The phenomenon you are referring to is called nuclear fission, which is the process of atomic nuclei capturing neutrons and splitting into two or more smaller nuclei. Nuclear fission can occur spontaneously in some isotopes, such as uranium-235, or can be induced by bombarding the nuclei with additional neutrons.
During nuclear fission, a heavy nucleus absorbs a neutron, becoming unstable and subsequently splitting into two or more smaller nuclei, releasing a significant amount of energy in the process. This process is accompanied by the emission of additional neutrons, which can go on to induce fission in other nuclei, creating a self-sustaining chain reaction. The released energy can be harnessed for various purposes, including electricity generation in nuclear power plants or as a destructive force in nuclear weapons.
It's important to note that the formation of heavy elements through the rapid capture of neutrons is a different process called neutron capture or neutron bombardment. Neutron capture can occur in various astrophysical environments, such as supernovae or neutron star mergers, where heavy elements are formed through successive neutron capture reactions.