KAHIBARO
12.6. Hadronic Physics
Table of Contents
Proton interactions
In GATE, hadronic physics is responsible for the way protons and other hadrons interact with matter. Proton interactions are especially important in proton therapy simulations, but they also matter whenever high energy protons are present, for example in secondary radiation from accelerators or cosmic rays.
When a proton travels through matter, it loses energy mainly through two broad mechanisms. The first mechanism is electromagnetic interactions with atomic electrons. These are already covered by electromagnetic physics lists and are responsible for the continuous slowing down of the proton along its path. The second mechanism involves nuclear interactions with the nuclei of the material and belongs to hadronic physics. Here you configure which nuclear models are used and how secondary particles are produced.
At clinical energies used in proton therapy, typically from about 60 MeV to 250 MeV, protons undergo a mixture of elastic and inelastic nuclear scattering. In elastic scattering, the proton interacts with a nucleus and both particles change direction but not their internal state, so the nucleus remains in its ground state and no nuclear fragments are produced. Elastic processes are important for describing angular spreads and the lateral penumbra of proton beams.
In inelastic interactions, the proton excites or breaks up the target nucleus, which then emits secondary particles. These secondaries can include neutrons, protons, deuterons, alpha particles, and heavier fragments, as well as gamma rays from nuclear de‑excitation. Inelastic processes change both the energy spectrum and the composition of the particle field, and they contribute to dose outside the primary field and to activation.
GATE uses Geant4 hadronic models that are valid over specific energy ranges. For protons, these models are often combined into physics lists such as QGSP_BIC or QGSP_BERT, or medical variants like QGSP_BIC_EMY, which are commonly recommended for proton therapy simulations. The low and intermediate energy regime for protons in tissue and water is typically modeled with intranuclear cascade or binary cascade approaches that reproduce experimental depth‑dose curves and secondary neutron production reasonably well. At higher energies, additional high energy models are activated, but this is usually outside the typical medical proton therapy range.
An important practical effect of proton hadronic physics is the shape and position of the Bragg peak. While the peak position is mainly determined by electromagnetic stopping power, nuclear interactions influence the tail beyond the peak, the production of a dose halo around the beam, and the out‑of‑field dose. If nuclear interactions are not modeled correctly, simulations can underestimate the dose delivered to healthy tissue around the target or misrepresent secondary neutron fields.
In a GATE hadronic configuration, you mainly control which hadronic physics list you use rather than selecting individual proton processes one by one. For medically oriented simulations, you will usually choose a preconfigured Geant4 list that already contains a consistent set of proton nuclear models, elastic and inelastic, transport parameters, and cross section data. You should then validate your proton depth‑dose and lateral profiles against measurements or reference data, since the chosen hadronic models directly affect these results.
For accurate proton therapy simulations, always use a hadronic physics list that includes low and intermediate energy proton nuclear interactions, and verify the simulated Bragg peak depth, width, and distal tail against experimental or reference data.
Neutron interactions
Neutrons play a special role in hadronic physics because they have no electric charge. They do not lose energy by ionization, so their interactions are almost purely nuclear. In GATE, neutron physics is automatically handled through Geant4 hadronic models once you select an appropriate physics list. Neutrons are produced in many simulations as secondaries, for example from proton or heavy ion interactions in proton therapy beams, from high energy photons in radiotherapy, or from activation processes.
Neutron interactions in matter depend strongly on neutron energy. At high and intermediate energies, neutrons undergo elastic and inelastic scattering on nuclei, similar in concept to proton interactions, but with different cross sections and kinematics. Elastic scattering can change the neutron energy and direction while leaving the nucleus in its ground state, whereas inelastic scattering excites the nucleus and may produce gamma rays or eject nucleons.
As neutrons slow down, they enter the epithermal and then the thermal energy ranges. In these regimes, nuclear cross sections can be very large and highly dependent on the target material. Thermal neutrons are especially prone to capture. In neutron capture, the neutron is absorbed by a nucleus, and the compound nucleus subsequently de‑excites by emitting one or more gamma rays. These capture gammas can carry significant energy and contribute to dose and activation.
In GATE hadronic lists, neutron interactions are modeled with energy‑dependent libraries and models. At low energies, high precision neutron data are often used, sometimes denoted in Geant4 physics lists as HP models. These rely on evaluated nuclear data libraries and are important when accurate low energy neutron transport and capture are needed, for example in detailed shielding studies or in proton therapy out‑of‑field dose calculations.
For medical applications, correct modeling of neutron production and transport is important even if you do not directly score neutron dose. Secondary neutrons generated in treatment room components and patient tissues can deliver dose to distant organs, affect shielding design, and determine staff exposure. In imaging contexts, neutrons are usually less prominent, but they can still appear in high energy accelerator facilities.
From a configuration perspective, you do not specify individual neutron processes in GATE. Instead, you choose a physics list that includes the neutron models required for your energy range and level of precision. For realistic out‑of‑field or shielding simulations, you may need a physics list with high precision neutron models and suitable production cuts that allow low energy neutrons and capture gammas to be transported and recorded.
Whenever secondary neutrons are relevant, use a physics list that includes high precision neutron models for low energies, and set production cuts and scoring so that neutron‑induced gamma radiation and dose are not artificially suppressed.
Nuclear interactions
Nuclear interactions are the core of hadronic physics. They describe how hadrons, such as protons and neutrons, interact with atomic nuclei, and how nuclei themselves can transform, emit particles, or become radioactive. In GATE, nuclear interactions are handled by the Geant4 hadronic framework using a combination of theoretical and data‑driven models.
Several types of nuclear interactions are important in medical physics simulations. Elastic scattering processes change particle directions and contribute to beam broadening and angular distributions. Inelastic interactions deposit energy inside nuclei, break them apart, or eject nucleons and light ions. These processes produce secondary particles that can travel far from the original interaction point and influence dose distributions and detector signals.
Nuclear fragmentation is especially important at higher energies and for heavy ions but also appears in proton therapy. In fragmentation, the projectile and sometimes the target break up into lighter fragments. These fragments can include protons, neutrons, deuterons, tritons, alpha particles, and heavier clusters. The fragments have their own transport and interactions and can deposit dose outside the primary beam path, which contributes to the dose halo and can affect organs at risk.
Nuclear de‑excitation processes produce gamma rays as excited nuclei return to their ground states. These nuclear gamma rays can appear as lines in energy spectra and can be detected by imaging systems or dosimetry detectors. In some simulations, such as prompt‑gamma imaging for proton therapy range verification, these nuclear gamma rays are the main signal of interest. Modeling them correctly requires consistent nuclear interaction models and de‑excitation schemes in the chosen physics list.
Nuclear capture and activation processes change the identity of nuclei and can create radioactive isotopes. These isotopes later undergo radioactive decay, emitting radiation over time. In many GATE simulations, long term activation is not followed explicitly, but for specific applications, such as room activation or isotopes created in tissues, this may need to be considered. Nuclear capture is also the basis of some therapeutic techniques, such as neutron capture therapy, where capture gamma rays and charged particles deliver local dose.
In practice, when you configure hadronic physics in GATE you select a physics list that combines nuclear elastic scattering, inelastic scattering, cascade models, precompound models, de‑excitation, and low energy data where needed. Each physics list is a particular combination and ordering of these components. Your choice should be guided by the type of particles, energy range, and required accuracy of nuclear effects in your simulation.
You should always be aware that nuclear interactions influence not only the local energy deposition but also the spectrum and spatial distribution of secondary particles. For example, in a PET or SPECT scanner, nuclear interactions in shielding and detector materials can create background events and extra gamma lines that appear in measured spectra. In radiotherapy, nuclear interactions in patient tissues and hardware change the dose distribution and generate out‑of‑field dose components.
Nuclear interactions control the production of secondary particles, nuclear gamma rays, and activation products. Choosing an inappropriate hadronic physics list can lead to incorrect dose tails, secondary particle fields, and background spectra, so always match the nuclear models to your particle types, energy range, and validation data.
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