[Gate-users] Detection of optical photons

[Vesna Cuplov]

Hello Pawel,

I have some questions connected with detection of optical photons. In my
> simulations I need to use information about optical photons from Hits tree
> because in OpticalData tree there is no information about time (I need this
> information).
>
> I simulate single scintillator with two photomultipliers attached at its
> ends. Beam of gamma particles is directed perpendiculary to the
> scintillator.
>
> I detect optical photons using perfect_apd surfaces between my
> scintillator and photomultipliers. Part of hits in the end of scintillator
> is reflected and part of it is transported through metal.
>
> My 5 questions:
>
> 1) Hits which have processName equal to 'Transportation' should be treated
> as optical photons absorbed by the surface?
>

Not really. All photons with processName "Transportation" do not exactly
end on the geometrical volume boundary. In Geant4, particle transportation
is a process by which particles interact with geometrical volumes
boundaries. The transportation moves the particle on the boundary by
calculating a safety distance to the boundary (see Geant4 Geometry
Navigator). As far as I understand, the G4Navigator finds the safety
distance from a given point to the nearest geometric boundary.

2) If these photons should be treated as absorbed in metal, why position of
> hits with 'Transportation' is not always equal to position of surface? It
> is visible on attached picture: phenomenon_in_function_of_location.jpg.
> Value 1 means OpticalAbsorption, 2 - Transportation, 3 - Scintillation.
> Transportation should be located just at -250 and at 250 but locations of
> transportation hits are blurred.
>

see previous answer.

3) After 'Transportation' photon may be reflected from surface? Or it is
> rather like photon is either reflected or transported?
>

Depending on the definition of your surface, the optical photon might be
reflected, refracted (transmitted) or even absorbed by the surface.

4) Accoding to
> http://geant4.web.cern.ch/geant4/G4UsersDocuments/UsersGuides/PhysicsReferenceManual/html/node66.html part
> of photons is reflected from surfaces (crystal_last_hit_pos_z_A.png,
> crystal_last_hit_pos_z_B.png). Why they are reflected if in surfaces.xml
> file reflectivity is set to 0?
>

Looking at your plots, what makes you think that optical photons are
reflected? The bump between 240 and 250? From what I understand, your
scintillating material is located between -250 and 250. How did you define
that material? Any absorption by the bulk?
In case of a dielectric metal surface, the probability of reflection
(REFLECTIVITY) is specified by the user. You set it at 0 so you should not
have any reflection and the photon is detected as the detecting efficiency
is set to 1.

5) Is there any way to connect information from Hits and OpticalData trees?
> Something like hitID?
>

If you prefer to work with Hits tree, you can make your own root analysis,
loop over the Hits and select only Hits located at the detecting surface
position, therefore you'll get the Hit position and Time.
You can also add the Time leaf in the OpticalData Tree yourself. Just
follow how it is done in the code
GateToRoot.cc

I hope this helps,
vesna
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