Questions about Cherenkov light

Dear Fluka expert,

Recently, I have tested many Cherenkov light models and found that there are always some problems that cannot be solved and have been bothered for nearly a month.

During the test process, I found that:

  1. Like the source I set in Inp, the generated Cherenkov light will be emitted on the cone at an angle of 60 ° with the beam direction. When I set the refractive index of the material in region 1 to 2 and the material in region 2 to 1, according to Snell’s theory, the critical angle of total reflection at the boundary between regions 1 and 2 is 30 °, which means that all the generated Cherenkov light can pass through the boundary from region 1 to region 2, But I found that at 60 ° (with respect to the space angle is Π) It is found that 40% of the photons enter the void region through the boundary from region 1, which is completely inconsistent with the total reflection theory.

2.No matter how I change the refractive index n of region 2, 40% of the photons are absorbed and 60% escape. I don’t understand the meaning of absorption,and these 40% of the photons are at 60 ° (space angle Π) Is recorded in usrbdx. When I change the material of region 2 to ‘blackhole’, 100% of the photons escapes, Does that mean that 40% of the photons in the previous case entered region 2 from region 1?

No matter whether you can put forward a solution to the problem, i would like to express my most sincere thanks!
Attachment is Inp file.
refraction.inp (1.8 KB)

Dear @Will,

In answer to your first question: The optical photons that you generate by switching on Cherenkov radiation should indeed be emitted at the specific angle of 60 degrees in this case, however this does not mean they are emitted at a single wavelength. The refractive index of the material is also dependent on the wavelength, hence not all optical photons will exactly satisfy the total internal reflection criterion. This would cause this particular amount of photons to still cross into the Air region.

In answer to the second question: the amount of absorption of photons does not necessarily have anything to do with the refractive indices of the respective materials, more with their absorption coefficients. For the materials you use these optical properties are not specified and FLUKA will assume defaults values. However indeed when implying the ‘BLACKHOLE’ material to the second region will kill all photons arriving there, hence they escape at a 100% rate.

You probably need to include another OPT-PROP card which associates the refractive index to a specific wavelength and also defines the wavelength range. (If you read the documentation and especially the notes carefully you can set a constant value of the refractive index for this entire wavelength range by using continuation cards with SDUM=&1, &2, &3 by specifying the derivatives). Th OPT-PROP cards can then also be used to set absorption and diffusion coefficients in addition to the refractive index. If you set the absorption coefficient to a different value you will evidently see a change in the amount of photons absorbed.

Using another strategy to get ‘clean’ reflection: I obtain the USRBIN plot below of the optical photon fluence by narrowing the generated Cherenkov spectrum using OPT-PROD to a very small range which corresponds to the central wavelength that I set using an additional OPT-PROP card. The other OPT-PROP card then associates the refractive index to this wavelength. As you can see there is no transmission from the Oxygen to the Air region and also the output file confirms this, no photons are absorbed in the Air region.


Hopefully this puts you on the way, let me know in case things are not clear, cheers,



Dear @anwaets ,

First of all, thank you very much for your reply, which helped me a lot.
I have the following questions to reconfirm with you:

  1. Do you mean that even if i use OPT-PROP for SDUM=blank to set the refractive index, it will still change with the wavelength? Is it not a fixed value? I used to think that this was a constant value,All light in the wavelength range had this refractive index.

  2. If OPT-PROD has set the wavelength range of light production of a material, will this material not transmit light in this wavelength range by default? Or is it necessary to use OPT-PROP again even if the wavelength range of light production and transmission is the same?

  3. The two materials I use ‘oxygen’ and ‘air’ have default optical properties in FLUKA, but I set their absorption coefficient and diffusion coefficient to 0 in OPT-PROP. Why is there still optical absorption of certain components?

Thank you again for your help. I wish you a happy life!

Dear @Will

Cherenkov light generated with a very relativistic particle in a medium with n=2 (region1) will indeed propagate at an angle of 60° relative to the direction of propagation of the particle. Then, that same light hits region2 at an angle of 60° relative to the surface normal. region2 having n=1, the critical angle for total internal reflection is 30°. This angle being smaller than 60° implies that ALL of the light is internally reflected.

I plotted the propagation of light in your simulation using USRBIN. As you can see, none of the light enters region2. Instead, all of it is internally reflected back into region1. Finally, it is stopped at the back edge of region1 as it enters the blackhole void.

Could you tell me which scorers you used exactly to derive those 40%/60%?


1 Like

Dear @blefebvre ,

Thank you for your reply!

First of all, I am sorry that I made a mistake in my initial description: according to the total internal reflection theorem, all Cherenkov light cannot cross the boundary.

Secondly, 40% absorption was obtained using USRBDX,The result really puzzled me.

Dear @Will ,

Just to clarify, you are using the integrals of the “re1-2” and “re1tovoid” detectors to calculate the 40:60 fractions?

Dear @blefebvre ,

Yes.Other USRBDXs(‘re-2de’ and’re1-2de1’) are used to count photons within a specific angle range. It is found that the refracted photons(40%) are all at the angle of 60 °, corresponding to the spatial angle Π.

The same result can also be obtained by using usrbin in the .out file.


Kind regards,

As Andreas was hinting, the 40% absorption you observed in the output file is explained by the wavelength transport range not encompassing the Cerenkov production range. To obtain a 0% absorption, you must either (1) reduce the wavelength range of Cerenkov production (like Andreas suggested), or (2) increase the wavelength limits in your optical materials using the OPT-PROP card (see below for an example). Note that the default wavelength limits are 2.5E-5 to 6.00E-5 [see card OPT-PROD for SDUM=WV-LIMIT for details], smaller than what you specified for Cerenkov light production (2E-5 to 9E-5), which explains the 40% loss you noticed.

Dear@blefebvre ,

It’s really the error that I didn’t set the wavelength transmission range.

Thank you for your help!