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

[Vesna Cuplov]

Hello,


>
>  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 ?
>

Yes, to define a surface you need to define both volumes: crystal and the
0.1mm reflector. In your definition of the surface, you only specified the
behavior of the surface for photons going from the reflector towards the
Lyso crystal. You need to define the surface behavior for photons going
from the Lyso towards the reflector. These are 2 different surfaces. See
the user's guide, there is a drawing.

>    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 1stclad is acrylic (n = 1.49) and the 2
> nd 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.
>

If you have all the specifications of your fiber as you seem to have, you
can simulate it with all these details. I don't know about the best
solution.
Regarding the trapping efficiency (which you know as it is derived from the
3 refractive index), I think this is actually related to the WLS absorption
length (you have the spectrum?). The WLS absorption/emission will tell
where inside the fiber the optical photon absorption/emission took place.
Maybe the safe way to make sure you get the right trapping efficiency is
actually to simulate the core, 1st and 2nd clads.

vesna
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