[Gate-users] Energy Distribution and positron annihilation (Gate 6.0.p01)

[Emmanuel BRARD]

Dear Gate users,

 

I am currently simulating a PET system with 4 planar detectors a phantom and
a positron source emitting randomly within this phantom. When I analyze the
resulting dat files, I can see that there is positron annihilation outside
the phantom, actually that's even worse than that because I can see randomly
annihilation inside the detector crystals. I thought this might occur
because of a wrong positron energy distribution and so I tried with a fix
energy value of 0 eV that is a positron created at rest :

 

/gate/source/addSource positron

 

/gate/source/positron/gps/particle e+

/gate/source/positron/gps/monoenergy 0. Mev

 

/gate/source/positron/gps/type Volume

/gate/source/positron/gps/shape Cylinder

/gate/source/positron/gps/radius 10. mm

/gate/source/positron/gps/halfz 2. Mm

 

But even in that case I had positron escaping the phantom. I took a look at
the G4 output and I saw that the positron never take a initial kinetic
energy of 0 eV :

 

[G4]
****************************************************************************
*****************************

[G4] * G4Track Information:   Particle = e+,   Track ID = 1,   Parent ID = 0

[G4]
****************************************************************************
*****************************

[G4] 

[G4] Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng
NextVolume                   ProcName

[G4]     0           2.67           0.184       0.447    0.0752
0               0             0                  NEMACylinder_phys
initStep 

 

How come I cannot have a null energy value ? Is there a known issue with
energy distribution ?

 

Thank you,

Best regards,

 

--

BRARD Emmanuel 

emmanuel.brard at alumni.u-psud.fr

Medical Physics - PhD Student

IPHC IMABIO Bat 25 - 23 rue du Loess

67037 Strasbourg Cedex 2

FRANCE