Unexpected peaks and high-energy tails in volumetric source simulations (Co-60 and Cs-137) with a well-type HPGe detector

Physics, Transport and Magnetic Fields
1

Dear FLUKA and Flair experts,
I am currently simulating gamma spectra for volumetrically distributed radioactive sources in FLUKA (specifically Co-60 and Cs-137). The source is uniformly distributed within a cylindrical volume, and the detector is a well-type HPGe. While the general behavior of the simulated spectra is reasonable, I am observing some unexpected features:

  1. Co-60 Spectrum
  • Along with the expected 1173 keV and 1332 keV peaks, there is a third peak around 2505 keV. I can tentatively explain this as a sum peak due to coincidence summation. However, I also notice a broad tail extending to higher energies that I cannot clearly interpret.
  1. Cs-137 Spectrum
  • In addition to the main 662 keV peak, there are several unexplained peaks at higher energies (above 662 keV).

These effects disappear if I switch to a point source instead of a volumetric (cylindrical) source. I tried implementing the cylindrical source distribution both via standard FLUKA cards and via a custom source subroutine, but the results are the same.

My questions are:

  • Is there a known effect in FLUKA that causes additional summation or unexpected interactions when sources are distributed volumetrically?
  • Could these extra peaks and tails be normal outcomes of coincidence summation, scattering, or other physical processes in large volumetric sources?
  • Are there any specific settings or physics cards that I might be missing or should double-check to handle volumetric radioactive decay simulations correctly?

I would greatly appreciate any insights or advice on how to interpret or address these additional peaks and the high-energy tail in my simulations.
Thank you in advance for your help!
D70.flair (4.0 KB)

137cs_spectrum
137cs_spectrum948×641 19.2 KB

60co_spectrum
60co_spectrum948×641 22.2 KB

3

Dear Gabriele,

what you observe is the contribution coming from the electrons created during decays.
While you raised the production and transport thresholds for electrons, it is not working in your case because:

  1. Your cylindrical source extends over the SAMPLE into the vacuum region of the HPGe.
  2. The production threshold is not applied to the source.
  3. The transport threshold is not applied to the VACUUM material.

This means you still get electrons in the sensitive region. If you reduce the radius of the cylindrical source (as you did with switching to a point source), the contribution from the electrons disappears.

Cheers,
David

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