KAHIBARO
12.4. Electron Physics
Table of Contents
Ionization
When an electron travels through matter it interacts mainly with the atomic electrons of the material. In GATE, this behaviour is described by ionization processes taken from Geant4’s electromagnetic physics models. For GATE users, the key role of electron ionization is that it controls how electrons slow down, how much energy they deposit locally, and how many secondary particles they create.
Ionization means that a moving electron transfers energy to a bound atomic electron, which can then be ejected from the atom. The incident electron loses part of its kinetic energy at each interaction. Many small ionization interactions along the track create a continuous loss of energy that is often summarized by the stopping power, usually written as $-\mathrm{d}E/\mathrm{d}x$. This quantity tells you how much energy an electron loses per unit path length in a given material.
In Geant4-based simulations, including GATE, ionization is modeled using a combination of continuous and discrete processes. Very small energy transfers are treated as a smooth, continuous slowing down. Larger transfers, where a secondary electron with significant energy is produced, are treated as explicit secondary particles. Which secondaries are actually created is governed by production cuts and transport parameters, which are described in the dedicated chapter on production cuts.
For medical physics, electron ionization is crucial in several applications. In dose calculations, almost all absorbed dose from electron and photon beams comes from electron ionization in tissue. In imaging detectors, such as scintillators or semiconductor detectors, ionization defines the initial energy deposition that eventually becomes a measurable signal. Because of this, the choice of electromagnetic physics list and production cuts directly impacts the accuracy of local energy deposition and thus the reliability of dose or detector-response simulations.
In practice, when you select a physics list in GATE that includes electromagnetic physics, an appropriate electron ionization model is already configured. For low-energy electrons in medical materials, these models include detailed cross sections that depend on energy and material composition. The continuous-energy-loss part controls the shape of electron tracks, while the discrete ionization events can produce extra electrons, called delta electrons, that may travel away from the primary track and broaden the region of energy deposition.
Ionization is also responsible for generating characteristic x rays and Auger electrons when inner-shell electrons are removed from atoms. Whether those secondaries are followed explicitly depends on your physics configuration and cut settings. For imaging or dosimetry at the millimeter scale, the default configurations in medical-physics oriented lists are usually sufficient. For microdosimetry or nanodosimetry, much smaller cuts and more detailed models are required, which increases computation time.
Ionization defines the stopping power $-\mathrm{d}E/\mathrm{d}x$ of electrons and is the dominant mechanism for electron energy deposition in tissue in medical physics simulations.
Bremsstrahlung
Bremsstrahlung is the radiation emitted when an electron is deflected by the electric field of an atomic nucleus or, to a lesser extent, bound electrons. In GATE, electron bremsstrahlung is handled by electromagnetic physics processes that simulate the production of bremsstrahlung photons with energies and directions sampled from appropriate distributions.
Physically, when an electron is accelerated or decelerated in the Coulomb field of a nucleus, it loses kinetic energy and emits a photon. The energy of this photon can range from very small values up to almost the full electron energy. The probability and spectrum of bremsstrahlung depend strongly on the atomic number $Z$ of the material and on the electron’s kinetic energy. In high-$Z$ materials and for high-energy electrons, bremsstrahlung becomes very important.
In medical applications, two regimes are especially relevant. In megavoltage photon radiotherapy beams, the primary photons are produced by bremsstrahlung of high energy electrons hitting a high-$Z$ target. In imaging detectors that use high-$Z$ crystals, such as LYSO or BGO, electron bremsstrahlung can create additional photons that may escape the detector or deposit energy in neighboring crystals, which influences the detector response and scatter.
From the simulation point of view, bremsstrahlung is a discrete process. When it occurs, an electron loses a portion of its energy, and a gamma photon is created with that energy. Whether this photon is followed as a secondary particle depends on your production cuts for gammas. If the gamma energy is below the cut, its energy will instead be added to the continuous local energy loss. If it is above the cut, GATE will track the photon and its subsequent interactions, such as Compton scattering or photoelectric absorption.
The electromagnetic physics lists available in GATE provide different bremsstrahlung models optimized for various energy ranges and materials. Standard models are suitable for radiation therapy energies up to several hundred MeV. Livermore or Penelope style models offer more detailed cross sections and spectra at lower energies, which can be relevant in precise detector simulations.
Bremsstrahlung has a direct impact on the spatial distribution of dose and on detector signals. In high-$Z$ materials, bremsstrahlung photons can travel farther than electrons, so the energy they carry may be deposited far from the original electron track. In radiotherapy beam modeling, properly including bremsstrahlung is necessary to reproduce the photon spectrum and dose distribution, especially in the build-up region and in the field penumbra.
To control the balance between accuracy and performance, you can tune production cuts so that only bremsstrahlung photons above a certain energy are simulated explicitly. Lowering the cut increases accuracy for low-energy photons but also increases the number of particles, so simulations become slower.
Bremsstrahlung production increases strongly with electron energy and atomic number $Z$. High-energy electrons in high-$Z$ materials create many bremsstrahlung photons, which must be modeled carefully in radiotherapy and detector simulations.
Multiple scattering
Multiple scattering describes the cumulative deflections that an electron experiences as it undergoes many small-angle elastic scatterings with atomic nuclei and electrons along its path. Instead of simulating every tiny angular deflection individually, GATE uses Geant4 multiple scattering models that approximate the net effect over a step.
For electrons, multiple scattering is particularly important because they are light and strongly deflected by Coulomb fields. As a result, electron tracks are not straight lines but curved and tortuous paths. This lateral spreading and angular dispersion influence how energy is distributed around the initial direction of the beam or particle track.
In practice, the multiple scattering process in Geant4 modifies the direction of the electron at each step, using probability distributions derived from theoretical and empirical models. The step length is limited by predefined criteria to ensure that the approximation remains valid. The result is a realistic description of the angular distribution and spatial spread of electrons after propagating through a given thickness of material.
In medical physics, multiple scattering affects both imaging and therapy simulations. In external beam radiotherapy, lateral broadening of electron and photon beams in tissue is largely controlled by electron multiple scattering, which influences field penumbra and dose gradients. In proton therapy, secondary electrons produced by proton interactions also undergo multiple scattering, affecting the microscopic dose distribution. In detectors, electron multiple scattering affects where ionization occurs and can spread energy deposition over several voxels or detector elements.
The choice of electromagnetic physics list in GATE determines which multiple scattering model is used and how aggressive the step limitation is. More detailed models and smaller step sizes can provide a more accurate description of lateral scattering, but at the cost of increased computation time. For most clinical dose and detector simulations, default configurations provide a reasonable compromise. For fine spatial resolution studies, such as microdosimetry or detailed small-field dosimetry, you may need to select more precise models and adjust transport parameters.
Multiple scattering also interacts with production cuts and local energy deposition. Because electrons follow longer and more tortuous paths when multiple scattering is modeled accurately, the effective path length inside a region may be longer than its geometric thickness. This affects the total energy deposited and the shape of dose profiles.
Finally, when interpreting simulation results, it is important to remember that multiple scattering inherently introduces spatial blurring of electron tracks. Some of this blurring is physical, representing the real behaviour of electrons, and some of it depends on how step limitations are configured. For consistent results and reproducible studies, keep your physics list and transport settings fixed across simulations that you intend to compare.
Electron multiple scattering controls lateral spreading and angular deflection of electron tracks. It plays a major role in shaping dose profiles and detector energy deposition patterns in medical physics simulations.
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