[fluka-discuss]: Low energy neutron cross-sections for different bondings

From: Joshua Albert <joalbert_at_imap.iu.edu>
Date: Thu, 21 Aug 2014 14:28:30 -0400

Dear experts,

I have (somewhat accidentally) discovered that some of the low energy neutron cross sections, which appear to be for very similar materials, differ quite a bit. This may be due to a misunderstanding of what the cross-sections represent, or perhaps a lack of knowledge about the physics.

The question involves cross-sections of the same material at the same temperature, but with different bonding. For example, let’s take hydrogen. I have plotted the total cross section (SIGT) as a function of energy for:

H H2O bound natural Hydrogen 296K ENDF/B-VIIR0 Y HYDROGEN 1 -2 296 Y (Green on plot)
H CH2 bound natural Hydrogen 296K ENDF/B-VIIR0 Y HYDROGEN 1 -3 296 Y (Magenta on plot)
H Free gas natural Hydrogen 296K ENDF/B-VIIR0 Y HYDROGEN 1 -5 296 Y (Black on plot)

So, the question is, why do these differ from each other so much? I would not expect the chemical bond to affect the nuclear cross-section.

An even more extreme example can be seen between:
C Free gas natural Carbon 296K ENDF/B-VIIR0 Y CARBON 6 -2 296 Y (Black)
C Graphite bound nat. Carbon 296K ENDF/B-VIIR0 Y CARBON 6 -3 296 Y (Red)

What is going on here? It is becoming clear that it is very important to select the correct bonding version of the materials to get a reasonable simulation.

Thank you,
Josh
Received on Thu Aug 21 2014 - 23:12:35 CEST