Re: How can I read out the position information about scintillation process?

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Although the G4ScintillationProcess inherits from G4VRestDiscreteProcess (for reasons I don't quite remember now - it should maybe more properly inherit from G4VRestContinuousProcess), the scintillation photons are emitted uniformly along the step. The code documents:

// This routine is called for each tracking step of a charged particle

// in a scintillator. A Poisson/Gauss-distributed number of photons is

// generated according to the scintillation yield formula, distributed

// evenly along the track segment and uniformly into 4pi.

What this means is that there is no such thing as the 'position of the scintillation process'. The origins are on a line between PreStepPoint and PostStepPoint.

You can get the origin of each and every scintillation photon with code such as the one exhibited in:

/examples/extended/optical/LXe/scr/LXeSteppingVerbose.cc:

        G4int tN2ndariesTot = fN2ndariesAtRestDoIt +
                              fN2ndariesAlongStepDoIt +
                              fN2ndariesPostStepDoIt;

        for(size_t lp1=(*fSecondary).size()-tN2ndariesTot;
                        lp1<(*fSecondary).size(); lp1++){
          G4cout << "    : "
                 << std::setw(6)
                 << G4BestUnit((*fSecondary)[lp1]->GetPosition().x(),"Length")
                 << std::setw(6)
                 << G4BestUnit((*fSecondary)[lp1]->GetPosition().y(),"Length")
                 << std::setw(6)
                 << G4BestUnit((*fSecondary)[lp1]->GetPosition().z(),"Length")
                 << std::setw(6)
                 << G4BestUnit((*fSecondary)[lp1]->GetKineticEnergy(),"Energy")
                 << std::setw(10)
                 << (*fSecondary)[lp1]->GetDefinition()->GetParticleName();
          G4cout << G4endl;
        }

As you can see, this assumes that the secondaries of a step are stored in the fSecondary vector such that the optical photons come last. I don't know if this should be expected in all cases. Of course, you can ask for the ParticleName first and then, if 'opticalphoton' print its origin position.

Hope this helps, Peter