[Gate-users] "Strange" true coincidences in the strip J-PET detector

[Pawe Kowalski]

Dear Gaters, 
I am simulating J-PET detector based on plastic scintillators. This scanner is based on axially arranged scintillating detectors (http://inspirehep.net/record/1346549/files/detector3Daxis.png).In contrast to traditional PET scanners, proper hit is not caused by the photoelectric effect but by the Compton phenomenon. For example true coincidence is an event in which there are two primary Compton scatterings in two different strips with energy deposition bigger than fixed energy threshold equal 200 keV. There is additional condition that in an event there must be eqactly two Compton scatterings with energy deposition bigger than 10 keV (noise threshold).
True coincidences are selected using following conditions on Gate variables:
  if (h1.eventID==h2.eventID) {    if (h1.nPhantomCompton==0 and h2.nPhantomCompton==0) {      if (h1.nCrystalCompton==1 and h2.nCrystalCompton==1) {        -> TRUE COINCIDENCE      }    }  }  ...
Algorithm of simple geometric recontruction:p1 = (h1.posX, h1.posY, h1.posZ)p2 = (h2.posX, h2.posY, h2.posZ)t1,t2 - times of two primary Compton scatterings from single annihilation
l=|p1-p1|dl=(t1-t2) * ca=dl(2 l)+1/2p = p1 (1-a) +p2 a // - position of "reconstructed" point
Unfortunatelly, after simple geometric recontruction of only true coincidences, I get some "strange" results. In the attachment there is result of simulation of sphere source with a diameter 1 mm with activity 370 keV placed in (1cm,0,0). The detector is placed in the vacuum. Source of back-to-back gamma quanta with energy 511 keV was used in the simulation.
One can see, that there are some points that lie outside the region with diameter 1 mm and even outside the region with radius equal to 5 cm.What may be the reason of formation of these coincidences? Number of these strange coincidences is much less than number of proper true coincidences but I would like to find a solution to improve my results. Thank you in advance for your time and your help.
Best regards,Paweł Kowalski,National Centre for Nuclear Research
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