Simulating Background Radiation and Uncertainties in FLUKA for isotopes

Dear Fluka Experts,

I’m doing a project to simulate various radioactive isotopes (with a HI-PROPE card and a BEAM card set as an isotope) placed 0.5 m away from a NaI detector and measuring DOSE-EQ using a USRBIN card while a RADDECAY card is set in semi-analogous mode. I need to find the uncertainties involved in the practical situation and include them in the simulation, which means I need to find the ground level background radiation and simulate it in FLUKA if possible. I saw this in a different thread here: Simulating scintillation response

“Natural background at ground level comes mainly from the uranium series, thorium series, and potassium 40. The ratio between these mainly depends on the location. You might try using such radioactive sources using the RADDECAY card. I would personally recommend trying U-238 as a source. In case you need to simulate cosmic radiation, there are tools to do that as well.”

“I would also recommend adding an OPT-PROP card with Type = “blank” to your .inp file to set the optical properties (such as the refraction index) of your scintillator.”

  1. I would like to know how to do this in FLUKA. Should I use the BEAM card set as an isotope to add U-238, Thorium, and K-40?

  2. If so, where should I place these U, Th, and K isotopes to simulate a real-life situation, assuming I include the detector and my main radioactive isotope in the middle of a 5m x 5m x 5m room?

  3. Or is there any other easier method to simulate ground level background radiation with different cards?

  4. How should I include cosmic radiation in the simulation? with what cards? An example would be very helpful. :pray:

  5. If I’m adding the optical properties with the OPT_PROP card, should I give the refractive index of the NaI crystal? (n = 1.839) Do I need to include the refractive index for other materials included in the detector, such as Al and Si?

  6. What about biasing? Do I need to consider mean free path biasing when photons move through air to NaI crystal?

  7. To simulate the environmental factor in Fluka, what else can I do?

Here is my code for your reference:
I_123.inp (3.0 KB)
I_123.flair (3.7 KB)


this kind of simulation might be extremely complicated since the major source of uncertainty is given by the experimental condition of your apparatus. FLUKA is not the correct approach when dealing with natural radioactivity: your model uncertainties will be the dominating factor in the results.
In any case, let’s try to go through this:

  1. Yes. Look at slide 33 in this lecture:

  2. This is the key point. The concentration of these elements might vary depending on the location of your experiment. They will be mostly in the concrete (floor, walls). Keep in mind that them (or some elements of the decay chain, notably Radon) are dispersed in air with complicated transport mechanisms.
    If you really want to do it, as an educated guessing, I would consider an uniform distribution of these elements taking the values from the most recent literature (e.g. Assessment of Natural Radioactivity in Cements Used as Building Materials in Poland - PMC)

  3. Unfortunately, I do not foresee easier approaches

  4. In the installation directory, you should have a ./example/gcr folder. It contains a working example of a cosmic ray generation simulation.

  5. It must be defined for every material where you want to transport optical photons. Look at this lecture:

  6. Considering the type of simulation, biasing might be needed to reduce the computational times. I would not use a mean free path biasing, instead I would do a region importance biasing (i.e. regions far away from the detector have lower importance).

  7. I would change the question in another one: “Can FLUKA simulate natural background radiation?” And the answer will be: probably not.
    To fully know the concentration of your radioactive elements in your setup, you would need to measure their activities (i.e. the background itself)!

I hope this clarified a bit the issue.

Thank you very much.