[Gate-users] optical-photon-simulation-result-interpretation

[Vesna Cuplov]

Hello Debendranath,

I am simulating optical photons in a 5cmX5cm LaBr(Ce) crystal in GATE 7.0.
> I have chosen a Beam type gps source of gamma ray (140 keV) 1 Bq for 1 sec
> which means *only one* incident gamma photon on the crystal.
>
The 64X64 anodes on the PSPMT detect the scintillation photons generated.
> However, I also get a lot of trajectories passing through the system. What
> do these trajectories correspond to?
>

The green trajectories are optical photons from the scintillation process.
In your Materials.xml file, I do not see the definition of G4_TEFLON.
I also looked at the Surface.xml file to take a look at the BLACK surface
definition. The sum of all reflective types should be 1:
specularlobe+specularspike+backscatter+lambertian = 1. (lambertian being
inplicit)
Also I see a typo in the Reflectivity definition: <ve energy="2.999"
value="o"/>  ==> should be '0' not 'o'.
If you look at the example_OPTICAL directory, you will see a Surface.xml
file with a black surface definition.

In the definition of the surface as perfect_apd permits, you need to have
the sum of reflection types = 1 otherwise I don't know what the code
does exactly with such a definition where all reflection types have a
probability of 1. ?

In your Geometry (see attached plot) I see an AIR gap of 0.2mm between the
teflon Cover and the LaBr. Is that ok?
A part of the PMT is outside the SPECThead volume and therefore there is a
large air gap in between the PMT and
the Crystal Glass. Is this geometry what you wanted to simulate?

Your surface called perfect_apd is between the 2 volumes: LaBr and teflon
Cover but these 2 volumes
do not touch as there is a 2mm air gap. Therefore,  I think that surface is
not used. It can not be used as there is no surface between LaBr and teflon
cover.

So your simulation does the following: The gamma photon travels towrads the
LaBr and in that volume it undergoes the scintillation process
following the definition of the LaBr material. So ~7000 optical photons are
generated. These photons travels towards the PMT by going through
an Air gap and through the PMT volume. Where is your detecting surface?

I think you need to zoom into your geometry and make sure volumes and
surfaces are in the right positions.

use this to look at the geometry:
/vis/viewer/set/viewpointThetaPhi    90. 0.
/vis/viewer/zoom                            3.0

Hope this helps,
Vesna
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