Unexpected fluence spike

Dear Experts,

I’m simulating 35 MeV protons (with a certain energy spread and a secondary lower-energy component) impinging at almost grazing angle (-0.18 deg) onto a LiF film (550 nm thickness) on a planar Si substrate (0.5 mm thickness). This sample is in air. The beam spatial distribution is rectangular in x and y; the y width is large enough to encompass the whole y height of the sample. The 2D fluence (average along x) I obtain has a strange spike, you can see it in the image – it’s obtained with 10M primaries, but the problem is still there if I use more primaries. This spike affects the resulting energy deposition within the film I’m interested in. I don’t know what could be its origin. Maybe a wrong setting in the simulation? I upload the Flair project.

Thank you in advance for your help.

NLiF85n8.flair (4.5 KB)
NLiF85n8.inp (2.4 KB)

Hello Enrico!

While we digest this particular spike in a bit more detail, and while celebrating your excellently placed FLUKAFIX cards to make sure proton steps are reasonably short such that no more than 1% of energy is lost to ionization, you may further benefit from abiding by the recommendation to switch to single scattering whenever thin layers (even 550 nm in your geometry!) of materials are present - these scenarios start to probe the limits of condensed-history Monte Carlo codes.

Indeed, passing a MULSOPT card to request single scattering for protons (SDUM=GLOBHAD ticking E<Moliere and putting a few single scatterings near boundaries) should help - of course at the expense of an extra bit of CPU time. Here goes the resulting plot respecting your original color scales:

Granted, the spike is not completely cured, but it is significantly suppressed. Finally, note that you’re plotting in a linear scale - the “spike” is a mere ~10-15% difference, though of course still deserving a bit more eventual analysis.

Incidentally, I took the liberty of addressing a majestic typo in the fluence units in your original plot.



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Hello Cesc,

thank you very much for the detailed reply! Would you mind sharing a screenshot of the MULSOPT card you used to obtain that plot, in case you haven’t deleted the Flair project yet?
Regarding the units of the projected fluence, thank you for the correction! That 3 exponent came from a copy-past of the energy-loss density plot of the same setup: I changed GeV to p but forgot to change cm^3 to cm^2.


P.S. Sorry for my late feedback, but this time I did not get a notification about a reply to my post, strange! (I also checked the spam folder).


Sure! I proceed in the atavistic stone-age ASCII way:

MULSOPT                                          1         1        10GLOBHAD



Hi Cesc,

thank you!