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Forum: Processes Involving Optical Photons
Re: Question Question about dielectric_LUT type optical surface (Young)
Re: Feedback Re: Question about dielectric_LUT type optical surface (Gumplinger Peter)
Re: None Re: Question about dielectric_LUT type optical surface (Young)
Date: 25 Mar, 2013
From: Gumplinger Peter <Gumplinger Peter>

Hello Young,

> I have found
> that the photons that escape the wrapped volume seem to be extremely
> near the edges, i.e. the boundaries of adjacent faces, right before they
> escape.
> 
> For example, for a 30x30x2mm box centered at the origin (0, 0, 0), an
> escaping photon could have coordinates (14.24862431017721, -15.0,
> 0.9999999999999727) mm at the step just before leaving the scintillator.

What is strange is that you have a photon step ending at such location. Or is it that you start a photon at that location? What causes a photon step to end inside your scintillator? Usually, optical photons are tracked from where they are 'born' to the next volume boundary. They only stop before if they are bulk absorbed or wavelength shifted.

In any event, what you are seeing is clearly a geometry issue in Geant4. You are at the limit of precision for the code and so the G4 Navigator "thinks" it is already outside the volume and never encounters the volume boundary which is the prerequisite for the G4OpBoundaryProcess to be called; e.g.

pPostStepPoint->GetStepStatus() == fGeomBoundary

> On a side note, I am having trouble interrogating which process was
> invoked in the stepping verbose class that I copied from the LXe
> example. I would have thought that
> "fStep->GetPostStepPoint()->GetProcessDefinedStep()->GetProcessName()"
> would either return Transportation or OpBoundary, but it seems to return
> Transportation for all steps. I am pretty sure I am missing something,
> but I can't seem to figure out what. Any comments on what I'm doing
> wrong or misunderstanding?

The G4OpBoundaryProcess never limits the step and so it never is the 'Process that Defined the Step'. It is always 'Transportation' that limits the step at a volume boundary. In fact, when a step ends on a volume boundary, it is the 'Transportation Process' that limited the step. G4OpBoundaryProcess springs always into action, at every step, since it is a 'Forced' process for some particles (optical photons) but only when fGeomBoundary does it actually do anything.

Peter

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1 None: Re: Question about dielectric_LUT type optical surface   (Young - 26 Mar, 2013)
(_ None: Re: Question about dielectric_LUT type optical surface   (Erik Dietz-Laursonn - 16 Apr, 2013)
(_ None: Re: Question about dielectric_LUT type optical surface   (Nick Calvert - 02 May, 2013)
(_ More: Re: Question about dielectric_LUT type optical surface   (Zach Hartwig - 09 May, 2013)
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