45.5 Configuring Electromagnetic Physics

Table of Contents

Understanding Electromagnetic Physics in the Detector Example

In the gamma ray detector example, you use electromagnetic physics to describe how photons and the charged particles they produce interact with your scintillator and surrounding materials. At this stage, you already have a world, a detector volume, and a gamma source. Configuring electromagnetic physics tells GATE which physical processes are active and which models to use for them.

In all that follows, we focus only on what is needed for this simple scintillation detector. More general information about physics lists and processes is handled in the dedicated physics chapters of the course.

What You Need from Electromagnetic Physics

In a basic gamma ray detector example, you care mainly about how incoming gamma photons transfer energy to the scintillator, because that deposited energy will later be turned into an energy spectrum.

For gamma detection with a scintillator, the following processes are essential:

Photoelectric effect
This is the process responsible for full energy absorption. A gamma photon is absorbed and its full energy is transferred to an atomic electron, which then deposits its energy locally. These events build the photopeak in your spectrum.
Compton scattering
A gamma photon scatters on an electron, losing only part of its energy and changing direction. The scattered photon can either escape the detector or interact again. Compton interactions create the continuum part of the spectrum.
Pair production
At sufficiently high energies (above 1.022 MeV in matter), a gamma photon can create an electron-positron pair in the field of a nucleus. In a simple scintillator tutorial you may not use such high energies, but the process can still be part of the default models.
Electron and positron transport
Once gamma interactions produce electrons or positrons, these charged particles slow down and deposit their energy in the detector. Even though you may think in terms of photon interactions, the actual energy deposition in your scintillator is largely due to these charged secondary particles.

A standard electromagnetic physics configuration in GATE already includes these processes, so your main task in the example is to select an appropriate physics list and optionally adjust its options.

In a gamma ray detector simulation, never omit electromagnetic physics for photons and electrons. Without it, your detector will see almost no energy deposition and your spectrum will be empty or unphysical.

Selecting an Electromagnetic Physics List for the Example

GATE uses Geant4 physics lists. For medical imaging and dosimetry, Geant4 provides electromagnetic physics options optimized for different energy ranges and levels of precision. In a simple gamma ray detector example with MeV-scale gamma rays, a common choice is the so‑called "standard" electromagnetic physics with some options, or one of the "option4" or "emstandard" variants that balance precision and speed.

In OpenGATE, you typically do not have to add individual processes manually. Instead, you select a predefined physics list in your Python script, for example:

python

sim.physics_manager.physics_list_name = "G4EmStandardPhysics_option4"

or a similar string, depending on the OpenGATE version and available lists.

The important points for the practical detector example are:

You must choose a list that includes electromagnetic physics for photons and electrons over the energy range of interest.
You should not try to mix many unrelated hadronic physics packages unless you actually need them. For a pure gamma spectrometry example, hadronic physics is not necessary.

For a simple gamma detector example with MeV photons, use a pure electromagnetic physics list. Do not enable hadronic physics unless your simulation actually involves hadrons, otherwise you will increase computation time without benefit.

Matching Physics to Gamma Detector Energies

Your gamma ray detector example will usually involve photons from a radionuclide such as Cs‑137 or Co‑60, or a monoenergetic beam. Typical energies are in the range of a few hundred keV up to a couple of MeV. Modern Geant4 electromagnetic physics lists are designed for this range and below, and many of them are tuned for medical applications.

In practice, for this example, you want to ensure the following:

Photon interactions are accurately modelled in high‑Z detector materials, such as NaI or LYSO. This affects the photopeak position and shape.
Electron transport parameters are suitable for the scale of your detector crystal. If electrons are tracked with a reasonable step size and production cut, the energy deposition will be spatially accurate enough to represent the interaction.

Some physics lists include more detailed low energy models, especially useful for energies down to a few keV. If your detector tutorial focuses on MeV gammas, using a high precision low energy package is optional, but it can improve the accuracy of the Compton continuum and edge features at low energy.

Production Cuts and Their Role in Detector Physics

The physics list defines which processes exist, but production cuts control which secondary particles are actually created and tracked. Even though a later chapter covers production cuts in detail, in the context of the gamma ray detector example it is important to understand their role in energy deposition in the scintillator.

A range cut converts a geometric range into an energy threshold. Electrons or gammas with expected ranges smaller than this threshold are not created as explicit secondary tracks. Instead, their effect is accounted for locally. This is very important for performance, but if the cut is too large compared to your detector dimensions, you may lose spatial precision or even under‑estimate energy escape from thin regions.

For a typical block scintillator that is several millimeters or centimeters in size, using default electromagnetic cuts is usually sufficient. If you choose very thin detectors or want to study fine spatial structure at the submillimeter scale, you may need to consider smaller cuts.

In the context of this tutorial, emphasize:

You do not need to tune cuts aggressively. Reasonable defaults should correctly reproduce the total energy deposition in the scintillator.
If in doubt, slightly reduce the default cut values in the detector region to ensure that low energy electrons are still produced and tracked until they deposit their energy.

In a gamma detector, do not set production cuts larger than the typical crystal thickness. Excessively large cuts can make low energy secondaries disappear too early and distort the low energy part of your spectrum.

Verifying That Electromagnetic Physics Works in the Example

Once you configure the electromagnetic physics list in your script, the most direct way to confirm that it behaves correctly is to look at very simple observables in the example:

You should see events where the total energy deposited in the crystal is close to the primary photon energy, which indicates photoelectric absorption.
You should also see a broad distribution of partial energy deposits, corresponding to Compton scattering and escape.

Later in the example, when you build the energy spectrum, the appearance of a clear photopeak at the correct energy is strong evidence that your electromagnetic physics and detector material configuration are working as intended.

If you see no energy in the detector or a spectrum strongly peaked at zero, it usually indicates that either electromagnetic physics is not properly enabled, the detector material is incorrect, or the source is missing the detector. In this chapter, the key point is that electromagnetic physics must be active and appropriate for photons and electrons.