Sorry to be bothering you again, but I have another question concerning the CSDA range. I am shooting a 40 MeV electron beam inside aluminum and use USRBIN → BEAMPART to track primary electrons. According to ESTAR the range is 6.63 cm, but in the picture below electrons can be seen reaching well above 8 cm. I would at least expect the range to be lower than 6.63 cm due to ESTAR not taking into account energy-loss fluctuations. My (false) explanation is that primary electrons follow the restricted stopping power which is lower than ESTAR’s (I use the default PRECISIOn settings), therefore their range is higher. But if that is the case then FLUKA calculates energy deposition of primaries in unrealistic ranges. Could you please guide me on what is going on?
As you intuitively guessed by dropping your false explanation, the appropriate use of restricted stopping power is meant to increase the simulation accuracy, and not to jeopardize the beam range. In fact, the respective reduction in continuous energy loss is duly compensated by the explicit Moller scattering events, contributing in turn to the primary electron energy loss.
As for the (low) tail exceeding the ESTAR reference value, this comes from the fluctuations related to the radiative part (bremsstrahlung). If you (artificially) inhibit explicit gamma emission by raising its production threshold above beam energy (with EMFCUT), you will get the ESTAR range, not accounting for the aforementioned fluctuations either. These play in both directions, allowing on a event-by-event basis for higher energy losses but also - alternatively - longer ranges (such as to reproduce globally the expected mean), as in reality.
Thank you for your reply. Inhibiting photon production indeed reduced the range, although still remained a bit higher than ESTAR’s, which as you said must be due to energy loss fluctuations. I suppose my concern was, that even though these fluctuations exist, the range should still be lower than ESTAR’s, as FLUKA includes multiple scattering, secondary production, energy straggling etc. After all, isn’t this the reason it is advised to consult ESTAR before creating a USRBIN mesh, since its range is always an overestimation (not accounting for secondary production)?
Dear Fitilis, not really: as displayed in your case and recalled above, fluctuations imply either larger AND smaller energy losses than average. On the other hand, not all processes have actually a sizeable impact on the beam range. Bremsstrahlung fluctuations do, and allow for the physical tail well beyond the ESTAR range. Note also that FLUKA modeling is independent of the latter, such as it does not exactly coincide.
Moreover, in the electron case, it’s questionable to call range the distance reached only by a tiny tail of the beam particle distribution.
What you say makes perfect sense. For some reason I had this notion that ESTAR should be considered as an upper limit. I will need to do some more reading on Bremsstrahlung fluctuations. Thank you for your patience!
