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Forum: Geometry
Re: Question Surface reflectivity (andreas battaglia)
Date: 23 Feb, 2006
From: Gumplinger Peter <Gumplinger Peter>

> 2) define a dielectric_dielectric groundfrontpainted surface: in this case....

I think you meant to write 'groundbackpainted' (you can also use 'groundfrontpainted' but the result is the same as your approach 1)

> Now, if all this is correct, ....

I see a problem, you need to specify:

OpSurface -> SetType(dielectric_dielectric);

for 'backpainted' (see Source listing 5.2.9 in the Application Developers Guide).

> how can I check the reflectivity, in particular in the second case?

Yes, counting reflections in 1) is easy. The photon reflects off the metal surface only once or it is absorbed. The probability of absortion is sampled from "Reflectivity".

In the second case, what happens is somewhat more involved. A photon encountering a 'backpainted' surface can do several things. It can be totally internally reflected on the interface of scintillator/air-gap. This 'reflectivity' has nothing to do with the wrapping and depends on the incident angle and the index of refraction of scintillator and the air-gap. It can also be Fresnel reflected. If the photon is Fresnel refracted out and into the air-gap it will reflect off the wrapping (backpaint) with set probability. It then encounters the air/scintillator interface again. If the wrapping was perfectly polished, I gather, it'll refract out of the air-gap and back into the scintillator. If not, the incident angle may have changed from the rough wrapping and a reflection occurs or it Fresnel reflects on that interface and bounces back to the wrapping where the reflectivity is sampled yet again. So, the set reflectivity for the wrapping is convoluted with the bounce history the photon takes and in general the simulated reflectivity will be lower than the set reflectivity for the wrapping.

theStatus is the last thing that happens to the photon as its bounces are simulated internally to the G4OpBoundaryProcess routine.

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