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Forum: Processes Involving Optical Photons
Date: 28 Nov, 2008
From: Estela Suarez <Estela Suarez>

Dear Peter,

thank you for your prompt reply. I have made the modifications that you suggested, and some other cross-checks. I explain you here their outcome.

Modifying option 1) as you suggested:


makes the effect we wanted: now the PMT collects almost the same amount of light as with option 2), about 40%. (By the way, I had implemented a dielectric-metal surface because I had understood from the post #882 on 27-03-2008 (this forum) that only dielectric-metal surfaces take the 'REFLECTIVITY' into account. I have checked with the dielectric-dielectric surface that changing the reflectivity does make a difference. I just note it here because it can be useful for somebody else.)

With your suggestion method 1) works well. This made me at the beginning to think about using method 1), since in this way I don't have to implement the physical volumes of AirGap and aluminum wrapping. But in fact there is an important difference in the overall output of the simulation: since the aluminum wrapping is not there anymore, ALL the X-ray photons that I launch interact in the scintillator. That is not the case in the real experiment, since low energy X-rays (5keV and below) will often be stopped by the wrapping. Therefor I decided to rather use method 2), which reproduces more accurately both the optical and the material properties of the detector.

To understand the whole optical surfaces subject I checked anyway if I could reproduce with method 2) the low light collection from my initial results from method 1). So I introduced the changes that you suggested:

 G4double polish=0.0;

first I didn't understand well why I need to put the surface of my reflector as completely unpolished, since I would rather consider it as perfectly polished. In any case I implemented it like this and I was very surprised to see that I get actually I higher light collection that ever before: about 45%. Do you know why should it be so? When defining polish=1.0 (or just SetFinish(polished)), I get back my 40% light collection. By the way, is it normal that I get more light the rougher the aluminum is?

Just as an additional note, the only way in which I could reproduce the low result (20%) using method 2) is when I implemented the Air-Aluminum surface as dielectric-dielectric using UNIFIED model. This is in any case not the result that I am searching, because in this way I am loosing the light by refraction across the aluminum. What I want is that the aluminum either reflects back (98% of the time) or absorbs (2% of the time) the optical photons.

During the checking process a couple of more general questions came to my mind. I would be very thankful if you could answer them:

a) You said in your e-mail that the wrapping can ONLY be a "diffuse" reflector. So, what is then the best way of simulating an specular reflector (like aluminum foil)? Is actually my method 2) doing the right thing? As I said before I just want that the aluminum either reflects back (like a mirror) or absorbs the optical photons.

b) It is said in the Application Developers Manual that: "The border surface is an ordered pair of physical volumes, so in principle, the user can choose different optical properties for photons arriving from the reverse side of the same interface". What happens if I want the same kind of behavior from both sides? Do I need to implement the same twice? (My question is because some of my photons come through the air to the aluminum, and come back, so I see Air->Alu and Alu->Air).

Thanks a lot for your attention. Your comments are very helpful.

Cheers, Estela.

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1 Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Wolfgang Lukas - 10 Dec, 2008)
2 Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Gumplinger Peter - 10 Dec, 2008)
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