26.4. Selecting the Physics List

Table of Contents

Why the Physics List Matters in Proton-in-Water Simulations

In a proton beam in water simulation, the physics list largely determines whether you obtain a realistic Bragg peak, accurate multiple scattering, and correct production of secondary particles such as neutrons and photons. Your geometry and source define what you simulate. The physics list defines how particles interact with water.

For this example, you do not need to design a custom physics list from scratch. Instead, you select and possibly lightly configure a reference physics list that already contains the electromagnetic and hadronic processes needed for proton transport in water.

For proton beams in matter, always use a physics list that includes hadronic physics for protons and neutrons and appropriate electromagnetic models. A pure electromagnetic physics list is insufficient.

Requirements for a Proton Beam in Water Physics List

A proton beam in water requires several groups of physics processes to be active. You can think of them in terms of what they influence in the final depth dose curve.

First, you need electromagnetic processes that model energy loss and angular scattering of protons. These processes control the location and shape of the Bragg peak. Geant4 provides specialized electromagnetic models for hadrons that are already included in modern reference physics lists. You should avoid disabling them unless you know exactly what you are doing.

Second, you need hadronic processes for protons and neutrons. Protons interacting with water can undergo nuclear reactions. These reactions remove energy from the primary beam, generate secondary particles, and influence dose outside the primary beam path. For a realistic simulation of patient dose or water phantom dose, these inelastic interactions are important, and the reference lists such as FTFP_BERT include suitable models.

Third, you need models for low energy neutrons if you plan to study neutron dose or you want a more complete description of secondary radiation. Reference lists already contain high precision neutron options in some variants, which improve low energy neutron transport in materials that have detailed evaluated cross sections.

Fourth, you need decay and capture processes, which are also part of standard physics lists. These are relevant for long lived secondaries and activation, although in a basic depth dose study they are usually less dominant than electromagnetic and hadronic energy loss.

For a realistic Bragg peak in water, ensure that hadronic inelastic processes for protons and proton multiple scattering and ionization are included. Otherwise, the peak position and tail will be incorrect.

Choosing an Appropriate Reference Physics List

Geant4 provides a set of built in reference physics lists. For therapeutic or high energy proton beams in water, several of them are suitable. The most commonly used general purpose choice is FTFP_BERT. This list combines the Fritiof high energy hadronic model at higher energies with Bertini intranuclear cascade at intermediate energies, and it includes a standard electromagnetic configuration appropriate for many applications.

In many proton beam in water simulations, users simply select FTFP_BERT without modification. This is sufficient for depth dose curves, Bragg peak studies, and many shielding or secondary dose studies, provided the energy range is within the coverage of the models.

There are also variants of FTFP_BERT tuned for different priorities. For example, some variants add high precision neutron data, and some focus on reduced CPU cost. If your primary interest is accurate neutron transport in water, for instance in shielding studies around a proton therapy room, a high precision variant that includes detailed low energy neutron data will be more appropriate than the plain list.

If your goal is a basic tutorial level proton in water simulation, FTFP_BERT without extra options is a good starting choice. It balances accuracy and performance and is widely used and tested.

For a beginner proton in water simulation, FTFP_BERT is a recommended default physics list. It already contains the electromagnetic and hadronic processes needed for a realistic depth dose.

Adding the Physics List in Your Application

In your Geant4 application, the physics list is created and registered in the user initialization phase. For reference physics lists, Geant4 provides a factory that simplifies this selection. You typically implement this inside your action or initialization class and pass the chosen physics list to the run manager.

The key idea for this example is that you do not write process registration by hand. Instead, you create an instance of a predefined list such as FTFP_BERT and let it configure all required processes internally. This keeps your example code cleaner and separates physics configuration from geometry and primary generation.

Within the context of this proton beam in water example, your code will therefore create three main user initialization classes. One is the detector construction that defines the water phantom, another is the primary generator that defines the proton beam, and the third is the physics list, selected from the Geant4 reference lists using the provided factory. All three are then provided to the run manager before the run is initialized.

Impact of Physics List Choice on the Depth Dose

Once you start comparing simulated depth dose curves in water, the impact of the physics list becomes clear. The position of the Bragg peak, its height, and the shape of the distal falloff all depend on how the physics list handles proton interactions in water.

If you used a list that does not include hadronic inelastic reactions for protons, you would obtain an unrealistically narrow Bragg peak with too little dose in the tail region. Conversely, using a physics list that includes detailed hadronic interactions will produce a more realistic tail due to nuclear fragments and secondary particles.

Multiple scattering models also affect the lateral spread of the beam and, indirectly, the depth dose if you score in small voxels. Different electromagnetic configurations can change the exact shape of the peak at the percent level. For a basic learning example this level of detail is usually acceptable, but for clinical quality simulations you may need to compare different lists and possibly tune parameters.

When you later record energy deposition per slice and generate a depth dose curve, you should remember that any discrepancies between simulation and measurements can be caused by geometry, beam model, or physics list. Physics list selection is therefore an essential part of validation, not only a technical configuration step.

When you compare your simulated Bragg peak to data, always consider that physics list choice is one of the main sources of systematic differences in the depth dose curve.

Practical Guidance for This Example

For the proton beam in water example in this course, the main goals are to visualize proton transport, obtain a Bragg peak, and compute a depth dose curve. For these goals you can safely rely on a standard reference physics list.

You should select a list that is widely used in proton therapy studies, such as FTFP_BERT, and keep its default configuration. This reduces the number of parameters you must understand at this stage. As you progress to more advanced topics, you may explore alternative lists or custom electromagnetic configurations, but for a first depth dose simulation that complexity is not necessary.

With the physics list selected and registered, you can proceed to record energy deposition in the water phantom, divide the phantom into slices, and compute the depth dose profile in the next steps of the example.