I use FLUKA for dosimetry with X-ray photons in the keV energy range, and I was wondering whether there is a recommended way to check if the assumptions underlying the multiple Coulomb scattering (MCS) treatment are adequately satisfied.
For example, could the electron transport step length be used for such an assessment? Is there a useful rule of thumb based on the step length relative to the elastic mean free path, or on the expected number of elastic scattering events within a transport step?
If the latter is relevant, how could the number of scattering events be estimated in practice?
Alternatively, would it be useful to compare the transport-step length with the electron range in the material, for example a CSDA-like range estimated from FLUKA stopping powers?
More generally, what would be the recommended approach to assess whether the Molière MCS treatment is appropriate for a given electron energy and material range?
I have attached the MGDRAW routine that I currently use to extract electron transport-step information, together with the corresponding FLUKA/Flair input files and user source routine.
Thank you for your question.
Before looking at the specific files (thanks!), let me provide you a quick feedback that might already answer some of your doubts.
Multiple Coulomb Scattering theories description, including the one from the Moliere theory used in FLUKA, are valid only when the charged particle goes through multiple elastic scatterings in the current (condensed) step. This statement can be traduced in a low-limit value for the particle step: usually, the Moliere MCS is assumed to be valid for step-lengths 20 times longer than the elastic mean free path.
This assumption could break in different situations:
Step in thick regions limited by geometry boundaries. FLUKA tracking algorithm has some logics to drop the MCS description of the particle track in favour of a detailed treatment following the single scattering theory.
Thin regions or diluted gas. The recommendation is for the user to activate the single scattering treatment with the MULSOPT card (manual). The FLUKAFIX and EMFFIX cards could also be used to limit the particle step in function of kinetic energy that can be lost to dE/dx along the step. This might be of particular interest (and necessary even) for dosimetry studies in thin materials.
Additionally, I suggest to follow the advance settings lesson where some common artefacts related to the MCS theory or to the ionization process are reported together with some similar suggestions on how to fix them.