i did a simulation of neutron bombing iron, there are two cross section selected:
one is “Fe. Natural Iron SelfShielded, 296K”,
the other is “Fe. Natural Iron, 296K”.
the results are different like below:
so what is the exact difference between normal cross section and self shielding cross section of low neutron?
many thanks for your question. In absence of the flair project I cannot really judge what was estimated and plotted but your question has a more general answer.
In the current version of the code, the transport of neutrons with energies lower than 20 MeV is performed by a multigroup algorithm with few exceptions (see section 10 of the manual for a more detailed explanation).
The group structure is necessarily coarse with respect to the resonances in the cross sections for some materials and if an isotope exhibiting large resonance is very pure or is present with a large fractional abundance it can act as a neutron sink causing dips in the neutron spectrum corresponding to each resonance: this leads to a lower reaction rate and the “self-shielding” effect of the material would be lost unless a special correction is taken into account. Self-shielded cross section have been included in the FLUKA libraries for a few important elements (Al, Fe, Cu, Au, Pb, Bi).
In the end it is up to you to decide what cross section you should use depending on your problem and with the following general recommendation: use the self-shielded cross sections for bulky (large) pieces that are pure or almost pure when the element is available, non-self-shielded cross section in the other cases.