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Feedback Re: Energy deposited is larger than the energy of the primary gamma beam 

Forum: Processes Involving Optical Photons
Re: Question Energy deposited is larger than the energy of the primary gamma beam (Martin Kocan)
Date: 28 Jan, 2014
From: Gumplinger Peter <Gumplinger Peter>

Hi Martin,

> -I am using unix version of geant4.9.6.

Can you please use geant4.9.6.p2 if you don't already use the 2nd correction patch.

> (i) The energy deposited in a NaI scintillator is larger than the energy
> of the primary gamma beam. As an example the energy of the gamma beam
> was set to 1MeV shooting straight into the NaI scintillator (defined in
> PrimaryGenerationAction.cc) however the energy deposited occured most
> often at about 1.01965MeV. I would expect the maximum energy deposited
> at 1MeV. Not sure where the stray energy comes from...

I have no explanation for this. I suggest that you run one of the /examples/extended/electromagnetic/TestEmX examples that deals with tallying the total energy deposit per event.

Your code is sufficiently complex that I can't spend the time to carefully verify how you are collecting energy deposit etc. is correct.

> (ii) The number of optical photons detected at the photon detector was
> ~6800 which is less than expected ~8000. The scintillation yield was set
> to 40000 photons per 1MeV. The efficiency of the PMT was 20% so the
> expected photon hits should be around 8000 per 1MeV.

You should verify that the number of scintillation photons created corresponds to your yield and the energy deposition. The number of detected photons can be less because you have absorption in the medium or at the walls or you have some leak.

If all else fails, you need to simplify your code to the bare minimum - just the NaI, the Al housing and the detector volume.

> -The surfaces of NaI cylinder and photon detector (metallic disc) are
> defined via G4LogicalBorderSurface. Refractive index is defined for
> AlHousing,

Why do you define a refractive index for the AlHousing? Define this surface as dielectric_metal and give 100% reflectivity.

> Reflectivity of the NaI
> surface is set to 94%.

Bingo, this explains why you see fewer than expected photons. And although you know that each time a photon hits the wall 6% will be absorbed. What you don't know is how often, on average, your photons hit a wall before arriving at your detector.

OK, you did define the G4BorderSurface to be dielectric_metal.

I agree with everything else you explained in your posting.

Peter

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1 None: Re: Energy deposited is larger than the energy of the primary gamma beam   (Martin Kocan - 30 Jan, 2014)
(_ Agree: Re: Energy deposited is larger than the energy of the primary gamma beam   (Gumplinger Peter - 31 Jan, 2014)
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