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Feedback Re: amount of produced photons in the scintillator 

Forum: Processes Involving Optical Photons
Re: Question amount of produced photons in the scintillator
Date: 24 Jan, 2011
From: Gumplinger Peter <Gumplinger Peter>

> I need some help concerning the produced number of photons in a
> scintillator after an interaction witha gamma ray. I set the resolution
> scale of the scintillator to zero, so that a gamma ray of a certain
> energy should produce always the same number of photons.

The code calculates the number of scintillation photons produced in a step according to:

        if (MeanNumberOfPhotons > 10.)

        {

          G4double sigma = ResolutionScale * std::sqrt(MeanNumberOfPhotons);

          NumPhotons = G4int(G4RandGauss::shoot(MeanNumberOfPhotons,sigma)+0.5);                  

        } else {

          NumPhotons = G4int(G4Poisson(MeanNumberOfPhotons));

        }

So, setting the ResolutionScale to zero does not necessarily mean that you are always going to produce the same number of photons. For steps in which the mean number of produced photons is small that number is always sampled from G4Poisson regardless of ResolutionScale. I admit, I didn't not foresee that someone would set the ResolutionScale to zero when a value close to 1.0 is more physical.

More importantly, note that:

MeanNumberOfPhotons = ScintillationYield*TotalEnergyDeposit;

where TotalEnergyDeposit is the total energy deposit during one simulation step of the charge particle. The photon you simulate to begin with, itself, does not produce scintillation. It has to first interact to produce a charge particle.

> But this is not
> always the case: - the number of produced photons is constant, if the
> the ProductionCut is set to 1mm; - the number of produced photon variies
> if the ProductionCut is set to 0.1mm or lower; Does someone know the
> reason herefore?

The total number of scintillation photons you see also depend on whether or not the electromagnetic shower is contained in you scintillator. Even if it is, you don't have control over the shower simulation by setting ResolutionScale. That is still a statistical process with, in general, three processes competing and, yes, the ProductionCut decides which secondaries are actually produced and tracked. Depending on the exact step-wise simulation of the shower, and rounding the numbers to the nearest interger for every step, all can conspire to produce slightly different number of scintillation photons you count.

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