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'''Spontaneous fission''' (SF) is a form of [[radioactive decay]] that is found only in very heavy [[chemical element]]s. The [[nuclear binding energy]] of the elements reaches its maximum at an [[atomic mass number]] greater than about 58 [[atomic mass unit]]s (u), spontaneous breakdown into smaller nuclei and a few isolated [[nuclear particle]]s becomes possible at heavier [[atomic mass unit]]s (amu).
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Constraints in forming the daughter fission-product nuclei, spontaneous fission into known [[nuclide]]s becomes ''theoretically'' possible (that is, energetically possible) for some [[atomic nuclei]] with [[atomic mass]]es greater than 92 amu, with the [[probability]] of spontaneous fission increasing as the atomic mass number increases above this value.
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The lightest natural nuclides that are hypothetically subject to spontaneous fission are niobium-93 and molybdenum-94 (elements #41 and #42, respectively). Spontaneous fission has never been observed in the naturally occurring [[isotope]]s of these elements, however. In practice, these are [[stable isotope]]s.
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Spontaneous fission is feasible over practical observation times only for atomic masses of 232 amu or more. These are elements at least as heavy as [[thorium-232]] – which has a [[half-life]] somewhat longer than the age of the [[universe]]. Thorium-232 is the lightest [[primordial nuclide]] that has left evidence of undergoing spontaneous fission in its minerals.
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The known elements most susceptible to spontaneous fission are the synthetic high-atomic-number [[actinide elements]] with odd atomic numbers: [[mendelevium]] and [[lawrencium]], and also some of the transactinide very-heavy elements, such as [[rutherfordium]].
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For naturally occurring thorium, [[uranium-235]], and [[uranium-238]], spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms, [[alpha decay]] or [[beta decay]] occurs instead. Hence, the spontaneous fission of these isotopes is usually negligible, except in using the exact branching ratios when finding the [[radioactivity]] of a sample of these elements.
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Mathematically, the criterion for whether spontaneous fission can occur in a time short enough to be observed by present methods, is approximately:
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:<math>\hbox{Z}^2/\hbox{A}\ge47.</math><ref>{{cite book
 
  | last = Krane
</ul>
  | first = Kenneth S.
  | title = Introductory Nuclear Physics
  | publisher = [[John Wiley & Sons]]
  | year = 1988
  | pages = 483–484 (Equation 13.3)
  | isbn = 978-0-471-80553-3 }}</ref>
 
where Z is the [[atomic number]] and A is the [[mass number]] (e.g., 235 for U-235).
 
Spontaneous fission gives much the same result as induced [[nuclear fission]]. However, like other forms of radioactive decay, it occurs due to [[quantum tunneling]], without the atom having been struck by a neutron or other particle as in induced nuclear fission. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a self-sustaining chain reaction. Radioisotopes for which spontaneous fission is not negligible can be used as neutron sources. For example, [[californium]]-252 (half-life 2.645 years, SF branch ratio about 3.1 [[percent]]) can be used for this purpose. The neutrons released can be used to inspect airline luggage for hidden explosives; to gauge the moisture content of soil in [[highway]] and building construction; or to measure the moisture of materials stored in silos, for example.
 
As long as the spontaneous fission gives a negligible reduction of the number of nuclei that can undergo such fission, this process can be approximated closely as a [[Poisson process]]. In this situation, for short time intervals the probability of a spontaneous fission is [[directly proportional]] to the length of time.
 
The spontaneous fission of uranium-238 and uranium-235 does leave trails of damage in the [[crystal structure]] of uranium-containing minerals when the fission fragments recoil through them. These trails, or ''fission tracks'', are the foundation of the [[radiometric dating]] method called [[fission track dating]].
 
==Spontaneous fission rates==
Spontaneous fission rates:<ref>{{cite book
  | last = Shultis
  | first = J. Kenneth
  | coauthors = Richard E. Faw
  | title = [[Fundamentals of Nuclear Science and Engineering]]
  | publisher = [[CRC Press]]
  | year = 2008
  | pages = 141 (table 6.2)
  | isbn = 1-4200-5135-0 }}</ref>
 
{| class="wikitable"
|-
! Nuclide
! Half-life
! Fission prob. per decay
! Neutrons per fission
! Neutrons per gram-second
|-
| {{SimpleNuclide|Link|Uranium|235}}
| 7.04x10<sup>8</sup> years
| 2.0x10<sup>−9</sup>
| 1.86
| 3.0x10<sup>−4</sup>
|-
| {{SimpleNuclide|Link|Uranium|238}}
| 4.47x10<sup>9</sup> years
| 5.4x10<sup>−7</sup>
| 2.07
| 0.0136
|-
| {{SimpleNuclide|Link|Plutonium|239}}
| 2.41x10<sup>4</sup> years
| 4.4x10<sup>−12</sup>
| 2.16
| 0.022
|-
| {{SimpleNuclide|Link|Plutonium|240}}
| 6569 years
| 5.0x10<sup>−8</sup>
| 2.21
| 920
|-
| {{SimpleNuclide|Link|Curium|250}}
| 6900 years
| 0.61
| 3.31
| 1.6x10<sup>10</sup>
|-
| {{SimpleNuclide|Link|Californium|252}}
| 2.638 years
| 3.09x10<sup>−2</sup>
| 3.73
| 2.3x10<sup>12</sup>
|}
 
In practice {{SimpleNuclide|Plutonium|239}} will invariably contain a certain amount of {{SimpleNuclide|Plutonium|240}} due to the tendency of {{SimpleNuclide|Plutonium|239}} to absorb an additional neutron during production. {{SimpleNuclide|Plutonium|240}}'s high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% {{SimpleNuclide|Plutonium|240}}.
 
The rarely used [[gun-type fission weapon|gun-type atomic bomb]] has a [[insertion time|critical insertion time]] of about one millisecond, and the probability of a fission during this time interval should be small.
Therefore only {{SimpleNuclide|Uranium|235}} is suitable. Almost all nuclear bombs use some kind of [[Nuclear weapon design#Implosion type weapon|implosion method]].
 
Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes [[superdeformation]].
 
== History ==
The first nuclear fission process discovered was the fission induced by [[neutron]]s. Because [[cosmic ray]]s produce some neutrons, it was difficult to distinguish between induced and spontaneous fission events. Cosmic rays can be reliably shielded by a thick layer of rock or water. The spontaneous fission was identified in 1940 by [[Soviet Union|Soviet]] physicists [[Georgy Flyorov]] and [[Konstantin Petrzhak]]<ref>{{cite journal |author=G. Scharff-Goldhaber and G. S. Klaiber |year=1946 |month= |title=Spontaneous Emission of Neutrons from Uranium |journal=Phys. Rev. |volume=70 |issue=3–4 |pages=229–229  |doi=10.1103/PhysRev.70.229.2|bibcode = 1946PhRv...70..229S }}</ref><ref>[http://www.nato.int/acad/fellow/94-96/sutyagin/01-02.htm Igor Sutyagin: The role of nuclear weapons and its possible future missions]</ref> by their observations of uranium in the [[Moscow Metro]] [[Dinamo (Moscow Metro)|Dinamo]] station, {{convert|60|m}} deep underground.<ref>[http://n-t.ru/ri/ps/pb092.htm K. Petrzhak: How the spontaneous fission was discovered] (in Russian).</ref>
 
[[Cluster decay]] was shown to be a superasymmetric spontaneous fission process.<ref>{{cite journal |author= [[Dorin N Poenaru]] et al. |year=1984 |title=Spontaneous emission of heavy clusters|journal=Journal of Physics G: Nuclear Physics|volume=10 |pages=L183-L189|DOI=10.1088/0305-4616/10/8/004}}</ref>
 
== Notes ==
<references/>
 
== External links==
* [[Image:Ndslivechart.png]] '''[http://www-nds.iaea.org/livechart The LIVEChart of Nuclides - IAEA ]''' with filter on spontaneous fission decay
{{Nuclear_processes}}
 
<!--Categories-->
[[Category:Nuclear physics]]
[[Category:Radioactivity]]

Latest revision as of 01:05, 11 January 2015

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