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Forum: Processes Involving Optical Photons
Re: None About volume boundary crossing precision (Michele Gabusi)
Re: Disagree Re: About volume boundary crossing precision (Gumplinger Peter)
Date: 16 May, 2013
From: Michele Gabusi <Michele Gabusi>

Hi Peter,

You have made everything clear!
thanks a lot!
Cheers.

Michele.




2013/5/15 Gumplinger Peter <gum@triumf.ca>

>
> *** Discussion title: Processes Involving Optical Photons
>
> Hi Michele,
>
> > suppose that you want to propagate optical photons through two
> > consecutive volumes, that are put in contact each other (they belong to
> > the same mother volume). In general I know that some issues can be
> > caused by decimal precision loss in volume definition if you don't keep
> > them slightly spaced.
>
> I don't know what codes you are talking about, but in Geant4, you have
> to absolutely put the two volumes abut next to each other!!! There must
> not be any space between them.
>
> > I think it's a quite common problem, but it's not
> > clear to me how and if propagation of optical photon is affected by
> > that.
>
> If the G4Navigator 'detects' that it is leaving the daughter into the
> mother (the space between your two volumes) you'll get the index of
> refraction of the daughter and the mother; e.g. you'll have a small gap
> made up of the mother's material.
>
> > I was wondering if in Geant4 there any options to set volume boundary
> > crossing precision to say: "if two volumes are closer that X*mm then
> > they really share a surface". In principle this could definitely avoid
> > any problem with optical photon propagation between different volumes.
> > Or Geant4 has a different approach for this kind of problems?
>
> One way to overcome issues with two volumes 'touching' each other is to
> make one volume the daughter of the other - if that's possible within
> your geometry definition. In that case, the boundary crossing is unique.
> A track leaving the daughter will enter the mother and a track
> encountering the daughter will go in. As well, when the daughter shares
> a surface with the mother, the G4Navigator will determine that the track
> - coming from the outside of the mother - will go directly into the
> daughter.
>
> If you want to know what is the navigation tolerance for surfaces in
> your simulation, print:
>
> G4cout << "Computed tolerance = "
>
> << G4GeometryTolerance::GetInstance()->GetSurfaceTolerance()/mm
> << " mm" << G4endl;
>
> See:
>
>
> http://www-geant4.kek.jp/lxr/source/global/management/src/G4GeometryTolerance.cc#L107
>
> The default is 1E-9*mm
>
> Peter
>
> -------------------------------------------------------------
> Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or
> unsubscribe) at:
>
> http://hypernews.slac.stanford.edu/HyperNews/geant4/get/opticalphotons/507/1.html
>



-- 
Michele Gabusi
UniversitÓ degli Studi di Pavia
Dipartimento di Fisica Nucleare e Teorica
via A. Bassi 6, Pavia
Tel. +39 0382 987593 (Office)
Tel. +39 0382  987637 (Lab)

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