I need to determine the best boundary shape for tracking photons being emitted from a cesium 137 DISK SOURCE that would be hitting a POINT DETECTOR 50 cm away. The cesium is not contained by anything, it is only traveling through air. What I mean by boundary shape is: the software tracks individual simulated photons (about 10 million of them) so in order to minimize computing power, I have to create a boundary to stop the tracking if that photon isn't going to hit the detector. I need to account for scattering so a cone is not optimal according to my professor. I am aware that incident photons from compton scattering may veer off in different directions. you can imagine the disk source to be in the yz plane and the point being along the x axis, completely perpendicular to the source. Sorry if I'm posting in the wrong forum or anything, this is my first time posting.