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Forum: Processes Involving Optical Photons
Re: None What is the "wrapping" in the dielectric_dielectric backpaint? (Guillaume)
Date: 29 Aug, 2013
From: Gumplinger Peter <Gumplinger Peter>

Guillaume, sorry for confusing you. I'll try to put you straight.

My discussion below is based on the premise that 'Tyvek' is a reflecting material, thick enough that it will not allow any photons to escape to the outside (air). And though Tyvek is technically not a metal, in this situation it behaves just like a metallic mirror. We can give it a reflectivity but we don't need to give it an index of refraction. Photons are either reflected or absorbed. With this in mind, the Tyvek BECOMES/IS the "backpaint" in a situation were it wraps your scintillator not perfectly snug but leaves some gap.

I hope you now better understand the 'groundbackpainted' option. And while you will specify the surface to be 'dielectric_dielectric', the model as UNIFIED, the dielectic_dielectric surface is that between your LYSO and the AIR (or some other gas/material that you specify the index of refraction for) which I presume is between LYSO and the not perfectly snug Tyvek. The Tyvek itself does not exist as a volume/material/surface.

The UNIFIED parameters you give are those for the interface of LYSO and the dielectric gap.

> I have trouble understanding what "wrapping" means in the
> groundbackpainted description.

The "wrapping" is your Tyvek.

> My understanding is that the Tyvek
> surface is a kind of wrapping around my LYSO crystal.

Yes

> The flowchart
> seems to imply that this surface that I view as a kind of wrapping
> itself has its own wrapping.

Yes, in your implementation, and so there is one wrapping too many!

> Furthermore, the description refers to a crystal-air gap
> interface.

If your wrapping is perfect you don't need to specify 'backpainted', you can specify 'frontpainted' and then there is no gap.

> My understanding was that
> the layers through which light would go through in my setup is:
> 
> LYSO -> Tyvek -> Air

If your Tyvek is perfeclty snug around the LYSO, and if it does not perfectly optically seal off the LYSO from the outside (air), then you need to specify a volume made from Tyvek (and with an refractive index n > 1), and a surface between that volume and LYSO that is dielectric_dielectric, ground (NO painted option), and you specify a finite "TRANSMITTANCE". In this case, your "TRANSMITTANCE" will overwrite the one calculated from Snell's Law.

> The slide on page 7 of this document:
> 
> http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2011/12/05/03.01-31709-optical_surface.pdf
>

This page has one error - all else seems ok - no light escapes a 'backpainted' surface.

> is not helping me, since all I see for the groundbackpainted model is a
> thicker green interface area than the non-backpainted models, rather
> than what I would have expected from the groundbackpainted description
> of the flowchart: some sort of additional wrapping layer.

The thicker green indicates a gap between the backpainted reflector/wrapping and the LYSO.

> I am obviously very confused at the way geant is modelling this. If
> someone could reply with an enumerated list of every layer that a photon
> originating in the LYSO crystal goes through, given my setup, I would
> appreciated it.

With the backpainted option, the photon simply arrives at the surface, the G4OpBoundaryProcess takes over and simulates what happens at the interface:

LYSO/gap, gap/back-paint(refector,wrapping), gap/LYSO

The photon can get multiple reflected in the gap (all computed inside OpBoundary) and G4Transportation only takes over again if the photon reenters LYSO.

> Some additional info about the tyvek surface:
> 
> Specular Lobe 1.00 Specular Spike 0.00 Backscatter 0 Index of Refraction
> 1.00 Reflectivity 0.98 Efficiency 0.00

The reflection properties in G4 do not refer to the 'wrapping' (your Tyvek) but to the LYSO/gap interface. Your Tyvek is a diffuse reflector; e.g. ground, and so you specify 'groundbackpainted'. The 2% of photons not reflected are absorbed ; e.g. they don't continue into the outside/air.

Hope this helps, Peter

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