Clarification about neutron kerma and heavy ion

Dear Fluka team,
I have a question regarding the “neutron dose”.
I am scoring the energy deposited in scoring volumes due to secondary particles from a neutron source.
In the process, for instance, I also score energy deposited by heavy ions.
I have made some comparisons with GEANT4 and the results are quite satisfying when I reconstruct the total dose from all particles.
As you can see, with FLUKA, I get a “neutron dose”. I know these particles are not directly ionizing and that a part of the dose originating from secondaries is included in the neutron dose by mean of KERMA factors.
However, I do not understand totally what this kerma factor represents. In my figure, you can see that for neutrons from 1E-09 to 1E-03 MeV, heavy ion dose is similar between GEANT4 and FLUKA. Above 1E-03 MeV, all happens like if a part of the heavy ion dose is included in the “neutron dose” scored by mean of kerma factors. Hence, I do not understand why the heavy ion dose is not totally included in the neutron dose.
My questions are the following:

  1. What represent the “neutron dose” provided by the kerma factors and why the heavy ion dose is not included in the neutron dose for the whole range of energy? If I add the heavy ion dose + the neutron dose, am I scoring two times the heavy ion dose? Which part of heavy ion dose is included in neutron dose? I really do not understand the “neutron dose”.
  2. What is the exact meaning of heavy ion particles? Do they include light ions such as deuteron, triton, helium or only particles heavier than helium ? Are they only ions or they can also be recoil nucleus not ionized?

Thanks a lot for your help.

The attribution of dose to one particle type is arbitrary, since it depends whether its energy transfer to the concerned material is modeled explicitly or not. In the first case, dose is attributed to its secondary particles, while in the latter case it is attributed to the original particle.
The group-wise treatment of low energy neutrons in FLUKA makes their energy transfer by nuclear reactions accounted through kerma factors, without generation of secondary particles other than neutrons and photons (no HEAVYION), with the respective dose attributed to neutrons. On the other hand, special point-wise treatment of low energy neutrons or nuclear reactions by neutrons of energy larger than 20 MeV can originate in particular nuclear recoils that, depending on their energy and the requested transport threshold, can be transported and become directly responsible for dose deposition. In this sense, the addition of the different contributions does not imply double counting.
The HEAVYION filter is limited to transported nuclei heavier than alphas, which can hardly be not ionized at all. They are considered as fully stripped ions for which, however, effective charge (lower than the nucleus one) is taken into account to ionization purposes.

Thank you Francesco for the clarification. That is very interesting. So, am I right when I say that the heavy ion dose above 1 keV (red curve) is included in the “neutron dose” (black curve)? However, there is still something I do not understand. If there is no production of heavy ion as you mentionned in your response, why do I score heavy ions?
Is there a paper I can refer to regarding the FLUKA treatment of neutrons by kerma factors?

Dear Thomas,
yes, the dose not explicitly attributed to heavy ions is included in the neutron dose (you do not seem to plot a dose in your above plot, though).
As for the observed heavy ions, I cannot see your input, does it include 10B?
For reference I’d suggest this lecture.

Thank you Francesco. I think it is now clear for me and the topic can be closed. You are right it is not a dose but it is proportional to a dose. I did not want to go in the details of the calculation as the description of my question was already long enough :smiley:
My heavy ions do not include boron.
Thank you again.

The presence of hydrogen, with the respective point-wise treatment, may explain the heavy ion appearance, due to proton induced reactions.