Annihilation reaction process

I-128 isotope decays with approx 93% beta and 6 % positrons. In our transport codes like Geant4, I believe the secondaries are kept in a stack and then transported one by one. Since e- and e+ are not being transported at the same time, How does it track Annihilation reactions from delayed particles that produce 0.511 MeV gamma?

I understand that positrons will more likely annihilate with the electrons already present in the medium. However, are we tracking the unlikely event where both the e- and e+ from the decay annihilate?


Geant4 will never annihilate two tracks. The toolkit has no provision for any kind of direct interaction between active G4Tracks. The annihilation process only applies to positrons, and involves the positron interacting with an “effective” electron in the material being traversed.

It would be an interesting exercise to compute what density of I-128 you would need in real life in order to have enough (nearly) simultaneous decays that you would see direct interactions between decay electrons and decay positrons.

1 Like


annihilation of e+ is enabled in all EM physics builders. Annihilation process may happens on fly or at rest. This is very standard classical process implemented inside Geant4.


1 Like