The positron decay of 15 o goes directly to the ground state

The positron decay of 15 o goes directly to the ground state of,15 N; no excited states of 15 N are populated and no Gamma rays follow the beta decay. Yet a source of,15 0 is found to emit Gamma rays of about 0.5 MeV. Explain the origin of these Gamma rays.

Solution

The positron is made of anti-matter and does not last very long once emitted. As soon as it meets an ordinary electron the two particles annihilate and the energy contained in their mass transformed into two gamma-rays of 0.5 MeV each, travelling in opposite directions. So, positron radioactivity is therefore always accompanied by the emission of gamma rays with an energy of about 0.5 MeV.