About accessing the energy of the particle

[weller]

you can access the primary vertex of each event as described here:

Plot primary energy spectrum

In your UserEventAction, you can get the information about the generated primary in each event via G4Event::GetPrimaryVertex(G4int i) (use G4Event::GetNumberOfPrimaryVertex() to get the count and loop). From each vertex, you can get the individual primary particles from G4PrimaryVertex::GetPrmary(G4int i) and G4PrimaryVertex::GetNumberOfParticle(). If you used GPS to throw a single particle per event, you could skip the looping and just get index 0 in each case, but then you’d have to modify y…

and from there you can access the energy the primary (“the photon at the beginning of the event”) was equipped with.