[Gate-users] Wavelength-shifting (WLS) fibres coupled to scintillators

Filipe Castro ifcastro at ua.pt
Wed Jan 29 18:53:35 CET 2014

Hello Filipe,
Hi Vesna, thanks for the reply.

 I want to simulate scintillation photons produced by 511 keV gammas and their optical path through a LYSO scintillator with mirror polished ends and reflective side surfaces. One end of the crystal is coupled to a wavelength-shifting fibre (double-cladded BCF-91A fibre), such that crystal and fibre make a 90 degree angle.
As I am not experienced with Gate (using vGate 2.1), I have many doubts regarding the proper physics and geometry necessary for the simulation, namely of the optical photons path, optical boundaries and fibre geometry. Is there anyone who has simulated similar experiments with this kind of fibres, for example in the AxPET project, who can help me out?
I've already recompiled Gate with optical ON.
In order to define the reflective surfaces of the crystal, do I need to first define the volumes with the reflective material or is it possible to define the surfaces only with the crystal volume?

If your crystal is the LYSO material, you just need to define your crystal volume with the LYSO material properties in Materials.xml. For the polished ends and reflective surfaces, you define the surfaces with the finish, reflectivity, .... parameters in Surface.xml.

                Don't I need to have 2 volumes of different materials to define a surface? For now I have only defined the LYSO volume. My question is if I need to define the volume with white reflective material (0.1 mm BaSO4) and then the surface, e.g.

/gate/LYSO/surfaces/name Surface-From-LYSO-To-Reflector
/gate/LYSO/surfaces/insert Reflector (previously defined?)
/gate/LYSO/surfaces/Surface-From-LYSO-To-Reflector/SetSurface surface_name (defined in Surface.xml)
                Or if these 3 lines are enough and I only need to define the surface properties in Surface.xml ?

 Is it necessary to define separate volumes with the different materials of the fibre (core, first clad and 2nd clad)? In this case, it is a square fibre, so what would be the best way to do it?
For the WLS fiber, you can describe it as one volume or more detailed volumes. I think one volume would be enough as long as you have its properties which are the absorption/emission wls spectra, mixture (in case you use one volume that is a mix of sub components) refractive index, ...

The thing is that the core of the fibre is polystyrene (n = 1.60), the 1st clad is acrylic (n = 1.49) and the 2nd clad fluor-acrylic (n = 1.42). These refractive index differences are designed to obtain a higher photon trapping efficiency. In the case of a square 1 mm fiber, the core has approx. 0.94 mm thickness, the 1st clad 0.04 mm and the 2nd clad 0.02 mm. I'd like to have the simulation as realistic as possible, so I wonder what is the best solution to achieve this. I can assume the whole volume of the fibre to have the same absorption/emission wls spectra.

Are these physics processes enough: Scintillation, PhotoElectric, Compton, GammaConversion, OpticalAbsorption, OpticalWLS, OpticalBoundary ?

You might also need OpticalRayleigh and OpticalMie which are the physics processes related to the optical photon scattering. In that case you would need the Mie and/or Rayleigh scattering lenghts, material anisotropy for Mie scattering. This depends on your crystal and fiber materials.

Thanks, I will look that up.

I hope this helps,
Thanks again! It sure helps. I hope you or someone can help with the remaining doubts.
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