Low Energy Activation, LOW-PWXS, Pointing to ENDF Files

Dear Experts,

I have a few questions regarding low-energy neutron transport. I’m concerned with the production and transportation of neutrons produced by muons. Particularly, I am concerned with the activation of xenon-136 for which there are ENDF and JEFF files.

Here are my questions:

  1. Is LOW-PWXS only for thermal neutron scattering events? Can it not point to capture cross section files?

  2. If there is an empty LOW-PWXS card in my .inp file, is this equivalent to having one for each material for which there is a corresponding thermal scattering file?

  3. How can tell FLUKA to pull data for a particular neutron capture from a chosen file?

  4. I use a fairly comprehensive MGDRAW file to track events. How are particle trajectories segmented? Is this a fixed distance?

Thank you for the help. This is a most useful forum.


  1. LOW-PWXS is NOT only for thermal neutron scattering. Is to enable the point wise treatment for neutrons below 20MeV and optionally for thermal neutron scattering below 4eV using the S(α,β)
  2. An empty LOW-PWXS is equivalent to activate the pointwise treatment for all isotopes in your problem, and the thermal scattering law ONLY for Hydrogen-1 assuming it is bound to WATER.
  3. You can add multiple LOW-PWXS cards.
  • First one empty LOW-PWXS card to activate the pointwise neutron treatment for all isotopes
  • Subsequent several LOW-PWXS cards filled with various options to select a the library: ENDF JEFF or others, for a specific isotope, select the temperature and select a thermal scattering data S(α,β) if available for the isotopes you are interested
  1. Tracks are segmented:
  • from interaction to interaction (variable random distance depending on the material, particle, energy, …)
  • when crossing a geometrical boundary
  • several other reasons (depending on particle type, tracking parameters, field, etc…).

Hello Vasilis,

Thank you, this is indeed helpful. However, I’m left with a few other questions.

  1. Is there a way to cap the step length in order to track the particle through finer geometries for instance?
  2. Despite having LOW-PWXS activated and PRECISIOn defaults, when I run a single neutron at 1e-10 GeV, the ETRACK variable returns 0.93… GeV— what might be causing this?

Thank you very much for the response.


Hi Regan,

2. ETRACK is the total energy, i.e. the sum of the particle mass (mc^2 in GeV) and its kinetic energy.

1. I understand you care about neutrons, for which stopping power segmentation, controllable by the FLUKAFIX card, does not apply. In this case, the step length is sampled with reference to the physical mean free path until the next interaction, as Vasilis indicated. A fine geometry implies anyway step segmentation at the region boundary. To increase the neutron interaction statistics, linked to the reliability of the activation result, you may consider some available biasing options. By implementing for instance a region importance biasing scheme (with the BIASING card), you can artificially multiply the number of neutrons traveling through your regions of interest.

This is all excellent. I’ll leave it to the forum moderators to select the solution— both were very helpful to me.

Thank you very much!