FLUKA vs. ESTAR stopping power

Dear FLUKA users,

I am trying to create a geometry where electrons are stopped inside an aluminum target. To figure out the length of the target, I consulted the ESTAR electronic stopping power of aluminum. I also used the EMFFIX card to print dE/dx in FLUKA and then divided it by the aluminum density (2.7 g/cm3) to get the stopping power ( MeV*cm2/g ). My understanding is that this value is the collisional stopping power and not the total. The graph below shows the collisional and total SP taken from the ESTAR database and from FLUKA. The FLUKA results differ significantly. Could you please explain why that is and which dataset should I use to create my geometry?


Thank you for you time,

Hello Fitilis,

ESTAR gives you the unrestricted electronic stopping power.

What you get in the FLUKA output file is the restricted electronic stopping power for whichever delta-ray production cuts Td are set in your simulation (100 keV if you’re using the DEFAULTS card with PRECISIOn). And for good reason: ionization losses are treated in a combined scheme in the code:

  • Collisions with target electrons transferring an energy T>Td are sampled explicitly: the resulting delta ray (e-) is tracked.
  • Collisions with target electrons transferring an energy T<Td are treated in an aggregate manner along the particle step on the basis of a restricted stopping power.

To obtain the unrestricted stopping power, on paper you’d like to raise the delta-ray production threshold (see EMFCUT card) to very high values. However, when you do that, FLUKA will raise the electron transport cut as well (to avoid having a range of energies where you could transport e- but not produce them, which would pose bookkeeping issues). This has the unfortunate effect of rising the first abscissa of the tabulated stopping power to higher energies.

Nevertheless, in the following plot you can gather a clear impression that as you raise the delta-ray production threshold (Td) the stopping power is essentially indistinguishable from that of ESTAR:

The output file table header should however be more explicit. Shall be enhanced.

With kind regards,



Dear @cesc,

Thank you for this wonderful explanation. It all makes sense now!

Best regards,