A GUI for Setting and Running Radiation Simulations for Complex CAD Geometries


RSim enables users to set up a simulation for modeling radiation effects via a user-friendly interface. Users can create a geometry from geometry primitives (spheres, cones, boxes, etc.) and boolean operations (intersections, unions) on primitives, or import a CAD file. After creating or importing a geometry, users can assign materials to the geometry, specify common radiation sources, and formulate tallies (dose, fluence, etc.). RSim translates the simulation setup into Geant4 input then runs the simulation.

RSim runs on Linux, macOS, and Windows.

Included examples help users quickly learn RSim so they can create their own new simulations.


RSim is an application providing a GUI for setting up and running radiation simulations. The current release RSim 1.0 is implemented for using GRAS [1] as its simulation engine. GRAS (Geant4 Radiation Analysis for Space) was developed by ESA as a wrapper for Geant4 [2] and offers a generic application, allowing users to specify the geometry, radiation environment and analysis purely by the extended input language of Geant4 (.mac file). In particular, one imports geometry using GDML (Geometry Definition Markup Language [3]) files, defines sets up the analysis using GRAS UI commands and use GPS [4] (General Particle Source) for defining radiation environment.

GRAS should be downloaded independently from RSim at obtained from A path to GRAS executable is communicated to RSim via the preferences of the application.

Users of RSim can import complex CAD geometries, define radiation sources and tallies for radiation analysis and run simulations (see Fig. 1) without a need to write C++ code or manually editing input files. In the next release of RSim, we will add support for PHITS.


Fig.1. Internal transformation of user input into Geant4/GRAS input.



Setting Simulations

Defining Geometry

Users can add Constructive Solid Geometry (CSG) primitives, apply Boolean operations, and create 1D, 2D, and 3D arrays of them (see Figs. 2, 3).

Fig.2. Adding primitives in RSim. The data is then translated to GDML for Geant4/GRAS, Tetgen for PHITS, Abaqus for MCNP6, and MOAB for DAGMC.


Internally RSim uses OpenCascade [5] to read CAD data in STEP format [6] then translates STEP into tessellated solids of GDML. RSim visualizes tessellated solids in its Setup tab (Fig. 4).



Fig.3. Using Boolean operations in RSim allows creation of a rich set of CSG elements.

Fig. 4: CAD geometry in STEP format is imported to RSim

Fig.4. CAD geometry in STEP format is imported into RSim then next can be exported to GDML for Geant4/GRAS.

Defining Radiation Sources in RSim

Setting up a source begins with type selection (see Fig 5). If a surface or volume source is chosen, the user next attaches a geometry primitive (created prior to selecting the source type), which serves as the origin of tracks.

Fig. 5.  Choosing the shape of the source.

Fig.5. Choosing the shape of the source.

Next users can set up a type of spectrum (see Fig. 6) and edit its parameters as shown on Fig. 7. A particularly useful type of spectrum is 2 Column File because the radiation environment is often specified in tabular data format expressed as differential fluences vs. energy.

Fig. 6: Choosing the energy spectrum of the source.

Fig.6. Choosing the energy spectrum of the source.

Fig. 7: RSim Spectrum Editor

Fig.7. Depending on the type of the spectrum, users are presented with different views of the spectrum editor.

On the backend, RSim translates radiation source data into GPS format for Geant4/GRAS. Sources can be visualized in RSim (see Fig. 8), which makes debugging of setups much easier.

Fig. 8: Visualizing targets, shielding and source in RSim setup.

Fig.8. Visualizing targets, shielding, and source in RSim setup.

Setting Scoring/Tallies

In the current release of RSim0.1 Tech-X implemented and tested volume and mesh tallies for energy deposition, and fluence/fluxes (see Fig. 9). Through the RSim GUI, users can choose a part of CAD geometry to which to apply the tally.


Fig. 9: Choosing a tally in RSim.

Fig.9. Choosing a tally in RSim.

Material Annotation

Users can choose materials from the RSim database, which contains more than 700 common materials,as well as create custom materials such as isotopes, molecules, and mixtures. Annotation of solids with material information can be applied interactively (manually) or automatically by using a file to associate CAD parts with common material names.

Future Directions

In the next release we will address (1) support for PHITS simulations and (2) visualization of Geant4 and PHITS simulation results for easy comparison and cross-validations of these radiation transport models.


[1] Santin, Giovanni, Vladimir Ivanchenko, Hugh Evans, Petteri Nieminen, and Eamonn Daly. "GRAS: A general-purpose 3-D modular simulation tool for space environment effects analysis." IEEE Transactions on Nuclear Science 52, no. 6 (2005): 2294-2299. DOI: 10.1109/TNS.2005.860749
[6] STEP format:

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