[Gate-users] direct count rates

[Hartmann Julia]

Hi Gate users,
I have a question about coincidence count rates. I simulated a dual-headed PET-system. The heads consist of 66*66 crystals (size: (0.1*10*0.1) mm^3) made of lead to get a "perfect absorber". During the acquisition, the scanner rotates around the z-axis from 0 to 360 deg. I simulated several point sources at different axial (z-) positions at the same transaxial (x,y=0) positions. Considering only the direct coincidences, which means coincidences between two detectors of the same axial position, one would expect to get approximately the same count rates for all sources, no matter what their axial positions are.
My problem is, that I get different count rates for different axial positions. I`ve attached a figure to make clear what I mean. The figure shows the direct counts of 12 sources. The sources 1-6 are located at axial positions from 0 to 3.1 mm; this are just the limits of one crystal, so one can investigate the count rates if the source position varies within one crystal. Sources 7-12 are again located between the limits of an axial crystal row, axial positions vary from 46.5 to 49.6 mm. As you can see in the figure, the count rates for the sources located near to z=0 are slightly lower than those near 50 cm. One could claim that the result is still o.k. if one plots the errorbars as well, but I found it a bit strange that this tendency can be seen at some axial positions, at others not.
Does anybody understand this?
Here is the geometry of the scanner and the sources:

#     D E F I N I T I O N  O F  T H E  S Y S T E M
#
#     C Y L I N D R I C A L

/gate/world/daughters/name cylindricalPET
/gate/world/daughters/insert cylinder
/gate/cylindricalPET/setMaterial Vacuum
/gate/cylindricalPET/geometry/setRmax 40 cm
/gate/cylindricalPET/geometry/setRmin 33 cm
/gate/cylindricalPET/geometry/setHeight 22 cm
/gate/cylindricalPET/vis/forceWireframe


#
#     H E A D
#
/gate/cylindricalPET/daughters/name rsector
/gate/cylindricalPET/daughters/insert box
/gate/rsector/placement/setTranslation 0 35.5  0 cm
/gate/rsector/geometry/setXLength 20.46 cm
/gate/rsector/geometry/setYLength 1. cm
/gate/rsector/geometry/setZLength 20.46 cm
/gate/rsector/setMaterial Vacuum
/gate/rsector/vis/forceWireframe

#
#                C R Y S T A L
#
/gate/rsector/daughters/name crystal
/gate/rsector/daughters/insert box
/gate/crystal/geometry/setXLength 3.1 mm
/gate/crystal/geometry/setYLength 10. mm
/gate/crystal/geometry/setZLength 3.1 mm
/gate/crystal/setMaterial Lead
/gate/crystal/vis/forceWireframe


#                R E P E A T    C R Y S T A L
/gate/crystal/repeaters/insert cubicArray
/gate/crystal/cubicArray/setRepeatNumberX 66
/gate/crystal/cubicArray/setRepeatNumberY 1
/gate/crystal/cubicArray/setRepeatNumberZ 66
/gate/crystal/cubicArray/setRepeatVector 3.1 0 3.1 mm

#                R E P E A T   H E A D
/gate/rsector/repeaters/insert ring
/gate/rsector/ring/setRepeatNumber 2


#                A T T A C H    S Y S T E M
/gate/systems/cylindricalPET/rsector/attach rsector
/gate/systems/cylindricalPET/module/attach crystal
/gate/crystal/attachCrystalSD

....

#                                                                 #
#          D E F I N I T I O N   O F  T H E   S O U R C E         #

##### simple particle source twogamma backtoback

/gate/source/addSource 2gamma1
/gate/source/2gamma1/gps/particle gamma
/gate/source/2gamma1/setType backtoback
/gate/source/2gamma1/setActivity 10000 becquerel
/gate/source/2gamma1/gps/energytype Mono
/gate/source/2gamma1/gps/monoenergy 511 keV
/gate/source/2gamma1/gps/angtype iso
/gate/source/2gamma1/gps/mintheta 89 deg
/gate/source/2gamma1/gps/maxtheta 91 deg
/gate/source/2gamma1/gps/minphi 0. deg
/gate/source/2gamma1/gps/maxphi 360. deg
/gate/source/2gamma1/gps/type Volume
/gate/source/2gamma1/gps/shape Sphere
/gate/source/2gamma1/gps/radius 0.5 mm
/gate/source/2gamma1/gps/centre {x} {y} {z} mm
#/gate/source/2gamma1/gps/confine NULL

Thanks in advance,
Julia



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