Frghns.f direction of reflection

Dear FLUKA experts,

As far as i understood from the manual, in order to create diffuse reflection on the surface for optical photons, I should use ‘frghns.f’ user routine. I’m a little bit confused about region definition, namely MREG and NEWREG variables. Let’s say I have a geometry (picture 1) where I have two regions filled with two media. The source of optical photons is situated in Region 1. Region 1 is a Vacuum. The source emits optical photons in the direction of Region 2. The Region 2 is a diffuse mirror (solid piece of metal). I want to create a diffuse reflection on the mirror surface. With respect to this case which region should be considered MREG and which region should be considered NEWREG?
Thank you in advance. I attached my version of ‘frghns.f’ user routine for this case. Is it correct?

frghns.f (1.2 KB)

Dear Alexy,

For your use case, you should have the current region (MREG) to be “Region 1” and the new region (NEWREG) to be “Region 2”. Altough it doesn’t seem to make sense as rays are not really transmitted to Region 2, this approach is such that the same convention is used in case Medium 2 would be transparent. If that were the case, transmitted (to region 2) and internally reflected (to region 1) rays would have their trajectory smeared according to the FRGHNS parameter of the routine. If medium 2 is a metal, like in your case, the same logic is applied, except that now only internal reflections and absorptions are possible.

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Dear Benoit,

Thank you for your response.