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Forum: Processes Involving Optical Photons
Re: Question wavelenth shifting and reflectivity (Stephan)
Date: 11 Sep, 2013
From: Gumplinger Peter <Gumplinger Peter>

> Hi, I'm simulating a water cherenkov detector with different inner
> linings. In the case of DF2000MA (3M), the reflectivity is 99% above
> 380nm and 10% below 380nm. I use the UNIFIED model with the option
> "dielectric-dielectric" and "polishedfrontpainted". Therefore, the
> absorption probability is calculated by "1-reflectivity".


> DF2000MA can
> also shift wavelengths below 350nm. But only 10% of the absorbed photons
> are reemitted. I know that there is "WLSABSLENGTH, WLSCOMPONENT,...",
> but I don't know how to specify, that photons are absorbed by
> WLS-process in 10% of the time and in 80% of the time by the absorption
> process calculated with the reflectivity. Is there any possibility to do
> that?

Good question!

Let me explain ... UNIFIED + "dielectric-dielectric" and "polishedfrontpainted" is basically a mirror that has the reflectifity you specified as a fuction of wavelength. Photons not reflected are absorbed. They are gone/killed and don't actually penetrated into your DF2000MA.

> Is there any possibility to do that?

I am afraid, perhaps not ....

However, if you have access to your source, you can change the code at:

        if (type == dielectric_metal) {


          // Uncomment the following lines if you wish to have
          //         Transmission instead of Absorption
          // if (theStatus == Absorption) {
          //    return G4VDiscreteProcess::PostStepDoIt(aTrack, aStep);
          // }


by uncommenting the lines.

In that case, you define the surface between the inside and DF2000MA as dielectric_metal (with your reflectivities). You then define the DF2000MA as a volume with finite thickness and a WLSABSLENGTH combination that will result in 10% WLS absorption probability.

Or, since you already have to fudge the source code, you can change the above by throwing a random number to only transmit 10% of the otherwise absorbed photons. You can then make your WLSABSLENGTH very short compared to the finite thickness but the DF2000MA lining has to remain a real volume in the G4 sense and defined to be a WLS.

I am afraid that no, currenlty, you can't define a surface that's also a wavelength shifter.

Hope this works out for you. Keep us posted, Peter

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