Photodisintegration

Photodisintegration is a physical process in which extremely high energy gamma rays interact with an atomic nucleus and cause it to enter an excited state, which immediately decays into two or more daughter nuclei. A simple example is when a single proton or neutron is effectively knocked out of the nucleus by the incoming gamma ray, and an extreme example is when the gamma ray induces a spontaneous nuclear fission reaction. This process is essentially the reverse of nuclear fusion, where lighter elements at high temperatures combine together forming heavier elements and releasing energy. Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton rich elements via p-process which takes place in supernovae.

Hypernovae

In explosions of very large stars (250 or more times the mass of earth's Sun), photodisintegration is a major factor in the supernova event. As the star reaches the end of its life, it reaches temperatures and pressures where photodisintegration's energy absorbing effects temporarily reduce pressure and temperature within the star's core. This causes the core to start to collapse as energy is taken away by photodisintegration, and the collapsing core leads to the formation of a black hole.