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

[Alexandre Dupas]

Dear Emmanuel,

What materials do you use in the definition of the geometry of the
phantom? In air, our simulation showed us a positron annihilation
distance of maximum 2m. In denser materials, the distance shrinks
(e.g. around 2cm in water). Thus, you can except much more positron
annihilation inside your detector since the material is denser, when
the positron enter the crystal it most certainly annihilate instantly.

Well, of course if you have defined a correct geometry with dense
material, the previous statements are not applicable and something
else must be wrong. If you have a Gate macro that shows the issue, we
can have a look.

I don't have any answer for the positron at rest, I actually never tried that.

Best regards,
Alexandre
--
Alexandre Dupas
Postdoctoral Research Fellow
INSERM U698, CHU X. Bichat, Médecine Nucléaire


2011/2/16 Emmanuel BRARD <emmanuel.brard at iphc.cnrs.fr>:
> 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
>
>
>
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