Spectrum of annihilation photons

Hi all,

I am working with geant4.10.02.p02 on Ubuntu. I developed a custom package for studying photons spectrum for cell irradiations using Varian Linac as the source - that is implemented through IAEA phase space files. It’s all going well but there is a feature of the photon spectrum I don’t understand.

I scored photons in the sensitive detector via ProcessHits() method and looked at primary and secondary photons. For primaries, I use G4Step aStep->GetTrack()->GetParentID()==0 && aStep->GetTrack->GetDefinition==G4Gamma::GammaDefinition().

For secondaries on the other hand, I check aStep->GetTrack->GetCreatorProcess()->GetProcessName() and I extract “annihil” as annihilation photons (511 keV at low energies). I also score bremsstrahlung and ionization photons.

How come I have photons in the annihilation spectrum going up to 2 MeV? Is there something I’m missing?

Thanks a lot in advance!

I’ll defer to the Physics experts, but v10.2.2 is obsolete and no longer supported.

Thank you. I am using the Penelope physics list, this should be consistent between the versions.
I think it’s the kinetic energy of the beta particle for some of these events that gives photons >511 keV energy.

Possibly you are observing examples of positron annihilation in flight. For that process, the two gammas do not have equal energy and do not have to be (in fact, generally are not) 511 keV each. That would wexplain the high energy and low energy tails.

Yes, thank you very much!
Closing this issue.

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