Minimum Energy Threshold for Neutron Beam in FLUKA (Below 1e-8 GeV Not Working)

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1

Hello FLUKA experts,
I’m a beginner using FLUKA for neutron transport simulations. I’m currently experimenting with very low-energy (cold/thermal) neutron beams and observed an issue I don’t fully understand.
I set up a simple geometry with a neutron beam directed at a material, and I vary the incident energy using the BEAM card.

When I set the beam energy to around 1e-7 GeV, I can still see results (particles are tracked and transported). However, as I go below 1e-8 GeV, say to 1e-9 or 1e-12 GeV, the beam appears to be ignored or killed — no tracks, no interactions, no particles show up in the scoring detectors. It’s as if FLUKA doesn’t recognize the beam at all.

I would like to ask:

  1. What is the lowest neutron energy FLUKA can simulate reliably?
  2. Is there a hard-coded lower limit for neutron transport or source energy?
  3. Are there known limitations or physics model cutoffs for cold or ultra-cold neutron simulation?

Thank you in advance for your guidance. Any suggestions or examples would be much appreciated.

Best regards,
Prakash S

3

Hello,

To specifically understand why your simulation fails, I would need to check your inputfile.

  1. In general, the transport thresholds are listed in the FLUKA manual: 2.3. Transport — FLUKA Manual

  2. You can specify the limit in the appropriate card: 7.22.57. PART-THRes — FLUKA Manual

  3. With the point-wise treatment of low energy neutrons, the cross sections are based on well known databases. Let me know if you have any ultra cold neutron process are you specifically referring to.

Hope this helps.
Cheers,
Daniele