[Gate-users] Potential Coincidence Sorter Bug

[Moritz Schaar]

Hi Ane,

thank you for your suggestion of this more or less undocumented parameter.
Following your explanation this indicates, that we always lose events at the end of the simulation as they are cleared instead of processed.
Is this intended?

However, I re-run my simulation using
	/gate/digitizer/Coincidences/setPresortBufferSize 32
and to my surprise nothing changed, same issue.
To be fair I have to mention that I did not fix the seed point, so the results are not identical and changes in detail are expected.
But, the overall outcome should have changed at least a bit, especially the time points should vary which they do not.
I tried different histogram bin sizes but this does not change the results.
Is there anything else to check?

Best,
Moritz


-----Ursprüngliche Nachricht-----
Von: Ane Etxebeste <ane.etxebeste at creatis.insa-lyon.fr> 
Gesendet: Donnerstag, 8. August 2019 14:38
An: Moritz Schaar <schaar at imt.uni-luebeck.de>
Cc: gate-users at lists.opengatecollaboration.org
Betreff: Re: [Gate-users] Potential Coincidence Sorter Bug

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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> Gate-users at lists.opengatecollaboration.org
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