I can not simulated C12(n,α)Be9 reaction

I build a 12C target , I want to research the C12(n,α)Be9 reaction. I record the deposit energy in every event, and get the deposit Energy Spectrum, but , I can not find a peak
of 8.299MeV which is the energy of the C12(n,α)Be9 reaction. I use the QGSP_BERT_HP model and I have tried different data library, but I can not make it.
I don not why, what should I do? Can you help me?

I am facing the same problem.
Up :smile:

It’s not obvious to me that there is anything wrong with the deposited energy plot. The Q value of the 12C(n,alpha)9Be reaction is -5.702MeV. “the energy of the reaction” is a very vague phrase, but I’m assuming that what the first poster meant is that the energy of the incident neutron is 8.299MeV (target at rest). Thus, the total deposited energy should be ~2.597MeV and I see a peak roughly there. The rest of the spectrum is not unreasonable, given all the 12C(n,…) reaction channels that are available at that energy.

A more detailed description of the geometry, reaction, energies, and what is trying to be measured is needed to assess if there is anything wrong with the physics.

Thanks for the reply @John_McFee
Actually I should have add more information about my problem.
I am studying several n,alpha reactions for several materials.
The kinetic energy distributions of the products are correct using QGSP_BERT_HP
However, I obtained one strange result with the n,alpha reaction on Carbon. Instead of having a distribution of the kinetic energy (Observed with other code like PHITS) I observed with Geant a cut at 2.5 MeV in my kinetic energy distribution for 14 MeV impinging neutrons (only for the n,alpha on carbon). Could it be a problem with the nuclear data ?

Hi All, I find this topic interesting and i want to place some suggestions which can be worth.

In Geant4 especially when dealing with neutron kinetic energy < 20 MeV the process undergoes in four ways. Elastic, Inelastic, Radiative capture, and fission. And all calculations done under environmental variable G4NEUTRONDATA with data libraries G4NDL4.6 and is based on JEFF-3.3

Previous data files were based on ENDF/B. I guess you please check it once and its version as JEFF provides closest agreement with MCNP results.

My other suggestion is to test your case using QGSP_BIC_HP → This certainly use binary cascade method. I am not sure this will increase accuracy in the calculations but will be worth to compare the results.

You may also compare the data libraries of both softwares’. I guess they also use the JEFF but I am not sure.


Here, a run of example Hadr03, with QGSP_BIC (not _HP) in PhysicsList.cc
The difference comes from the dissociation C12 → 3 alphas, which does not exist in HP

name.mac.txt (381 Bytes)
name.bic.out.txt (1.7 KB)

Hi Michel, There is a slightly puzzling thing going on in your (or my) output using QGSP_BIC. My plot and terminal output is attached. From the latter, it is clear that mine has the neutron + C12 → neutron + C12 reaction whereas yours has the neutron + C12 → neutron + 3 alpha.

I am running Geant4 11.0.2 and the Hadr03 example from that distribution recompiled with QGSP_BIC (not _HP) with your (unmodified) name.mac. I assumed we would get the same result. Is there something different in the (apparently) newer version of Geant4 that you are running?


name.mybic.out.txt (25.9 KB)

Thank you @drvijayraj , @John_McFee, @maire for your reply.
I have changed my physicslist from QGSP_BERT_HP (although it is intended for the purpose of simulating neutron with E<20MeV which is what I am interested in) to QGSP_BIC (not _HP).

I find this time the right kinetic energy distribution for the alphas produced in C(n,alpha)9Be.
However :

  • I don’t get the kinetic energy distribution of the gamma produced in C(n,a)Be anymore, while I still get it for the other capture reactions on other nuclei.
  • I get a lower count rate for this C(n,alpha) than the one obtained with BERT_HP (seems normal) but, I now obtain alpha+8Be+n produced in some events (with higher count rate than C(n,alpha)9Be), which was not expected.
  • Finally, I get a kinetic energy distribution this time, in another (n,alpha) reaction on another nuclei, with lower energy boundaries than the ones expected.

Thank you again for the discussion.
N.B. : I am using geant4 11.0.2

HI John, thanks you have noticed this difference. I have sent a mail to the maintener.