I’m trying to moderate a primary neutron distribution only to around 1 MeV (not fully thermalize it). I have just modified example Hadr04 to modify the geometry and scoring.
Geometry is an isotropic neutron source with custom energy distribution; generated neutrons pass through a 10 cm thick D20 moderator and finally histogram the KE of the ones that reach a virtual detector. The world volume and virtual detector are made of G4Galactic.
Unfortunately, from the below output spectra, I only think the neutrons are getting absorbed and not moderated (like the peak does not shift).
Any help is highly appreciated to just shift the peak of my initial neutron distribution without too much absorbtion.
Black - original distribution; Red - moderated distribution
I think it would be useful for you to explore the verbosity of your simulation using something like /tracking/verbose 1 in your macro file and confirm the processes that your neutrons are going through. Run your simulation with low statistics but with “high” verbosity.
To confirm your observation, here a macro for example Hadr06.
An isotropic, mono energetic (1 MeV) neutrons source at the centre of an heavy water sphere (radius 10 cm).
Plots of emerging neutrons, for 3 different HadronElasticPhysics constructors : local-HP (left), G4-HP (right), G4-XS (bottom). The conclusion is simple: use HP
ps. In PhysicsList::SetCuts(), to save time, put cut for proton = 10 km
Thanks a lot for sharing these, and sorry for the delay in getting back.
I basically compared the above plot with the Hadr04 example that I am working with. The spectrum looks fairly close, though the second low energy peak is quite shifted between the two examples. Do you have an idea of what is causing this behavior?
Also, my goal is to just moderate the initial neutron distribution (Maxwellian distribution peaking at around MeV) to a final neutron distribution (peaking at around 1 MeV) and prevent it from thermalizing to the eV range. I know this is kind of a physics question, and I do not know how to achieve this, but I’ll highly appreciate any ideas you may have.
Finally, not relevant for my purpose but what I find weird is that no matter if
I use thermalscattering model or not, I still get the same spectra, which contradicts my understanding that the thermal scattering should have a pronounced effect, especially at the low energies.
Hallo K, in your last plot (Hadr06) is the shift, redistribution not in line with a moderation? What happens if you choose a larger thickness. Also maybe your requirements are too stringent, the physics can’t work. What happens if you try a composite structure of moderators.
Finally you may not be able to avoid some thermalization?
Many thanks; yes, that shift in Hadr06 corresponds to moderation. However, the neutrons that are fully thermalized (a few eVs).
I was not sure if I could just moderate enough that the peak at 1 MeV itself shifts to a lower energy. Maybe it’s putting a limit on the physics of moderation.
Thanks for this idea! I’ll give it a try and get back !
