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Forum: Geometry
Re: Question overlapping volumes with same material (Jean-Pascal Laedermann)
Date: 06 Aug, 2013
From: Michael H. Kelsey <Michael H. Kelsey>

On Tue, 30 Jul 2013 13:21:38 GMT, Jean-Pascal Laedermann wrote:
> It seems that Geant 4 doesn't allow overlaps of two volumes in the same
> mother.

That's correct. Overlapping volumes are forbidden. If one volume is fully contained within another, it should be made a daughter volume.

> But if the volumes are made from the same material, this should
> not be a problem, isn't it?

It is a problem, because the volume boundaries serve purposes other than just to separate materials.

For optical processes, they are the actors for reflection/transmission/refraction effects. Two blocks of "the same material", with different surface properties, may have quite different optical photon propagation results than a singule uniform volume.

In general, particle tracking is always "stopped" at a volume boundary. The particle is then stepped into the new volume and material, and tracking starts up again. With overlaps, this action may not be consistent, nor reversible.

The nested, non-overlapping structure of volumes is a necessary assumption for the tracking (G4Navigator) system to optimize its internal map of the World. The "voxelization" process is able to collapse your hierarchical geometry into a 3D table mapping locations in space uniquely to volumes, thereby dramatically improving the tracking speed. Overlapping volumes makes the unique mapping impossible (or in practice internally inconsistent).

Geant4 provides a set of "boolean" volume classes to allow you to merge overlapping simple shapes (our so-called "CSG solids") into more complex structures, with overlaps, removed chunks, and so on. Please see the Application Developers' Guide for details.

  -- Michael Kelsey

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