[Gate-users] Potential Coincidence Sorter Bug

[Moritz Schaar]

Hi everyone,

I got a very strange output of my PET simulation and I am not sure if this is a bug or me doing something wrong.
The setup is quite simple: cylindricalPET with a point source and without any phantom, 4 ns time window of coincidences.
My backtoback source is emitting 1 Bq activity over a time slice length of 1e8 s (very low activity to keep the number of randoms as low as possible).
Looking at the coincidences I can see:
    1) a drop in the number of coincidences over time to approx. 1/2 of the previous magnitude, happening roughly at 3.6e7 s and
    2) that after a point in time no coincidences are found at all, happening at 7.2e7 s.
Especially point 2) indicates that nothing happens from 7.2e7 to 1.e8 s.
Checking the singles shows that the simulations works as expected, so I get a homogenous emission without any drops over the whole measurement time.
In a second attempt I halved the simulation time and doubled the activity: 5e7 s with 2 Bq.
Here I do see the same drop after the same amount of time: 3.6e7 s.

I also checked what happens if I split the simulation into several runs but this does not help at all, as the time stamps are not resetted for each run, only the event id's.
The same behavior can be seen for the second coincidence sorter method:
/gate/digitizer/Coincidences/allPulseOpenCoincGate true

I attached an image showing the different time histograms for singles and coincidences in the 1 Bq and the 2 Bq case as well as my simulation macro.

I would like to understand what's happening here as this makes longer simulations unreliable. Any help is appreciated.

Best,
Moritz

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