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Question Re: Does GEANT4 accurately model the trapping efficiency for a scintillating optical fiber? 

Forum: Processes Involving Optical Photons
Re: Question Does GEANT4 accurately model the trapping efficiency for a scintillating optical fiber? (matthew)
Re: Feedback Re: Does GEANT4 accurately model the trapping efficiency for a scintillating optical fiber? (Gumplinger Peter)
Date: 25 Mar, 2015
From: matthew <matthew>

Peter,

thanks for your quick response.

> > Problem in brief: I am trying to model the trapping efficiency of a
> > scintillating optical fiber and the efficiency I measure is more than
> > 10x the manufacturer's value.
>
> You trap photons that are produced inside the fiber. The Cherenkov
> photons even have a specific cone distribution relative to the radiating
> particle's direction. To understand your situation it would be best to
> eliminate the Cherenkov photons (no such process) in your simulation for
> the moment.

I can certainly turn off Cherenkov. That could reduce the number of variables for now.

>
> > I am trying to model the scintillation and Cherenkov light for a Bicron
> > BCF-10 scintillating fiber. factory values for the refractive indices of
> > the fiber are n2 = 1.49 (cladding), and n1= 1.6 (fiber core).
> > Calculating the trapping efficiency using the equation eff =
> > 0.5(1-n2/n1) gives eff = 3.44%, which is the manufacturer stated value.
>
> Real fibers aren't perfectly smooth. A little roughness introduced
> changes the situation dramatically.
>
> > Strangely, when I calculate eff = (# photons absorbed in PMT)/(# of
> > scintillation photons + # of Cherenkov photons produced), i get eff =
> > 37%. (steppingAction used to keep track of produced and absorbed photons
> > is attached.)
>
> Looks ok.
>
> > Do I misunderstand something in the opticalPhysics process? Shouldn't
> > the number of photons trapped in the fiber and transported to the PMT be
> > equal to the value calculable based on the ratio of refractive indices
> > of the cladding and the fiber? I understand that value only describes
> > the meridial rays, but the skew rays only add a few percent.
>
> I would think that almost all of your rays are skew rays. Why do you
> think the opposite?

I agree the rays are mostly skew. I'm just confused as to why I'm detecting so many of them. Shouldn't the detection efficiency I'm calculating be closer to 1/2 the manufacturer stated trapping efficiency?

matt

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