[Gate-users] Moving ion source is not producing desired coincidence data at low activity

[Satyajit Ghosh]

Dear Gaters,

                     I am working with a *moving ion source*. The movement
is implemented through the *'attachTo'* command. The scanner is a PET
scanner. I am acquiring coincidence data. For 1000 Bq activity, I got
expected coincidence data. But for activity 100 Bq and lower than that, the
coincidence data is not expected. Plotting source points in root through
this command


*Coincidences->Draw("sourecPosX1:sourcePosY1:sourcePosZ1")*
                    I found that the end part of the trajectory is cut down
(see trajec_1000Bq.png and trajec_100Bq.png files). This cut happens early
in the trajectory for a lesser activity source (see trajec_10Bq.png file).
Interestingly, I found that in all these low activity cases the Singles
data tree is showing a complete trajectory. So, maybe the problem is
related to the digitizer. But it is hard to believe that digitizer is
dependent on activity. Hereunder I am pasting the coincidence part of
digitizer implementation.





*/gate/digitizer/Singles/insert
timeResolution/gate/digitizer/Singles/timeResolution/setTimeResolution 2.0
ns/gate/digitizer/Coincidences/allPulseOpenCoincGate
false/gate/digitizer/Coincidences/setWindow 3.4
ns/gate/digitizer/Coincidences/minSectorDifference 2 *

                    I came to know from a paper that GATE has some *bugs*
for low activity and small time slice cases. So, I tried coarser time
slices. But the problem remains as it is. Also, half life can't be an
issue. Scan time is 128 sec, whereas half life is 4063 sec. It is also not
a problem that trajectory moves outside scanner FOV, and hence coincidences
can't be found, but singles still can be acquired. I tried a bigger axial
length scanner. The problem remains as it is.

Regards
Satyajit Ghosh
12-Oct.-2021


-- 
*Satyajit Ghosh, M.Sc.*
*PhD Student*

*Nuclear Spectroscopy and Molecular Imaging Lab,*
*Physics Department, IIT Bombay,*
*Mumbai-400076, INDIA*
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