Gap in spectrum with low energy-photons

Hello FLUKA experts,

i am simulating scintillators (LaBr3 & NaI) using photons with different energies / isotopes. The spectrum of photons with an energy of 400keV looks like this:

But when using low energy Photons with 50 keV there will be a gap in the spectrum:

What is the cause for this gap? Do i have to activate something for the correct simulation with low energy photons?

Thanks in advance

Dear Moritz,

We’d have to see the input file for a closer assessment of your problem.

You might be using a DEFAULTS card with SDUM=PRECISIOn. In this case, the photon transport/production thresholds are set to 33 keV by default. Try lowering the photon production/transport threshold to, say, 1 keV using the EMFCUT card.

Use the same card to reduce also the delta-ray production threshold (and electron transport threshold) from the default value of 100 keV (assuming DEFAULTS PRECISIOn) to, say, 1 keV.

Let us know if the problem persists.



Dear Cesc,

yes, you are right, i am using the DEFAULTS with SDUM=PRECISIOn. I tried to implement your solution with the EMFCUT-Card, but i get errors using it. Seems like i don’t know how to use this card properly as you mentioned. I will send the input-file in the attachment this time, maybe you can correct my mistakes.
0.5m_NaI_Vacuum_50keV_1100012.inp (3.7 KB)

Thank you very much!


Dear Moritz,

Thanks for your input file.

Upon first global inspection one spots:

  • You request a total (rest mass + kinetic energy) e- transport
    threshold of 1 keV, hence the crash. Select kinetic energy instead of
    total energy. It may be meaningful to apply these thresholds
    not just for your detector region, but for all nearby regions for good

  • You pass one EMFCUT card for the transport thresholds (which is
    fine), but you are missing an EMFCUT card for production
    thresholds (SDUM=PROD-CUT). There is logics in place to set them for
    e- as a function of the transport threshold, but for good measure
    let’s just add it. Regarding WHAT(3), labelled as FUDGEM, please

    EMF-CUT problems about what(3) - #2 by cesc

    i.e. for such low thresholds you should put FUDGEM say 1e-5.

Let us know if the troubles persist.

With kind regards,


1 Like

Dear Cesc,

the gap has gone after implementing your solution. It worked fine.

Thank you very much! :slight_smile: