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Question Re: Optical Photons Not Propagating Down WLS Fibre 

Forum: Processes Involving Optical Photons
Re: Question Optical Photons Not Propagating Down WLS Fibre
Re: Feedback Re: Optical Photons Not Propagating Down WLS Fibre (Gumplinger Peter)
Date: 19 Dec, 2016
From: <Emile>

Hi Peter, thanks for the quick reply, sorry for my delay.

> Hi Emile,
> 
> > I would expect more photons to be hitting
> > the detector for water than for air.
> 
> What makes you expect this?

I would expect this result based on Snells law. The scintillator has a refractive index of ~1.52. Air has a refractive index of 1, and water of ~1.33. For photons to enter the fibre, they must be entering at an angle less than the critical angle of the interface, determined by arcsin(n2/n1), which for air is ~41 degrees, and for water is ~61 degrees.

This would lead me to expect that more photons would be entering the fibre for water than they would for air, which we do see in the simulation.

What we're not seeing is the photons then staying in the WLS fibre, which I would expect based on the outer layers having successively larger refractive indices than the inner layers (leading to no total reflection when the photon is entering the fibre, and probable total internal reflection once the photon is inside, I would assume).

Is my logic above correct? If so, are there any other factors that might be allowing the photons to escape the fibre as they are?

> 
> > I currently have the volumes nested (gap within scintillator, fibre
> > within gap), and am not implementing any optical surfaces between them.
> > I'm a relatively new user of Geant4, is there something else I have to
> > do to see the expected photon behavior?
> 
> No, what you are doing is fine. Except, of course, no real surface is so
> perfectly, mathematically smooth as is the case with your geometry.
> 
> Best regards, Peter
> 

Thanks! Emile

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1 Feedback: Re: Optical Photons Not Propagating Down WLS Fibre   (Gumplinger Peter - 06 Jan, 2017)
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