Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray is absorbed by an atomic nucleus and causes it to enter an excited state which immediately decays by emitting a subatomic particle. A single proton, neutron or alpha particle is effectively knocked out of the nucleus by the incoming gamma ray. Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and sometimes exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton-rich elements via the p-process in supernovae.
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Photodisintegration of deuterium
A photodisintegration reaction
was used by James Chadwick and Maurice Goldhaber to measure the proton-neutron mass difference. This experiment proves that a neutron is not a bound state of a proton and an electron, as had been proposed by Ernest Rutherford.
Photodisintegration of beryllium
The photodisintegration of beryllium by gamma rays emitted by antimony-124 is used as a source for thermal neutrons.
Hypernovae
In explosions of very large stars (250 or more times the mass of the 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. A portion of mass escapes in the form of relativistic jets, which could have "sprayed" the first metals into the universe.
Photofission
Photofission is a similar but distinct process, in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission (splits into two fragments of nearly equal mass).