When atoms of an element are bombarded by neutrons, the atomic nuclei are (artificially) disintegrated and emit lighter particle ($eg$. $\alpha -\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e},\mathrm{ }\mathrm{\beta }-\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e}$,proton etc.).

Sometimes a neutron is observed by the nucleus which is converted into its heavier isotope and energy is emitted in the form of $\mathrm{\gamma }$-photons.

This process in which heavy nucleus is broken into two nearly equal fragments is called nuclear fission.