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Forum: Processes Involving Optical Photons
Re: Question Strange behavior of the OpWLS process (vesna cuplov)
Date: 21 Jun, 2012
From: Gumplinger Peter <Gumplinger Peter>

Dear Vesna,

> I notice that the fluorescent photon has an energy greater than the
> excitation photon. I haven't seen this with Geant4.9.4.

Wave length shifting in a material is not simulated in G4 from first principles. All that G4 does is sample the wave length shifted photon spectrum you've provided with:

AddProperty("WLSCOMPONENT",PhotonEnergy,EmissionFib,nEntries);

If EmissionFib[E] > 0 at an energy, E, that is above the excitation photon's energy, you'll get some WLS photons at that energy E. True, also, that event by event there is no energy conservation in optical processes. So, when the excitation energy spectrum and the WLS spectrum overlap, the emitted photon may be higher in energy than the original. That was true for 9.4 as well. So, while we have unphysical events, statistically, the WLS spectrum is reproduced as you've requested.

You may find that the program produces a slightly different WLS spectrum in 9.5 vs 9.4 because the interpolation of EmissionFib = f(E) was linear before and is now spline. This could result in a sampled spectrum, from the interpolation, that hasn't gone to zero quite yet with the new fit by a spline.

Peter

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1 Ok: Re: Strange behavior of the OpWLS process   (vesna cuplov - 25 Jun, 2012)
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