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

From: Alberto Fasso' <fasso_at_slac.stanford.edu>
Date: Fri, 22 Aug 2014 01:21:05 -0700 (PDT)

Dear Josh,

you must not forget that the neutrons, for which a different molecular
bonding affects the cross section, are called "thermal" because their
energy is so low that they are in thermal equilibrium with the atoms of
matter. And the thermal equilibrium depends on molecular recoils, which
are different for different chemical bonding.

In a book by the famous physicist Rober Serber ("Serber says: About
Nuclear Physics", World Scientific Lecture Notes in Physics Vol. 10,
World Scientific, Singapore 1987) there is this very clear explanation:

That the binding of the proton in a molecule could affect the
scattering at first seems surprising, because of the short
range and large magnitude of the nuclear force, but it must be
remembered that if the neutron energy is less than the energy
required to excite the proton to its first vibrational state in
the molecule then energy conservation requires the proton to
remain in the ground vibrational state after the collision. As
a result, the molecule recoils from the collision as a whole
and the kinematics are quite different than in a free collision.
Typical molecular vibrationalenergies are a few tenths of an
electron volt. Fermi* showed that, in the limiting case of low
energy and heavy molecules,
                   sigma_bound = 4 sigma_free

* E. Fermi, Ricerca Sci. 7, 364 (1936)

Alberto



On Thu, 21 Aug 2014, Joshua Albert wrote:

> 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)
> [IMAGE]
> So, the question is, why do these differ from each other so much?  I would n
> ot 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)
> [IMAGE]
> What is going on here?  It is becoming clear that it is very important to se
> lect the correct bonding version of the materials to get a reasonable simula
> tion.
> Thank you,
> Josh
Received on Fri Aug 22 2014 - 11:50:49 CEST