KAHIBARO
12.1. What Is a Physics List?
Table of Contents
Particle interactions
In GATE, all radiation transport and interactions are provided by Geant4. A physics list is the way you tell Geant4, and therefore GATE, which types of particles and which interaction processes are active in your simulation. Without a physics list, particles would fly through your geometry without interacting, so no dose, no detector response, and no imaging signal would be produced.
At the most basic level, a physics list defines which particle types exist in your simulation and which interaction processes apply to them. For medical physics, the most important particles are photons, electrons, positrons, and various hadrons such as protons and neutrons. Each of these particles can undergo different kinds of interactions when they move through matter.
In GATE, you do not have to enable each interaction manually for every particle. Instead, you select or configure a physics list that internally registers the relevant processes. Conceptually, these processes fall into broad categories.
For gamma photons, the key electromagnetic interactions are the photoelectric effect, Compton scattering, and pair production. These are the interactions that produce energy deposition in detectors and tissue, and that shape the spectra and image quality in PET, SPECT, CT, and radiotherapy simulations. At low energies, the photoelectric effect dominates. At intermediate energies, Compton scattering is dominant. At high energies, pair production becomes important. The physics list decides which models are used to represent these interactions and for which energy ranges.
For electrons and positrons, important processes include ionization, bremsstrahlung, and multiple scattering. Ionization is primarily responsible for continuous energy loss and local dose deposition. Bremsstrahlung leads to the emission of secondary photons, which can then transport energy further away. Multiple scattering describes the many small angle deflections that gradually change the particle trajectory. Accurate modeling of these processes is critical for dose calculations, electron beam therapy, and the description of secondary electrons produced by photon interactions.
For hadrons such as protons and neutrons, there are both electromagnetic and hadronic processes. Protons lose energy mainly through ionization, but they can also undergo nuclear interactions that create secondary particles. Neutrons interact almost exclusively through nuclear processes such as elastic and inelastic scattering and capture. Hadronic processes are particularly important in proton therapy and in shielding calculations where neutron production and transport must be considered.
A physics list in GATE must describe which particles are present and which interaction processes are active for them. If a process is not included by the physics list, that physical effect will not appear in your simulation.
Because medical physics applications often involve several particle types in the same setup, typical GATE physics lists are combinations of electromagnetic and hadronic packages that together cover all relevant interactions. Later chapters on specific physics (gamma, electron, positron, hadronic) will detail the underlying processes. Here, it is enough to understand that a physics list is the master configuration that ties these processes to the particles in your simulation.
Transport processes
Once particles and their possible interactions are defined, Geant4 must move them through your geometry and decide when and where interactions occur. This movement is called particle transport. A physics list tells Geant4 not only which interactions are allowed, but also how to transport the particles between interactions.
Transport is handled by stepping the particle through the geometry in small increments called steps. At each step, Geant4 checks whether an interaction should happen, whether the particle crosses a material boundary, or whether some other condition like energy falling below a threshold is met. The physics list determines which transport-related processes are active and how they influence the step size and particle behavior.
For charged particles such as electrons, positrons, and protons, multiple scattering and continuous energy loss are modeled as transport processes. Multiple scattering changes the direction gradually as the particle moves, while continuous slowing down describes energy loss due to many small interactions that are not simulated one by one. In the background, Geant4 uses the physics list to decide the step length based on material, energy, and the desired accuracy.
Production cuts are closely tied to transport. A production cut is a threshold that determines whether a secondary particle is created explicitly or its effect is handled as part of the continuous energy loss. For example, a physics list may specify that electrons below a certain range are not produced as new tracks. Instead, their energy is deposited locally. This choice strongly affects the balance between simulation accuracy and speed.
Transport processes are also responsible for stopping particles when they are no longer relevant. When a particle leaves the world volume, reaches a user defined energy limit, or falls below the production cut range, the transport process will terminate its track. The physics list includes the configuration that defines these conditions, in particular through default range cuts that are applied globally or to specific regions.
From the perspective of a GATE user, transport processes are usually not configured individually, but rather through selecting an appropriate physics list and then optionally adjusting cuts and step limits as needed. For example, a physics list for proton therapy will use transport models that give an accurate Bragg peak position for protons, and you may tune production cuts in the patient or phantom region to get a good compromise between accuracy and computation time.
Transport in GATE is controlled by the physics list through:
- Which transport-related processes are active (for example multiple scattering).
- How step size and production cuts are defined, which determines where and when interactions are simulated explicitly.
If cuts are too loose, results can be inaccurate. If cuts are too strict, simulations can become very slow.
The detailed configuration of production cuts and transport parameters is covered in a dedicated chapter. Here you should retain that the physics list is the central place where these transport choices are made available and connected to your simulation.
Physics models
Every interaction or transport process in Geant4 must be described by a physics model. A model is a mathematical and numerical description of how a certain physical process occurs, based on cross sections, empirical fits, or theoretical calculations. The role of the physics list is to decide which models to use for each process, in which energy range, and in which materials.
For example, gamma Compton scattering can be described by different models that may be optimized for speed or for accuracy in a given energy range. Low energy electromagnetic physics lists often use detailed models that include atomic effects and are validated down to a few keV, which is important for diagnostic imaging and dosimetry in soft tissues. Higher energy lists may favor models that are efficient above a few MeV and are suitable for megavoltage beams in radiotherapy.
Electrons and positrons also have several possible models for ionization, bremsstrahlung, and multiple scattering. Some models handle very low energy electrons and fine details of scattering, which improves spatial accuracy of dose and energy deposition, but at the cost of more CPU time. Others use approximations that are faster and can still be adequate for certain applications where fine detail is less critical.
Hadronic physics is described by yet another set of models, which can be quite complex. For example, for protons there are models that predict nuclear interactions and the production of secondary particles over a wide energy range. For neutrons, specific models describe elastic scattering, inelastic scattering, and capture. A physics list for proton therapy will select hadronic models that are validated for the clinical energy range of interest and for common patient and beamline materials such as water, bone, and metals.
A complete physics list is therefore a collection of physics models assembled in a coherent way. In GATE, you usually choose from pre defined physics lists that ship with Geant4 or that GATE recommends. These combinations are named according to their intended use, for example lists optimized for general electromagnetic physics, for low energy electromagnetic interactions, or for hadron therapy. When you select such a list in GATE, you are effectively choosing a specific set of models rather than specifying each model directly.
A physics list is not a single model. It is a consistent combination of many physics models, one or more for each relevant process, chosen and validated for:
- A specific energy range.
- A specific type of application (for example imaging, dosimetry, hadron therapy).
Choosing an inappropriate physics list can lead to systematic errors in your results, even if the simulation runs without technical errors.
For a beginner, the main task is to understand that different applications need different physics lists. PET and SPECT focus on accurate modeling of gamma interactions and positron transport in the energy range of nuclear medicine radionuclides. CT simulations need good descriptions of x ray attenuation and scatter in the tens to hundreds of keV energy range. Radiotherapy and proton therapy need models that are reliable at megavoltage energies and for hadron beams. GATE simplifies your work by exposing recommended physics lists for these use cases, while still allowing advanced users to customize or extend the model combinations when needed.
Later chapters on electromagnetic, gamma, electron, positron, and hadronic physics, as well as on choosing physics models for specific applications, will help you select and tune a physics list that matches your simulation goals. At this stage, you should understand that the physics list is the central configuration object that links particle types, interaction processes, transport behavior, and the underlying physics models into one consistent description of the physics in your GATE simulation.
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