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A fusion process that produces nuclei lighter than [[iron-56]] or [[nickel-62]] will generally release energy. These elements have relatively small mass per nucleon and large [[binding energy]] per [[nucleon]]. Fusion of nuclei lighter than these releases energy (an [[exothermic]] process), while fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is [[endothermic]]. The opposite is true for the reverse process, [[nuclear fission]]. This means that the lighter elements, such as hydrogen and [[helium fusion|helium]], are in general more fusible; while the heavier elements, such as [[Uranium 235|uranium]], [[thorium]] and [[Plutonium 239|plutonium]], are more fissionable. The extreme [[astrophysics|astrophysical]] event of a [[supernova]] can produce enough energy to fuse nuclei into elements heavier than iron. | A fusion process that produces nuclei lighter than [[iron-56]] or [[nickel-62]] will generally release energy. These elements have relatively small mass per nucleon and large [[binding energy]] per [[nucleon]]. Fusion of nuclei lighter than these releases energy (an [[exothermic]] process), while fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is [[endothermic]]. The opposite is true for the reverse process, [[nuclear fission]]. This means that the lighter elements, such as hydrogen and [[helium fusion|helium]], are in general more fusible; while the heavier elements, such as [[Uranium 235|uranium]], [[thorium]] and [[Plutonium 239|plutonium]], are more fissionable. The extreme [[astrophysics|astrophysical]] event of a [[supernova]] can produce enough energy to fuse nuclei into elements heavier than iron. | ||
In 1920, [[Arthur Eddington]] suggested hydrogen-helium fusion could be the primary source of stellar energy. [[Quantum tunneling]] was discovered by [[Friedrich Hund]] in 1929, and shortly afterwards [[Robert d'Escourt Atkinson|Robert Atkinson]] and [[Fritz Houtermans]] used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in artificial [[nuclear transmutation]] by [[Patrick Blackett]], laboratory fusion of [[Isotopes of hydrogen|hydrogen isotopes]] was accomplished by [[Mark Oliphant]] in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars was worked out by [[Hans Bethe]]. Research into fusion for military purposes began in the early 1940s as part of the [[Manhattan Project]]. Self-sustaining nuclear fusion was first carried out on 1 November 1952, in the [[Ivy Mike]] [[hydrogen bomb|hydrogen (thermonuclear) bomb]] test. | |||
Research into developing controlled fusion inside [[fusion reactors]] has been ongoing since the 1940s, but the technology is still in its development phase. | |||
==Process== | |||
[[File:Deuterium-tritium fusion.svg|thumb|Fusion of [[deuterium]] with [[tritium]] creating [[helium-4]], freeing a [[neutron]], and releasing 17.59 [[Electronvolt|MeV]] as kinetic energy of the products while a corresponding amount of mass disappears, in agreement with ''kinetic E'' = ∆''mc''<sup>2</sup>, where ''Δ''m is the decrease in the total rest mass of particles.<ref name=Shultis> | |||
{{cite book | |||
|author1=Shultis, J.K. |author2=Faw, R.E. | |||
|name-list-style=amp |year=2002 | |||
|title=Fundamentals of nuclear science and engineering | |||
|url=https://books.google.com/books?id=SO4Lmw8XoEMC&pg=PA151 | |||
|page=151 | |||
|publisher=[[CRC Press]] | |||
|isbn=978-0-8247-0834-4 | |||
}}</ref>]] | |||
The release of energy with the fusion of light elements is due to the interplay of two opposing forces: the [[nuclear force]], which combines together protons and neutrons, and the [[Coulomb force]], which causes protons to repel each other. Protons are positively charged and repel each other by the Coulomb force, but they can nonetheless stick together, demonstrating the existence of another, short-range, force referred to as nuclear attraction.<ref>[http://www.ck12.org/flexbook/chapter/1903 Physics Flexbook] {{webarchive|url=https://web.archive.org/web/20111228011150/http://www.ck12.org/flexbook/chapter/1903 |date=28 December 2011 }}. Ck12.org. Retrieved 19 December 2012.</ref> Light nuclei (or nuclei smaller than iron and nickel) are sufficiently small and proton-poor allowing the nuclear force to overcome repulsion. This is because the nucleus is sufficiently small that all nucleons feel the short-range attractive force at least as strongly as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles. [[iron peak|For larger nuclei]], however, no energy is released, since the nuclear force is short-range and cannot continue to act across longer nuclear length scales. Thus, energy is not released with the fusion of such nuclei; instead, energy is required as input for such processes. | |||
==References== | ==References== | ||