Overlap problem

[Kim_Myeongho]

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Geant4 Version: 11.0.4
Operating System: windows 10 (64-bit)
Compiler/Version: MSVC (Visual Studio Build Tools 2022, MSBuild 18.0)
CMake Version: 4.1.1

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Hello,

I am currently working on importing a TCAD structure into Geant4 using CADMesh. My workflow is:

• Export geometry from TCAD as a .sat file

• Convert the .sat file to .obj

• Import the .obj file into Geant4 using CADMesh

However, I am encountering the following warning messages from G4PVPlacement::CheckOverlaps().

The issue is that even when the geometries are visually not overlapping, Geant4 still reports overlap warnings.

I have two questions:

1. Why does Geant4 report overlaps even when the geometries do not actually overlap?

2. How can I resolve or suppress these warnings?

I suspect this might be related to:

• tessellated geometry precision (from CADMesh)

• floating-point tolerance

• or the internal sampling method used by CheckOverlaps()

Has anyone experienced a similar issue when importing CAD/TCAD geometries?

Any advice would be greatly appreciated.

Thank you!

[evc]

Hello Kim,

@Kim_Myeongho

Why does Geant4 report overlaps even when the geometries do not actually overlap?

Please trust Geant4 overlap check, if it reports an overlap, then the overlap really exists. In your case the first overlap is very tiny, 9.61774 And = ~1e-6 mm, so it, indeed, could be a problem related to precision.

The second overlap is more serious, it reports a defect in the geometry, namely, one volume is mistakenly placed inside another volume.

@Kim_Myeongho

How can I resolve or suppress these warnings?

If to set the last parameter in G4PVPlacement to false, then the overlap check will not be applied. However, it is better to fix all existing overlaps.

Importing geometries from CAD is always tricky, please see the following topic:

Importing CAD geometries into Geant4: practical notes and common pitfalls

Hi all, I would like to share some practical experience on importing CAD geometries into Geant4, as this question comes up regularly and many of the problems are not obvious from the documentation. Geant4 official guidance Geant4 already provides some guidance on importing CAD models as tessellated shapes. It is highly recommended to read this first: https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/Detector/Geometry/geomSolids.html#importing-cad-models-as-tessellate…

Tessellation (generation of mesh structure) may introduce artificial overlaps. For example, on the image below there are two tubes (black), one inside another. The tubes do not intersect, but their meshes (red) are clearly do.

So, if the geometry is not very complex, I would recommend to describe it directly in Geant4. Please also note that in this case the simulation will be much faster.