10.2. Particle Types

Table of Contents

Gamma

Gamma particles in GATE represent high energy photons with no electric charge and no rest mass. In simulations they are commonly used to model nuclear medicine photons from SPECT radionuclides or annihilation photons in PET. Since they are uncharged, gammas interact only through electromagnetic processes such as the photoelectric effect, Compton scattering, and pair production, and they travel in straight lines between interactions.

When you define a gamma source in GATE you select the particle type as a photon, typically using the name "gamma" or "photon" depending on the interface, and then specify its energy, position, and direction. Because gammas deposit energy only when they interact, not continuously along their path, you often need many events to obtain smooth energy spectra or dose distributions. Gammas are the natural choice for simulating detector response in PET and SPECT, imaging with gamma cameras, and photon beams in diagnostic x ray studies once the beam has been converted to individual photons.

In medical imaging simulations, use gamma photons whenever you want to represent nuclear medicine photons or x ray photons after they have been sampled from the source spectrum. Do not confuse gamma particles with generic optical photons, which are a different particle type with different physics.

Electron

Electrons are light, negatively charged particles that play a central role in energy deposition and dose. In GATE they are produced both as primary particles as in electron therapy beams and as secondary particles created when photons interact with matter. Because electrons are charged they lose energy continuously through ionization and excitation, and they undergo multiple scattering that bends their paths.

In most imaging simulations you do not need to create a primary electron source, but you must enable appropriate electron physics so that secondary electrons are modeled correctly. This is crucial for realistic energy deposition in detectors and in tissue. For radiotherapy or electron beam studies, electrons can be chosen as the primary particle type, with energy and beam shape set to match a clinical electron beam. You typically control how far low energy electrons are transported using production cuts and range cuts defined elsewhere in the simulation.

Electrons dominate local energy deposition in tissue. Even if your primary particles are photons or protons, ensure that electron transport is correctly configured in the physics list when you care about absorbed dose.

Positron

Positrons are the antiparticles of electrons. They have the same mass as electrons, but a positive electric charge. In GATE, positrons are central to PET simulations because many PET radionuclides emit positrons which then travel a short distance in tissue before annihilating with electrons. This annihilation produces two 511 keV gamma photons that are detected by the PET scanner.

When simulating PET you typically select a radioactive source that emits positrons, rather than creating a simple monoenergetic positron source. The transport of positrons before annihilation is handled by the physics list. Their finite range introduces a small blurring in PET images, especially for high energy positron emitters. In some test studies you might create a pure positron source, for example to investigate positron range, by explicitly choosing the positron particle type and setting its initial energy.

In PET simulations, the annihilation photons come from transported positrons. If you artificially generate 511 keV gammas directly without simulating positron transport you will underestimate physical effects such as positron range and non collinearity.

Proton

Protons are heavy, positively charged particles that are important in proton therapy simulations. As they traverse matter protons lose energy mainly through ionization, leading to a characteristic depth dose distribution with a pronounced Bragg peak near the end of their range. GATE uses specialized hadronic and electromagnetic physics to describe proton interactions with tissue and other materials.

When you choose protons as the primary particle type you configure the beam energy, lateral size, divergence, and direction to represent a treatment beam or an experimental beam. The proton range in a given material depends strongly on the initial energy, so small changes in energy can shift the Bragg peak position noticeably. Secondary particles, including secondary protons and neutrons, are also generated and may contribute to dose outside the target.

For proton therapy simulations, always select an appropriate proton physics configuration and verify that the Bragg peak position and shape agree with reference data for the chosen beam energy and materials.

Neutron

Neutrons are neutral particles found in many hadronic interactions and are especially relevant for shielding, accelerator studies, and secondary radiation in proton therapy. Since they carry no electric charge neutrons do not lose energy through ionization. Instead they interact through nuclear reactions, such as elastic scattering, inelastic scattering, and capture. These interactions can produce secondary charged particles that deposit energy.

In GATE you seldom use neutrons as a primary source in medical imaging, but they appear naturally as secondaries when simulating high energy photon beams, proton therapy, or accelerator components. When neutrons are important to your study you must ensure that suitable hadronic physics is enabled, and that you have enough events to capture their often rare interactions. If you explicitly define a neutron source, you choose the neutron particle type and then assign an energy distribution that matches the neutron field you want to study.

Because neutron interactions are stochastic and sometimes rare, simulations that include neutron effects often require many events and suitable hadronic physics lists. Neglecting neutron physics can lead to underestimation of out of field dose and shielding requirements.

Ions

Ions in GATE represent charged nuclei heavier than protons, such as alpha particles and therapeutic ion beams like carbon ions. They carry positive charge and large mass, which leads to very dense energy deposition along their track, particularly near the end of their range. This makes heavy ions useful in therapy, but it also requires specialized physics models for accurate simulation.

To define ion sources, GATE relies on Geant4 ion definitions that specify the atomic number $Z$ and mass number $A$. For example, you can create alpha particles, which are helium nuclei, or carbon ions with specific charge states. In radiobiology and ion therapy studies you choose the appropriate ion type, set the beam energy per nucleon, and configure the beam geometry just as you would for protons. The resulting depth dose curves show a sharp Bragg peak similar to protons, but with different linear energy transfer characteristics.

When simulating ions, always use an ion capable physics configuration and specify the ion species correctly by its atomic and mass numbers. Misconfiguring the ion type or energy can completely change the depth dose curve and invalidate therapy related results.