I am doing my first simulation in FLUKA/FLAIR, trying to score the dose in a person from a photon beam of 9 MeV with 3 x 100 000 primaries in vacuum.
However, I am not getting the result to match with the theoretical value.
The scoring is for the person - as region - to measure DOSE-EQ. AUXSCORE connected to the BIN-card is set for AMB74 and the assigned material for the person is TISSUEIC.
In FLUKA the measured value in the person is 245 pSv/primary=73,5 microSv in total, but theoretically I’m getting 5,6 nSv.
The person weighs 77 kg.
I might have gotten a tunnel vision, missing something here, but I can’t figure out what it is.
Why is the FLUKA result differing from the theoretical value? Do I need to consider the volume even though the person is set as region?
It seems to me that you are multiplying the FLUKA result with the number of primaries used for the simulation, which is wrong.
You need to multiply with the actual beam intensity. Furthermore, you need to divide the result by the volume of the region in the case of a USRBIN scoring with Region type.
If you multiply with photon/s, then you indeed get a dose rate value.
In general, you need to use the same normalization as it was used for the theoretical calculation.
Ok, then something is still wrong here. My aim was getting the total dose using DOSE-EQ for a region. Not the rate.
The approach was:
Get the value of concerned region from simulation. This was found to be 245 in units pSv/primary .
Multiply with quantity of primaries (since the unit in 1. is pSv/primary) to get total dose in Sv.245*E-12 Sv/primary x 300 000 primaries = 73,5*E-6 Sv
Since the radiation weighting factor for photons is 1, the equivalent dose from answer in 2. equals absorbed dose.
Then to compare this to the theoretical value.
Calculate the total amount of energy for the region (the person):
9 MeV/primary= 1,44*E-12 Joule/primary
1,44*E-12 Joule/primary x 300 000 primaries=4,32*E-7 Joule
Convert energy to absorbed dose:
4,32*E-7 Joule / 77 kg = 5,6*E-9 Gy
Multiply with quality factor 1 to get equivalent dose:
First, you did not divide the DOSE-EQ USRBIN value by the region volume, as recalled by David above: in case of USRBIN scoring by region, the division by the region volume is up to you (this means that the simulation result is in pSv cm^3 / primary and becomes a dose equivalent in pSv/primary only after the division by the region volume). Hence your simulated value is actually 0.96 nSv.
Then, you missed the fact that in your simulation the 77 kg ‘person’ actually absorbs much less than 9 MeV/primary, since half of the beam is passing below it (at negative y), without impacting it, and the other half (at positive x) is not totally absorbed either, as you can realize looking at the actual energy deposition in the region of interest, reported in the region table towards the end of the standard output file of each cycle. Hence your ‘theoretical’ value is actually slightly less than 1 nSv too.
Finally, note that the multiplication by the simulated number of primaries is totally pointless: you can perfectly do the intended comparison without it. It has to be kept in mind that, in order to eventually obtain a meaningful value, what matters is the real beam intensity, and not the simulated number of primaries, which has only an impact on the reliability of the simulation result, in terms of statistical quality.
