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Forum: Processes Involving Optical Photons
Date: 02 Aug, 2006
From: Gumplinger Peter <Gumplinger Peter>

> OpSurface -> SetSigmaAlpha(sigma_alpha);

> SMPT -> AddProperty("SPECULARLOBECONSTANT",pp,specularlobe,NUM);

> SMPT -> AddProperty("SPECULARSPIKECONSTANT",pp,specularspike,NUM);

> SMPT -> AddProperty("BACKSCATTERCONSTANT",pp,backscatter,NUM);

> concern the interface between your detector and the thin dielectric
> layer between it and the wrapping,

Yes, correct.

> while:

> SMPT -> AddProperty("REFLECTIVITY",pp,reflectivity,NUM);

> and 'polishedbackpainted' or 'groundbackpainted' concern the

> wrapping. "

No, not correct as written and as you understand my remark. What I meant is that the part 'backpainted' means there is a wrapping with intermediate thin layer (as opposed to 'frontpainted' where there is no such layer)

Also, in my same reply to http://geant4-hn.slac.stanford.edu:5090/HyperNews/public/get/geometry/448/1.html I incorrectly wrote:

> SetFinish(groundbackpainted) or SetFinish(polishedbackpainted)
>
> depending on whether your wrapping is a diffuse or polished reflector.

The 'ground' and 'polished' part actually refers to the interface scintillator/dielectric layer. The wrapping itself is ALWAYS assumed to be Lambertian reflector; an "external diffuse reflector" for those photons which are refracted (transmitted) through the surface interface into the thin dielectric layer:

At:

http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/TrackingAndPhysics/physicsProcess.html#5.2.5
:

In case the surface is painted or wrapped (but not a cladding), the table may include the thin layer's index of refraction. This allows the simulation of boundary effects at the intersection between the medium and the surface layer, as well as the Lambertian reflection at the far side of the thin layer.

> If revetment reflectivity is zero, all photons are absorbed by the

> wrapping.

Yes, correct

> If sigma_alpha is a property of the interface detector-dielectric

> layer

Yes, the interface of your dielectric scintillator and the thin dielectric layer between it and the wrapping.

> and groundbackpainted is a property of the wrapping, and if photons

> are all absorbed by the wrapping, then why sigma_alpha acts only if

> the wrapping is grounbackpainted and not polishedbackpainted?

Now, here 'polished' doesn't refer to the wrapping, as I corrected myself above, but to the initial dielectric interface and since it is perfectly 'polished' the sigma_alpha is ignored. It acts only if you specify "groundbackpainted"

I'll post an errata to my posting from Oct. 2005 which was confusing as it was formulated.

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