Hi, Seb, Thank you for your reply. I got these questions because that from the gate.root we can see the acolinearity angle distribution if the positron source are used, however, if a isotope source was used, then we can not get the acolinearity distribution from the gate.root output. In Geant4, there is one chapter describe the process of positron and electron annihilation, with can be found here: http://wwwasd.web.cern.ch/wwwasd/geant4/G4UsersDocuments/UsersGuides/PhysicsReferenceManual/html/node43.html From this chapter, it seems that Geant4 will calculate the final states of the two gamma photons according to the kinetic energy of the positron, which means the acolinearity should be included. But it also mentioned a method called AtRestDoIt, in that case the kinetic energy is taken as 0. I am not quite familiar with Geant4, so I don't know what's the real model they are using in Geant4 for the e+e- annihilation. Maybe you can find it out by checking the source code. Best regards, Yuxuan <tt>Hi Yuxuan, >  1. If a backtoback gamma source is used, is the acolinearity be simulated? No, the acolinearity class is called only for e+e- annihilation >  2. If a isotope source is used, (F18, O15, etc), is the acolinearity be > simulated? Yes, in this case the acolinearity is always simulated. > but in the Physics Reference Manual of Geant4, the acolinearity should > be calculated in > Geant4. Could you send me the page where you have read this information in the G4 guide ? (or the class name which simulates this process) Thanks Seb > > Anyone can give some explainations about that? Thanks. > > > Yuxuan ZHANG > > > ------------------------------------------------------------------------ > > _______________________________________________ > gate-users mailing list > gate-users@lphe1pet1.epfl.ch > http://lphe1pet1.epfl.ch/mailman/listinfo/gate-users _______________________________________________ gate-users mailing list gate-users@lphe1pet1.epfl.ch http://lphe1pet1.epfl.ch/mailman/listinfo/gate-users </tt>