Based on the Geant4-DNA (Geant4-v11.0.0) dnaphysics example I use the option 2 physics list to simulate the production of secondary electrons by incident ions. I noticed a strange behavior when moving to higher kinetic energies (>100 MeV/u) of the primary ion, e.g. for 56Fe.
When increasing the kinetic energy of the primary I also expect an increasing maximum energy of the secondary electrons. Everything seems to be fine up to an energy of approximately 100 MeV/u (5550 MeV for 56Fe) where I expect a maximum secondary electron energy of about 200 keV. However, when I switch to slightly higher ion energies and add just a few MeV (5560 MeV), the maximum secondary electron energy drops by two orders of magnitude down to only a few keV. It seems like the energy spectrum is just cut off at a certain point. The rest of the spectrum seems fine.
I simulate a water box of 200x200x200 micrometer^3 and in its center I create a beam of ions. I follow the primaries along a track of only 1 micrometer to ensure that the energy loss of the primaries can be neglected along its track. I score the energy of all secondary electrons produced by the ions in a histogram. Note that the histogram bins in the figure are logarithmic and not yet normalized in any manner. I simulated 100 56Fe ions. I found the same behavior for 12C ions.
Can someone please try to reproduce this problem? I tried to understand the details of the G4DNARuddIonisationExtendedModel, which is applied in this case and tried to figure out the source of the problem. I took a look into the cross section data applied by the model (geant4-v11.0.0-build/data/G4EMLOW8.0/dna/sigma_ionisation_fe_rudd.dat) and saw that the first entry is 5.6E3. Is this value given in MeV? If yes, is there maybe a switch between some models at 100 MeV/u?
My file “dnaphysics.in”:
/gun/ion 26 56
/gun/energy 5550 MeV
#/gun/energy 5560 MeV