|Message: Re: What is the||Not Logged In (login)|
Click on the Forum title, e.g. on the "Forums by Category" page, to read a sequence of postings to the Forum and its threads all in one page. If you are only interested in one thread or the thread following a specific posting, click the thread or the posting, which takes you to a smaller page, which contains only the part you are interested in and may be easier to navigate.
Messages are "chained" if there are only replies at the first level, i.e. 1/1.html, 1/1/1.html etc. In case of "chained" messages the message number is replaced by the icon and there is no indentation.
Inline: Display the subject line only or also the text of the posting(s); for the choice "All" the "Outline" choices are switched off.
|1||0||1||no text / full text of posting|
|2||1||All||text for level 1 only / text for All postings|
Outline: Choose the depth of the posting thread, successive toggle controls provide increasing detail.
|1||2||1||2 levels / 1 level (original posting)|
|2||3||2||3 levels / 2 levels|
|3||3||All||3 levels / all levels (all postings)|
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.
> 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
|Inline Depth:||Outline Depth:||Add message:|