Photodisintegration

Photodisintegration (also called phototransmutation, or a photonuclear reaction) 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 (absorbs energy) if performed on elements lighter than iron, and exothermic (releases energy) if performed on elements heavier than iron. In an endothermic reaction, the resulting nuclei has a higher total mass, according to the mass-energy relation, and vice versa in an exothermic reaction.

In γ-process, a form of p-process nucleosynthesis, photodisintegration acts on pre-existing seed nuclei (usually heavier than iron) and converts them into lighter, neutron-deficient, proton-rich isotopes, through sequences of (γ,n), (γ,p), and (γ,α) reactions. These reactions shift material toward lighter, neutron-deficient, proton-rich isotopes. The γ-process is thought to contribute to the production of stable p-nuclei, especially in supernovae.

In some hypernovae, endothermic photodisintegration can convert so much radiation energy into mass-energy that it causes the star to collapse.