Total reflection

Dear Fluka experts,

I have a B4C-coated silicon mirror and 1keV x-rays as the source. At a grazing angle of 9 mrad, 96% of the rays are reflected and only 4% absorbed, I want to calculate the penetration depth and/or absorption length.
With the “Beampart” card, I am unable to see the reflected beams from the surface. I would appreciate it if you could tell me how to ensure that the simulation takes into account the total reflection.

Absorbed Energy.flair (4.1 KB)
Absorbed Energy.inp (3.9 KB)

Best regards,

Dear marziyeh.tavakkoly,

With the beam card you can only see the primary particles before they interact once. In your case, I suppose you wish to see the albedo of the backscattered radiation. It means the primary X-rays would interact with the surface and the USRBIN detector will not see any backscattering. Try scoring the particle “PHOTON” instead.

Also, since you simulated particles of very low energy, I recommend that you also lower the production cuts with the card EMFCUT. As for the transport cuts, they should start from region “VOID” so that you cover all relevant regions of your geometry. For example:


Finally, you are using a very large number of bins for your USRBIN detectors. Make the binning coarser or you might experience instabilities when running the simulation.

Dear Benoit,

Thanks a lot for your kind help and quick response.

About the “very large number of bins for your USRBIN detectors”, do you mean Unit:69BIN or (Nx, Ny, Nz)? Should I use a smaller value?

I was asking about reflection, since penetration depth (Attenuation length) should not differ much at a grazing angle of 9 and 19 mrad.
In experiment results, it is about 14 and 14.3nm for 9 and 19mrad, but in fluka, it is 25 for 9mrad and 51nm for 19mrad). My first thought was that the reflection was not included.

I would appreciate any suggestions you may have regarding how to correct it.

Dear Marziyeh,

Your USRBIN detectors have of the order of 1000 bins in all 3 dimensions, which in total makes up to one billion cells. Considering that each stores a double precision number weighting 8 bytes, you would be using 8GB of memory just to store it! Then consider that you may have multiple of such detectors, and run parallel simulations. This way you fully load the memory of your computer and overwhelm the software. Simply keep this in mind when declaring multi-dimensional detectors.

I suppose you are wondering whether FLUKA handles X-ray specular reflectivity and the answer is no. Unfortunately I would advise using another software, or to write a user routine which handle this special case (usrmed.f might work for that). Keep in mind that FLUKA generally does not use wave-like models for particle propagation. Hence, if the physical process you have in mind uses the wave properties of the radiation (uses Maxwell’s equations for example), it is likely not included in FLUKA.

Dear Benoit,

Thanks a lot for your help.