Geant4

GEANT4 is a toolkit for developing detector simulations. Before starting the exercise, create a GitHub repository and work from there. It is good practice to commit your progress whenever you verify that a step is working (e.g., expected output and successful header imports).

Exercise 0: Hello World

Run a Hello World program (see main.cc) to check if you can use GEANT4 (G4ios). This requires adding the missing pieces to the CMakeLists.txt file. It is recommended to create a directory called build, run cmake .. from there, and then use make -j (-j utilizes all CPU cores). You'll then be left with executable which you run like .HelloWorld . Geant4 has some prerequisites, if it's telling it can't find EXPAT I'll lend you mine: use -DEXPAT_LIBRARY=/user/selmorab/libexpat/lib/libexpat.so while cmake-ing.

Once this step is complete, you will have set up the compilation process. This part is tedious, but understanding it fully will be beneficial for debugging and maintaining good coding practices. Physicists rarely write full GEANT4 simulations in isolation after all. Also, having a .gitignore file to exclude extraneous files from your repository is good practice.

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Developing the Simulation

Now that you have access to GEANT4, we can start coding! There are many ways to structure a GEANT4 simulation, and in this exercise, we (well you*) will:

• Rewrite main.cc to include a G4RunManager. After registration and initialization, it should take an integer value set by the user (runManager->BeamOn(numberPrimaries)) to run a simulation.
• Organize the project by setting up an include folder for headers and a src folder for source files.
• Implement ActionInit, which initializes:
  • PrimaryGenAction to spawn particles.
  • SteppingAction to print stuff
• Output results into a ROOT/CSV file or print them to the command line.
• Implement DetConstruction to define the detector's geometry and materials.
• Ensure that all files communicate with main.cc, where the physics list is imported.

Task Distribution

At a minimum, there are four roles to distribute among team members:

1. Input/Output:

   • Ensure useful output from the program. Don't track too much stuff (slows down system) or too little. CSV file or print?
   • Verify that particle spawner inputs and material properties are well-sourced (e.g., where you get that model/measurement?).
   • Visualize data. Can be done via Geant4 or Python.
   • Start by examining SteppingAction and PrimaryGenAction.

2. Geometry:

   • Create a realistic physical model (physics lists) while balancing performance and accuracy.
   • Model both the source and the detector.
   • With Main lead ensure communication with other parts.
   • Start by examining DetConstruction.

3. Material and Physics List:

   • Associate material properties with the geometry.
   • Input/Output gives you the numbers, you choose how to implement refraction for example.
   • Work closely with the Input/Output role to validate physical accuracy.
   • Start by examining DetConstruction.

4. Main Lead:

   • Develop main.cc and make executive decision on the set up. Maybe separate .cc for material and geometry? You'll require a broad perspective of everything.
   • Establish communication between components (e.g., geometry and particle spawning).
   • Ensure consistency and efficiency across the project. Primary gen people aren't printing stuff that Input/Output people can do better?
   • Write the header files.

Expanding the Scope

Just because I cut things in four does not mean you should, you can for example

• Split Input and Output tasks to speed things up.
• Assign additional team members to geometry, materials, and physics implementation to increase simulation detail. Could we use or do we need an electric field?

Planning your simulation is part of the exercise. While excessive planning can delay coding, poor planning can lead to inefficient work distribution or difficult-to-maintain code.

Resources

I am not going to appeal to Plato's anamnesis, especially if you are not too familiar with C++. You can check GEANT4's own documentation for examples and definitions, and GEANT4 is also open source. So if you recently discovered that in the photon electric effect, photons are also generated (wait, how?), you can explore how GEANT4 handles this quantum phenomenon in its source code.

• Mostly Application Developer's Guide & Physics Reference Manual: GEANT4 Documentation
• GEANT4 Open-Source Repository: GEANT4 GitHub

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GEANT4 is used in Dark Matter/neutrino experiments, extremely high energy CERN experiments, and even more energetic events in cosmic ray experiments (also medical/biophysics). While there are many other simulation packages in particle physics such as CST studio, COMSOL, or Pythia, this exercise will show you the general considerations involved in developing a particle physics simulation in general, and also the type of group work/organization that is required in real world research.