[Gate-users] Potential Coincidence Sorter Bug

[Ane Etxebeste]

Hi Moritz,
The coincidence sorter is designed so that the incoming singles are 
placed in a buffer. Once the buffer exceeds a size (256 by default), the 
earliest single is extracted from the buffer and tested for 
coincidences.
You can change the size of the buffer with the following command
  /gate/digitizer/Coincidences/setPresortBufferSize 32
At the end of the simulation the singles in the presort buffer that have 
not been checked for coincidences are discarded.

That may explain why even if your simulation lasts 1e8 s you do not have 
coincidences after 7.2e7 s.

I hope that helps.

Ane

On 2019-08-08 14:09, Moritz Schaar wrote:
> 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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