Editing Nuclear physics (section)

===Nuclear fission===
[[Nuclear fission]] is the reverse process to fusion. For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones.

The process of [[alpha decay]] is in essence a special type of spontaneous [[nuclear fission]]. It is a highly asymmetrical fission because the four particles which make up the alpha particle are especially tightly bound to each other, making production of this nucleus in fission particularly likely.

From several of the heaviest nuclei whose fission produces free neutrons, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a [[chain reaction]]. Chain reactions were known in chemistry before physics, and in fact many familiar processes like fires and chemical explosions are chemical chain reactions. The fission or [[Nuclear chain reaction|"nuclear" chain-reaction]], using fission-produced neutrons, is the source of energy for [[nuclear power]] plants and fission-type nuclear bombs, such as those detonated in [[Hiroshima]] and [[Nagasaki, Nagasaki|Nagasaki]], Japan, at the end of [[World War II]]. Heavy nuclei such as [[uranium]] and [[thorium]] may also undergo [[spontaneous fission]], but they are much more likely to undergo decay by alpha decay.

For a neutron-initiated chain reaction to occur, there must be a [[critical mass]] of the relevant isotope present in a certain space under certain conditions. The conditions for the smallest critical mass require the conservation of the emitted neutrons and also their slowing or [[neutron moderator|moderation]] so that there is a greater [[Neutron cross section|cross-section]] or probability of them initiating another fission. In two regions of [[Oklo]], Gabon, Africa, [[natural nuclear fission reactor]]s were active over 1.5 billion years ago.<ref>{{cite journal |last=Meshik |first=A. P. |date=November 2005 |title=The Workings of an Ancient Nuclear Reactor |journal=Scientific American |volume=293 |issue=5 |pages=82–91 |url=http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor |access-date=2014-01-04 |doi=10.1038/scientificamerican1105-82 |pmid=16318030 |bibcode=2005SciAm.293e..82M |archive-date=2009-02-27 |archive-url=https://web.archive.org/web/20090227134554/http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor |url-status=live |url-access=subscription }}</ref> Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the Earth's core results from radioactive decay. However, it is not known if any of this results from fission chain reactions.<ref>{{cite web |last1=Biello |first1=David |title=Nuclear Fission Confirmed as Source of More than Half of Earth's Heat |url=https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/ |website=Scientific American |access-date=25 January 2023 |date=July 18, 2011 |archive-date=25 January 2023 |archive-url=https://web.archive.org/web/20230125171628/https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/ |url-status=live }}</ref>