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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)
Date: 20 Aug, 2014
From: Michael H. Kelsey <Michael H. Kelsey>

On Thu, 21 Aug 2014 00:22:32 GMT, Nathan Murtha wrote:
>    Attachment:


> 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]
[ etc. ]

Okay, these calls are just getting the mean free path for the process in each material. What you want to look for are "::PostStepDoIt" lines (use |grep| on your log file, for example, to find them). But see below; you're not going to find any...

> On Wed, 20 Aug 2014 21:33:39 GMT, Michael H. Kelsey wrote: [...] > > Your physics list looks okay, as does your geometry.

I spoke too soon about your geometry -- I was looking to see whether you attached material properties, and whether you created optical surfaces. I didn't notice that you have inadvertently stomped on yourself :-)

You store two different material pointers:

    G4Material* FiberCoreMaterial = NISTmanager->FindOrBuildMaterial("G4_POLYSTYRENE");
    G4Material* DetectorMaterial = NISTmanager->FindOrBuildMaterial("G4_POLYSTYRENE");

and later in the code you attach different material properties tables to them:


Unfortunately for you, those two material pointers are the same (since they are referencing the same G4NistManager material definition). Consequently, that second assignment _replaces_ the first, with the result that your fiber core is just the same as your little "detector".

Optical photons in the core will never get shifted, because you removed all of the WLS-related parameters from the material. This is one of the nasty dangers of G4's optical physics system: if you assign properties to a given material (i.e., a particular G4Material*), they apply to _every_ use of that material, everywhere in your geometry.

To do what you intended, what you need to do is to create a new, different G4Material* as a copy of the NIST "G4_POLYSTYRENE". Then you can attach different optical properties to the two "different" materials (different objects in memory). G4Material has a fancy "copy" constructor which takes a G4Material* as input to do this; see G4Material.hh.

> [...] 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.

Yup; see above.

> 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.

You set the absorption length in your detector to 1 cm. That same absorption length got applies to your WLS fiber, with the result that any photons which make it into the fiber disappear rather quickly :-/

There is another way to do what you want. You can make your detector out of a _different_ material, to which you assign a (energy dependent) quantum efficiency ("EFFICIENCY"), which you could set to 100%. Create a "dielectric_metal" optical surface between the fiber and the detecttor, and any photons which hit the boundary will be recorded as "detections".

  -- Michael Kelsey

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1 Question: Re: Problem with WLS fiber   (Nathan Murtha - 21 Aug, 2014)
(_ 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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