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Forum: Processes Involving Optical Photons
Re: None Scintillation Process invoked in Non Scintillators (Marcus O'Flaherty)
Date: 09 Jun, 2016
From: Gumplinger Peter <Gumplinger Peter>

Hi Marcus,

> I have a simulation involving various components and materials,
> including an empty hall of air, a tank of water, and some scintillating
> paddles. I've found that the Scintillation process appears to being
> invoked in regions where the material is not a scintillator - that is,
> code in UserSteppingAction identifies steps with
> PostStepPoint->GetProcessDefinedStep()->GetProcessName() =
> "Scintillation", even when the
> PostStepPoint->GetPhysicalVolume->GetName() indicates a physical volume
> "tank" - i.e., my water tank, which should not be scintillating. (my
> tank is in no direct contact with scintillator, so boundary events
> shouldn't matter). I see the same in the "Hall" volume, which I have
> defined as being full of Air, and for which I don't even define an index
> of refraction.

The G4Scintillation process sets *condition = StronglyForced in GetMeanFreePath and *condition = Forced in GetMeanLifeTime and return DBL_MAX i. e. the process does not limit the step, but sets the condition for the DoIt to be invoked at every step. G4Scintillation::IsApplicable returns false only for "opticalphoton" and short-lived particles. So, what this means is that the G4Scintillation process is invoked at every step in the simulation of all particles (except - see IsApplicable) and no matter where the step is taken in the geometry. It is the G4Scintillation::PostStepDoIt that checks whether or not the step is taken inside a G4Material with scintillation properties defined in its G4MaterialPropertyTable.

> But other investigations make it appear that in these steps no
> scintillation is really occurring: I tried printing out the energy
> deposited for these steps from theStep->GetTotalEnergyDeposit(), and it
> returns 0eV. I found comments that indicated it should be possible to
> find the number of secondaries produced in a step too, which would allow
> me to count photons generated, but I couldn't find the relevant
> method/property.

G4int G4Step::GetNumberOfSecondariesInCurrentStep()

> Finally, in my PostUserTrackingAction I record the ProcessDefinedStep()
> for the last step in the track - which should be the step that stopped
> Geant4 tracking it. Again i see scintillation in non-scintillators;
> plotting the distribution of particles for which the final step process
> was scintillation shows points throughout my detector system. On the
> other hand, plotting the start position of photons generated by
> scintillation indicates they are indeed only generated in the
> scintillators.

ProcessDefinedStep is not a good indicator - for some processes - of whether they actually did anything. G4Scintillation is a 'parasitic' process (forced) that may define the step in the absence of any other process defining it even though the step length returned is DBL_MAX.

Why don't you set /tracking/verbose 1 together with your ProcessDefinedStep to find out which particle at what point triggers this.

> So, what's going on here? Should scintillation be being invoked as a
> process, even when the material isn't a scintillator?

Yes, in Geant4 the tracking knows nothing about the physics processes. So, some processes need to be always invoked!


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1 Question: Re: Scintillation Process invoked in Non Scintillators   (Marcus O'Flaherty - 10 Jun, 2016)
(_ Feedback: Re: Scintillation Process invoked in Non Scintillators   (Gumplinger Peter - 10 Jun, 2016)
(_ None: Re: Scintillation Process invoked in Non Scintillators   (Marcus O'Flaherty - 11 Jun, 2016)
(_ Feedback: Re: Scintillation Process invoked in Non Scintillators   (Gumplinger Peter - 13 Jun, 2016)
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