Photodisintegration

Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray is absorbed by atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron or an alpha particle^[1] 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 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 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.^[2] This experiment proves that a neutron is not a bound state of a proton and an electron,^[3] 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.^[4][5]

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. A portion of mass escapes in the form of relativistic jets, which could have "sprayed" the first metals into the universe.^[6][7]

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). Very high energy gamma rays have been shown to induce fission in elements as light as tin.

References

1. ↑ Clayton, D. D. (1984). Principles of Stellar Evolution and Nucleosynthesis. University of Chicago Press. p. 519. ISBN 978-0-22-610953-4. 
2. ↑ Chadwick, J.; Goldhaber, M. (1934). "A nuclear 'photo-effect': disintegration of the diplon by γ rays". Nature 134 (3381): 237–238. Bibcode:1934Natur.134..237C. doi:10.1038/134237a0. 
3. ↑ Livesy, D. L. (1966). Atomic and Nuclear Physics. Waltham, MA: Blaisdell. p. 347. LCCN 65017961. 
4. ↑ Lalovic, M.; Werle, H. (1970). "The energy distribution of antimonyberyllium photoneutrons". Journal of Nuclear Energy 24 (3): 123–132. Bibcode:1970JNuE...24..123L. doi:10.1016/0022-3107(70)90058-4. 
5. ↑ Ahmed, S. N. (2007). Physics and Engineering of Radiation Detection. p. 51. ISBN 978-0-12-045581-2. 
6. ↑ Fryer, C. L.; Woosley, S. E.; Heger, A. (2001). "Pair-Instability Supernovae, Gravity Waves, and Gamma-Ray Transients". The Astrophysical Journal 550 (1): 372–382. arXiv:astro-ph/0007176. Bibcode:2001ApJ...550..372F. doi:10.1086/319719. 
7. ↑ Heger, A.; Fryer, C. L.; Woosley, S. E.; Langer, N.; Hartmann, D. H. (2003). "How Massive Single Stars End Their Life". The Astrophysical Journal 591 (1): 288–300. arXiv:astro-ph/0212469. Bibcode:2003ApJ...591..288H. doi:10.1086/375341.