Geant4: Electromagnetic Processes 3 V.Ivanchenko, BINP & CERN Low energy package X-ray emission Optical processes V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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. V.Ivanchenko 01.10.03 EM physics

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. V.Ivanchenko 01.10.03 EM physics

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. V.Ivanchenko 01.10.03 EM physics

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 V.Ivanchenko 01.10.03 EM physics

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 01.10.03 EM physics

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 01.10.03 EM physics