DPA Discrepancy Between Mesh and Region Based Scorings

Dear FLUKA experts,

A colleague, who is having login issues to the FLUKA forums, has noticed an unexpected output in calculating DPA. Ideally, the results of DPA as calculated by a region-based scoring should match a mesh-based scoring of 1 bin with equivalent volume of that region. However, we see that this is not the case. The DPA as calculated by the region-based scoring is higher than the mesh-based scoring by a factor of exactly the volume of the region. We could not find an explanation for this in the manual. Is this behavior expected and understood? Displacements Per Atom is already a volume-normalized quantity, so it should not be corrected by total volume when used with region-based scoring.

Thank you!

Dear @Adobe_Achromat,

Thank you for your question. What you describe is exactly what you should expect.
In order to get a proper estimate of the displacements per atom, the number of atoms in the considered volume plays a role by definition (i.e. a division by the volume is implied).
Now, in the case of USRBIN mesh scoring, the volume of each bin can be computed by FLUKA and is used to automatically to obtain a proper estimate of the DPA. In the case of USRBIN region scoring (or other region based scorings in general), instead, the code does not know the region volume. This is why in the second case you should divide the result by the volume of the region (in cm3).

Cheers,
Davide

Ah, I see; the number density in the material is known, but without the volume (which regional scoring is ALWAYS agnostic to) it is not possible to actually normalize by number of atoms. Thank you very much, that explains the difference!

Now, we were already familiar with this phenomenon for scoring of other quantities (e.g., energy deposition), but for DPA we did not see this in the documentation (which we checked in advance). So, we were wondering if it might be worth adding a short statement to the documentation to clarify this point, since technically Displacements Per Atom is not actually being scored in a DPA-SCO or ARCDPA regional scoring…

For energy deposition the value of the regional scoring is still sensible: it is the total energy deposited in the entire region! But for a DPA value, dividing by the number of atoms is implicit to the name (Per Atom), and here we see that in the case of a regional scoring the total calculated displacements are divided by a number density appropriate for the material, but the division by volume is missing (or I suppose, taken to be 1 cubic centimeter). So the situation is slightly more complicated than the usual case, and maybe warrants a note?

Thanks again for your time and help,
Will

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Dear @Adobe_Achromat,

Indeed, we have reviewed the manual page for USRBIN (note 13 in particular), which has been reformulated to better clarify this aspect. Please note that the revised manual page will be available from the next release of the code.

Best,
Davide

Thank you very much Davide! I appreciate your help, and am glad that our suggestion was helpful.