KAHIBARO
12.6. Hadronic Physics
Table of Contents
Proton interactions
In Geant4, hadronic physics describes how strongly interacting particles such as protons, neutrons, pions, and nuclei interact with matter. For protons, this includes both their electromagnetic energy loss and their specific nuclear reactions with target nuclei. The details of which models are used, and in which energy ranges, are controlled by the chosen reference physics list, but the main ideas are the same across lists.
When a proton travels through a material, it first loses energy continuously through electromagnetic interactions with electrons. This is handled by electromagnetic processes and is not unique to hadronic physics. Hadronic physics becomes important once you care about nuclear reactions: inelastic scattering that creates secondary particles, elastic scattering that changes direction and energy, and nuclear capture or fragmentation in special cases.
Geant4 handles these proton nuclear interactions by assigning a set of hadronic processes to the proton particle definition. At a basic level, the important Geant4 hadronic processes for protons are:
Elastic scattering, where the proton scatters from a nucleus without changing the internal state of the nucleus. The target nucleus may recoil, and the proton can significantly change direction, especially at low energies. This affects beam broadening and spatial dose distributions.
Inelastic scattering, where the proton transfers enough energy to the nucleus to excite or break it apart. This often produces secondary particles such as neutrons, gamma rays, pions, light ions, and nuclear fragments. Such secondaries can contribute to dose away from the primary beam, activation of materials, and background in detectors.
At low energies, nonelastic reactions may lead to compound nucleus formation, evaporation of neutrons and light ions, and emission of gamma rays as the residual nucleus de-excites. At higher energies, intranuclear cascade models are used, where the incoming proton initiates a cascade of nucleon nucleon collisions inside the nucleus, followed by precompound and evaporation stages.
For typical reference physics lists such as FTFP_BERT or QGSP_BERT, Geant4 automatically chooses suitable models for different proton energy ranges, for example, Bertini or Binary cascade at intermediate energies and string models at higher energies. As a beginner, you usually do not select these models manually. Instead, you choose an appropriate reference list and rely on it to provide realistic proton interactions in the relevant energy domain of your application.
In detector simulations, proton hadronic interactions are important in several contexts. In hadron therapy, they determine nuclear fragmentation and the tail of the depth dose curve beyond the Bragg peak. In shielding studies, they define the production of secondary neutrons and other particles that might penetrate or activate the shield. In space applications, they help model how cosmic ray protons interact with spacecraft materials and detectors.
When you analyze results, look for the contribution of secondary particles produced by proton inelastic interactions. For example, neutrons produced in a proton beamline may deposit little energy where they are generated, but can travel farther and interact in sensitive volumes. You can inspect these effects using tracking information, parent IDs, and energy deposition in your sensitive detectors.
In Geant4, proton hadronic interactions are provided by the physics list, not by user geometry or source code. For realistic results, always choose a physics list that is appropriate for your proton energy range and application, and do not try to emulate proton nuclear reactions with only electromagnetic processes.
Neutron interactions
Neutrons have no electric charge, so they do not lose energy through ionization like charged particles. Their entire transport in Geant4 is governed by hadronic interactions with nuclei. This makes the hadronic physics models for neutrons especially important. The type of neutron interaction, and the models used, depend strongly on energy, from fast neutrons at MeV and above, down to thermal and even subthermal energies.
For fast and intermediate energy neutrons, Geant4 uses intranuclear models that describe elastic scattering, inelastic scattering, and spallation. Elastic scattering changes the neutron direction and energy by transferring kinetic energy to the target nucleus. This is a key mechanism for moderating neutrons, for example in hydrogenous materials where the target mass is similar to the neutron mass. Inelastic scattering excites the target nucleus, and the nucleus de-excites by emitting gamma rays or nucleons. These secondaries can contribute to dose and detector signals.
At lower energies the description changes. In the so called high precision or HP region, down to thermal energies around $0.025\ \text{eV}$, Geant4 uses evaluated nuclear data files, such as ENDF based libraries, to provide detailed cross sections and final state distributions. These are enabled by special neutron HP models that you activate through specific physics lists or options. If you require accurate thermal neutron behavior, for example in reactor physics, shielding design, or neutron detector studies, you should select a physics list variant that includes these high precision neutron data.
Neutrons can also be absorbed by nuclei. At certain energies this may happen through radiative capture, where a gamma ray is emitted, or through other reaction channels that produce charged particles or light ions. In neutron detectors, these capture reactions are often what you measure. For example, $^{10}\text{B}(n,\alpha)$ or $^{3}\text{He}(n,p)$ capture reactions convert a neutral neutron into detectable charged particles. Geant4 simulates these processes as hadronic inelastic or capture interactions, and their probability is again determined by the physics list and associated data libraries.
In many applications, secondary neutrons are generated by other particles. For example, proton or heavy ion interactions in a target or patient can produce abundant neutrons. The subsequent neutron transport and capture can contribute substantially to dose away from the primary beam. In shielding problems, you may be interested primarily in how neutrons propagate through thick walls and how many reach a protected area. In both cases, the accuracy of neutron interaction modeling is central to credible results.
When you set up a simulation that involves neutrons, pay particular attention to the choice of physics list and any modifiers that enable or disable high precision neutron options. Some reference lists come in variants that include thermal neutron data. You should also ensure that the necessary neutron data libraries are properly installed with Geant4, because the high precision models rely on them.
Finally, neutrons often require longer simulation times because they may scatter many times before being absorbed or leaving the geometry. Production cuts, region definitions, and scoring strategies can help control performance without sacrificing the neutron physics that is important for your study.
For neutron simulations in Geant4, the correct physics list and data libraries are critical. For problems involving thermal or epithermal neutrons, always use a physics list that includes high precision neutron models and verify that the corresponding neutron data libraries are installed and configured.
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