Photodisintegration is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. It therefore decreases the mass number of the affected nucleus. The reactions are called (γ,n), (γ,p), and (γ,α), respectively. It is generally endothermic if performed on elements lighter than iron, and exothermic if performed on elements heavier than iron. In an endothermic reaction, the outcome nuclei has a higher total mass, according to the mass-energy relation, and vice versa in an exothermic reaction.