You've wrongly assumed that all gamma rays are from positron annihilations. In fact most of them should be from an ordinary EM effect: http://en.wikipedia.org/

You've wrongly assumed that all gamma rays are from positron annihilations. In fact most of them should be from an ordinary EM effect: http://en.wikipedia.org/wiki/Bremsstrahlung http://en.wikipedia.org/wiki/Bremsstrahlung The graph in the article is not counting positrons, but gamma photons. A fraction of those photons (not quantified) are from electron-positron annihilations: they infer this because their instrument [1] measures the energy of individual photons, and there is a cluster at/above the lowest possible energy for e-/e+ annihilation photons (511 keV). [1] http://en.wikipedia.org/wiki/Scintillation_counter http://en.wikipedia.org/wiki/Scintillation_counter