CBSE NOTES CLASS 12 PHYSICS
CHAPTER 13 NUCLEUS
γ-rays are electromagnetic rays.
When a γ-particle is emitted by a nucleus its atomic number and mass number remain unchanged.
When a nucleus in an excited state spontaneously decays to its ground state (or to a lower energy state), a photon is emitted with energy equal to the difference in the two energy levels of the nucleus. This is called gamma decay.
A gamma ray is emitted when an α or β decay results in a daughter nucleus in an excited state, which returns to the ground state by a single photon transition or successive transitions involving more than one photon. For example
Ni is in excited state, which results in successive emission of gamma rays of energies 1.17 MeV and 1.33 MeV.