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

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

Solution

Since the gamma rays are emitted in positron decay of O-15 when the O-15 is in the excited state.

Hence the origin of the gamma rays is the excited state of O-15.