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2.5. Geant4 Example Applications
Table of Contents
Basic examples
The Geant4 distribution includes several sets of ready‑to‑build example applications. For a beginner, the basic examples are the best starting point because each one focuses on a single concept and the source code is short and readable.
The basic examples are located under the examples/basic directory in your Geant4 installation. Each subdirectory contains a small, self‑contained application that you can configure, build, and run. They are designed to illustrate how to construct a minimal but complete Geant4 simulation with geometry, physics, primary generation, and simple output.
A typical workflow with a basic example is straightforward. First, you inspect the CMakeLists.txt file and source files to see the project structure. Then you create a build directory inside the example, run CMake to configure it against your Geant4 installation, and compile the code. After building, you can run either interactively or using macro files that come with the example.
Most basic examples demonstrate one particular aspect of Geant4. For instance, some focus on geometry construction and simple particle tracking, others on simple scoring of energy deposition or track length, and others on basic visualization. The code is heavily commented, which helps you connect the concepts from this course to real Geant4 classes. It is useful to compare the example’s main() function and user classes with the structure you will create in your own first project.
In practice, a beginner should pick one basic example, build it, and then experiment. You can modify the geometry dimensions, change the primary particle type or energy in a macro, or switch visualization options. This is an effective way to see how small changes in the code or in macro commands affect the simulation behavior.
Basic examples are meant as learning tools, not as ready‑to‑use physics simulations for scientific results. Always treat them as templates and adapt them carefully for your own application.
A good exercise is to compile two or three different basic examples and compare how each one organizes the same building blocks of a Geant4 application. This will make the structure of user initialization classes and macro usage much clearer before you move on to more complex examples.
Extended examples
Extended examples are larger, more realistic applications that demonstrate how to use Geant4 features in practical scenarios. They are located under examples/extended and cover a wide set of domains, such as detector design, medical physics, hadronic physics, electromagnetic physics, and visualization techniques.
Compared to the basic examples, extended examples usually contain more source files, more complex geometries, and more detailed physics configurations. They show how to implement complete workflows that are closer to real research or development tasks, for example full detector setups, scoring meshes, more elaborate primary sources, or custom analysis code.
Many extended examples are organized by topic. For instance, there are examples that focus on medical applications such as dose calculation or imaging setups, others that focus on calorimetry or tracking detectors, and others that illustrate specific physics models or field propagation. Within each topic, the example code is structured to highlight the relevant components, for example specialized detector construction classes, advanced sensitive detectors, or particular physics list configurations.
For a beginner, the extended examples can look overwhelming at first because of their size. A good strategy is to start from the README file that usually accompanies each example. This file explains the purpose of the example, how to build it, which macro files to run, and what kind of output to expect. You can then trace from main() into the detector, physics, and action classes to see how the components are assembled.
It is helpful to use extended examples as references when you start adding features to your own project. For example, if you need a scoring mesh or a particular type of detector array, you can look for an extended example that uses a similar feature and adapt the relevant code segment into your own application, rather than starting from scratch.
When copying patterns from extended examples, always check that the choices of physics, geometry, and cuts match your own application goals. Extended examples illustrate techniques, they are not automatically suitable as is for every problem.
By studying an extended example that is close to your field of interest, you will also see common coding conventions, directory structures, and macro usage that make larger Geant4 applications easier to manage.
Advanced examples
Advanced examples are the most sophisticated applications included in Geant4. They are intended primarily for experienced users and typically represent realistic detector simulations or complete experimental setups. You can find them under examples/advanced in the distribution.
These examples often implement detailed geometries with many components, complex material definitions, and carefully tuned physics lists that correspond to real experiments or reference simulations. They also tend to include advanced features such as custom event generators, specialized scoring and analysis logic, or integration with external tools and data formats.
The code structure in advanced examples is closer to production applications. You will see more elaborate class hierarchies, configuration mechanisms using macro files or external configuration data, and in some cases support for multithreading and high‑performance execution. They are valuable as design references once you understand the basics of how a Geant4 simulation is organized.
For absolute beginners, advanced examples are not the best place to start reading code, but they are very useful to run and observe. You can build an advanced example, execute provided macro files, and study the produced output and visualizations. This gives a sense of what a mature Geant4 application can do and what level of detail is achievable.
As your skills grow, you can return to the advanced examples to study specific topics. For instance, you might look at how an advanced example organizes sensitive detectors and hit collections in a large detector, or how it structures analysis output into histograms and ntuples. The patterns used in these examples can be adapted to your own large projects.
Advanced examples are complex and optimized for specific use cases. Do not try to copy them entirely into a new project. Instead, identify and reuse only the parts that illustrate a particular technique you need.
In summary, the three layers of examples basic, extended, and advanced together form a progression. Start with basic examples to learn the core structure, use extended examples to see realistic but focused applications in your domain, and refer to advanced examples as models of full‑scale Geant4 simulations once you are comfortable with the fundamentals.
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