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 beta decay. Yet a source of ^15 O is found to emit gamma rays of about 0.5 MeV. Explain the origin of these rays.

Solution

gamma rays almost always accompanies other radioactive emmissions because it represents energy loss when the remaining nucleons reorganise into more stable arrangements. thus here in this problem the origin of gamma rays arise due to mass defect as per Einesteins theory of relativity.