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Forum: Processes Involving Optical Photons
Re: Question Geant4 handle optical process vs. optical process in reality (Josh Xie)
Date: 16 Aug, 2013
From: Gumplinger Peter <Gumplinger Peter>

Hi Josh,

I'll try to answer your questions as best as I understand them.

> 1. For scintillating process: any particle that can deposit energy into scintillating material will trigger scintillating process,

Please, see the /examples/extended/optical/OpNovice (formally, /examples/novice/N06). You still have to:

    if (fScintillationProcess->IsApplicable(*particle)) {
      pmanager->AddProcess(fScintillationProcess);
      pmanager->SetProcessOrderingToLast(fScintillationProcess, idxAtRest);
      pmanager->SetProcessOrderingToLast(fScintillationProcess, idxPostStep);
    }

If you did this (or used G4OpticalPhysics - physics_list), then yes.

> and the emission photon distribution will follow emission spectrum we defined.

yes

> This suggests that, the number of photons got generated will only depend on the total energy of the particle in every step deposited in the material.

yes. It is also possible that the yield depends on the particle type and energy for non-linear scintillators.

> Somehow I believe there is a threshold for the minimum energy deposition to trigger the process, which correspond to some constant of the material.

No

> Since "opticalphoton" will not deposit any energy until it dies, it will not trigger scintillating process before it dies.

yes

> 2. For WLS process: any incoming "optical photon" will be absorbed in respect of 
> WLS-Absorption-Length

yes

> (which defines the average travelling length of a photon before it gets absorbed- is this same as Absorption-Length?)

no - you can have absorption without WLS - in this case the original optical photon is simply 'killed'.

. Once a photon got absorbed, it will emit a photon with different energy in respect of the distribution of emission spectrum we defined.

yes (WLS)

> Therefore, this suggests that any optical photon that overcomes the minimum threshold for WLS

(no minimum)

> will be shifted into different wavelength regardless what the absorption spectrum is.

The absorption spectrum is a distance

> Following above conclusions, however, there is one thing I don't understand:
> Is WLS process decent for the process that involves one higher energy photon (which is still Geant4's "opticalphoton") causes excitation of multiple lower energy photons?

No, currently in G4, 1 optical photon will produce only 1 WL shifted photon. With a recent correction to the code, the shifted photon will have longer wavelength than the first.

Peter

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