Hi, I need to do some simulation about the resolutions of PET. There are three types of sources in GATE: backtoback gamma pairs; predefined F18, O15 and C11 positron sources, and all kinds of isotope sources defined in Geant4. According to the GATE developer, the positron sources of F18, O15 and C11 are the only sources that simulate the acolinearity. However, using these three sources, the positron range effect are not included. If we use the isotope sources, then the acolinearity are not include. So is there anybody knows a way that I can include the positron range and acolinearity in one simulation setup?   Also according to the GATE manual, there are some command like /gate/source/gps/mintheta /gate/source/gps/maxtheta /gate/source/gps/minphi /gate/source/gps/maxphi can be used to limit the range of the direction of the source which can reduced the simulation time if the detector angle are small. Using these commands with backtoback gamma source is very helpful. However, if I am using the positron or isotope sources, looks like there's no physics meaning to using these commands, since the direction of the gamma ray pairs should be independed from the original particles. If this is true, then is there any other method to set the range limit to direction of the gamma ray pair? Yuxuan