1. Geant4: Electromagnetic Processes 2
Geant4 cuts
Stopping powers and ranges
Fluctuations
Multiple scattering
V.Ivanchenko
Salamanca

2. Electromagnetic processes
Energy spectrum of -electrons  1/T2
Energy spectrum of Bremsstahlung  1/
Huge number of low energy e- and gammas cannot be tracked by any Monte Carlo
Cuts should be used
V.Ivanchenko
Salamanca

3. Geant4 cuts No tracking cuts, only production thresholds
Cuts can be established to any particle but mandatory cuts on gamma and electrons
Thresholds for production secondaries are expressed in range, universal for all media,
Range 10 keV gamma in Si  2 cm
Range of 10 keV electron in Si  2 microns
Energy thresholds for a material is calculated from this cut in range.
In 2002 a possibility to set a cut by logical volume will be implemented
V.Ivanchenko
Salamanca

4. G4 cuts
Production cuts for a typical G4Ionisation Tc subdivides continues and discrete part:
Energy loss
-electron production
Energy loss can be used for generation of -electrons under the threshold (subcutoff) and for fluorescence and Auger–electrons emission
V.Ivanchenko
Salamanca

5. G3/G4 cuts in Pb/Ar calorimeter
Protons 500 MeV G3
Cut 450 keV Ar Pb Ar G4
1.5 mm keV LAr
2 MeV Pb
V.Ivanchenko
Salamanca

6. Geant4 validation
Everyday task for Geant4 team is the validation of the toolkit
Comparison with data
G3/G4 comparisons
Comparison experiments (BaBar, SLAC, Stanford; ATLAS, CERN, Geneve; others)
Testing procedures
V.Ivanchenko
Salamanca

7. G3/G4/Data comparison
V.Ivanchenko
Salamanca

8. Remarks about G4 cuts
The use of production threshold is mandatory only for ionisation and bremsstahlung process
Other processes can use or ignore G4 cuts
Alternative mechanism is UserLimits, which can be defined in a given G4LogicalVolume:
Maximum step size;
Maximum track length;
Maximum track time;
Minimun kinetic energy;
Minimum range;
V.Ivanchenko
Salamanca

9. Stopping powers and ranges
At initialization stage dE/dx, R(E), and E(R) tables are calculated for all materials and basic particles e-, e+, -, +, p, pbar
At run time dE/dx, R(T), T(R) for hadrons and ions are calculated using scaling Tp=T·mp/m
Deuterons
V.Ivanchenko
Salamanca

10. Stopping powers and ranges
Scaling is applicable to all types of ions
Initialization should be redone if run conditions are changed:
New basic particle
New material
New cut
V.Ivanchenko
Salamanca

11. Straggling
At each step of charged particle after calculation of average energy deposition the sampling of energy is performed
Three fluctuation models are used
Bohr fluctuations
Vavilov fluctuations
Landau fluctuations
V.Ivanchenko
Salamanca

12. Multiple Scattering Provides step limit
Sample transverse displacement at end point of the step
Sample scattering angle
Model is used in which scattering angles are not small
V.Ivanchenko
Salamanca

13. Conclusion remarks
Geant4 electromagnetic model based on new conception: universal cut in range
Geant4 is not a frozen program – it is a free toolkit, which allows to implement any variant of cuts and thresholds
Geant4 electromagnetic physics is well tested and demonstrates a good quality for HEP applications
In some cases Geant4 provide physics which is absent in Geant3, in others – more precise models
V.Ivanchenko
Salamanca