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Forum: Processes Involving Optical Photons
Date: 10 Dec, 2008
From: Wolfgang Lukas <Wolfgang Lukas>

Dear Estela,

thank you very much for sharing your problem in such great detail. I was surprised to realize that you and I are apparently doing something very similar! I am trying to simulate the optical photon transport in a scintillation detector tube with a side-on PMT. The task is to optimize the light collection at the photocathode by using different geometries and materials.

In order to get a more thorough understanding of all the photon statistics, I am also using what you describe in "method 2": different physical volumes for scintillator, gap and wrap. My "coupling window" between the scintillator tube and PMT is quite small (an unfortunate constraint!), and therefore I can see big differences between e.g. diffuse paint (groundfrontpainted), diffuse wrap (like Teflon) with air gap, polished aluminum foil (I use "common aluminum" with a reflectivity of 91%) with/without gap, ...

I am counting things like photon absorptions, total internal reflections, the number of photons going from one volume to another, etc. in "SteppingAction", add them up in "EventAction", write these values into log files and calculate average values & statistics in "RunAction".

I can see that the critical angle plays an important role in the "with gap" setup, especially with a diffuse reflector. The total internal reflections reduce the absorption losses, and the photons can propagate more easily. The diffuse reflector introduces some "randomness". Consider these scenarios:

1) gap + polished reflector: The gap does not change the propagation of photons, it only reduces absorption losses for reflections at shallow angles. Imagine some photons bouncing from one side to the other - even if they slowly approach the PMT before they get absorbed, they will be reflected back at the Glass->Vacuum surface (critical angle!), and thus never reach the photocathode.

2) no gap + diffuse reflector: All reflections will be Lambertian. Imagine a photon coming from the "top" of the scintillator, hitting the "side" in a shallow angle: it will most likely lose its favourable direction and become reflected sideways. Chances are low for the photon to recover its favourable direction; if it approaches the PMT, it will most likely be reflected back at the Glass->Vacuum surface (see above).

3) gap + diffuse reflector: This setup combines the advantages of both worlds: fast propagation at shallow angles (behaviour like a polished surface), and some randomness when Lambertian reflection occurs (so that photons will not "bounce sideways forever", but eventually find a favourable direction).

--> Perhaps this also explains your higher light collection with the rougher aluminum surfaces?

I found that for each geometry there is an optimal choice of reflectors (or reflector combinations). It is also very important to consider the total internal reflection at the Glass->Vacuum surface of the PMT, rather than "just count" all photons that reach the PMT.

ad b) I think you will not really see Alu->Air, because the photons are reflected at the Alu surface without entering the physical Alu volume. (Peter, is this correct?) But in the case of total internal reflection of e.g. Sci->Air, my photon statistics suggests that the photons really enter the physical Air volume before they are reflected back into the physical Sci volume (like evanescent waves?).

I hope this was helpful to you and not too confusing. :-)

Best regards, Wolfgang

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