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Forum: Processes Involving Optical Photons
Re: Question How to define the WLS quantum yield (vesna cuplov)
Re: Feedback Re: How to define the WLS quantum yield (Gumplinger Peter)
Re: None Re: How to define the WLS quantum yield (Sarah Eno)
Date: 10 Jul, 2012
From: Gumplinger Peter <Gumplinger Peter>

Hi Sarah,

> When I look at data shifts for WLS fibers, such as:
> 
> http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Brochures/Scintillating-Optical-Fibers-Brochure.pdf
> 
> second to bottom figures on page 6 (BCF-91A and BCF-92), for
> "absorption", there are unit-less numbers.

For the emission spectrum, the unit-less numbers are ok. Now, I don't have an idea how to interpret the Absorption part of those graphs, except that at one wavelength all of the photons are absorbed. There must be some distance scale(?) Or that scale may be very short compared to the dimensions of the fiber and then it may refer to the quantum efficiency of a fiber to absorb and re-emit.

If you want to implement an efficiency of WLS, you'll need to implement it in your StackingAction. The WLS process will by default emit one WLS photon for every absorbed photon. In StackingAction you can then discard some of these WLS photons as a function of their wavelength. This efficiency is not implemented as part of the WLS process.

> In the GEANT4 routine WLSABSLENGTH, this should be a number with units
> of length.

Yes, its what you would normally say is an absorption length/mean free path. For trapping WLS photons in fibers it is somewhat important to simulate correctly exactly where inside the fiber the absorption and emmission of individual photons take place. If you had a very short length, all of the emitted photons would originate near the surface of the fiber.

> How did you, for example, have other people gotten
> the values used in this routine?

The routine relies on user input.

> I think this says that a photon is wave length shifted when the binder
> absorbs the photon, and the energy is transferred by non-radiative
> dipole-dipole interactions to excited levels of the solute, producing
> fluorescence. From this, I assume that the photon is absorbed by the
> binder, and then there is a probability of this non-radiative transfer,
> given by those figures in the saint-gobain brochure that do not have
> units of length, but look like probabilities (a "quantum efficiency").
> This is why I think you multiply the binder absoption length by the
> numbers in this figure to get the numbers needed by your routine.

See my comment about efficiency in StackingAction above.

> However, then I have a question: there are photons that are absorbed
> within the fiber, but that do not result in a wave-length shifted
> photons. In this case, I guess the absopbed photon's energy is
> dissipated through phonons or some other mechanism. How is this handled?

not handled - use StackingAction

> Is there some other place where you put in the absorption length of the
> binder * (1- the quantum efficiency) to represent photons absorbed but
> not shifted?

In G4 you can specify two types of absorption. The one that leads to WLS emission and one that will simply absorb the photon. Both are in units of length. I am with you - it is not necessarily easy to come up with the required numbers from manufacturers' data tables.

Peter

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