# Scoring proton range and range straggling in LiF

Hi! I wish to estimate with FLUKA the range R and the range straggling S of 4 MeV protons in LiF. Assuming a Gaussian distribution of the final particle positions along the penetration depth z, I score the proton fluence with a USRBIN and plot its 1D Projection along z. To my understanding, at this energy the 1D-projected fluence per primary should approximately follow this law:

1/2 - (1/2) * erf [(z-R) / (sqrt(2)*S)]

By best fitting the obtained FLUKA simulation with the above function, I get R = 118.0 um and S = 1.801 um. I compare these values with values found with SRIM: while the ranges agree within a small error (118.0 um vs. 118.8 um), the range stragglings differ much more, as from SRIM I get S = 2.081 um, 16% larger than 1.801 um of FLUKA.

It must be said that the value of S found from SRIM seems to be correct at least when using an analytical model for LET curve calculation: the SRIM S value works well to replicate the LET curve calculated by FLUKA, besides that calculatd by SRIM itself. However, analytical formulas such the one I used (derived from Bortfeldâ€™s) could give uncorrect results below say 10-12 MeV.

Nonetheless, I would like to check if there is something wrong in my way to estimate S from the FLUKA simulation of fluence. Do you spot any bug in it? Alternatively, is there a more direct way to score range and range straggling in FLUKA?

Thank you a lot in advance for your help and kind regards,
Enrico

P.S. I upload the FLUKA files.
fluenza_LiF.flair (2.8 KB)
fluenza_LiF.inp (1.9 KB)

Hello Enrico,

Thanks for this post.

Your input file looks OK. Indeed, thereâ€™s no direct way to score range/straggling in FLUKA.

The first impulse would have been to suggest lowering the proton-transport threshold, but this turns out to have minimal impact in your problem. 100 keV (default cuts with DEFAULTs PRECISIOn) and 1
keV protons (lowest possible) have a range of about 1 um and 4.8e-2 um respectively in LiF (quick estimate from NISTâ€™s PSTAR website). However, in your problem it looks like after ~120 um the proton fluence is zero: a further 4.8e-2 um wonâ€™t make much difference.

Switching off the few nuclear reactions that do occur, the difference is also minimal for straggling.

I also did not see an effect lowering the delta-ray production thresholds for protons (as was reasonably expected, since the discrete and continuous energy loss schemes of FLUKA are consistent/robust under changes in the delta-ray-production threshold).

It might be that the differences you observe are due to different ionization-loss fluctuation models in SRIM than in FLUKA.

With kind regards,

Cesc

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Hi Cesc,

thank you very much for your detailed reply and for the time you spent in checking my input file and all those other things that could have influenced the result!

Kind regards,
Enrico

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