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<p class="MsoNormal"><span lang="EN-US">Hi everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The setup is quite simple: cylindricalPET with a point source and without any phantom, 4 ns time window of coincidences.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Looking at the coincidences I can see:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> 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<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> 2) that after a point in time no coincidences are found at all, happening at 7.2e7 s.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Especially point 2) indicates that nothing happens from 7.2e7 to 1.e8 s.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Checking the singles shows that the simulations works as expected, so I get a homogenous emission without any drops over the whole measurement time.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In a second attempt I halved the simulation time and doubled the activity: 5e7 s with 2 Bq.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Here I do see the same drop after the same amount of time: 3.6e7 s.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The same behavior can be seen for the second coincidence sorter method:<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:35.4pt"><span lang="EN-US">/gate/digitizer/Coincidences/allPulseOpenCoincGate true<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I would like to understand what’s happening here as this makes longer simulations unreliable.
</span>Any help is appreciated.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Best,<o:p></o:p></p>
<p class="MsoNormal">Moritz<span style="color:#145A6E;mso-fareast-language:DE"><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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