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Idea Re: Single solid with different optical boundary properties 

Forum: Processes Involving Optical Photons
Re: Question Single solid with different optical boundary properties (Bertrand Echenard)
Date: 04 Sep, 2014
From: Michael H. Kelsey <Michael H. Kelsey>

On Thu, 04 Sep 2014 17:05:54 GMT, Bertrand Echenard wrote:
> I am trying to simulate the propagation of optical photons in a crystal
> where one of the faces is ground, and the others are polished (this can
> improve the longitudinal response uniformity). The crystal is surrounded
> by a reflector, and has a readout at one of its end.
> As far as I understood, the UNIFIED model can't simulate this case,
> since the crystal faces must either be all polished or ground. What
> would be the best way to do so?

I don't think it's the UNIFIED model per se which has that restriction. The problem is that you can only assign one optical surface to a logical volume (G4LogicalSkinSurface), and that surface applies _everywhere_ (including at boundaries with internally placed daughter volumes).

You want to make use of G4LogicalBorderSurface, which is instantiated between two adjacent (or nested) physical volumes, i.e., G4PVPlacement. In your case, you'd make some very thin flats of some material, and create the border surface between the crystal (as the first PV) and the flat (the second PV). For each surface, you can assign polished or ground independently.

If you have a complex, well-written geometry model, doing this might be really awkward. You may not have access to the crystal's PV at the time you need it. Or you may have many crystals, and you only want to implement the optical surfaces once, then do multiple placements.

The way I solved a similar problem was to create an "envelope volume" the size of my scintillator, place a daughter volume of the actual scintillator inside that envelope to get a PV, then place the wrapper material around the surface of the scintillator (as daughters) to get the second PV. With that structure, I can place the logical "enelope" multiple times into my geometry, and the daughter PVs would remain the same, along with their associated surfaces.

  -- Michael Kelsey

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