The problem about thermal scattering

The graphite was used to moderate neutron,
when the thermal scattering cross section is not used, the result is like:

when the thermal scattering cross section is used, the result is like:

it seems odd, but this is the right one?

What do you mean with thermal scattering cross section used/not used? How do you use/not use it? The second spectrum looks indeed odd.

To call thermal scattering cross section:

not to use thermal scattering cross section:

Could you send us your input to check?

Scatter.inp (1.4 KB)
in this .inp file, the thermal scattering cross section is called.

Thank @Newconcept_1979 for the interesting question:

what you observe indeed is a physical effect which is due to the Elastic Coherent scattering of neutrons with the graphite lattice structure. The first Bragg Edge on graphite according to the database is at E_0 = 1.82\ meV, the energy where you observe the step.

The thermal scattering cross section for graphite is divided into Inelastic and Elastic Coherent.
In the TSL Inelastic event the neutron can lose or gain energy while in the TSL Elastic event the neutron energy remains the same (since we assume a collision with a crystal of infinite mass wrt to the neutron), only the direction is changing. For the Coherent Elastic the scattering cosine is given as
\mu = 1 - E_i/E
Below the energy of the first Bragg Edge, only the Inelastic channel remains open. This provokes an apparent increase to the neutron flux due to the much lower inelastic cross section. The neutrons travel longer paths before being scattered leading to an increase in the track length.

FLUKA cross sections

The data are coming from the JEFF library. For more information look the INDC report on Graphite

figure 6.5

General Comments:

  • Your example shows the importance of the S(α,β) treatment at thermal energies, which when ignored one can get wrong result with respect to the free gas cross section (yellow line) on the above figure.
  • Always add an empty LOW-PWXS card to enable the point wise treatment for all materials in the problem.
    Then add a second LOW-PWXS card to bind the S(a,b) for graphite
  • Adding an empty LOW-PWXS card it will inform FLUKA that all materials are point wise (including VACUUM), therefore it will honor the true binning requested in your the USRxxx cards, otherwise as is your case it will use the Group wise binning
  • Your plot shows a very pronounced effect, due to the way that is plotted (not in lethargy). When plotting neutron fluences use the isolethargic units <X>*Y in order to have a more correct visual representation of the neutron integrals. The surface of the plot will be proportional to the neutron fluence.

@vasilis, thanks for your reply.