Cluster Decay
Cluster decay is a type of nuclear decay in which an unstable atomic nucleus emits a cluster of protons and neutrons that is larger than an alpha particle, but smaller than a typical binary fission fragment.
Cluster decays have very small branching ratios. This makes them very rare events, and difficult to observe experimentally.
An Example of Cluster Decay
Barium-114 is an isotope of barium that can decay via cluster decay to tin-102, producing a carbon-12 nucleus. The nuclear equation below represents this process.
$$_{56}^{114}\textrm{Ba} \, \rightarrow \, _{50}^{102}\textrm{Sn} \, + \, _{6}^{12}\textrm{C}$$
A carbon-12 nucleus contains six protons, so the atomic number of the original nucleus is reduced by six. A carbon-12 nucleus also contains six neutrons, making for a total of twelve nucleons, so the mass number of the original nucleus is reduced by twelve.