What is gamma decay ? Explain by proper example. 

The process of $\gamma$-ray (photon) emission during the disintegration of a radioactive nucleus is called gamma decay.

$\gamma$-ray emitted in gamma decay has no mass and charge so the mass number of the daughter nucleus does not change. Its general equation is as follows:

${ }_{\mathrm{Z}}^{\mathrm{A}} \mathrm{X} \rightarrow{ }_{\mathrm{Z}}^{\mathrm{A}} \mathrm{X}+{ }_{0} \gamma^{0}$

$\text { Excited }$ $\text{ state}\text { Ground }$ $\text { state }$

where left side nucleus $\mathrm{Z}^{\mathrm{A}}$ is in excited state.

right side nucleus $\mathrm{Z}^{\mathrm{A}}$ is in ground state.

Atomic energy level spacing are of the order of $\mathrm{eV}$, while the difference in nuclear energy levels is of the order of $\mathrm{MeV}$.

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 the so called gamma decay.

The energy $(\mathrm{MeV})$ corresponds to radiation of extremely short wavelength, shorter than the hard $X$-ray region.

Typically a $\gamma$-ray is emitted when a $\alpha$ or $\beta$ decay results in a daughter nucleus in an excited state.

This daughter nucleus then returns to the ground state by a single photon transition or successive transitions involving more than one photon.