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Geant4 Electromagnetic Processes Overview

Learn about energy loss, stopping powers, ranges, and multiple scattering in Geant4 simulation framework for high-energy physics applications. Includes ionization, fluctuations, and validation.

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Geant4 Electromagnetic Processes Overview

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  1. Geant4:Electromagnetic Processes 2V.Ivanchenko, BINP & CERN Geant4 cuts Stopping powers and ranges Hadron and Ion ionization Fluctuations Multiple scattering G4 validation 01.10.03 G4 Energy Losses

  2. Introduction remarks • 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 01.10.03 G4 Energy Losses

  3. Geant4 cuts • No tracking cuts, only production thresholds • Cuts can be established now only for gamma, electrons, and positrons • Thresholds for production of 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. • From G4 5.1 a possibility to set a cut by G4Region have been implemented 01.10.03 G4 Energy Losses

  4. G4 cuts • For a typical process G4Ionisation production threshold Tc subdivides continues and discrete part of energy loss: • Energy loss • -electron production • By default energy is deposited at the step • Energy loss can be used optionally for generation of -electrons under the threshold (subcutoff) and for fluorescence and Auger–electrons emission 01.10.03 G4 Energy Losses

  5. G3/G4 cuts in Pb/Ar calorimeter Protons 500 MeV G3 Cut 450 keV Ar Pb Ar G4 1.5 mm - 450 keV LAr 2 MeV Pb 01.10.03 G4 Energy Losses

  6. Remarks about G4 cuts • The use of production threshold is mandatory only for ionization and bremsstahlung processes • 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 01.10.03 G4 Energy Losses

  7. Energy loss and range tables • 2 processes contribute to e+ or e- continues energy loss: • Ionization • Bremsstrahlung • 3 process for ± energy loss: • Ionization • Bremsstrahlung • e+e- pair production • To achieve CPU performance dE/dx tables are calculated as a sum of all contributed losses • Range and inverse range tables are calculated from dE/dx tables 01.10.03 G4 Energy Losses

  8. Deuterons 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 • For heavy particles ionization is a function of only velocity, so scaling relation is applicable • At run time dE/dx, R(T), T(R) for hadrons and ions are calculated using scaling Tp=T·mp/m 01.10.03 G4 Energy Losses

  9. 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 01.10.03 G4 Energy Losses

  10. Straggling • At each step of charged particle after calculation of average energy deposition the sampling of energy is performed • Two fluctuation models are used • Bohr fluctuations for “thick” absorber regime • Urban fluctuations for “thin” absorber regime • Small cut – Bohr fluctuations 01.10.03 G4 Energy Losses

  11. Multiple Scattering • Long evolution from G4 3.2 to G4 5.2 • L.Urban is developing a combine model which applicable for case of big scattering angles • Both multiple Coulomb scattering and hard Rutherford scattering • Boundary algorithm for backscattering 01.10.03 G4 Energy Losses

  12. Multiple Scattering • Provides step limit near the boundary of the volume • Sample transverse displacement at end point of the step • Sample scattering angle 01.10.03 G4 Energy Losses

  13. Electron ionization in media Geant4/Sandia data • Backscattering and energy deposition profiles in semi-infinite media are directly connected • Correct energy profile and backscattering are achieved when boundary algorithm is applied 01.10.03 G4 Energy Losses

  14. G3/G4 data comparisons • From the beginning of G4 the comparisons G3/G4 have been doing permanently • Usually G4 is close to the data or is close to G3 predictions • Currently focus on comparison with the data 01.10.03 G4 Energy Losses

  15. GEANT4 GEANT4 GEANT3 GEANT3 data data 0.2 0.3 0.4 0.5 9 9.2 9.4 9.6 Geant4 electron response in ATLAS calorimetry EMB Electron Energy Resolution • Geant4 reproduces the average electron signal in all ATLAS calorimeters very well ( ±1%) • Signal fluctuations are very well simulated • stochastic term: G4 and data similar Tilecal 5 prototypes electrons at =20o 01.10.03 G4 Energy Losses

  16. 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 new model or alternative process • 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 01.10.03 G4 Energy Losses

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