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Forum: Geometry
Re: None Beginner Question : overlap of mother (David Sarria)
Date: 16 Aug, 2016
From: John Apostolakis <John Apostolakis>

Dear David,

As you see from the documentation we recommend that the geometry is free 
of overlaps.  Since the geometry modeller is a complicated beast, if 
there is a way to avoid unnecessary overlaps, it could improve the 
robustness of the simulation.  This concerns overlaps which involve a 
non-microscopic distance (e.g. an overlap of a few microns in a setup 
measuring 10 meters in diameter is tolerable, and likely will also occur 
due to imperfect floating point operations on rotation matrices. )

It is possible that creating non-overlapping versions of a few volumes 
will results in one or more solids (shapes) which are much more 
complicated.  Then an argument can be made that it would be better (more 
robust) to live with a few small overlaps at a single level of the 
geometry. And when all volumes which overlap are filled with the same 
material (vacuum), it is reasonably likely that the simulation results 
would be unaffected by the overlap.

If what you are asking though, is whether it is ok to introduce overlaps 
at several levels of the geometry (potentially for convenience or 
simplicity), then the answer is NO.  The geometry modeller can become 
confused, and its answers cannot be guaranteed.  And when you get 
navigation errors, then the geometry modeller (G4Navigator) will check 
whether your geometry has overlaps - and you will get error messages.  
We are not able  to diagnose or help in case of problems caused by 
overlaps which the user has intentionally introduced into their geometry 
setup.

I hope that this gives a measure of clarity.

Best regards,

John Apostolakis

On 11/08/16 17:10, David Sarria wrote:
> *** Discussion title: Geometry
>
> Hi all,
>
> I have a basic question about geometry.
>
> I want to place several copies of a detector unit. The parts of one
> detector unit are daugther of a logical volume, that is a box slighly
> larger than all the detector unit elements and filled with vacuum.
>
> When I place several times the detector units, some small parts of the
> vacuum boxes overlap (but not the detector parts). Is it a problem ?
>
> And if some part of the vacuum box (not the detector parts) is
> overlapping with some other physical surrounding elements ?
>
> Is there a way to place only the daughters but not the vaccum box ?
>
> Thanks in advance.
>
> David
>
> -------------------------------------------------------------
> Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at:
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1 None: Re: Beginner Question : overlap of mother   (David Sarria - 22 Aug, 2016)
(_ None: Re: Beginner Question : overlap of mother   (David Sarria - 22 Aug, 2016)
(_ None: Re: Beginner Question : overlap of mother   (John Apostolakis - 24 Aug, 2016)
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