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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)
Date: 20 Aug, 2014
From: Nathan Murtha <Nathan Murtha>

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/08/20/17.00-61716-ScintSensitiveDetector.ccOn Wed, 20 Aug 2014 21:33:39 GMT, Michael H. Kelsey wrote:
> On Wed, 20 Aug 2014 15:03:47 GMT, Nathan Murtha wrote:
> 
> > I'm a very new user to Geant4, and I'm trying to make a WLS fiber
> > embedded in a scintillating plastic bar. I *think* that I have my
> > detector built properly, and I *think* that I've properly set up my
> > physics list - but I'm having problems, meaning that I've made a mistake
> > that I can't see.
> > 
> > Optical photons are hitting my WLS fiber, but the fiber is not
> > re-emitting any photons. I'm not familiar enough with Geant4 to know
> > where to start looking for this problem, so I'm hoping that someone here
> > might have an idea! I've attached my physics list (heavily cannibalized
> > from an example) and my detector construction files to this post.
> 
> Your physics list looks okay, as does your geometry. I'm not sure why
> you set a reflectivity for the fiber, rather than just treating it as
> transparent and letting the two RINDEX values determine reflection vs.
> refraction; but that shouldn't be a problem.
> 
> How do you know there is no re-emission? If you have tracking verbosity
> enabled, then you should see steps where G4OpWLS is the stopping process
> and a new secondary photon is created. Alternatively, you could run in
> the debugger and set a breakpoint at G4OpWLS::PostStepDoIt(). You could
> also turn on verbosity for G4OpWLS, and it should report each time it's
> invoked.
> 
> if you've made the fiber a sensitive detector, what does your SD code
> look like?
> 
>   -- Michael Kelsey

Hi Michael,

Thank you very much for taking the time to look over my code, I appreciate that!

The reason I set a reflectivity for my fiber was simply out of desperation, I couldn't get things to work and I hoped that setting a reflectivity would help. I initially only had RINDEX values set, so I'll revert back to that.

As for re-emission; I turned on verbosity for the OpWLS process, and got a lot of output that looks like the following:

G4VDiscreteProcess::PostStepGetPhysicalInteractionLength [ OpWLS]
 Particle type - opticalphoton
   mass:        0[GeV]
   charge:      0[e]
   Direction x: -0.37971, y: -0.630631, z: -0.676849
   Total Momentum = 3.04666e-09[GeV]
   Momentum: -1.15685e-09[GeV], y: -1.92132e-09[GeV], z: -2.06213e-09[GeV]
   Total Energy   = 3.04666e-09[GeV]
   Kinetic Energy = 3.04666e-09[GeV]
 MagneticMoment  [MeV/T]: 0
   ProperTime     = 0[ns]
 in Material  G4_PLASTIC_SC_VINYLTOLUENE
InteractionLength= 1.79769e+307[cm] 
G4VDiscreteProcess::PostStepGetPhysicalInteractionLength [ OpWLS]
 Particle type - opticalphoton
   mass:        0[GeV]
   charge:      0[e]
   Direction x: -0.214698, y: 0.704116, z: -0.676849
   Total Momentum = 3.04666e-09[GeV]
   Momentum: -6.54113e-10[GeV], y: 2.1452e-09[GeV], z: -2.06213e-09[GeV]
   Total Energy   = 3.04666e-09[GeV]
   Kinetic Energy = 3.04666e-09[GeV]
 MagneticMoment  [MeV/T]: 0
   ProperTime     = 0[ns]
 in Material  G4_AIR
InteractionLength= 1.79769e+307[cm] 

Unfortunately, I don't know if this is good or bad news, I'm still trying to learn the ropes of Geant4. Does this mean that the OpWLS process was actually called? When I turn on the tracking verbosity, I see lots of transportation processes in my bar and world volumes, and occasionally in the WLS fiber cladding, but never the core. I don't see OpWLS called in the tracking verbosity output.

My sensitive detector (code attached) is actually a very small plastic cylinder that's flush with the end of my WLS fiber. My idea here is that any photons that are emitted from the fiber and escape from that end should be seen by the sensitive detector - like a crude photomultiplier tube if you will. I just want to count photons that make it out the end of my fiber. Unfortunately, no photons ever make it into my sensitive detector, which is another reason I think that photons might not be being produced in my WLS fiber. Hopefully I'm wrong about that though!

Thanks again, Nathan

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