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| Light–matter interaction |
|---|
 |
| Low-energy phenomena: |
| Photoelectric effect |
| Mid-energy phenomena: |
| Thomson scattering |
| Compton scattering |
| High-energy phenomena: |
| Pair production |
| Photodisintegration |
| Photofission |
Photodisintegration (also calledphototransmutation, or aphotonuclear reaction) is anuclear process in which anatomic nucleus absorbs a high-energygamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The incoming gamma ray effectively knocks one or moreneutrons,protons, or analpha particle out of the nucleus.[1] The reactions are called (γ,n), (γ,p), and (γ,α), respectively.
Photodisintegration isendothermic (energy absorbing) for atomic nuclei lighter thaniron and sometimesexothermic (energy releasing) for atomic nuclei heavier thaniron. Photodisintegration is responsible for thenucleosynthesis of at least some heavy, proton-rich elements via thep-process insupernovae of type Ib, Ic, or II.This causes the iron to further fuse into the heavier elements.[citation needed]
A photon carrying 2.22 MeV or more energy can photodisintegrate an atom ofdeuterium:
James Chadwick andMaurice Goldhaber used this reaction 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,[why?][3] as had been proposed byErnest Rutherford.
Aphoton carrying 1.67 MeV or more energy can photodisintegrate an atom ofberyllium-9 (100% of natural beryllium, its only stable isotope):
Antimony-124 is assembled with beryllium to make laboratoryneutron sources andstartup neutron sources. Antimony-124 (half-life 60.20 days) emits β− and 1.690 MeV gamma rays (also 0.602 MeV and 9 fainter emissions from 0.645 to 2.090 MeV), yielding stable tellurium-124. Gamma rays from antimony-124 split beryllium-9 into two alpha particles and a neutron with an average kinetic energy of 24 keV (a so-calledintermediate neutron in terms of energy):[4][5]
Other isotopes have higher thresholds for photoneutron production, as high as 18.72 MeV, forcarbon-12.[6]
In explosions of very large stars (250 or moresolar masses), photodisintegration is a major factor in thesupernova 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 ablack hole. A portion of mass escapes in the form ofrelativistic jets, which could have "sprayed" the firstmetals into the universe.[7][8]
Terrestrial lightnings produce high-speed electrons that createbursts of gamma-rays asbremsstrahlung. The energy of these rays is sometimes sufficient to start photonuclear reactions resulting in emitted neutrons. One such reaction,14
7N(γ,n)13
7N, is the only natural process other than those induced bycosmic rays in which13
7N is produced on Earth. The unstable isotopes remaining from the reaction may subsequently emit positrons byβ+ decay.[9]
Photofission is a similar but distinct process, in which a nucleus, after absorbing a gamma ray, undergoesnuclear fission (splits into two fragments of nearly equal mass).