Surface reflection of Tungsten

Dear FLUKA experts,

I am interested in understanding the surface reflection of electrons caused by Tungsten. In particular, I would like to obtain the reflected angle, and reading the different topics in the forum I understood that reflection is specular by default, so the electrons are reflected with the same angle, is this true?
In addition, I have made some runs to understand the electron reflection through the USRBIN card and trace the electrons.

Near the surface, the electrons seem to be reflected by the same angle (in this case 10 degrees) however I do not get the reason why electrons are reflected in all directions.
Finally, I suppose that the reflection takes place on the same plane so in this case Y= constant for all the electrons, is it correct?

Best regards,

Dear @Singh,

Could you please post your .flair file?

Dear @amario

Thanks for your reply. Here is the .flair file
Reflection_10degree_10MeV.flair (1.9 KB)


Hello Lovepreet,

I do not spot any anomaly with your simulation. Indeed, 10 MeV electrons are impacting with a grazing angle on a tungsten slab.

  1. Electrons interact with the tungsten via various mechanisms (ionization, Compton scattering, bremsstrahlung etc). Nobody forbid these electrons to have different paths after the interactions, that’s why you see lot of electrons flying with different angles.
    Notice that, at 10 MeV, you produce photons above the pair production threshold energy, i.e. generate more electrons (and positrons) isothropically.
  2. For the same reason, y is not constant for all electrons.

Now a few comments on your plot:

  • You are plotting the projection of the energy fluence in the plane xz. You might want to select only the electrons around y = 25, or at least the right bins to see the electrons bouncing back with minimal interaction.
  • You are spanning over a conspicuous amount of order of magnitudes in your plot. This visually enhance the most uncommon events. With your simulation I can also show you the following plot:


If you want to manually flip the electrons trajectory when they travel in the tungsten slab, you need to use usrmed.f user routine, but this is another topic.

Thank you, @dcalzola, for your answer. Just a couple of doubts:

  1. Following your point 1. I would think that in FLUKA it is not possible to get information about the average angle of the reflected electrons, is it true? or is there a scoring card that could give me some insights about it?
  2. When you say ‚Äúmore electrons are generated isothropically‚ÄĚ what does isothropically mean in poor words? I know that once the primary electrons hit W I have the generation of secondary electrons so you might be saying that these electrons once generated travel in all directions without any ‚Äúpreference‚ÄĚ.


  1. You have many possibilities to study the angular distribution of the deflected electrons. You can, for instance, use USRBDX or USRYIELD scoring.
  2. When the electron generates a high-energy photon, this can create an electron-positron pair. Perhaps ‚Äúisotropically‚ÄĚ was a strong word, indeed I meant that your secondary electron could have a broader angular spectrum. It was just an example of the main processes that can alter your angular distribution.

I encourage you in reading the manual of the two aforementioned cards and giving it a try. Let me know in case of trouble.

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Thanks a lot!