Validation of EM Part of Geant4 February 22, 2002 @ Geant4 Work Shop - PowerPoint PPT Presentation

Validation of EM Part of Geant4 February 22, 2002 @ Geant4 Work Shop Tsuneyoshi Kamae/Tsunefumi Mizuno 1

Purpose and Plan of this Talk We have validated EM processes in Geant4 important for gamma-ray satellite GLAST. Energy range of GLAST is 20 MeV - 300 GeV. So we need to validate physics processed down to ~10 MeV and lower. • Particle Ionization Energy Loss in Matters • Bethe-Bloch Formula (pp. 3-5) • Landau Distribution (pp. 3-5) • Range (p. 6) • Processed related to electromagnetic shower Tracker (Si, Pb) • Pair creation (pp. 8-9) • Bremsstrahlung (p. 10) • Moller scattering/BhaBha scattering (pp. 11-12) • Comparison of shower profile with experimental data (pp. 13-14) • Comparison of shower profile with EGS4 (pp. 15-17) Calorimeter (CsI) • We use Geant4 2.0 2

Ionization Energy loss in matters • Shoot proton/electron in Pb and Si. • Cutoff Length: 0.4mm(e-), 0.1mm(others) Thin absorber: Thick absorber: Landau distribution Bethe-Bloch formula Check mean energy loss (thick absorber), shape and 3 most probable energy loss (thin absorber).

Bethe-Bloch formula for protons Geant4 well reproduces ionization energy loss in thick Si and Pb of proton down to 10 MeV 4

Landau Distribution for protons and electrons protons electrons Fluctuation of energy loss and the most probable value in thin Si and Pb are appropriate. 5

Range of protons Shoot protons of 200 MeV and 1 GeV in CsI, Pb and W Compare the results with NIST data: (http://physics.nist.gov/Ph ysRefData/Star/Text/conte nts.html) Show good agreement within 0.6%. 6

Processes related to EM shower • Pair creation (pp. 8-9) (and compton scattering and photo-electric effect) • Bremsstrahlung (p. 10) • Moller scattering (pp. 11-12) • Bhabha scattering (pp. 11-12) We need to validate these processes, • Cutoff especially in low-energy, since 0.04mm(e-) • Pair creation and EM shower are key 0.01mm(others) processes to determine gamma-ray energy and direction in the GLAST. • Low energy electrons may suffer large-angle scattering and cause trigger in the GLAST. Tracker (Si-Strip detectors) Calorimeter (CsI scintillator) e- 7

Pair creation – cross section Compare the cross section in Pb calculated by Geant4 with that of a reference (http://physics.nist.gov/Phy sRefData/Xcom/Text) Geant4 correctly calculates cross section down to ~100 keV 8

e- 4 MeV Pair creation-angular distribution of emitted Events/Bin electrons • Shoot 20 MeV gammas into Pb absorber (10% RL) • Compare the angular distribution 0 0.125 0.25 0.375 0.5 of generated e- with that of Theta [rad] theoretical formula e- 18 MeV Events/Bin • We found discrepancies for low energy electrons and fixed the code 0 0.125 0.25 0.375 0.5 9 Theta [rad]

Bremsstrahlung -- angular distribution of generated photons • Shoot 20 MeV e- into Pb absorber (10% RL) • Compare the angular distribution gamma 2 MeV of generated photons with that of theoretical formula gamma 6 MeV Geant4 well reproduces the angular distribution gamma 10 MeV of photons generated via bremsstrahlung down to ~1 MeV 10

Moller/Bhabha scattering (scattering with e-/e+) • Shoot e- of 20 MeV and 100 MeV into Pb absorber (1% RL) • Compare the angular distribution of scattered electrons with that of theoretical formula 11

Moller scattering (with e-) Bhabha scattering (with e+) Theoretical furmula Geant4 20 MeV 20 MeV 100 MeV 100 MeV G4 well reproduces the theoretical formula down to 20 MeV 12

EM shower profile – Crannell experiment (1) 28 cm • Cranell experimtns: top view Vacuum 10 rings in each layer Carol Jo Crannell, et al., Phys. Rev. 184 (1969) 426 Ring 0 Ring 10 Ring ID width 0 ~ 3 1 cm 4, 5 2 cm • shoot 1GeV e- into water tank 6 ~ 10 4 cm (10 rings and 11 layers) side view Layer ID thickness 11 Water Layers electron 0 ~ 3 20 cm Material : Water 4 ~ 11 40 cm Layer 0 20 cm Layer 3 • Compare the G4 results 40 cm Layer 4 with that of experimental 400 cm data and EGS4 (Professor Nelthon and Liu). Layer 11 13

EM shower profile – Crannell experiment (2) longitudinal profile lateral profiles Shower profile of G4 is narrower than that of experiment and EGS4 14

EM shower in Pb layers (1) To investigate the EM shower profile in the GLAST Tracker, we constructed 10 Pb layers(0.1RL Geant4 each) with air (3cm). The radius is 0.2 Moliere radius (core of EGS4 shower) 20 MeV, longitudinal profile Pb lateral profiles air 15

EM shower in Pb and air (2) Geant4 Geant4 EGS4 EGS4 500 MeV, longitudinal profile 5 GeV, longitudinal profile lateral profiles lateral profiles • Energy deposition is higher in Geant4 (low energy region) • Geant4 shows narrower shower profile (and could not be 16 solved by fixing the angular distribution of pair creation)

Effect of cutoff energy (length) 0.4mm, 0.04mm (e-), 0.1mm, 0.01mm(others) • The discrepancy can Geant4 not be attributed to cutoff length (energy) EGS4 The reason of the narrower shower profile is unrevealed. 17

Summary • We have validated EM part of Geant4 (2.0). • Energy loss in matters is well simulated down to ~10 MeV. • Angular distribution of pair-created e- was found to be inappropriate and fixed. • Cross section of pair creation, angular distribution of bremsstrahlung/Moller scattering/Bhabha scattering were validated down to ~10 MeV. • Shower profile of G4 is slightly narrower than that of experimental data and EGS4. The reason of this is unknown. 18