|Message: Re: Relation between physical particles and G4Tracks||Not Logged In (login)|
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> I have noticed that when I toggle between > opticalPhysics->SetTrackSecondariesFirst true and false for kCerenkov > and kScintillation, it drastically changes the later muon interactions. > For example, when they are set to "false" there are muPairProduction > events in addition to muIoni; however when they are set to "true" there > is only muIoni and no pair production.
> I expected that the muon interactions should be the same regardless of > whether or not secondaries are tracked first. Why does the tracking > order affect the physics?
When you toggle between SetTrackSecondariesFirst true and false, you are also affecting the sequence of the random number generator that applies to the muon track. With other words, you should not expect that the muon simulation for the muon track will be identical whether or not secondary optical photons are tracked first or not. Some random numbers that apply to the muon in the false case, will apply to the optical photon instead. So, the sequence gets out of sync and so it appears to you as if the physics has changed. However, statistically, over many muon histories you should see the same physics events. I am guessing, you don't see muPairProduction in the few events you simulate in one case, while you do see it in the other case.
I suggest you modify your output and only output when a muPairProduction happens, while your output is silent otherwise. You then run many muon histories and compare how often pair production happened.
Best regards, Peter