0) G4Exception("G4VUse
  rPhysicsListAddTransportation no process
  manager!") else // add
  transportation with ordering ( -1, "first",
  "first" ) pmanager-gtAddProcess(theTransportationP
  rocess) pmanager-gtSetProcessOrderingToFirst(theT
  ransportationProcess, idxAlongStep) pmanager-gtSe
  tProcessOrderingToFirst(theTransportationProcess,
  idxPostStep)

9
Gamma processes

• Discrete processes - only PostStep actions
• Use function AddDiscreteProcess
• pmanager is the G4ProcessManager of the gamma
• Assume the transportation has been set by
  AddTransportation
• Code sample
• // Construct processes for gamma
• pmanager-gtAddDiscreteProcess(new
  G4GammaConversion())
• pmanager-gtAddDiscreteProcess(new
  G4ComptonScattering())
• pmanager-gtAddDiscreteProcess(new
  G4PhotoElectricEffect())

10
Electron processes

• // Construct processes for positron
• G4VProcess theMultipleScattering new
  G4MultipleScattering()
• G4VProcess eIonisation new G4eIonisation()
• G4VProcess eBremsstrahlung new
  G4eBremsstrahlung()
• // add processes
• pmanager-gtAddProcess(theMultipleScattering)
• pmanager-gtAddProcess(eIonisation)
• pmanager-gtAddProcess(eBremsstrahlung)
• // set ordering for AlongStepDoIt
• pmanager-gtSetProcessOrdering(theMultipleScattering
  , idxAlongStep, 1)
• pmanager-gtSetProcessOrdering(eIonisation,
  idxAlongStep, 2)
• // set ordering for PostStepDoIt
• pmanager-gtSetProcessOrdering(theMultipleScattering
  , idxPostStep, 1)
• pmanager-gtSetProcessOrdering(eIonisation,
  idxPostStep, 2)
• pmanager-gtSetProcessOrdering(eBremsstrahlung,
  idxPostStep, 3)

11
Positrons processes

• G4VProcess theeplusMultipleScattering new
  G4MultipleScattering()
• G4VProcess theeplusIonisation new
  G4eIonisation()
• G4VProcess theeplusBremsstrahlung new
  G4eBremsstrahlung()
• G4VProcess theeplusAnnihilation new
  G4eplusAnnihilation()
• pmanager-gtAddProcess(theeplusMultipleScattering)
• pmanager-gtAddProcess(theeplusIonisation)
• pmanager-gtAddProcess(theeplusBremsstrahlung)
• pmanager-gtAddProcess(theeplusAnnihilation)
• pmanager-gtSetProcessOrderingToFirst(theeplusAnnihi
  lation, idxAtRest)
• pmanager-gtSetProcessOrdering(theeplusMultipleScatt
  ering, idxAlongStep, 1)
• pmanager-gtSetProcessOrdering(theeplusIonisation,
  idxAlongStep, 2)
• pmanager-gtSetProcessOrdering(theeplusMultipleScatt
  ering, idxPostStep, 1)
• pmanager-gtSetProcessOrdering(theeplusIonisation,
  idxPostStep, 2)
• pmanager-gtSetProcessOrdering(theeplusBremsstrahlun
  g, idxPostStep, 3)
• pmanager-gtSetProcessOrdering(theeplusAnnihilation,
  idxPostStep, 4)

12
Hadrons EM processes

• Hadrons (pions, kaons, proton,)
• Light ions (deuteron, triton, alpha)
• Heavy ions (GenericIon)
• Example
• G4VProcess theMultipleScattering new
  G4MultipleScattering()
• G4VProcess hIonisation new G4hIonisation()
• pmanager-gtAddProcess(theMultipleScattering)
• pmanager-gtAddProcess(hIonisation)
• pmanager-gtSetProcessOrdering(theMultipleScattering
  , idxAlongStep, 1)
• pmanager-gtSetProcessOrdering(hIonisation,
  idxAlongStep, 2)
• pmanager-gtSetProcessOrdering(theMultipleScattering
  , idxPostStep, 1)
• pmanager-gtSetProcessOrdering(hIonisation,
  idxPostStep, 2)

13
PhysicsList

• In contrary to other codes Geant4 requires from
  users to build PhysicsList
• Novice examples
• N02 Simplified tracker geometry with uniform
  magnetic field
• N03 Simplified calorimeter geometry
• N04 Simplified collider detector with a readout
  geometry
• Extended examples
• Advance examples

14
Optical Photon Processes in GEANT4

• Concept of optical Photon in G4
• ? gtgt atomic spacing
• G4OpticalPhoton wave like nature of EM
  radiation
• G4OpticalPhoton ltgt G4Gamma
• (no smooth transition)
• When generated polarisation should be set
• aphoton-gtSetPolarization(ux,uy,uz) // unit
  vector!!!

15
Optical Photon Processes in GEANT4

• Optical photons generated by following processes
  (processes/electromagnetic/xrays)
• Scintillation
• Cherenkov
• Transition radiation
• Optical photons have following physics processes
  (processes/optical/)
• Refraction and Reflection at medium boundaries
• Bulk Absorption
• Rayleigh scattering
• ExampleN06 at /examples/novice/N06

16
Optical Photon Processes in GEANT4

• Material properties should be defined for
  G4Scintillation process, so only one type of
  scintillators can be defined in the run
• G4Cherenkov is active only if for the given
  material an index of refraction is provided
• For simulation of optical photons propogation
  G4OpticalSurface should be defined for given
  optical system

17
Conclusion remarks

• Using Geant4 examples novice user can design
  his/her PhysicsList without detailed studying of
  interaction of particles with matter
• Default values of internal model parameters are
  reasonably defined
• To estimate the accuracy of simulation results
  indeed one have to study Geant4 in more details
• It is true for any simulation software!