I am currently simulating a neutron detector in CERN FLUKA. In my model, neutrons are moderated and then captured in a He-3 counter. My incident neutron energy range is from 1 keV to 10 MeV.
I would like to ask for clarification about the treatment of low-energy neutron physics, especially thermal scattering in moderator materials.
At present, I am using the global card:
LOW-PWXS ENDF-VIII0
My understanding is that this enables point-wise low-energy neutron cross sections globally, but does not automatically assign the correct bound thermal scattering law S(alpha,beta,T) for all moderator materials.
In my geometry, I use polyethylene and heavy water as moderator materials, and He-3 gas in the proportional counter. I would like to confirm the following points:
Is my understanding correct that the global LOW-PWXS ENDF-VIII0 card alone is not sufficient for accurate thermal moderation in polyethylene and heavy water?
If I want to model the moderators more accurately, do I need to add specific LOW-PWXS cards for:
H bound in polyethylene (H in CH2)
D bound in heavy water (D in D2O)
My main concern is whether my current setup may describe neutron moderation only approximately, rather than with the proper bound thermal scattering treatment in the moderator.
by default all materials are treated as unbound except HYDROGEN which is always assumed to be bound to water.
Your understanding is correct, the global definition LOW-PWXS ENDF-VIII, will ask the code to use the ENDF-VIII0 library for all low-energy neutrons, without affecting use of TSL
Also correct, you need to add at least two extra cards as you mention to bind H to CH2 and D to D2O. Now if you are using Hydrogen in some other material is better to create a new Hydrogen material with a different name than the default HYDROGEN
Not using the proper TSL materials, it will have an effect especially on the thermal peak and below. However the effect depends a lot on your setup. In most cases I dealt with it was ranging on the order of few 10%. If you are interested in the thermal neutrons (which I guess you are) it is safer to enable it.
