Missing neutron transport cross section

Dear FLUKA experts

I’m interested in photonuclear interactions of Ra-226. However, FLUKA is missing the low energy neutron cross section for this material. In the attached example you’ll see that the simulation does not even start and gives the error:

**** Low energy neutron xsec not found for some media 5 6 ****

This question came up some time ago already and it was suggested to just raise the neutron transport threshold with PART-THR. This works. However, for my case I need the neutron transport in the rest of the geometry. Hence, is there a way to:

  • get somehow the low energy neutron transport cross section for Ra-226 into FLUKA?
  • or to set a high threshold for the neutron transport only in a specific region?

I tried already with LOW-BIAS but that did not help.
Thanks for your help!


20200117_minimal_FLUKA_Forum_example.inp (3.1 KB)

Hi @lorenzo.mercolli
I do not have great news:

  • embedding the Ra-226 low energy neutron cross sections is unfortunately far from being straighforward;
  • hadron thresholds are set globally and not on a region basis. By the way, the solution you think of would transform the Ra-226 region into a neutron sink.
    Since you are interested in photonuclear reactions, would you benefit from scoring - in the presence of the 20 MeV neutron transport threshold - the respective products by USRYIELD (EMERGING) or in more detail by a customized mgdraw (usdraw entry), in view of a possible second step simulation?
1 Like

Dear @ceruttif

Thanks for your answer. Indeed I didn’t consider that with regional transport thresholds one would create a neutron sink.

What will work for my case is to score all particles in front of the target and then compute the activation products ‘by hand’ with the cross sections from TENDL, ENDF, etc. (like doing a two-step simulation as you suggested but without the second step).