1. During the annihilation of positron and electron AT REST, two photons are generally created at opposite direction - angle between them being 180 degres. However, an electron that is orbiting around an atom can sometimes react with a positron. The orbital electron in that case has therefore a momentum, hence not at rest, and the resulting photons are therefore emerging at an angle that is NOT 180 degrees - See the file attached! The momentum of the electron around the atom can be rougly obtained with the equation of the quantized angular momentum 2. L = p×r = nh/2pi 3.I am trying to get the maximum angle θ that appears when the vector of the momentum of the electron (black) is perpendicular to the horizontal line (red) Since writing the equation here could be quite fastidious to understand, I attached a written version of my equations a a .gif file Thank you very much, Vincent. ps: I am absolutely new to this forum so please tell me if I need to add more details or wrote something wrong.