Strange LET values

Scoring
1

Hello,

I am coming back from few weeks ago when I asked a question regarding the scoring of LET and Fluence.
I managed to perfom the simulations but after analyzing the results it seems to me that the LET values are much smaller than expected - few orders of magnitude. Did I oversee a normalization or did I get the units wrong?

I am attaching the input and flair project alongside with the plots that I have.

Many many thanks in advance!

24godeuri.flair (28.3 KB)
24godeuri.inp (9.6 KB)

FLUENCEvsDEPTH-24plates
FLUENCEvsDEPTH-24plates840×750 38.7 KB

LETvsDEPTH-24plates

3

Hi Irina,

It seems to me that the LET plotting you’re obtaining is directly produced from the 1-D projection plotting of this card:

USRBIN            10    ENERGY       -22        56         2         2EneDep
USRBIN            54        -2       -2.       200       200       200 &

However, I’m afraid this is not the correct way to proceed. In general, scoring LET in FLUKA is a non-trivial task, and I think these two posts might be very useful for you: Strange LET shape for 800 keV protons in LiF - #5 by msabateg and Difficulty scoring LET, Dose and Energy for protons in water.

Regarding your approach, you’re attempting to observe the LET deposited energy by performing a transversal integration of the USRBIN result. There are two key elements missing in your normalization:

  1. The surface area of the USRBIN slice (since you’re doing a projection, you should multiply the results by the transversal dimensions of your USRBIN, i.e., 4*4 = 16).
  2. The normalization factor: GeV/cm → keV/μm = 100.

With these adjustments, I ran a quick simulation for your 18 MeV case, and the results seem to agree with the NIST stopping power table (https://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html)

LET_18MeV_proton

I also noticed that you organized your run using several if/elif conditions. While this works, it’s not the most efficient approach. I’ve reorganized the different energy cases using Flair’s function evaluation capability (F3.6} Function evaluation). You can find an updated flair file here:

24godeuri.flair (10.2 KB)

Have a nice day!
Cheers,
Daniele

4

Dear Daniele,

The conversion from GeV/cm to kev/um I just figured it out but it’s so good to have a confirmation on that.
For the surface area I thought that in this case it is done automatically but thank you so much for the clarification.

I will check the input that you sent when I get into the office. The if/elseif trick was the only idea that I had so that I don’t have 9 almost identical simulations :joy:

Thank you so so much for your help! :heart:

Best,
Irina

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