EM Physics I. Geant4 Tutorial, Marshall Space Flight Center April 2012

Transcription

1 EM Physics I Geant4 Tutorial, Marshall Space Flight Center April 2012 Daniel Brandt (based on slides by T. Koi and D. Wright) based on Geant4 v9.5-p01

2 Outline EM physics assumptions and processes Physics list and EM process ordering Energy loss processes Process implementation Multiple scattering Ionization Production thresholds Summary 2

3 Basic Assumptions Projectile energy 1 kev Atomic electrons are quasi-free, their binding energy is negligible (excpet in the photoelectric effect) The atomic nucleus is fixed, its recoil momentum is negligible Matter is homogenous, isotropic and amorphous 3

4 List of processes - I 1. Common to all charged particles Ionization (> ~kev) Coulomb scattering from nuclei (> ~kev) Cherenkov emission Scintillation Transition radiation 2. Muons (e+, e-) pair production (>100 GeV) Bremsstrahlung (>100 GeV) Nuclear interaction (> 1 TeV) 4

5 List of processes - II 3. Electrons and Positrons Bremsstrahlung (> 10 MeV) e+ annihilation 4. Photons Gamma conversion (> 10 MeV) Coherent (< 100 kev) Incoherent scattering (100 kev 10 MeV) Photoelectric effect (< 100 kev) 5. Optical Photons Reflection and refraction Absorbtion Rayleigh scattering 5

6 The EM Physics List Each type of particle has its own list of processes There are three ordered lists per particle: AtRest action AlongStep action PostStep action These lists of processes are registered in an implementation of the G4VUserPhysicsList class Unless explicitly specified, processes occur in the order in which they were registered 6

7 The EM Physics List - II When building EM Physics lists from scratch, many processes have to be considered Different EM physics lists are provided by Geant4 MyPhysicsList::MyPhysicsList():G4VModularPhysicsList() {... G4int ver = 1; SetVerboseLevel(ver); RegisterPhysics( new G4EmStandardPhysics(ver) );... } The pre-made physics list builders can be found at $G4INSTALL/source/physics_list/builders 7

8 Energy loss processes - I Charged particle range inside material: Unfortunately dε/dx is a strong function of ε. In the case of muons, the following processes contribute to continuous energy loss: Ionisation Pair production Bremsstrahlung Nuclear scattering 8

9 Energy loss processes - II The maximum step length is limited and following each step the particle energy is reduced by the energy loss processes Since dε/dx varies strongly with ε, can t use Instead, use tabulated values 9

10 Energy loss processes - III Need to implement all relevant loss processes to get correct range For muons: Ionisation e+/e- pair production Bremsstrahlung Nuclear scattering These processes must occur in the correct order! 10

11 Building EM Physics Lists It is possible to specify process priority. The priority determines the order in which DoIt methods are called. OrdAtRestDoIt OrdPostStepDoIt if(particlename== e+ ){ pmanager->addprocess(new G4MultipleScattering,-1, 1,1); pmanager->addprocess(new G4eIonisation, -1, 2,2); pmanager->addprocess(new G4eBremsstrahlung, -1,-1,3); pmanager->addprocess(new G4eplusAnnihilation, 0,-1,4); } Each DoIt (AtRest, AlongStep, PostStep) has its own priority A priority of -1 indicates DoIt = inactive. OrdAlongStepDoIt 11

12 Building EM Physics Lists - II If priority is not specified explicitly, processes will occur in the order in which they are added to the physics list: if(particlename== gamma ){ pmanager->adddiscreteprocess(new G4PhotoElectricEffect); pmanager->adddiscreteprocess(new G4ComptonScattering); pmanager->adddiscreteprocess(new G4gammaConversion); } In newer versions of Geant4, process ordering is handled by the physics list helper. 12

13 Multiple Scattering Charged particle scattering by the potentials of nuclei Coulomb Scattering or Rutherford Scattering. Geant4 estimates cumulative effect of many scatters. 13

14 Multiple Scattering Figure shows histogram of angular distribution of electrons passing through foil Geant4 simulation (line) is in good agreement with real data (circles, squares) 14

15 Ionization Process An energetic charged particle can transfer energy to the atomic electron system and liberate an electron µ + atom µ + atom + + e - The energy transfer from a single ionization event is small. The majority of ionization events are just dealt with as average energy loss de/dx (slides 7 9). Occasionally enough energy is transferred to warrant tracking the ejected electron explicitly 15

16 Production thresholds - I Below the threshold energy T cut energy loss is continuous where E=energy of impactor, T=energy of ejected electron Cross section for δ-ray production above T cut : 16

17 Production thresholds - II Production thresholds are set in terms of the minimum range below which a particle will not be tracked Thresholds are set in the physics list SetCuts() method MyPhysicsList::SetCuts(){ SetCutValue(1.0*mm, gamma ); SetCutValue(0.8*mm, e- ); SetCutValue(0.8*mm, e+ ); } If no threshold cut is set, the default cut is 1 mm. 17

18 Processes with different models For some processes different models are available If a model different from the default is required, this can be set in the physics list implementation if (particlename== gamma ){ } G4PhotoElectricEffect* pef = new G4PhotoElectricEffect(); G4PenelopePhotoElectricModel* amodel; amodel = new G4PenelopePhotoElectricModel(); amodel->sethighenergylimit(limit); pef->addemmodel(0, amodel); pmanager->adddiscreteprocess(pef); 18

19 Validating EM Physics 19

20 The EM Physics calculator Geant4 calculates many physically relevant quantities like scattering cross sections Those quantities can be retrieved using the EMCalculator class #include G4EmCalculator.hh... G4EmCalculator emcalc; G4double density = material->getdensity(); G4double masssigma = emcalc.computecrosssectionpervolume (energy, particle, procname, material)/density; G4cout<< G4BestUnit(massSigma, Surface/mass ) << G4endl; 20

21 Summary A wide variety of physics processes is provided and can be registered using pre-made physics list builders Production thresholds prevent infrared divergences. They are set using the G4VUserPhysicsList::SetCuts() method Geant4 EM physics is well validated. Online validation plots are available at www-zeuthen.desy.de/geant4/web/ Physics quantities can be extracted using the G4EmCalculator class 21