Photonuclear reaction 14N(gamma, n) at 20MeV and above

Physics Processes, Models and Cross Sections
1

Hello everyone,

I’m new to Geant4 and I’m working photonuclear reactions for 14N in the range of 10MeV to 25MeV photons and I’ve been noticing something weird with the model that’s responsible for these reactions. I’m using the FTFP_BERT_HP physicslist, I ran 1E9 for each test and the nitrogen having an artificial density of 1g/cm3.

I’m interested in the energies of the neutrons coming out of these reactions. Given that the Q value is around 10.5MeV for 14N, the energy of the neutron coming out of this reaction (if we take the ground state for 13N) should be around 8.5 MeV (conservation of energy and momentum).

The problem I think is that the model treating the photonuclear reactions under 20MeV and above 20MeV is not the same. I’ve done several tests using 20MeV and 20.01MeV monoenergetic photons and the results are really not the same.

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What is most concerning is that the number of neutrons around 8.5MeV coming from the 14N(gamma, n_0)13N reaction is much higher in the case of 20.01MeV (2 orders of magnitude) than the 20.0MeV.

I also ran the same test with 21MeV photons and ended up having this kind of spectrum where all nuclear structure disappeared from the simulations. Also the peak from the 14N(gamma, n_0)13N reaction is not there (it should be around 9MeV).

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This is weird because under the GDR region (up to 30MeV), the physics behind the photonuclear reactions should be exactly the same but it’s not here. Does anyone have any explanation for all this ? Or maybe some recommendations about the physics list that should be used in this context.

I know that a lot of people are using Geant4 for radioprotection purposes in the medical field, so irradiation accelerators up to 23MeV photons (but not surrounded by Nitrogen) and it seems like there should be a model or a physicslist capable of correctly simulating the photonuclear reactions under the GDR zone.

2

There are two different models used around 20 MeV, you are correct. You can “patch” them together or disable/enable them. Several examples do this utilizing what is generally GammaNuclearPhysics files.

Here is one example and how you can tweak it accordingly. Yes, it is quite confusing.

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Thanks, I’ll look into it.

4

I was able to modify a bit the different models but the problem persists. The models change (I think … totally not sure) the cross section values of the interaction. My problem is the fact that the physics from which these neutrons are emitted are completely difference, the second case that I showed with 21MeV where there’s a continuum of neutrons up to 7MeV is not physical because it doesn’t take the nuclear structure into account and thus we have a situation where the conservation of energy and momentum is not at all respected.

I don’t know which part of the physicsList I should change to bring up these neutrons at the correct energies, all I know is that the “default” settings for this type of reactions is completely wrong. I’ll update if I were able to come up with something new.

Thanks again for your help.

5

I don’t think any HP list extends beyond 20 MeV because of a lack of data. If you do you will just get the value at 20 MeV extrapolated upwards. You could try to use the shielding physics list in hadr04, but in general you will have to compromise with a “worse” physics list at some range.

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