|Message: Bias first, but only first, interaction?||Not Logged In (login)|
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I would like to simulate a population of MeV-scale gammas that undergo photoelectric effect or Compton scatter at a particular location. In other words, I would like to control the first interaction of these gammas, forcing it to be at a particular location, but would like the simulation to proceed normally after that first interaction.
Is there a good way to go about this? Things I have considered:
G4PhotoElectricEffect::SetCrossSectionBiasingFactor(1e12); (similar for the Compton process) This will force the gamma to interact essentially instantly. I can generate gammas at the desired location and they will definitely interact there. However, this also affects the physics after the first interaction (relevant for Compton scatters, Bremsstrahlung, etc.).
Geometric biasing: kill all gammas that leave the desired interaction location before the first interaction. This has the disadvantage that I will simulate many events and keep few of them. It saves time compared to no biasing, but probably not enough.
Custom primary generator: Do not use GPS to make gammas, but instead write a custom generator that will begin the event with the correct products of the first gamma interaction. This would be the most desirable outcome, but my understanding is that I'd have to write my own code for the photoelectric/Compton behavior of the first interaction before letting the existing G4 p.e./Compton processes take over once tracking begins.
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