1. Geant4: Electromagnetic Processes 3 V.Ivanchenko, BINP & CERN
Low energy package
X-ray emission
Optical processes
V.Ivanchenko
EM physics

2. Geant4 low energy EM physics
Proton stopping power
When energy transfer become close to energy of atomic electrons atomic shell structure should be taken into account
Problems with theory, so phenomenology and experimental data are used
V.Ivanchenko
EM physics

3. Geant4 low energy EM physics
Validity down to 250 eV
250 eV is a “suggested” lower limit
data libraries down to 10 eV
1 < Z < 100
Exploit evaluated data libraries (from LLNL):
EADL (Evaluated Atomic Data Library)
EEDL (Evaluated Electron Data Library)
EPDL97 (Evaluated Photon Data Library)
Photon transmission, 1mm Pb
shell effects
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EM physics

4. Geant4 low energy EM physics
Compton scattering
Polarised Compton
Rayleigh scattering
Photoelectric effect
Pair production
Bremsstrahlung
Electron ionisation
Hadron ionisation
Atomic relaxation
Set of Penelope models (new)
It is relatively new package
Development is driven by requirements which come from medicine and space research
There are also users in HEP instrumentation
V.Ivanchenko
EM physics

5. Geant4 low energy EM physics
To use G4 lowenergy package user has to substitute standard process in the PhysicsList by corresponding lowenergy:
G4hIonisation  G4hLowEnergyIonisation
G4eIonisation  G4LowEnergyIonisation ……
The environment variable G4LEDATA should be defined
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EM physics

6. Geant4 low energy EM physics
antiproton
proton
Energy loss in Silicon
Ionization is different for particles and antiparticles (Barkas effect)
Ionization at low energy depends on molecular shell structure
Chemical formula can be assign to the material – will be effective for heights of the Bragg peak of ionization
V.Ivanchenko
EM physics

7. Geant4 space applications
Cosmic rays,
jovian electrons
X-Ray Surveys of Solar System Bodies
Solar X-rays, e, p
Geant3.21
ITS3.0, EGS4
Courtesy SOHO EIT
Geant4
Induced X-ray line emission:
indicator of target composition
(~100 mm surface layer)
C, N, O line emissions included
V.Ivanchenko
EM physics

8. Geant4 low energy EM physics
Atomic relaxations are implement for ionization processes and photoelectric effect
Cross sections of shell ionization are used
Fluorescence and Auger electrons are produced G4
Marc rock
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EM physics

9. Radiation dose distribution
The simulation of radiation dose distribution is required for medical, space, and other applications
If hadronic processes are ignored in simulation, then the width of the Bragg peak of ionization cannot be reproduced by Geant4
Taking into account hadronic processes the experimental data can reasonably reproduced
12C +H2O
V.Ivanchenko
EM physics

10. Optical Photon Processes in GEANT4
Concept of “optical Photon” in G4
λ >> atomic spacing
G4OpticalPhoton: wave like nature of EM radiation
G4OpticalPhoton <=|=> G4Gamma
(no smooth transition)
When generated polarisation should be set:
aphoton->SetPolarization(ux,uy,uz); // unit vector!!!
V.Ivanchenko
EM physics

11. 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
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EM physics

12. Optical Photon Processes in GEANT4
Material properties should be defined for G4Scintillation process, so only inside the scintillator the process is active
G4Cerenkov is active only if for the given material an index of refraction is provided
For simulation of optical photons propagation G4OpticalSurface should be defined for a given optical system
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EM physics

13. Cherenkov Process
Cherenkov light occurs when a charged particle moves through a medium faster than the medium’s group velocity of light.
Photons are emitted on the surface of a cone, and as the particle slows down:
(a) the cone angle decreases
(b) the emitted photon frequency increases
(c) and their number decreases
Cherenkov photons have inherent polarization perpendicular to the cone’s surface.
When the particle has slowed below the local speed of light, the radiation ceases and the emission cone has collapsed to zero.
V.Ivanchenko
EM physics

14. G4Cerenkov: User Options
Suspend primary particle and track
Cherenkov photons first
Set the max number of Cherenkov
photons per step
in ExptPhysicsList:
#include “G4Cerenkov.hh”
G4Cerenkov* theCerenkovProcess = new G4Cerenkov(“Cerenkov”);
theCerenkovProcess -> SetTrackSecondariesFirst(true);
G4int MaxNumPhotons = 300;
theCerenkovProcess->SetMaxNumPhotonsPerStep(MaxNumPhotons);
The need to suspend the primary charged particle track arises in the production of Cerenkov photons because the number of such photons generated during the length of a usual step, as defined by other physics processes, is often very large. Hence, it is common practice to suspend the Cerenkov radiating particle and put it on its own stack of generated secondaries, so that its proteges are tracked in turn before it is revived and transported further.
The user may want to limit the step size by specifying a maximum average number of Cerenkov photons created during the step.
This is an example how you’d instantiate the G4Cerenkov process.
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EM physics

15. G4Scintillation
Number of photons generated proportional to the energy lost during the step
Emission spectrum sampled from empirical spectra
Isotropic emission
Uniform along the track segment
With random linear polarization
Emission time spectra with one exponential decay time constant.
A Poisson distributed number of photons is generated according to the energy lost during the step. The emission spectrum for the material has to be supplied by the user, again, by way of the G4MaterialPropertiesTable.
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EM physics

16. Rayleigh Scattering
The cross section is proportional to cos2(α), where α is the angle between the initial and final photon polarization.
The scattered photon direction is perpendicular to the new photon’s polarization in such a way that the final direction, initial and final polarization are all in one plane.
Rayleigh scattering attenuation coefficient is calculated for water but in all other cases it must be provided by the user:
The implementation of optical photon bulk absorption is trivial in that the process merely ‘kill’s the particle. The procedure requires the user to fill the relevant material property table with empirical data for the absorption length.
The differential cross section in Rayleigh scattering is proportional to the square of the cosine of the angle between the initial and final photon polarization. The Rayleigh scattering process samples this angle accordingly and then calculates the scattered photon’s new direction by requiring that it be perpendicular to the photon’s new polarization in such a way that the final direction, initial and final polarization are all in one plane.
The Rayleigh process includes a method, which calculates the attenuation coefficient of a medium following the Einstein-Smoluchowski formula.
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EM physics

17. Boundary processes Dielectric - Dielectric Dielectric - Metal
Depending on the photon’s wave length, angle of incidence, (linear) polarization, and refractive index on both sides of the boundary:
(a) total internal reflected
(b) Fresnel refracted
(c) Fresnel reflected
Dielectric - Metal
(a) absorbed (detected)
(b) reflected
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EM physics

18. Boundary Process A ‘discrete process’, called at the end of every step
never limits the step (done by the transportation)
sets the ‘Forced’ condition.
preStepPoint: is still in the old volume
postStepPoint: is already in the new volume
Conceptual class: G4LogicalSurface (in the geometry category) holds:
(i) pointers to the relevant physical or logical
volumes
(ii) pointer to a G4OpticalSurface
V.Ivanchenko
EM physics

19. Conclusion remarks
Geant4 standard package the optimal for most part of HEP applications
Geant4 lowenergy package provide a possibility to apply toolkit to variety of applications for which atomic shell structure is essential
Nuclear interactions of hadrons should be taken into account even at low energies for precise dosimetry
Optical photons generation and tracking can be simulated inside the same geometry
V.Ivanchenko
EM physics