The nuclear fusion is responsible for energy production in stars via fusion of hydrogen nuclei into helium nuclei. In sun, the fusion takes place dominantly by proton-proton cycle, $$4_1^1H \to _2^4He + 2{e^ + } + 2\nu + 2\gamma $$. The neutrino ($$\nu $$) and $$\gamma$$-rays emissions are parts of this fusion reaction.

The uranium based fission reactions involve absorption of thermal neutrons by $${}_{92}^{235}$$U nuclei to produce the highly fissionable $${}_{92}^{236}$$U nuclei. This nuclei then fissions into two parts e.g., $${}_{92}^{236}$$U $$\to$$ $${}_{63}^{137}$$I + $${}_{39}^{97}$$Y + 2n. The fission fragments are unstable and undergo $$\beta$$-decay to reduce their neutron to proton ratio. The fragments are generally formed in excited states and consequently emit $$\gamma$$-rays. The heavy water (D₂O) is used as a moderator to slow down the fast moving neutrons.

In $$\beta$$-decay, a neutron is converted into a proton. In this process, an electron and an antineutrino are created and emitted from the nucleus, n $$\to$$ p + e^$$-$$ + $$\overline v $$. The $$\beta$$-decay in $${}_{27}^{60}$$Co $$\to$$ $${}_{28}^{60}$$Ni + e^$$-$$ + $$\overline v $$. The daughter nuclei $${}_{28}^{60}$$Ni is formed in excited state and comes to ground state by $$\gamma$$-ray emission.

The $$\gamma$$-rays are high energy electromagnetic rays. These rays are generally emitted when a nuclei in excited state (high energy) makes a transition to a lower state (low energy).