Strange pattern in Electron-Induced Ionization Processes

edit :
Dear all,

I noticed something unusual when plotting the energy of secondary electrons generated through ionization :

The energy of the ionized electrons stops at about half the energy of the primary electrons, which is around 10 MeV (for a 20 MeV electron beam).
I expected to see a continuous range of energies going up to the maximum energy of the primaries, but it didn’t happen.

  • I also experimented with different physics lists, including “emstandard_opt4” and “empenelope,” but got the same behavior.
  • When I increased the energy of the primary electrons, the limit increases to reach half energy of the primary.
  • I highlight that everything works fine with secondary Bremsstralung.

Any ideas of what is causing this ?

Thank you!

Geant4 version Name: geant4-11-01

This is due to indicernability of the final state : you cannot say who is the electron-beam and who is the electron-target. In this situation, it is enough to generate the second electron in the range (0, Ebeam/2)
See Physics Reference Manual → Electromagnetic Interactions → Electron and Positron incident → ionisation

If you start with a positron beam, you will see a different situation.

It appears that not only I am unable to observe secondary electrons with energies in the range of [Eprim/2, Eprim[, but I’m also not seeing the secondary electrons generated by secondary electrons with energies falling in the range of [Esec/2, Esec[.

Could you please elaborate on the choice of Tcut;

  • The indiscernibility of the final state is caused by the explicit production of secondaries with kinetic energies above the production threshold?
  • Or, a continuous production of secondary electrons will cause an indiscernibility in the final state?

Thank you very much,

" The indiscernibility of the final state is caused by the explicit production of secondaries with kinetic energies above the production threshold " yes

What is done is equivalent to the following. When an ionisation process happens :

  • stop and kill the incident e-
  • create 2 secondary e- : scattered primary and ejected e-, with energy-momentum conservation
  • track immediately the most energetic of the two

It is not done in this way, but it is equivalent. This is why you seem to see only the less energetic as secondary. in fact, the most energetic appears as the scattered primary …
But this is arbitrary, as the 2 particles are identical.
It is a general feature of Monte Carlo transport codes to try to track most energetic particles in priority, in order to minimize the size of the waiting stack.

Secondaries of secondaries are treated in the same way. If your production cut is small enough, you should see them.

edit (29-09-23) : this morning, I realize that my “explanation” is, perhaps, too complicated …
Shortly : include the scattered primary in the plot of secondaries.