Photodisintegration

Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the incoming gamma ray. 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.

Photodisintegration of deuterium

A photodisintegration reaction

2 1D

+

γ

→

1 1H

+

n

was used by James Chadwick and Maurice Goldhaber to measure the proton-neutron mass difference.^[1] This experiment proves that a neutron is not a bound state of a proton and an electron,^[2] as had been proposed by Ernest Rutherford.

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.

Photofission

Photofission is a similar but different process, in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission (splits into two fragments of nearly equal mass). Very high energy gamma rays have been shown to induce fission in elements as light as tin.

References

1. ^ James Chadwick and Maurice Goldhaber, "A nuclear 'photo-effect': disintegration of the diplon by rays", Nature,134, 237-38 (1934).
2. ^ Derek Livesy,Atomic and Nuclear Physics, Blaisdell Publishing Company, Waltham, Mass. 1996, p. 347