[Gate-users] possibility for getting pencil beam from back to back source

[Nafise Ghazanfari]

Dear gate-users,
 I want to know, Is it possible in the GATE to have pencil beam from
back-to-back source for   PET, because I have a pixellated PET and want to
irradiate just one pixel (and the same one in the opposite side for having
coincidence) along the x-axis, I have used following commands but it dose n'
t work.Any kind of information is appreciated

/gate/source/addSource twogamma
/gate/source/twogamma/setActivity 100000. becquerel
/gate/source/twogamma/gps/particle gamma
/gate/source/twogamma/setType backtoback
/gate/source/twogamma/gps/centre 0.0 0.0 0.0 mm
/gate/source/twogamma/gps/energytype Mono
/gate/source/twogamma/gps/monoenergy 0.511 MeV
/gate/source/twogamma/gps/type Volume
/gate/source/twogamma/gps/type Point
/gate/source/twogamma/gps/angtype iso
/gate/source/twogamma/gps/mintheta 90.0 deg
/gate/source/twogamma/gps/maxtheta 90.0 deg
/gate/source/twogamma/gps/minphi 180.0 deg
/gate/source/twogamma/gps/maxphi 180.0 deg
/gate/source/twogamma/gps/confine NULL


             Best regards
          Nafise.Ghazanfari