ncrystal-geant4

This package faciliates the usage of low energy neutron scattering physics provided by NCrystal, in the Geant4 simulation framework. Specifically, it allows users of HP physics models, to initialise selected G4Materials based on NCrystal cfg-strings, and to let NCrystal take over the scattering physics of low energy (<5eV) neutrons in those volumes (other particles and physics will still be handled by the usual Geant4 physics list).

The current implementation assumes that the user is using one of Geant4's HP physics lists, and unfortunately does not yet support multi-threaded Geant4 simulations (UPDATE June 2025: The example_bias/ shows how to work with multi-threaded Geant4!!).

To use the NCrystal-Geant4 bindings, one must:

1. Install NCrystal and Geant4. Although it is possible to build both projects manually, it should be noted that both are available on conda-forge. If not using conda, note that NCrystal is also available on PyPI, so assuming Geant4 has already been installed in another manner, it might be possible to simply complete the setup by a pip install NCrystal.

2. Install ncrystal-geant4. This can be done by pip install ncrystal-geant4, or alternatively by cloning the repository at https://github.com/mctools/ncrystal-geant4 and setting the CMake variable NCrystalGeant4_DIR to point at the src/ncrystal_geant4/cmake subdir of the cloned repo.

3. Edit your projects CMakeLists.txt to add a find_package(NCrystalGeant4) statement, and adding the NCrystalGeant4::NCrystalGeant4 target as a dependency of the Geant4 application you are building.

4. Edit your C++ code and include the header file G4NCrystal/G4NCrystal.hh. Then create NCrystal materials based on NCrystal cfg-strings like "Al_sg225.ncmat;temp=80K" using code such as:

   G4Material * myMat = G4NCrystal::createMaterial("Al_sg225.ncmat;temp=80K");
   

   Finally, one must inject the NCrystal physics process into the physics list by placing a statement like:

   G4NCrystal::install();
   

   Crucially, such a statement must be placed after runManager->Initialize() and before runManager->BeamOn(..) is called.

   Alternatively, inject NCrystal physics via the Geant4 biasing framework. See the example in the example_bias/ folder next to this README file (or find it online at https://github.com/mctools/ncrystal-geant4/tree/HEAD/example_bias )

For actual examples using the recipes above, see the example/ and example_bias folders next to this README file (or find them online at https://github.com/mctools/ncrystal-geant4/tree/HEAD/example and https://github.com/mctools/ncrystal-geant4/tree/HEAD/example_bias ).