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?