KAHIBARO
4.2. Creating a Simulation
Table of Contents
Simulation configuration
A GATE simulation always starts from a single central object that stores every setting and controls the run. In OpenGATE with Python, this object is usually created with a call such as sim = gate.Simulation(). Once this object exists, you configure it step by step. The important idea is that you never call many independent functions scattered everywhere. Instead, you always modify properties of this one simulation object before you run it.
At this stage, you do not need to define geometry, sources, or actors in detail. That will be done in later chapters. Here, focus only on what it means to configure the simulation itself. Typical global configuration items are the random seed, the number of events or the acquisition time, and the main output directory. These parameters affect the whole run, no matter which volumes or sources you later add.
The random seed controls the pseudo random number generator that drives all Monte Carlo sampling. To reproduce a simulation, you must either fix this seed to a known value or save it. Changing only the random seed, while keeping every other setting identical, produces statistically independent runs. This is useful when you want to reduce statistical uncertainty by combining several independent simulations.
Another important part of the configuration is how long the simulation will run. In GATE you can usually specify this either by the number of primary particles or by a physical acquisition time. For a simple test, you typically choose a small number of events. For realistic imaging or dosimetry, you may specify a longer duration, which connects more directly to the source activity. Later chapters about sources and activity explain the physical relation in detail.
Configuring the simulation also involves choosing a working directory for all outputs. It is good practice to create a dedicated folder for each run. Inside, GATE can write log files, statistics, images, and data such as hits or singles. By keeping each simulation in its own directory, you avoid mixing results and you make it easier to document and repeat your work.
Even at this basic level it is useful to think about structure. Many users separate the initial configuration into a small block of Python code at the beginning of the script, where they set the seed, the number of events, the output path, and other global options. Later, this makes it easier to change the simulation conditions without touching the rest of the code.
Basic simulation structure
A minimal GATE Python script follows a simple and consistent structure. First, you import the necessary Python modules, in particular the OpenGATE package. Next, you create the simulation object. Then you describe the world, the geometry, the particle sources, the physics list, and the actors. Only after all these parts are defined do you start the simulation and then inspect the results.
You can think of this structure as a sequence of stages: configuration, building, and execution. Configuration is what you have already seen, where you set high level parameters on the simulation object. Building is where you attach objects to the simulation, such as volumes, sources, and actors, using dedicated methods. Execution is a single call that tells GATE to run, at which point the configuration becomes fixed and particles start to be transported.
In code, this structure appears as a linear script. First, you create sim. Then you call methods on it to add the world and additional volumes, which are covered in later geometry chapters. After that, you attach at least one particle source. You select a physics configuration appropriate to your application, which is discussed in the physics list chapters. Finally, you add actors to record data such as statistics or dose.
After all this is in place, you call a run method of the simulation object. During the run, GATE uses the configuration you gave to generate events, track particles, and record the requested information. When the run finishes, you can read summary information directly from the simulation object or from the output files created on disk.
The important point for an absolute beginner is that the simulation script progresses in a clear order from top to bottom. You should avoid interleaving geometry, sources, and actors in a confusing way. Instead, keep one logical block for each aspect. A simple and common layout is: basic imports, creation and configuration of the simulation object, world and geometry, sources, physics, actors, then finally the command that runs the simulation and a short section that prints or inspects the most important results.
By keeping this basic structure constant across all your projects, you make it easier to debug issues and easier to extend simple test scripts into more complete simulations later in the course.
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