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Dear Peter, thanks a lot for you informative answer. I had a look at some other posts again, and I wondered whether one of them seemed to be in conflict to your answer. If I understand your answer correctly, you say that there is no problem with shared surfaces between mother and daughter volumes. When reading the following post, I got the impression, that this was only true if the surfaces are along planes in the X,Y or Z axis. Am I wrong or has maybe something changed in the implementation in Geat4 in the meantime? From this forum here: Re: Question Daughter volume sharing a surface with its mother (Andrea Conti) Date: 07 Dec, 2014 From: Michael H. Kelsey <Michael H. Kelsey> http://hypernews.slac.stanford.edu/HyperNews/geant4/get/geometry/1384/1.html?inline=-1 >> But this way the daughter will share a surface with the mother (that is, >> the mouth of the hole, on the same plane as the host material's surface) >> and nobody addressed this problem: is it acceptable ? Or are there going >> to be problems with the consistency of the geometry ? > > If the surfaces are simple planes along X, Y or Z, then you can do > this safely as the coordinate of the surface will be floating-point > identical in the two cases. If the surface (even a plane) is not > exactly on-axis, you are likely to have situations where the computation > of the two surfaces do not coincide. In these cases, particles can get > "trapped" between the surfaces, or you can have unexpected boundary > interactions, etc. This is why we generally discourage shared surfaces, > because the cases where they are safe are quite limited.