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Feedback Re: Confusion on simulating a foil wrapping 

Keywords: optical processes, optical surfaces
Forum: Processes Involving Optical Photons
Date: 17 Nov, 2006
From: Gumplinger Peter <Gumplinger Peter>

> I've read that using 'groundbackpainted' always results in lambertian
> reflection from the wrap.

Yes, this is correct. Also, note that the 'groundbackpainted' finish applies only the 'dielectric_dielectric' surface types.

> Is this the case even with a dielectric-metal interface?

If your surface type is 'dielectric_metal' then there is no implied narrow dielectric layer between your scintillator and the metal reflector.

> Is it possible wrap a volume in a specular reflector?

Yes, but you must specify the thin air layer as a separate dielectric volume space in G4. What you have then is a dielectric_dielectic surface between the lead glass block and the air gap, and a second surface 'dielectric_metal' between the air gap and the Al wrap. The finish of the second surface can be perfectly polished resulting in a specular reflector.

As an aside, at the moment, the unified model's list of options: SpikeReflection, LobeReflection, BackScattering and LambertianReflection, in the case of 'groundbackpainted' refer to the interface of the scintillator with the implied dielectric (air) gap. The reflection of the wrapping is always lambertian. As things are coded, it is not possible to specify these choices for both this interface and for the wrapping. Allowing both choices can be considered for a future extended functionality.

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