23.6. Selecting Electromagnetic Physics

Table of Contents

Why Electromagnetic Physics Matters for a Gamma Detector

A gamma ray detector simulation depends almost entirely on electromagnetic physics. Gammas interact through electromagnetic processes, and the energy that ends up in your scintillator crystal comes from those interactions. If you choose an inappropriate electromagnetic physics configuration, you can get wrong spectra, wrong peak shapes, and wrong efficiencies, even if your geometry and source are perfect.

In the gamma detector example, your main goal is to reproduce how incoming gamma rays deposit energy in the scintillator. That means you must include all the important photon interactions, as well as the transport and interactions of the secondary electrons and positrons that carry the energy into the detector material.

Relevant Electromagnetic Processes for Gamma Rays

For a simple scintillation detector, the key photon processes are:

Photoelectric effect. A gamma is absorbed and an electron is ejected from an atom. This is the main process that produces a full energy peak at low and medium energies, especially in high Z materials like NaI or CsI.

Compton scattering. A gamma scatters from an electron, transfers part of its energy, and leaves with reduced energy and a changed direction. Repeated Compton scatterings create the Compton continuum in the energy spectrum.

Pair production. At gamma energies above $2 m_e c^2 \approx 1.022 \,\text{MeV}$, a gamma can convert into an electron-positron pair in the nuclear field. If the annihilation photons escape, you see single and double escape peaks. If everything is contained, you get a full energy peak.

These processes create electrons and positrons, which then deposit their energy in the scintillator through ionization and, at higher energies, bremsstrahlung. Even if your focus is “photons,” you must still transport charged particles accurately, because they are the ones that actually lose energy in the detector volume.

For a basic gamma detector simulation you normally rely on Geant4’s standard electromagnetic physics, which already includes these processes and their secondary electrons and positrons. You do not have to add individual processes by hand if you use a reference physics list that contains electromagnetic physics.

Choosing a Suitable Physics List for a Gamma Detector

In the practical gamma detector example you are interested in:

Accurate gamma interaction probabilities in the detector material.

Realistic energy deposition of electrons and positrons inside the scintillator.

Reasonable simulation time for many events.

You can satisfy these requirements with one of the standard reference physics lists that already include an electromagnetic package. For a general-purpose detector simulation with gamma rays, a very common choice is:

FTFP_BERT. This is a full reference physics list that includes both hadronic and electromagnetic physics. For purely electromagnetic problems it is often more than necessary, but it is convenient, widely tested, and works well.

For a gamma detector you only need the electromagnetic part. If you want to focus explicitly on electromagnetic physics and avoid unnecessary hadronic models, you can use a reference that is electromagnetic-only. A typical choice is:

FTFP_BERT plus an explicit electromagnetic option.

For example, you might instantiate the reference list and then replace or configure its electromagnetic physics with a specific option such as “emstandard” or one of its variants. At the beginner level, it is enough to know that you want a list that includes:

Photon interactions: photoelectric effect, Compton scattering, pair production.

Electron and positron interactions: ionization, multiple scattering, bremsstrahlung, and positron annihilation.

You should avoid using a hadronic-only or incomplete list for a gamma detector, because you would miss crucial electromagnetic processes and end up with very low or zero energy deposition in the scintillator.

For a gamma ray detector, always use a physics configuration that includes:

Photoelectric effect, Compton scattering, and pair production for photons.
Ionization and multiple scattering for electrons and positrons.
If these are missing, your energy spectrum will be physically wrong.

In many Geant4 examples, you will see a simple physics list name used in a macro, for example:

/physics_list/select FTFP_BERT

For the gamma detector example, selecting such a list, or an electromagnetic-only variant provided by Geant4, is usually sufficient and recommended for beginners.

Using Electromagnetic Physics Options

Geant4 provides several electromagnetic “options” that adjust how precisely electromagnetic processes are modeled and how low in energy particles are tracked. These options trade off accuracy and speed. They are especially relevant for detector simulations where you care about fine details of energy deposition.

For a simple scintillation detector, the most common electromagnetic configurations are:

Standard electromagnetic physics. This is the default, designed for a broad range of applications. It provides good accuracy for many gamma energies and materials that are typical in detector simulations.

“Option” variants of the standard physics. These modify step size control, multiple scattering, and other details to improve accuracy in certain applications or to improve performance. You might encounter names like “Option3” or “Option4” in Geant4 documentation. They are aimed at better precision or specific use cases.

If your aim is to learn the basic workflow and obtain a physically reasonable energy spectrum, the default standard electromagnetic physics is already appropriate. You can extend to more specialized options later if you need more precise modeling, for example for low-energy medical imaging or microdosimetry.

In the gamma detector example, the important point is that you deliberately choose an electromagnetic physics configuration instead of relying on an empty or unrelated physics list. When creating or configuring your physics list class for this example, you should ensure that electromagnetic processes are added and activated for gamma, electron, and positron transport.

Impact on the Gamma Detector Response

Your choice of electromagnetic physics directly affects the shape and details of the simulated spectrum:

The height and position of the full energy peak depend on photoelectric absorption and pair production modeling, and on whether secondary particles are tracked until they stop.

The Compton plateau and edge positions depend on the Compton scattering cross sections and energy transfer.

Escape peaks and continuum structure at higher energies depend on how secondary photons and electrons propagate and possibly escape from the detector volume.

If the electromagnetic physics is too simplified, or if production cuts are too coarse, low energy secondaries may be suppressed, and you can lose part of the energy that should be deposited. This can shift or distort peaks and reduce the apparent detector efficiency.

Later chapters will cover production cuts and performance tuning in more detail, but at this stage it is enough to understand that:

Accurate electromagnetic physics is essential for a realistic gamma detector.

Using a recommended reference physics list with its standard electromagnetic component is the safest choice for beginners.

If your simulated peaks do not match expectations, electromagnetic physics configuration is one of the first places to check.