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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)
Re: Feedback Re: Question about dielectric_LUT type optical surface (Gumplinger Peter)
Date: 26 Mar, 2013
From: Young <Young>

Peter,

Thank you for your comments.

> 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.

The photons are stopped by bulk absorption (absorption length is set to around 5 meters) or manual termination within stepping action if their track length exceeds 1000mm. (My actual simulation does not have the scintillator wrapped on all sides, so the photons simply escape from the unwrapped face, but I have the scintillator wrapped on all sides right now to replicate this bug.)

I thought that maybe the reason that photons end up at the edges, is just because the photon is reflecting many times inside the scintillator, but you are right that it is a little strange given that a typical photon only reflects a few hundred times. One theory I have is that the look up table gives a non-zero probability of the photon reflecting close to perpendicular to the normal, i.e. along the surface. This seems to supported by the verbose output, as the momentum direction of the photon right before escaping the scintillator can be something like (-0.6156614753256581, 0.788010753606722, -2.082445306335849e-16).

> 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

As per your comments, I added the StepStatus to the verbose output, and I do get fGeomBoundary at each step before the photon escapes the world volume.

Any thoughts?

Cheers, Young

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1 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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