Interesting border interactions with OPTIPHOT

Dear FLUKA experts,

I’ve applied an OPT-PROD card to a 40x40x40 cm box of air and OPT-PROP cards to the box of air and the surrounding 100cm radius spherical void. In both OPT-PROP cards I’ve stated the refraction index to be exactly 1. As such, I would not expect there to be any border interactions occurring between the air and void.

Despite this, I’m observing the following graph.

I just wanted to ask why these border effects are occurring.

Many thanks,
Talha Shameem

Dear Talha Shameem,

I would indeed expect no interaction between the box of air and its surrounding. The exact geometry of the optical photon fluence will, however, also depend on your primaries and on other optical properties. You may not always have perfect symmetry.

Would you mind sharing your input?

Dear @blefebvre,

Thank you for your reply! I wasn’t sure if the flair file would be useful or not but I’ve included that as well. Let me know if there’s any more information you require.

Scintilair.inp (2.2 KB)
Scintilair.flair (2.6 KB)

Dear Talha Shameem,

I see in your input that you provided an absorption length of 10 cm in your cube of air. Given the scale of your simulation, it is not surprising that you see asymmetries. Try again using an absorption coefficient of 0 (or with something closer to reality) and you might get the results expected.


That said, looking at your OPT-PROD card, you specified a time constant of 5 seconds for scintillation. I don’t think you meant that. Remember that the default units in FLUKA are GeV, seconds and centimeters.


Dear @blefebvre,

Thank you for your reply. Indeed the absorption length was the issue! However, I was lead to believe that the absorption parameter should be put as 0.1 in order to prevent trapped photons and infinite loops?

Also, regarding the time constant, is this the decay time constant for the light produced by OPT-PROD?

Kind regards,
Talha Shameem

Indeed you provide an exit path to your optical photons, either from absorption in the material or on reflectors, to avoid having them bounce forever in the optical material. But considering your geometry with a cube of air, not azimuthally symmetric, you would then see asymmetries in the optical photon fluence for any non-zero absorption. It depends what you want to achieve.