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Question Re: Problem with WLS fiber 

Forum: Processes Involving Optical Photons
Re: None Problem with WLS fiber (Nathan Murtha)
Re: Feedback Re: Problem with WLS fiber (Michael H. Kelsey)
Re: Question Re: Problem with WLS fiber (Nathan Murtha)
Re: Warning Re: Problem with WLS fiber (Michael H. Kelsey)
Date: 21 Aug, 2014
From: Nathan Murtha <Nathan Murtha>

Hi Michael,

Again, thanks for looking at this!

I've attached an updated version of my geometry; I changed the detector material to an entirely different material (G4_Al). I didn't realize that Geant would consider both material pointers to be the same if I declared them both to be the same type of material, but that completely makes sense in hindsight. I haven't looked into making a detector with a quantum efficiency yet, I'm hoping that I can get away with my crude detection method until I have all the WLS physics worked out (that should be okay, right?)

I also made a change to the WLSABSLENGTH parameters that I assigned to my WLS fiber. Before, I had all the absorption lengths equal to 3.5*m. Now, I have any absorption length where the emission spectrum is 0.0 (for that photon energy) set to be 0.5*mm. Is this the proper way to use the WLSABSLENGTH? The way I'm thinking about this is that the fiber will absorb all the higher energy photons, and re-emit them at a lower energy (thus shifting their wavelengths). I've included that snippet of code here:

 G4double AbsorptionLengthFiberCore[nEntries] =
              { 3.5*m, 3.5*m, 3.5*m, 3.5*m, 
              3.5*m, 3.5*m, 3.5*m, 3.5*m,
              3.5*m, 3.5*m, 3.5*m, 3.5*m,
              3.5*m, 3.5*m, 3.5*m, 3.5*m,
              3.5*m, 0.5*mm, 0.5*mm, 0.5*mm,
              0.5*mm, 0.5*mm, 0.5*mm, 0.5*mm,
              0.5*mm, 0.5*mm, 0.5*mm, 0.5*mm,
              0.5*mm, 0.5*mm, 0.5*mm, 0.5*mm,
              0.5*mm };

    G4double EmissionFiberCore[nEntries] = // relative emission spectrum 
              { 2.0, 7.0, 10.0, 15.0,
              20.0, 30.0, 40.0, 50.0,
              80.0, 77.0, 75.0, 75.0, 
              98.0, 100.0, 37.0, 8.0,
              1.0, 0.0, 0.0, 0.0,
              0.0, 0.0, 0.0, 0.0,
              0.0, 0.0, 0.0, 0.0,
              0.0, 0.0, 0.0, 0.0,
              0.0};

I tried running my code with these new revisions, and my detector is seeing a few photons at the end of the fiber (which is better than I had before, thanks for the help!). Furthermore, these photons are at a lower energy than those that escape the bar, which is a good sign. But I can't find any reference to OpWLS in the tracking output; maybe I'm just looking in the wrong place? I'm a little skeptical that everything is working properly right now, because I expect to see a lot more photons make it down the fiber (I say this, because I actually made these measurements in real life, and now I want to try and simulate them).

Once I get everything working with my crude sensitive detector, and I'm convinced that the WLS physics is working, I'll look into setting up a detector with some quantum efficiency, thanks for that suggestion.

Thanks! Nathan

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/08/21/07.40-25297-ntDetectorConstruction.cc

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1 Feedback: Re: Problem with WLS fiber   (Michael H. Kelsey - 21 Aug, 2014)
(_ Question: Re: Problem with WLS fiber   (Nathan Murtha - 22 Aug, 2014)
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